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Ferromagnetic resonance studies of DC magnetron sputtered CO-CR films Ma, Changlin 1987

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FERROMAGNETIC RESONANCE OF DC MAGNETRON SPUTTERED  STUDIES CO-CR  FILMS  by CHANGLIN  MA  B.SC, 1984, Peking University A THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR  THE DEGREE OF  MASTER OF SCIENCE  in THE FACULTY  OF GRADUATE  STUDIES  Department of Physics  We  accept this thesis as conforming to the required standard  THE UNIVERSITY OF BRITISH COLUMBIA July 1987  ® ChangLin Ma, 1987  OF  In  presenting  degree  this  at the  thesis  in  partial fulfilment  of  University of  British Columbia,  I agree  freely available for reference copying  of  department publication  this or of  and study.  this  his  or  her  representatives.  Department of  p R ^  I  C  The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date  DE-6(3/81)  that the  may be It  thesis for financial gain shall not  permission.  requirements  I further agree  thesis for scholarly purposes by  the  for  an  advanced  Library shall make it  that permission  for extensive  granted  head  is  by the  understood be  that  allowed without  of  my  copying  or  my written  ABSTRACT The  X-band  resonance  FMR Fields  has been of  DC  temperatures. This angular  employed sputtered  to investigate the angular Co-Cr  2  with  different  substrate  variation has been Fitted with the classical uniaxial  anisotropy crystal model and yields the values 4K =-0.8-0.8 KOe  Films  variation of  of 2K /M — 4jrM= — 4  and g-factor = 2.3~2.8. The FMR  1  7  measurments of the first  anisotropy constant are quite different from the counterparts measured with This discrepancy is interpreted as a result of the formation of two phases. With this simple anisotropy is explained will  improve  VSM.  ferromagnetic  model, the substrate temperature dependence of First  and it is predicted that a lower substrate temperature  the Co-Cr  magnetic recording  KOe,  Films  devices. The  for their angular  discussed.  iii  potential application in perpendicular dependence  of the FMR  linewidth is  T A B L E OF  CONTENTS  Abstract  iii  List of Figures  vi  Acknowlegement Chapter  •  •  1. INTRODUCTION 1.1. General Introduction 1.2. The Motivation to Choose Cobalt-Chromium Films 1.3. Thesis Outline  •  v  i " 1 1 3 4  Chapter 2. PREVIOUS INVESTIGATION ON Co-Cr FILMS 2.1. Preparation Techniques of Co-Cr Films 2.2. Structure of Co-Cr Films 2.2.1. Texture Structure and Morphology 2.2.2. Phase Segregation and Grain Boundaries 2.2.3. Initial Layer and Substrate Material 2.3. Magnetic Properties 2.3.1. Magnetization 2.3.2. Anisotropy 2.4. Effects of Sputtering Parameters 2.4.1. Argon Gas Pressure 2.4.2. Substrate Temperature 2.5. DC Magnetron Sputtered Films 2.6. Ferromagnetic Resonance and Our Motivation 2.6.1. FMR Measurements 2.6.2. Our Motivation  6 6 7 7 9 11 13 13 14 15 16 16 17 17 18 19  Chapter 3. SAMPLES AND EXPERIMENTAL TECHNIQUES 3.1. The Preparation of the Samples 3.2. The Properties of the Samples 3.3. General Description of the Apparatus 3.4. Experimental Procedures  20 20 20 21 25  Chapter 4. FERROMAGNETIC RESONANCE THEORY 4.1. Justification of Classical Resonance Theory 4.2. Propagation of Electromagnetic Wave in a Metal 4.3. Theoretical Formula For the Resonance Field 4.4. FMR Width of Conducting Single Crystals 4.5. FMR Width of Polycrystals Chapter 5. RESULTS AND DISCUSSION 5.1. Numerical Analysis of Angular Dependence Fields 5.2. Linewidth and Lineshape 5.3. Domain Wall 5.4. The Interpretation of Other Resonance Peaks iv  27 27 Ferromagnetic 28 31 36 37  of  40 Resonance 40 45 53 55  5.5. Discussion 5.5.1. "Transition Layer" 5.5.2. A Phase Segregation Model 5.5.3. Substrate Temperature Dependence 5.5.4. Comparsion With Other Studies  57 58 59 62 63  Chapter 6. Conclusions and Suggestions  64  Bibliography  65  v  LIST OF FIGURES  Fig3.1 A typical T =199°C  M~H  curve. This  curve  corresponds  to  the  sample  with 22  measured  with 22  S  Fig3.2 VSM  The  substrate temperature  dependence  of the  coercity  Fig3.3 The substrate temperature measured with VSM  dependence  of the  remanent  Fig3.4 The substrate temperature measured with VSM  magnetization 23  dependence of the effective anisotropy constant 23  Fig3.5 The block diagram of the X-band ESR  spectrometer  24  Fig4.1 Orientation of the dc magnetic field H and of the magnetization M with respect to the coordinate system used in the calculations 33 Fig5.1a The FMR spectrum of the sample with narrow resonance is due to the calibration sample  T =157°C  at 0 J J = O°.  The 41  Fig5.1b The FMR spectrum of the sample with narrow resonance is due to the calibration sample  T = 217°C  at 0 = O ° .  The 41  at 0 = 9O°.  The 42  S  S  Fig5.1c The FMR spectrum of the sample with T = 217°C narrow resonance is due to the calibration sample S  H  H  Fig5.2 The angular dependence of resonance field for the sample with T =217°C. The solid line represents the fitted curve with 11! = — 4.5 KOe H = —0.05 KOe, and g=2.36 43 S  2  Fig5.3 The substrate temperature constant measured with FMR  dependence of the  first  effective anisotropy 46  Fig5.4 The substrate temperature dependence of the g-factor measured with Fig5.5 The substrate temperature measured with FMR Fig5.6 A with  typical unsymmetrical  dependence of the second  FMR  FMR. 46  anisotropy constant 47  spectrum. This corresponds to the sample  vi  T<, = 1 9 2 ° C , measured calibration sample  at  0JJ=84°.  The  narrow  resonance  is due  to the 49  Fig5.7 Measured and .calculated absorption lines for ha H<3 ^^x 2O2 2 ( M 9 2 ) measured at 15 967 Mc. The theoretical curve is valid for g=2, 2K /M=-3800 Oe, 2K /M=-420 Oe, 2K = 120 Oe, and K = 0 Oe 50 Y  2  2  1  2  4  3  Fig5.8 The measured and calculated angular dependence of peak-peak linewidth for the sample with T = 232°C. The solid curve is valid for 6H =0.0 KOe, 6H.j=0.0 KOe, and 60 =3° 51 S  o  H  Fig5.9 The substrate temperature measured with FMR  dependence  Fig5.10 Magnetization process  of the c-axis  dispersion  angles 52 54  Fig5.11 The influence of domain structure on resonance. a- the alternating magnetic field h is perpendicular to the static magnetization and the boundary layers. b- the alternating magnetic field h is perpendicular to the static magnetization and parallel to the boundary layer 55 Fig5.12a The irreversible resonance due to the irreversible domain wall movement. The arrows indicate the direction of dc field scanning. The narrow resonance is due to the calibration sample 56 Fig5.12b The reversible resonance after the complete saturation. The arrows indicate the direction of dc field scanning. The narrow resonance is due to the calibration sample 56  vii  A C K N OWLEGEMENT  This without  project was  whose  completed.  Special  guided and  assistance  and  supported by  patience this  Prof. Charles Schwerdtfeger  thesis  would  never  have  been  -  J  thanks  are given to Mr.  supplied the samples and the VSM  Z. Li and  Dr. R.  R.  Parsons  diagrams generously.  The research was supported through a National Research Council grant.  viii  who  CHAPTER 1. INTRODUCTION  1.1. GENERAL  INTRODUCTION  Magnetic materials have figured heavily in the spectacular growth of the . electronic  industry.  information  in  The  reason  is that  computers—ranging  from  the preeminent personal  method  computers  for storing to  large  mainframes—is magnetic recording. In one form of the technology, a rigid 14-inch aluminum disk coated with iron oxide is spun about its axis at 3,000 revolutions per minute. Such a rate of spin corresponds to a speed of more than 100 miles per hour at the edge of the disk. As the disk rotates, a ring-type "head" with which to "read" and "write" data is brought near it. The head consists of a coil of wire wrapped around a magnetic core, which is typically nickel-iron alloy.  By  passing an electric current through the head coil, one can record data  on the disk. The current generates a magnetic field in the coil, which magnetizes a particular area  of iron oxide on the disk parallel to the disk surface. That  area retains its magnetization and so can "remember" information. The process by which data are encoded on the disk is known as the write cycle. Information on the disk is read out with the same head by reversing the procedure. As the head moves over the disk, magnetized regions on the disk induce a current in the coil. By measuring the current as a function of time, the stored information is retrieved.  The  system described  above is dominant today and is called longitudinal  1  INTRODUCTION / 2 magnetic recording in the sense that the magnetization  of the stored information  is parallel to the recording medium surface.  Even though the processes  of reading and writing are based on simple  principles, attempts to pack more information onto disks have proved somewhat problematical with  the present  system. It has been demonstrated  high density recording of the present system, demagnetization only decreases vector  the remanent magnetization,  to establish  decrease  a circular  Even though it is understood the  formation  of this  in the medium not  mode, resulting  in a significant  signals (Iwasaki and Takemura,  1975).  that the merit of using a thin film is to prevent  circular  magnetization,  there  are a  few  unavoidable  obstacles if a future higher density recording system is to be pursued in same manner. The obstacles all stem magnetization  mode recorded  the  but also rotates the magnetization  magnetization  of the strength of reproduced  that, in  from  the property  the  of the longitudinal  by the ring-type head, that is, the demagnetizing  field in the mode increases and approaches a maximum value of 4TTM , as the s  recording density increases.  Iwasaki et a\{Iwasaki and Ouchi, 1978) first published their work in 1978, describing the interesting magnetic properties of cobalt-chromium films and their potential use as media for perpendicular magnetic  system, in which  recording. The proposed perpendicular  the magnetization  is perpendicular  to the medium  surface, has the unique property that the demagnetizing field basically approaches zero  in higher  Nakamura,  recording  densities.  It was  also  1977) that a high saturation magnetization  pointed  out (Iwasaki and  and a high coercive force  INTRODUCTION / 3 as well as perpendicular anisotropy of the medium are necessary to obtain high output  voltage  and high  recording resolution.  Furthermore,  mechanical and  chemical stability and high productivity are desired. These properties being taken into account, the cobalt-chromium  1.2.  THE  At  MOTIVATION  room  TO  CHOOSE  temperature,  close-packed lattice  film is the best candidate.  (hep) (See,  a  COBALT-CHROMIUM  cobalt  single  crystal  for example, Allibert  cobalt has a large magnetocrystalline uniaxial  FILMS  has  et al.,  an  1978).  anisotropy energy  hexagonal In addition  (Paige et al.,  1984), and the easy direction is along the c-axis. To meet the requirement for perpendicular magnetic recording, the film must have an anisotropy energy which surpasses  the demagnetization  unfortunately,  the  demagnetation  energy  anisotropy  energy energy  2jrMg (12.6X10* g  2  s  For pure  #(4X10'  single  erg/cc)  crystal  is less  cobalt,  than  while keeping the c-axis oriented perpendicular to the  film surface. When another nonmagnetic metal is added, the magnetization will  decrease. Roughly  the  erg/cc). Therefore it is necessary to add  2  other metals to reduce M  2jrM .  speaking, the anisotropy energy  is proportional  M  s  to M  g  while the demagnetization energy 27rM is proportional to Mg as long as the film 2  2  g  has at least a cluster structure with the c-axis along the normal to the film. As a result, the anisotropy energy will eventually exceed the demagnetization energy when the magnetization is sufficiently small.  There are two obvious reasons to choose chromium as the added metal. One  is that the Co-Cr alloy has a relatively stable hep phase at a low content  INTRODUCTION / 4 of Cr. Another one is that the saturation magnetization is expected to decrease dramatically when a small amount of Cr is added, since Co is antiferromagnetic at room temperature. Other metals such as Ti, V, Mo, Rh, Pd and W been  investigated  experimentally (Kobayashi and Ishida,  1981;  Iwasaki  have et  al.,  1980), but it was found out that Cr was the most promising candidate.  1.3.  THESIS  Chapter  OUTLINE  2 generally reviews the recent development  in the Co-Cr films  including the preparation techniques, the structure and the magnetic  properties.  Also a brief review over the effect of sputtering parameters, particularly of the substrate temperature, is presented.  Chapter  3  contains the properties  of our samples  as well  as the  experimental procedure. A general description of the ESR apparatus is included.  Chapter  4 gives an account  of detailed  ferromagnetic resonance theory  with first and second anisotropj' constants in uniaxial crystals. Special attention is paid to the polycrystallinity of the samples. The linewidth formula in FMR due to the spatial dispersion of the crystallites is also discussed.  Chapter magnetic  5  describes the procedure  properties are derived. A new  interpret the discrepancy between FMR linewidth is also discussed.  of numerical  fitting  and how the  phase segregaion model is proposed to and VSM. The angular dependence of  INTRODUCTION / 5 Chapter  6 summarizes  investigations are proposed.  the main  conclusions  of this  work and further  CHAPTER 2. PREVIOUS INVESTIGATION ON CO-CR FILMS  After the pioneering work of Iwasaki  et al. in 1978, an ever increasing  number of papers are appearing on this topic. However it is not the intention to review  this topic comprehensively  here. Therefore only pertinent works will be  discussed.  2.1. PREPARATION  Almost prepared vacuum  TECHNIQUES  OF CO-CR  all of the Co-Cr films reported  FILMS  in the literature  have been  by sputtering techniques. Other methods have been attempted, such as evaporation. Sugita et al(1981)  have pointed  out that evaporation in  vacuum can be used for very high rate depositions. However these authors have used a single source of Co-Cr alloy for vacuum deposition which indeed was not an  ideal  situation.  Co  and  Cr have different  evaporation  temperatures  and  evaporation rates, resulting in a deposit of different composition than that of the original alloy target, and even worse inhomogeneities this  disadvantage,  Krishnan  et al.(1985)  employed  could result. To overcome an  two  E-beams  vacuum  co-evaporation technique to prepare the films, which yielded the remarkable result that the crystalline orientation in the initial deposited layer in the co-evaporated Co-Cr films was the same as in the bulk of the film. However, since the two E-beams  vacuum  co-evaporation  method  is not sufficiently  developed  and the  avaibable films were sputtered, this thesis will concentrate on sputtering methods.  RF  sputtering has been widely used in the preparation of Co-Cr films,  6  PREVIOUS INVESTIGATION ON  Co-Cr FILMS / 7  since this method is suitable to prepare films of a high melting point alloy such as Co-Cr, and is superior to the other methods for the adhesion of the deposited magnetic  layer to the  substrate, and  important  factor is that Cr  sputtering process. It has large perpendicular  has  in addition it is reproducible. Another  the same sputtering yield  been found  that the RF  magnetic anisotropy, a  high  as Co  in the  sputtered Co-Cr film has  coercivity, and  RF a  other favorable  properties for high density magnetic recording. In the rest of this chapter the samples are assumed to be RF  In the past few and  sputtered unless otherwise indicated.  years, Hoffmann et al.(1985), Ouchi and  more recently Ravipati et al.(1986) and  deposited by high rate DC  Iwasaki(1985),  Li et al.(1986) have studied films  Magnetron sputtering. This will be discussed in more  detail later.  2.2.  STRUCTURE  OF  CO-CR  FILMS  2.2.1. Texture Structure and Morphology  The  film  microstructure  properties:  the  crystallographic(preferred)  morphology(crystal determined  by  Microscopy(TEM), SEM,  TEM  crystals. The  and  size  and  Scanning X-ray  is  usually  shape). The Electron  diffraction  described  orientation (or  its two texture)  structure properties are  Microscopy (SEM), and  by  rocking  curve  important and  the  experimentally  Transmission  Electron  methods. For  example,  X-ray are used to determine the morphology properties of the  rocking curve represents the anglular distribution of the intensity of  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 8 X-rays diffracted from a certain plane of the crystal, and consequently, expresses the axis dispersion corresponding to that plane. In all studies LB  5 0  of Co-Cr Films,  has been deFmed as the half angle width of the rocking curve: the larger  the LB s o, the broader the angular dispersion. Thus the rocking curve method is mainly devoted to determining the preferred orientation of crystal.  The  microstructure  was First reported  by Iwasaki et al.(1979).  paper, and subsequently most of all other papers, only structure has been identified by electron diffraction (002) peaks. The LB  i0  most of all sputtered exception  In that  hexagonal close-packed  and X-ray  diffraction  with  are usually a few degrees. So it is safe to conclude that Co-Cr Films  is the reported  exhibit only  hep crystallization. A  possible  appearance of the fee phase due to the presence of  mtrogen(Coughlin et al, 1982) in the RF sputtering chamber.  The two  morphology of Co-Cr Films is not clear at present time. There are  contrasting points of view about this. Iwasaki et al.(1979) determined that  the films had a columnar structure from a cross-sectional SEM view. Since then, a columnar structure viewed by a cross-sectional SEM has been accepted as one of the basic properties for perpendicular for a large part  anisotropic Films and this has accounted  of literature on Co-Cr Films. (Iwasaki et al,  Kabayadshi and Ishida,  1981; Honda  et al., 1983; Haines,  should be noted that electroplated Co Films exhibited  clear  Cavcllotti, 1982).  columnar  structure  1984b). In addition it  with perpendicular  in a cross-sectional  1980a; 1980b;  SEM  anisotropy  also  view(Chen and  On  PREVIOUS INVESTIGATION ON  Co-Cr FILMS / 9  the other hand, in studies of Co-Cr films deposited  by opposing target  cathode sputteringfKadokura and on  the  cross-sectional  magnetic recording charactristics. A al.,  1983;  SEM  SEM  Naoe, 1982), round grain stucture was  of  the  film  which  whereas films with a  is suitable  columnar stucture  conically shaped grain crystal also has  Hwang et al.,  for  observed  perpendicular  showed  undesirable  been observedfLocMer et  1986). However a cross-sectional structure viewed by a  is not always an indication of film structure, that is, SEM  is believed only  to be able to give evidence of intergranular fracture in films. In another words, in  the  case of the  fracture  in the  existence  film  with  of both transgranular  hep  phase only,  columnar structure, which would be inherent insensitivity. This was also was  strongly  that almost all the recording  are  supported by  missed by  is capable  intergranular  of viewing  cross-sectional SEM  the result reported  by  due  to its  be  concluded  properties of perpendicular  magnetic  believed to consist of columnar structure whether they can  cross section SEM Honda et al,  can  the  Sagoi et al.(1984), which  tensile test measurments. It can  samples with favorable  viewed in cross section SEM  1982:  TEM  fracture and  been  or not. Another point is that columnar structure in  not rule out poor c-axis oriention. (Kadokura  and Naoe,  1983).  2.2.2. Phase Segregation and Grain Boundaries  Experimental results regarding the presence of various phases in the films, which  are  summarized  thought to be as  responsible  follows. (1) The  M  g  for the and  the  high T  Q  recording  density,  can  be  are higher than those in the  bulk alloy with the same average composition as the films. (Fisher et al.,  1984)  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 10 (2) The M substrate  g  of the film  changes depending et  temperaturefS/m'te  al.,  drastically following annealingCMoeda phase  segregation  1984).  on preparation (3) The M  g  conditions such as  of the film  changes  et al., 1982). This behavior suggests that a  at temperatures  below  400°C.  (4) The  films  exhibit  a  particle-like rotation magnetization reversal mechanism in most samples. (Iwasaki. 1980a; Wuori and Judy,  1984)  Noting from the Co-Cr phase diagram(Mo/fatt, 1978) that an intermetallic compound  Co Cr  can coexist  3  thermodynamic equilibrium that  the  film  could  precipitation around  with  an e —Co(hcp) phase  in the bulk alloy, Chen  have  a  two  phase  the grain boundary.  under  a condition of  and Charlen(1983) reported  segregation  with  possible  The substrate temperature  Co Cr 3  was around  600°C when this phenomenon appeared in the TEM micrographs.  Another paper was published by Jhingan(1986) with a chemical analysis from a nanoprobe. For the as-deposited samples with 95 °C substrate temperature, three  types of grain  lower  Co/Cr ratio  boundary  segregation were observed: higher Cr/Co ratio,  and non-segregated. However after annealing treatment with  temperatures over 400°C, only the last type was observed. Furthermore, there was  no segregation within the grains at all temperatures.  The  results  from  all these experiments  seem  to suggest  a segregated  microstructure model whereby the ferromagnetic Co-rich region inside a columar particle is surrounded by a nonmagnetic Cr-rich region near the grain and  boundary  the pseudocircular Co-rich particles show a rotational magnetization reversal  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 11 mechanism. This  model  probably  can be safely used  below  400°C  substrate  temperature or annealing below 400°C.  A finer  rather powerful T E M study(Maeda  microstructure  in crystallites  than  and Asahi,  1987) has suggested a  the grains. This  study  revealed by  selective wet etching shows a definite striped pattern in each crystallite. These stripes,  identified  ferromagnetic  by  SEM,  were  caused  by  dissolution  of the Co-rich  regions.  2.2.3. Initial Layer and Substrate Material  Several recent experiments have shown that the films are often composed of two layers—an initial transition layer deposited on the top of a substrate with the  thickness  10 nm  to 100 nm  in a 1000 nm  structurally oriented bulk layer having  the intended  thick film, followed  by a  magnetic properties. (Haines,  1984b)  The  first  magnetization  Nakamre(1977) exhibited  hysteresis  an abrupt  loops  presented  by  range or anomalous jump  Iwasaki and  in the in-plane  loops. This effect was attributed to a fine grained transition layer separating the substrate and the columnar structure(7u;asa£i, 1980a; Ouchi and Iwasaki, 1982; Byun, 1985).  A  series of reports have been published  1986; Hwang et al,  recently. (Mitchell et al., 1985;  1986) A second resonance peak in FMR was identified as  PREVIOUS INVESTIGATION ON the transition layer with the magnetocrystalline result  was  composed  correllated to of  small,  TEM  and  randomly  oriented  temperature of 365°C after 45 well  oriented  bulk  the  anisotropy  transition grains.  h effectively turned  film. Application of a  averaged to zero.  layer  The  Co-Cr FILMS / 12  was  vacuum  thought  The  to  annealing  be  at  a  the transition layer into the  magnetic  field  during  the  annealing  process improved this result, particularly when the as-deposited film only consisted of a transition layer.  Such a initial transition layer is not desirable in magnetic recording  so  that the control of this layer could optimize the performance of this media.  The  nucleation of Co-Cr crystals in various underlayers  which are formed  on a substrate prior to Co-Cr alloy deposition has been studied. (Futomoto, An  amorphous  Ge  was  the  best  in  including  hep  structure metals such as  explained  by  a  new  growth  a  large  Ti and  number Cr. The  mechanism. However it was  of  sublayer  reason  1985)  materials  for this  noted by  was  Lodder et  al.(1983) that the electrical charge in the electrically insulating substrate from the sputter plasma should  be taken into consideration and  of the  a  1981;  substrate Leu  et al.,  had  strong  influence on  the  furthermore the clearness  crystal  habitfCoughlin  et  al.,  1985). Hence it is most likely that substrate materials are not  very critical as long as they generally satisfy the sputtering conditions.  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 13 2.3.  MAGNETIC  PROPERTIES  A large positive effective anisotropy and a high perpendicular coercive field are the favourable properties for perpendicular high density magnetic recording. Magnetic  characterization commonly involves Vibration Sample Magnetometer(VSM)  and Torque(balance) Magnetometer(TM). Each of these instruments can be used to determine both the magnetization and the anisotropy. However, VSM is preferred when finding  the magnetization  while TM  is preferred when  determining the  anisotropy.  2.3.1. Magnetization  Basically,  the magnetization  loop  of ferromagnetic  films  like  Co-Cr is  rectangular along the easy axis. Almost all saturation magnetizations are derived from the VSM measured magnetization loops. The saturation magnetization of the films decreases almost linearly with an increase of the Cr content and agrees with that of bulk Co-Cv(Bozorth, 1951) when the Cr content is less than 15 at.%. Between 15% and 30% atomic content, discrepancies in the magnetization begin to show between the bulk alloy and the films , and also among different authors(7u>asa&i and Ouchi,  1978;  Kobayashi  and Ishida,  1981;  Bolzoni  et  al.,  1983; Fisher et al., 1984). This latter inconsistency could be explained by the different phase segregations in the samples deposited under different conditions. (Hains,  1984a) These discrepancies are not large and almost  all the samples  have saturation magnetizations around 400 emu/cc at about 20 at.% Cr.  PREVIOUS INVESTIGATION ON The  microstructure  magnetization  but rather magnetization  considerably sputtering  inhomogeneities  from  2  KOe  to  400  conditions, reasonably  do  not  process. The Oe.  large  Through  Co-Cr FILMS / 14  influence  the  saturation  known coercive fields vary careful  adjustment  coercive fields can  be  of  the  achieved.  The  optimum parameters of sputtering will be discussed in the section 2.4.  2.3.2. Anisotropy  There  are  two  kinds  magnetocrystalline  anisotropy,  magnetocrystalline  anisotropy  of  another  is  is intrinsic  spin-orbit-lattice interaction. The effect. These two  anisotropy  in  shape to  the  the  Co-Cr  anisotropy. films.  This  shape anisotropy comes from  films: Of  one  these,  is due  is the  to  the  the demagnetizing  anisotropies cannot be separated in all measurements owing to  the fact that they possess the same angular dependence so that sometimes an effective anisotropy is used to represent their combination. may  be  measured  by  torque  The  magnetometer, magnetization  anisotropy energy curve  or  magnetic  resonance, although the common method in this case is torque magnetometer.  The hep uniaxial anisotropy energy  E = K  + K  0  in  which  6  is the  anisotropy is  -2ITM  angle sin e 2  2  E = K  x  0  + [K  sin 6 + K 2  2  4  between the M  vector and  the  c-axis. The  shape  so that - 2nM ) sin 6 + K sin 0 + higher order 2  x  (2.1)  sin 6 + higher order  2  4  2  (2.1a)  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 15 Kj-ZrrM  2  is called  accurately,  K =K +K -2JTM U  The K  x  and K  and  the effective  1  2  2  anisotropy  constant!  in most  More  should be used.  anisotropy constants depend strongly on the sputtering conditions. The 2  of pure bulk cobalt have been investigated extensively (Paige, 1984  references therein). At room temperature, K  is about 5.0X10' emu/cc, K ,  x  2  1.0X10' emu/cc. The effective anisotropy constant is negative The  cases.  experiments confirmed that the effective anisotropy  for pure Co films.  become  positive within  the  range of Cr 15-30 at.%(Iwasaki, 1978; Fisher, 1984). Experimentally the K  are  distributed among  1.1 — 1.9X10*  evgs/cc(Iwasaki, 1978; Honda  et al., 1983;  Bolzoni et al., 1983; Fisher et al., 1984; Sagoi, 1984) and the only K  were  obtained  by Fisher  et al.(1984) with  (—0.05 — 0.6X10' ergs/cc). Both values  a careful torque  x  2  curve  values  analysis  are dramatically different from the pure  bulk Co values, and the reason for that remains undetermined at present.  2.4. EFFECTS  OF SPUTTERING  PARAMETERS  It is well known that the magnetic properties in RF-sputtered Co-Cr films vary considerably depending on the substrate material, substrate temperature and argon  gas pressure,  nonmagnetic  phase  etc. To prevent from  occuring,  the oxidation  the initial  and nitrogen  background  pressure  induced fee should be  reduced to not less than 10" r(Coughlin et al., 1982; Honda et al., 1984). 7  PREVIOUS INVESTIGATION ON 2.4.1. A r g o n Gas  Co-Cr FILMS / 16  Pressure  Ar pressure has significant effects not only on c-axis orientation, but also on  articles (Honda  mechanical strength for Co-Cr sputtered films. Two  1983;  Sagoi et al,  pressure  1984)  decreasesdowerst  et  al,  presented similar results. They found that as the Ar 1  mr),  the  films  deposited have  superior  c-axis  orientations, the anisotropy constants become large due to the reduction of the demagnetizing fields caused by the change of the magnetic domain structure and, in  addition, the films have larger coercive fields and  strength. Lower Ar pressure was result that 0.5-1 mr  2.4.2. Substrate  The parameter  tested by  a very high mechanical  Niimura and  Naoe(1986) with the  was deemed the best range for a favorable Co-Cr film.  Temperature  most important parameter, but unfortunately  also the most elusive  in the case of sputtering, is the substrate surface temperature. In  RF-sputtering, the surface temperature, i.e.,  the temperature  at which the film  formation takes place, is predominantly determined by three factors which are of same order of magnitude: temperature  produced  temperature  produced  by by  (1) the temperature of the substrate holder, (2) the heat  from  heating from  the the  condensing sputter  atoms  plasma,  and mainly  (3)  the  by  the  electrons. This complex mechanism not only makes it difficult to determine an accurate surface temperature but also implies that this temperature will not be stationary during the sputter run. The surface temperature in RF-sputtering was determined by measuring  the hcp-fcc transition temperature  and  comparing  this  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 17 with bulk data for Co-Cr. The temperature was found to be 550°C higher than the measured substrate temperature (Iwasaki and Ouchi, 1980b), though such a comparison  is questionable.  Despite  all these  difficulties,  the dependence of  substrate temperature has been examined and found to be ambiguous(Coughlin et al., 1981; Wieldinga and Lodder, 1981). It is not easy to control the surface temperature or even to be able to compare the different results.  2.5. DC MAGNETRON  Although the RF  SPUTTERED  sputtered  FILMS  Co-Cr films are highly satisfactory  for  the  perpendicular recording medium, its deposition rate is usually less than 1 nm per second. The industrial production  needs a higher deposit rate to be viable. DC  Magnetron Sputtering may be introduced to meet such a requirement.  The  general features of these high rate depositions are similar to the RF  sputtering deposition. But the high rate deposition seems to result in a more inhomogeneous film, which in turn has a lower anisotropy  cons tan tfOuc/u and  Iwasaki, 1985). In these films more phase segregation can be identified and the effective anisotropy constant turns negative at a smaller percentage content of Cr than that in RF sputtered films.  2.6. FERROMAGNETIC  RESONANCE  AND OUR  MOTIVATION  PREVIOUS INVESTIGATION ON Co-Cr FILMS / 1 8 2.6.1. FMR Measurements  At the present time, there are only a few ferromagnetic resonance(FMR) reports on Co-Cr Films. VSM and TM are static measurements so that only the volume average magnetic quantities can be retrieved. FMR, on the other hand, is a sensitive dynamic technique which can provide detailed information about the magnetic inhomogeneities and, ultimately, the different regions.  One advantage was shown by Krishnan et a l . ( 1 9 8 5 ) that when the film is too thin to be measured with VSM, FMR still can faithfully be trusted. Other situations have also been successfully handled, such as the detection of the transition  layer (Mitchell et al,  1986)  and the surface  effect(Cofield et al, 1986).  An extensive comparsion between FMR and VSM/TM measurements was made by Mitchell et  al.(1985).  They found that when two layers existed in the  film, there was a discrepancy, while when the film had onty single layer there was agreement.  Resonances explicitlyfCb/je/a  7  at  different  et a.l, 1986).  microwave  frequency  were  compared  The resonance features changed little, except that  the broad resonance observed in the 9 . 8 GHz spectra had a more symmetrical lineshape and a smaller linewidth at 3 5 GHz. This broadened resonance was attributed to the domain wall resonance. As long as the sample was saturated, there seemed excellent agreement between the results of these two frequencies.  PREVIOUS INVESTIGATION ON  Co-Cr FILMS / 19  2.6.2. Our Motivation  This work was Resonance(ESR)  done at X-band(9 GHz)  spectrometer.  Certainly  with a conventional Electron Spin  Q-band(35  GHz)  FMR  has  some  advantages over X-band, among others, Q-band is more sensitive, the analysis is easier owing to the fact the the sample is most likely to be saturated at the resonance fieldsQO KOe). But, on the other hand, we related more to the inhomogeneities  believe that X-band can be  of the samples since the resonances  at relatively low magnetic fields. The  happen  numerical analysis is manageable with the  help of a computer.  Our  objective was  Sputtered Co-Cr films and  to study  some  charateristics  of the  to compare with the results of VSM  films made by Li et al.(1986). The  DC on  Magnetron the same  variation of the magnetic properties with the  change of substrate temperature are especially emphasized. Such a investigation should  be  quite valuable to the  charaterization of these  inhomogeneous films.  Furthermore, there is a lack of information about the angular dependence of the resonance  for  these  Co-Cr  films.  This  angular  dependence  should  allow  determination of the second anisotropy constant, which is an important parameter in magnetization studies.  CHAPTER 3. SAMPLES AND EXPERIMENTAL TECHNIQUES  3.1.  THE PREPARATION  OF THE  SAMPLES  All our samples are courtesy of Prof. R. R. Parsons and Mr. Z. Li in Dept., UBC.  Physics  The samples  were  prepared  with  DC  Planar  Magnetron  Sputtering with floating glass as the substrates.  down  A composition  target CoCr(20 at.%) was used. The chamber was pumped  to less than  10"'  T . Then  3  chamber. The target-substrate distance  mr was  Argon 9.5 cm  gas was while  flushed  into the  the substrates were  biased at -200 — 0V. The sputtering power was driven high (350 W) so that a high deposition rate of 100 nm  per minute was obtained. The substrates were  heated by a halogen lamp and as a result of this a series of samples with substrate temperatures from 150°C to 320°C were achieved.  3.2.  THE PROPERTIES  The  thickness  OF THE  was  SAMPLES  controlled to about  samples were investigated with  a SEM  500  nm.  The  structure of the  and an X-ray spectroscope.  The X-ray  diffraction results indicate clear (001) peaks so that the films consist of equiaxed grains rather than columnars. The diameters of the grains range from 10 nm to 100  nm  and  the grains  seemed  to "stick"  pictures.  20  together  according  to the  SEM  SAMPLES AND The  magnetization  EXPERIMENTAL TECHNIQUES / 21  curves have been characterizated by  typical example is illustrated in Fig3.1.  a VSM,  and  a  The coercive fields and the ratios of the  remanent magnetization to the saturation magnetization are plotted in Fig3.2 and 3.3,  respectively,  constants K area  against the  in Fig3.4  u  substrate temperature.  The  effective anisotropy  were determined by measuring, on the M~H  included between  the  magnetization  curves  for easy  and  graph, the hard  crystal  directions. The interesting thing here is that there are some anomalous behaviors, particularly  of the  anisotropy  constants, at  a  substrate temperature  around  200°C.  3.3.  GENERAL  The  DESCRIPTION  block diagram  of ESR  work is shown in Fig3.5. klystron(Varian connected  VA-273),  The a  to the input arm,  output. The  third arm  OF  THE  APPARATUS  X-band spectrometer which was  microwave bridge utilized  one  way  isolator,  and  a  used in this  a circulator. A flap  reflex  attenuator were  a crystal detector with a slide-screw tuner to the  ended in a T E  1 0 2  resonance  cavity coupled through  an  adjustable iris.  The  klystron was  frequency locked to the resonance cavity by  the reflector voltage with a sensitive  detected  magnetic  field  was  output  from  modulated  attached to the pole caps.  10 KHz the  signal and crystal  at 2.3  KHz  modulating  using the corresponding phase  detector as through  a  an  error  signal.  The  pair of modulation  coil  SAMPLES AND  EXPERIMENTAL TECHNIQUES / 22  400 - - - - in plane —  u  perpendicular  <v  //  200  /  S  ' 1 / / ' 1 / / ' 1/ / •' '/ / 1  ^  S  «s X  f  /  i  0  A J  / 1  / /  V /  1  //>'>'  -200  / A ' '  -400 h i  i  -8  -6  Fig3.1 A typical M ~ H T =199°C.  i  -4  i  -2  ,  0  1  2  1  1.  4  6  Appl Field (KOe) curve. This curve corresponds  I.  .  ._  8 to the sample with  S  O in plane  _ L2 O  A perpendicular  0.8  0.4  0.0 100  150  200 ,  Fig3.2 The substrate temperature VSM.  Ts(°C)  250  300  dependence of the coercity measured with  SAMPLES AND EXPERIMENTAL TECHNIQUES / 2 3  200  o o>  o  •200  -400  200  250  300  350  Ts(°C)  Fig3.4 The substrate temperature dependence of the effective anisotropy constant measured with VSM.  klystron  AFC  bias current  match load  isolator  crystal detector  frequency counter  variable attenuator  3~ scope  variable attenuator  >  way  directional circulator  % *d rw w  variable coupler  > a  w x W JO  chart recorder  lock-in  dc field supply  preamplifier modulat ion coil I  cavity  M H > t-  1  H W O  Fig3.5 The block diagram of the X-band ESR spectrometer. W CO N3 •0-  SAMPLES AND The  pre-amplified output  detected at 2.3 KHz(Ithaco was  from  391A  EXPERIMENTAL TECHNIQUES / 25  the crystal  Dynatrac  detector was  391A  phase  sensitive  Lock-in Amplifier). The  output  then connected to a chart recorder.  The  frequency of the microwaves was  meter (Hewlett-Packard  5245L  Electronic  measured with a digital frequency  Counter  with  Converter). The measurment range is 3 — 12.4 GHz  The  experiments  were  performed  on  plug-in  525 5A  Frequency  with an accuracy 1  a  12"  Magnion  KHz.  Magnet  with  HS-1365B Precision Magnet Supply with the FFC-4 Field Control Regulator. The maximum output current is 65 Amperes which gives about field. The  set accuracy is 0.1  Oe  with the stability  17 KOe  IX10"  5  dc magnetic  after three hours  warm up. The sweep linearity is within 0.1%.  3.4.  EXPERIMENTAL  The  PROCEDURES  spectrometer was  operated at 9.46  low and estimated to be —10  mW.  The time constant of lock-in was  The for  the  CuS0 «6H 0 4  the  2  bottom  amplitudes.  field was of  was  easily  The  cavity  met  to  2.3  was KHz.  usually set at 1.25 msec.  50 Oe/Min. The  without  the  permit  accuracy requirement  calibration  employed as a standard sample and the  microwave power  The field modulation frequency was  scanning rate of DC field was  dc  GHz.  estimation  of  sample.  However,  simultaneously placed on the  relative  resonance  SAMPLES AND EXPERIMENTAL TECHNIQUES / 26 The resonance  cavity  operated in the T E  1 0 2  mode and was made of  brass. The measurements were conducted at room temperature. The samples with the size  1.0X0.5 cm  2  were placed on a rotatable lucite holder, so that the  angular dependence of resonance could be measured.  CHAPTER 4. FERROMAGNETIC RESONANCE THEORY  4.1. JUSTIFICATION  The  OF CLASSICAL  specific  determined  features  primarily  RESONANCE  of resonance  by the fact  THEORY  phenomena  that in these  in ferromagnetics are  materials we deal not with  individual isolated atoms or with the comparatively weakly coupled moments of paramagnetic  systems,  but with  electrons. The exchange  nature  a complicated of this  system  interaction  of strong  causes  magnetic moments of the ions in the ferromagnetic  interacting  the unpaired  crystal  spin  lattice to assume  parallel orientation below the Curie temperature. The exchange interaction creates a substantial total magnetization and consequently a large internal magnetic field, and  this  internal field may alter  the usual paramagnetic  resonance condition  considerably.  A  macroscopic  description can be used  the quantum numbers 10  15  corresponding to the energy  levels  are of the order of  and higher. (See, for instance, Lutinger and Kittel, 1948).  the correspondence the  for ferromagnetic resonance since  problem  J.H.Vleck(1950) interaction  yield  It follows from  principle that classical and quantum-mechanical treatments of identical  results.  to the extent  and the ordinary  that dipolar  This  conclusion  his Hamitonian interaction  improved by  includes  the exchange  as well  anisotropy though minor approximations should be taken.  27  has been  as the crystalline  FERROMAGNETIC RESONANCE THEORY / 28 4.2.  PROPAGATION  OF  FERROMAGNETIC  METAL  The  to be  question  ELECTROMAGNETIC  addressed  microwave cavity as used in ESR it is in an  is that of a  IN  A  ferromagnetic  metal  in a  which interacts with the microwave field while  applied dc magnetic field H .  It is a formidable task to have a  0  complete analytic solution of such  WAVE  a problem, which fundamentally  differs from  the usual situation in that the induced magnetization is not linearly proportional to the applied field . To elucidate the specific features of the phenomenon in its pure form, we  should first consider an idealized problem, namely 1) the damping  processes should not be allowed, and 2) shape  anisotropics.  electromagnetic  In  waves  this  case  in a  the  we  shall neglect the crystalline and the  equations  conductor(Ament  ferromagnetic  include an equation of motion for the magnetization  - ^  7 dt  describing the and  propagation Rador,  of  1955)  M(r)  = -Mxff + ^ M x V M M;  v  (4.1) '  and Maxwell's equations:  1  V x  E=  d - — (H + 4nM)  (4.2) (4.3)  VxH=^-j  the first and the second terms in the right-hand side of eq4.1 the  moment  densities  effective exchange field  of the  force exerted  by  the  are, respectively,  magnetic fields and  the  FERROMAGNETIC RESONANCE THEORY / 29 the latter can be obtained from the expression for the exchange energy, which is proportional to the square of the magnetization gradients.  In microwave  eq4.3, the displacement frequency  current  it is small compared  has been with  neglected  the conduction  since  at the  current in a  metal. When the penetration depth 8 of the electromagnetic field into the metal is large in comparsion with the mean free path I of an electron, then the j is related to the electron field intensity E by Ohm's law(normal skin effect):  (4.5)  J=oE  We shall assume that inside the sample H = H + h,  M  0  here  M  0  microwave  is the induced components  =M +m  saturation magnetization  |m|<<M  0  and  (4.6)  0  \h\ <<H . a  caused  by  Certainly  H  0  and the  the boundary  condition for the fields are the continuity of the tangential componets of e and h:  *± l+=  e±  I-,  h± |+= h±  (4.7)  |-  The boundary condition for m can be obtained from the condition that the total moment of the exchange force should vanish over the entire sample, i.e.,  L  rn x V mdT 2  =  0  sample  (4.8)  Obviously, all of these equations have to be solved simultaneously with Maxwell's equations outside the sample but within the cavity  and under the  FERROMAGNETIC RESONANCE THEORY / 30 corresponding boundary conditions.  Stronger  approximations  solved. To get a flavor  must  be adopted  for this kind  before  the equations  can be  of electromagnetic propagation, a plane  incident microwave and an infinitive cavity are assumed. Let an electromagnetic wave be incident on a plane plate perpendicular to its surface, and the applied field H  parallel  0  to the surface of the plate. If the skin  effect is  neglected  (a=0), then _ M L  1  ~ IETTTZJIZTZ'  (4.9)  = 0  so the resonance condition is: 1  where u q= Ui~iu , e  With  2  ,w.  u angular frequency of microwave.  o*0,  and  therefore  the electromagnetic  absorbed. In this case the resonance field H , 0  u, 2  field  energy is  determined from the maximum of  is equal to .,  ,u).o  1 4wM  where  6 is the classical  exchange  interaction,  the  skin skin  0  4 UTTA  7  depth(with effect  u=l).  causes  Thus, a  as a  resonance  result line  of the shift in  ferromagnetic metals towards weaker fields by  AH. = f  (4.12)  FERROMAGNETIC RESONANCE THEORY / 31 Practically such a rigorous derivation in most cases has never been done and usualty is unnecessary. For our case, the skin effect can be ignored since the  skin effect basically comes from the microwave tangential electric field, which  is almost zero at the middle of the cavity where the samples cobalt at room temperature has a skin depth of about tangential microwave electric field E~E l/a, e  where E  0  1.5MIII  are placed. The at 10 GHz. The  is the maximum electric  field in the cavity. In our case, /-~0.4 cm the dimension of the sample and a—2 cm  the cavity size. Approximately this can give a factor of 5 increase to the  skin depth, which is then about 7.5 um whereas the sample thickness is 0.5 Atm. Both the  theoretical and experimental work have confirmed that a solution without consideration of Maxwell's  equations and the boundary  conditions is justified  as far as just the ferromagnetic resonance frequency is concerned.  4.3. THEORETICAL  FORMULA  FOR THE RESONANCE  FIELD  Suppose we are dealing with a single domain ferromagnetic crystal with the  magnetic  crystalline anisotropy and the shape anisotropy, but free from the  stress. The various interactions can be considered phenomenologicalfy, provided it is assumed that the spins responsible for the ferromagnetism  process with the  frequency CJ, not in the external field H, but rather in a certain effective field H ff e  , whose effect is equivalent to that of an external field. In this case, the  equation of motion of the magnetization vector M becomes  ^ = 7 at Another  method  was independently  M  «  proposed  %  (4J.)  by Smit and Beljers(1955)  and  FERROMAGNETIC RESONANCE THEORY / 32 Suhl(1955),  which is more convenient for our present purpose  and also permits  extensive generalizations. We shall dwell on it in some detail.  The  coordinate system  used  in our calculation of the magnetostatics and  the fields for resonance is shown in Fig4.1.  = M  sin 0 cos <f>  My = M  sin 0 sin <j>  M  cos 0  M  x  z  —M  hence the radial H^f, polar HQ and azimuthal  JJ  = H  M  H  s  x  sin 0 sin <f> +  H  (4.13)  components are  sin 0 sin <j> +  y  cos 0 cos <j> + H cos 0 sin  = H  x  y  /f^ = -H sin ((> + x  H  y  H  z  cos 0  - # sin 0 2  (4.14)  cos <^  and the eq4.1a in our spherical coordinate system assume the form 0 = *iHt,  <f>sm0=-iH  0  (4.16)  where we take into account that M=constant.  In the state of thermodynamic equilibrium, the components HQ and H^ of H fj- vanish, i.e., at the equilibrium position B q, <p g e  e  e  FERROMAGNETIC RESONANCE THEORY / 33  /  \  c axis y  4, Fig4.1 Orientation of the dc magnetic field H and of the magnetization M with respect to the coordinate system used the in the calculations. If a small perturbation exists, such as a microwave field, the orientation of the vector M varies due to the influence of the nonvanishing field componets H  = -Fg/M,  e  Hj, = -F^/M  (4.16)  sin 0  If the deflection from equilibrium is 6${t)  = 9{t) - Oeq,  64>{t) = <f>(t) - <f>  eq  (4.17)  then we may limit ourselves to the linear terms in the expansion of Fg and F^ F = F 60 + F t,6<f>, e  e9  e<  = F^69  + F 6<t> H  (4-18)  where the second derivative of the free energy with respect to the angles FQQ,  FERROMAGNETIC RESONANCE THEORY / 34 F^Q  and  FQQ  are calculated from the equilibrium position. Now, using  eqs4.1b,  4.16, 4.17 and 4.18, the small free oscillations of M about the equilibrium position are obtained -^MsmdegSd  l' M  F^gSe + F 6<j>  =  H  sin0 6<j> =  l  eq  + F ^6<f>  F 6e S9  (. )  e  4  which have the periodic solutions 66,60~exp(iut) if the determinant of  19  eq4.19  vanishes: + V  f}+ - " +* F  w  F  s i n 2  <i  0  =  Mi  ( - °)  0  4  2  whence for the eigen- or resonance- frequency of oscillation we obtain 7 M  sin  0  {F„F» -  (4.21)  eq  The free energy density function appropriate to a crystal with hexagonal structure, up to the second-order term in magnetic anisotropy, is given by the expression sin 0cos{<j>  F = -EM  -{K  y  where  H  + 2K ) sin  the first term  2  2  - <f>) + ^(4TTM ) 2  0 sin2 4> + K  represents  the  2  sin  Zeeman  4  0  sin 0sin  sin  2  4  2  <f>  energy,  <b  (4.22)  the  second the  demagnetization energy, and the last two terms the axial anisotropy energy with  FERROMAGNETIC RESONANCE THEORY / 35 the c-axis parallel to the y axis. anisotropy constants. For  and K  are the first- and second- order  2  the easy magnetization direction is in the x-z  K <0, l  film plane.  Written in the units of M, f — = -H  \ sin 6cos{<f> -<t>) -{-{Hi H  H - 4nM)  + -*-) sin 0sin 2  2  \ <f> + -H  £  £t  sin 0 sin 4  2  4  <j>  £t  (4.23) where  H = 2KJM,  F  1  eq  + 2H  H  sin  2  0  + 2# ) sin 0 cos0 , 2  eq  sin <p  eq  eq4.15  - 4> ) - ((Hi - 4nM)  = cosd {-Hcos((j>  e  From the  H =2K,/M.  1  eg  £  eq  =0 F4  =  sin 6  -H  + H  2  sin  4  s'm(<j) - <f> ) - {{H  eq  0  H  tq  sin  3  eq  - 4nM)  x  sin  + H) 2  2  B  eq  sin <f> cos <f> eq  <f>  eq  =0  (4.24)  since the dc field eq4.24,  eq  is applied in the x-y plane,  H  we obtain the equation for  H s'm(<f> - <[> ) = - {{H H  eq  x  and substituting  6 =vl2 eq  4> :  - 4TTM) -f  # ) cos <£ sin </>, + # 2 sin 2  e9  £  3  <f>  cos <^ (4.25)  + 2H  sin  eq  eg  The resonance fields can be calculated with the aid of eq4.21, (- )  = {H  2  res  cos(<t> - 4>eq) + ({Hi -4nM)  + H)  H  2  cos <f> sin <t> eq  x{H cos(4>H-<f>e )-((Hi-4nM)+H )cos2<f> +2H (3sm  2  Tes  q  2  eq  2  eq  2  3  4>  eq  cos 4>  4> cos 4> ,-sin <j> )} 2  eq  e  4  eq  (4.26)  e  FERROMAGNETIC RESONANCE THEORY / 36 and the g-factors are related to 7 by  <7 = IfiB  where u =2*X1.40  GHzlKOe.  B  4.4. FMR WIDTH OF CONDUCTING  For relaxation  pure  (4.27)  single  processes  crystal,  SINGLE  single  CRYSTALS  domain  ferromagnetic  are due to the spin-spin, spin-lattice  conductors the  and spin-conduction  electrons interactions. These relaxation processes can be taken into consideration formallj' by introducing a damping term in eq4.1a:  —  at  Where  = - 7 M x H - - % M x (M x H)  (4.1c)  M  a is a dimensionless  factor,  and in most  cases  it is quite small.  Repeating the procedure in section4.3 with the addtional damping term, yields out the resonance frequency „2>  M ' s m fl.  1  'eq  and the resonance absorption width  (^) Finite  relaxation  resonance  2  =^ { ^ +^/sin ^}  (a*0) leads to, for small  frequency  u  r e s  (4-28)  2  a, an insignificant  defined by eq4.21, and a few Oersted  usually results from the effective anisotropy field —10 Oe. s  shift width  in  the  which  FERROMAGNETIC RESONANCE THEORY / 37 Moreover, the line width and shape may change due to the finite penetration depth and the resulting spatial nonuniform magnetization. But in our case, this effect could be totally ignored, as has been discussed earlier. In spin-spin relaxation the energy of the uniform procession of the magnetization excited at resonance is transmitted to other spin waves with k&O, and in spin-lattice relaxation it is transmitted to lattice vibrations, and in spin-conduction electron relaxation it is transmitted to the nonmagnetic electrons. These relaxation processes will give an intrinsic linewidth, which can be assumed to be of the order of tens Oe and possess some symmetric lineshape such as Gaussian or Lorentzian. The detail is not quite meaningful here, since the inhomogeneities would broaden this intrinsic width greatly and cause the true lineshape to have little connection with the intrinsic lineshape.  4.5. fmr  width  of  polycrystals  Certainly in real crystals the various structural inhomogeneities would influence the lineshape dramatically. This implies that the line width usually is much larger than the intrinsic width, particularly in polycrystals. Hence in this case unless a be taken into account, an erroneous g value could be concluded.  Frequently one has to include the polycrystallinity of samples, surface defects and domain structure. The influence of these inhomogeneities on the line width can be described in many cases with the aid of a nonuniform internal effective anisotropy field, whose spatial fluctuation  6H^ determines  the line  FERROMAGNETIC RESONANCE THEORY / 38 broadening.  In ferromagnets with relatively low anisotropy, H-<<M, interactions  between  the various  crystallites  may  lower  strong dipole-dipole the effect  of the  nonuniformity of if; appreciably by coupling the crystallites together. In strong anisotropic  crystals, H->>M,  the resonance  conditions  in individual crystallites  appear to be independent, and the line width is determined by the dispersion of the  resonance  frequency  in the various  crystallites  due  to the different  orientations of the anisotropy field Hj. Here the resonance curve is generally asymmetric and even possesses a definite structure(two or more resonance peaks).  It seems reasonable to employ the rough estimation of the line width resulting  from  the inhomogeneities of the anisotropy  field  due to the spatial  dispersion of the c-axis and the magnitude fluctuation of the internal field, or in another words the internal field is treated as a vector. In our case we can denote H^=H —4nM+H 1  1  and treat H  2  as constant since Hi  is relatively small.  So the half width at resonance  AH = AH + A t o ( ^ ) + A ^ ( ^ f )  (4-29)  0  where the first and second terms are, respectively, due the spatial dispersion of the c-axis and the effective internal field fluctuation.  Our case is closer to the strong anisotropic crystal one, and with the help of eqs4.25 and 4.26, and remembering that the H variables of H- and 4>tf.  res  and 4> q are the dependent e  FERROMAGNETIC RESONANCE THEORY / 39  dH  res  dtf^ d  H  {(fi  =  =  {(F  2  <i> ) -  J 2 ) B i n ( ^ -  cos0  e 9  - F, sin  2  + ^)cos(^ -  ii*  i  +  eq  (jf f g  ^ ) - ( g i g + 4&) ^ )  +( ^  ffiWfrr  +  - <M>  - 4> )} e?  + f&Jsinfo* -  <f> )}  ( 4  eq  -  3 1 )  here Fi = H  res  cos((/) - <j) ) - (Hi - 4wM + H ) cos 20 + J7 (3 sin <f> cos 2  H  eq  2  e?  2  - sin <j> )  2  4  eq  eq  (4.32) F  2  - #  fM  cos(^# - 4> ) + (#j eq  + H ) sin <^ - H sin 2  2  eo  (4.33)  4  2  and noting that symbolically  dF d(peq  (4.34)  h2  Such computer,  a complicated which  will  sort  °<Peq  calculation only can be attacked numerically by the out  the  best  fitting  Qualitatively these two fitting parameters may  parameters  A#  H  and  AHj_.  reveal some microscopic structure  of the films and can be compared with the data of other measurments such as  «...  CHAPTER 5. RESULTS AND DISCUSSION  The general features of the resonance are complicated. Generally there are more than one peak but always one of these is much stronger than the rest. The  resonances  are broad  with the linewidths of about  1 KOe  and are also  unsymmtrical.  There  are some  variations  positions, the samples with T =157 g  peaks(Fig5.1a) additional  between  the different  samples.  In parallel  and 192 °C have three obvious resonance  while the other samples have only two obvious peaks with an  weak  peak(Fig5.1b).  As  for the perpendicular positions,  all these  samples have two peaks with the second one not quite visible(Fig5.Je).  5.1.  NUMERICAL  RESONANCE  The  ANALYSIS  OF  ANGULAR  was  investigated  DEPENDENCE  OF  FIELDS  spectrum  of each  film  as  a  function  of the  orientation of the applied dc field H in a plane perpendicular to the plane(angle 0  H  in Fig4.1).  The signal detected corresponds to the field  derivative  of the  absorbed power.  In first. The  the case of more than one peak, the strongest one was considered resonance  field  was  chosen  as the cross over  spectrum and the estimated baseline.  40  point  between the  RESULTS AND  DISCUSSION / 41  RESULTS AND DISCUSSION / 42  6  8 10 Applied field (KOe)  Fig5.1c The FMR spectrum of the sample with T = 217°C attf = 90°. The narrow resonance is due to the calibration sample. S  The  H  angular dependence of the resonance fields were obtained for each  sample with different substrate temperature T . One of them is illustrated in g  Fig5.2 with T =192°C. The uncertainty of the resonance field is -0.1 KOe. S  It is seen from eqs4.25, 4.26, and 4.27 that the resonance field H  r e s  depends on three parameters, i.e., the first and second anisotropy field H , H , 4  and the g factor. The saturation magnetization M  2  was obtained from the VSM  measurments as 400 emu/cc for all the samples.  In needed with  the numerical fitting  computation, however, an additional  parameter  to be considered, i.e., the misalignment angle. The sample was mounted the parallel  position  adjusted by hand, which  unavoidably introduced a  Fig5.2 The angular dependence of resonance field for the sample with T = 217°C. The solid line represents the fitted curve with H = —4.5 KOe H, = -0.05 KOe, and g=2.36. S  l  CO  RESULTS AND DISCUSSION / 44 misalignment angle. The fitting computation with four parameters simutaneously turned out to be rather lengthy and tedious, consequently these parameters were treated seperately.  First of all, the misalignment angle was determined by the curve fitting method  around  the perpendicular  position.  The angle  corresponding to  the  maximum of the curve fitting represented the true perpendicular position because of the symmtry of the resonance fields with respect to the y axis. Then all of the  data points including the resonance fields and the linewidth were shifted by  this angle.  Secondly, positions H p of  a  the adjusted  resonance  fields  in parallel  and perpendicular  and Hp , respectively, were combined to further reduce the number e  the dependent fitting parameters. When ^=0°,  0 q=O° under the e  condition  (Hp +H!-4jrM+H )>0 , so the resonance equation 4.26 becomes a  2  (-)  2  = H (H pa  - [Hi - 4nM) - H )  pa  2  (5.1)  1  Similarly 0 =9O°, tf> q=90° when (H +H -4ffM)>0, hence H  e  pe  - = H 1  pe  1  + H  x  - 4TTM  (5-2)  These two inequalities were forced to be satisfied through the fitting procedure. The eqs5.1 and 5.2 would reduce the number of the dependent parameters into one, which was chosen as  A  special  programme  thereafter.  was written  to solve  the eqs4.25 and 4.26  RESULTS AND DISCUSSION / 45 simultaneously. For a given riband angle  consequently  H  and g factor) and given  2  there are usually two solutions, which can be understood  in the sense  that the number of the stable equilibrium positions from eq4.25 could be more than one. In the procedure of calculating the second resonance peak, it should be noted  that the stable condition of the magnetization  vector M is not the usual  one, i.e., the free energy is minimum. For a general case, the stable condition may  be complicated. But one can have some  feeling  of this from  a simple  example. Assuming 6<f>=constant=0°, the stable condition from eq4.19 is clearly seen as F^-CO if 7 is positive. Fortunately for our case, this term is zero and there are not explicit conditions except for the case in eq4.26 that Fgg?  Thereafter the curve fitting procedures  could be accomplished  by varying  the parameter H . The fitted parameters of all the samples are illustrated in l  Fig5.3, Fig5.4,  and Fig5.5, respectively. The H  2  is one tenth to one Fifth of H , l  which indicates in some instances the second anisotropy is important  in Co-Cr  films.  5.2. LINEWIDTH  AND  LINESHAPE  As has been pointed out before, the g-factor determined 4.27, where the influence of relaxation was not taken prove to be anomalously large(Be/son and Kriessman,  from eqs4.26 and  into consideration, may  1959). According to eq4.28,  which was assumed to be valid only for single crystal, the linewidth should be much larger for parallel directions than for perpendicular directions. This is not consistent with our data. The angular dependence of the linewidth is similar to  RESULTS AND  100  150  200  Ts (°C)  300  250  DISCUSSION / 46  350  Fig5.3 The substrate temperature dependence of the first effective anisotropy constant measured with FMR.  100  250  300  350  T.(°C)  Fig5.4 The substrate temperature dependence of the g-factor measured with FMR.  RESULTS AND  DISCUSSION / 47  'v O  i  1.2-  I  0.80.4  "  0.0  -  -0.4  -  -0.8  "  -1.2  -  1 150  100  Fig5.5 The substrate constant measured with  some FMR  data  1 200  temperature FMR.  Ts l°C)  I 250  dependence  before: it is greatest for the  of  such  a  the  easy  smallest for the hard magnetization axis(Yager et al,  Obviously  I  I 300  Landau-Lifshitz relaxation  350  second  anisotropy  magnetization  axis and  1955).  model  is not  suitable to  polycrystals. However qualitatively the influence of the broadening to the g factor can  still  be  estimated  with  this  model. To  minimize  this estimate  from  the  inhomogeneous broadening, the smallest linewidth should be chosen which in this case is the perpendicular directions.  From eqs4.28 and  5.2,  DISCUSSION / 48  RESULTS AND where 6 H  De  is the linewidth in the perpendicular direction and  the true g factor  (5.4) In our case, the a  is too small to make any  significant shift in the g factor.  For example, for the sample with substrate temperature does not  alter g from  2.76.  Here and  192°C, a = 0.07,  below, the linewidth is defined  peak to peak difference in magnetic field as usually defined in ESR,  which as  the  abd referred  to as peak-peak linewidth.  Generally the signal is unsymmetrical and  the low  side of the  absorption  line is more prominent as shown in Fig5.6. This is even true for the case when the sample has been saturated completely, which certainly excludes the possibility that this unsymmetry  is due  to the domain walls. The  account for this phenomenon. A impacts on FMR.  One  large anisotropy  large anisotropy  usually can  have two  may strong  is that this can lead to a second resonance peak, and the  other is that in most cases a unsymmtrical peak is characteristic. It is known that under the condition of weak coupling between the crystallites, the absorption as a function of magnetic field should be essentially proportional to the number of crystallites that go through resonance at a given applied field. Because of the finite linewidth of a crystallite the observed absorption exact  but  such an large  rather  a  smeared-out image of the  linewidth  would not  be  calculated distribution. However,  approximation is certainly valid for a qualitative comparision. So for  anisotropic polycrystals, the  from the dc field with an  magnetization  vector  will  inclination towards the nearest  deviate  appreciably  easy direction. As  a  consequence, the anisotropy field preferentially assists the dc field such that the  RESULTS AND DISCUSSION / 49  I  I  I  0  2  4  I  I  6 8 Applied Field (KOe)  I  I  10  Fig5.6 A typical unsymmetrical FMR spectrum. This corresponds to the sample with T =192°C, measured at 0jj = 84°. The narrow resonance is due to the calibration sample. S  low  field  side of the absorption curve becomes more prominent. Very  similar  absorption peaks have been detected by Schlomann and «/ones(1959) as depicted in Fig5.7.  The next step is to study the angular depenence  of linewidth. From the  model of independent grains, this function can be generated according to eq4.31 and compared to the experiment. This procedure is not completely satisfactory as shown in Fig5.8.  Some relaxation  mechanisms other than pure inhomogeneous  broadening must exist and it is only possible to do a rough fit. This, in turn, makes it impossible to distinguish between the second and third terms in eq4.29. Since there is not available any information  about the third  term, this was  neglected. Only the data around the perpendicular direction were attempted to be  RESULTS AND DISCUSSION / 50  CCT5TID  Fig5.7 Measured and calculated absorption lines for Ba2Mg2Fei 2 0 2 (Mg Y) measured at 15 967 Mc. The theoretical curve is valid for p = 2 2K /M=-3^00 Oe, 2K /M=-420 Oe, 2K = 120 Oe, and K = 0 Oe.' (Schlomanms and Jones, 1959) 2  1  2  4  Tit{Fig5.8), anj| a series of values of 60  H  2  3  are plotted in Fig5.9. The 6H„ were  found to be zero for all the samples. ui  Let  us now  consider  the discrepancies  as seen  in Fig5.8.  To further  clarify the problem of the eddy current loss, two sets of particular experiments were arranged. First,  a small piece of the film cut from  the previous film,  which was sinall enough to eliminate eddy current loss if any, was tested under the same expermental conditions as before with  the result that no appreciable  change in lineji^idth was found. Second, the spectrum was taken where the film was placed on the bottom of the cavity with the substrate touching the bottom n  and  also upside  down with  the film touching  the bottom. The film was thus  exposed to two^ different strengths of the microwave eletric field, and if the eddy  T  2.50-  a> 2.25 O 2£  S  2  00 -  i <u c  -  1  o <u  1.75 "  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  r  DISCUSSION / 52  RESULTS AND  8  <  6  4  2  150  200 Ts C C )  Fig5.9 The  250  300  350  substrate temperature dependence of the c-axis dispersion angles  measured with  current loss was  FMR.  responsible for part of the linewidth, the linewidths should have  been significantly different. The  results were found to be the same. Therefore, it  is safe to conclude that the skin effect is not important in our case.  Maybe the only possible consistent explanation is due the  domain  reached,  walls. Principally  the  domain  significantly. At relatively  low;  the  walls, angle  whereas as  when the if they  0^ one  resonance occurs  existed, could  far away  from  approaches 90°,  90°, the  to the scattering by before  broaden the  the  saturation is FMR  peaks  resonance fields are  resonance fields move to  rather high field values. There are more domain structures at low fields and consequently more domain scattering. This certainly results in broader resonance peaks as is evident in our case. A  quantitative analysis must be associated with  RESULTS AND the detailed  domain  structure which is not available  serious explanation of this  angular  DISCUSSION / 53  at the present time. A  dependence of the linewidth needs to be  examined further, particularly Q band FMR  should be employed to eliminate the  effect of domain walls.  5.3. DOMAIN  The  WALL  magnetization  process  in crystals  with  domain  structure can be  classified into three parts as shown in Fig5.10.  The existence of domain Certainly  the first  walls may  is that they  have a few implications on  can behave like  a relaxation  mechanism  FMR. as  already discussed above.  Secondly, the phenomenon known as domain resonance could happen. In general, one resonance peak is expected for each type of domain. Any component of  the static  magnetization  perpendicular to the domain  wall  is continuous.  However, the time-varying magnetization perpendicular to the wall may  be out of  phase and thus leads to uncompensated poles at the wall. These poles produce a demagnetization magnetization peaks  field  with  some demagnetization  factor  which couples  vectors together(PoZcter and Smit, 1953)(Fig5.11).  as indicated  in Fig5.1  are believed to correspond  the two  These resonance  to the domain wall  resonances.  The  irreversible  domain  wall movement  can  lead  to irreversible  FMR  RESULTS AND  DISCUSSION / 54  Fig5.10 Magnetization process.  signals. To determine decisively the existence of domains, the scanning direction of dc magnetic field has been reversed, i.e., higher  value  to lower  value. The reversibility  the dc field was  scanned  from  at small resonance fields was  generally high. This can be easily understood since these fields correspond to the reversible magnetization procedure in Fig5.10. But when the resonance fields were close to the fields in the range of irreversible domain wall movement, the large lineshapes  took  p\ace(Fig5.12a).  difference  of the two  This  can be  readily  understood  since the film should be in a state of more order in the domain  structure when the applied field is scanned from high to low values than when the applied field is increased from low to high. It is well known that the FMR signals should be narrower and more sj'mmtrical in the ordered state. Another consistent fact is that when the dc field was set even higher to reach the range  DISCUSSION / 55  RESULTS AND  a  b  Fig5.ll The influence of domain structure on resonance. a- the alternating magnetic field h is perpendicular magnetization and the boundar} layers. b- the alternating magnetic field h is perpendicular magnetization and parallel to the boundary layer.  to  the  static  to  the  static  7  of  reversible rotation  processes,  the  signal  was  reproducible  again  in the  precedure described above as shown in Fig5.12b.  In conclusion, the films are most likely to have some kind  of domain  structure. Additional information is needed to be more specific in order to deal with this problem.  5.4.  THE INTERPRETATION  OF OTHER  There is one more peak unexplained  RESONANCE  PEAKS  in the parallel direction. This still  can be possibly attributed to the domain wall resonance. But, on the other hand, it is almost certain that all the Co-Cr films have some kind of transition layer,  RESULTS AND  Applied  Field  DISCUSSION / 56  (KOe)  Fig5.12a The irreversible resonance due to the irreversible domain wall movement. The arrows indicate the direction of dc field scanning.The narrow resonance is due to the calibration sample.  I  0  I  2  I  4  I  I  6  8 Applied  Field  I  10  1  12  (KOe)  Fig5.12b The reversible resonance after the complete saturation. The arrows indicate the direction of dc field scanning. The narrow resonance is due to the calibration sample.  RESULTS AND DISCUSSION / 57 •which has been identified by FMR(Mitchell  et al,  1985).  Suppose the transition layer is thin and its magnetocrystal  anisotropy is  averaged to vanish as is usually done. Then it turns out that the resonance at a  field  of about  magnetization assumption  6.3 KOe  of this  layer  can not accounted  for this  layer  is assumed  negative.  However,  to be  unless the if  the  that the magnetocrystal anisotropy averages to be zero does not hold,  then these resonances can be perfectly interpreted as due to the transition layer. (The concept of transition layer will be elaborated later). As an example, consider the  analysis of the film with T = 217°C. From S  according to eq5.1 with the neglect of H bulk layer, H =5.3 1  why  2,  H =6.3 pa  and the assumption  KOe, and  g=2.76 as the  KOe. This in turn gives Hp =—2.8 KOe, which explains e  the corrsponding resonance is not detected in the perpendicular direction as  shown in Fig5.1c. from the FMR  The relative volume of this transition layer can be estimated  curve. The area underneath  a j constituent absorption curve is  propotional to 4jrMjVj, where Vj is the relative volume. The derivative absorption curve may be roughly calculated by multiplying the vertical separation of the absorption derivative peaks by the square of the linewidth 6H. The magnetization is reasonably assumed to be uniform. Then from the resonance curve, the volume of the transition layer is one third of the whole film.  5.5.  DISCUSSION  RESULTS AND  DISCUSSION / 58  5.5.1. "Transition Layer"  The  primary  question  which  arises  is why  inconsistency between the measurements of K j Figs3.4  and  5.3.  The  in VSM  compared  from  determined  fairly accurately and the volume averaged  Then the effective anisotropy energy the unit of magnetic fields by while the counterparts in FMR  there  is such  and  FMR  saturation magnetization  M  a  huge  as can  be  can  be  g  value is about 400 emu/cc.  as defined in Fig3.4  can be converted into  dividing 0.5M , which would be  — 2~2.2  g  are — 7~—3.5 KOe.  KOe  In order to understand this  discrepancy, a few possibilities will be discussed.  One  of these  derived from  defined by Li et al(1986), the each  of which  parallel  and  is an  averge  perpendicular  the  so-called  in VSM of the  directions,  an  c  corresponding respectively.  and the ratio M /M r  g  g  magnetization When  the  loop in the  samples  become larger), the method of taking perferable to use anhysteretic  magnetization  which eliminates the contribution of irreversible processes  magnetization  energy.  the  show  is beyond 200°C(In Figs3.2 and  average field becomes unreliable. It is then  By  As  is the area between the two curves,  considerable hystersis as in our case when T 3.3, the coercivity H  anhysteretic magnetization.  application of the  to the  alternating field the film is  allowed to choose the state of lowest potential energy from the many metastable states that cause the hysteresis. Certainly this can not account for all of the discrepancy. This correction is estimated to be at most 1  Secondly, the VSM  tends  KOe.  to measure the whole averaged  magnetization  RESULTS AND DISCUSSION / 59 and anisotropy while FMR can literally distinguish the different constituents. For example, the effective anisotropy is reduced KOe  from  -6.0 KOe in FMR  to -4.0  in VSM/TM owing to this transition \ayer(Mitchell et al, 1985). In our case  the film with T = 217°C is an good example. The volume of the transition layer S  was  estimated  to one third  of the whole  considered to be uniform. The K and  u  from  FMR  film,  and the magnetization is  is -4.5 KOe for the bulk layer  5.3 KOe for the transition layer. Then the volume averaged  anisotropy is  calculated to be -1.6 KOe, fairly close to the value in VSM. This agreement with such a crude calculation is gratifying. The distorted weak signals prevent us from proceeding along this direction to treat other films.  5.5.2. A Phase Segregation Model  Now we should summarize the physical picture of phase segegation and the transition layer.  As  discussed  in chapter 2,  it is generally  believed  that  some  phase  segregation can exist in the Co-Cr films. From the structure analysis (sec2.2.1) it is  accepted  that  nonferromagnetic  the films consist  was  structure surrounded  by a  Co-rich region. The columnar shape is a very important factor  which partially causes suggested  of columar  the positive effective anisotropy. The T E M  investigation  a similar model but a finer microstructure(sec2.2.2). In that study, it  suggested  that phase  segregation into  phases in the hep crystallites causes  ferromagnetic  and nonferromagnetic  the compositional fluctuations along the  grain boundaries as film growth proceeds. Finally, coherent ferromagnetic Co-rich  RESULTS AND regions  are formed  perpendicular  in a  crystallite  with  a  wall-like  DISCUSSION / 60 structure  which is  to the substrate. This model is different from above in that the  phase segregation occurred in each crystallite having  a single phase and results  in a finer structure.  In order to interpret some previous results, an additional constituent was introduced, namely the transition layer. The existence of a transition layer located between the substrate and the bulk layer is certain from such results as SEM fracture  cross  patternsCr/iucmg  section (Ouchi  and  Iwasaki,  1982)  and  electron  diffraction  et al., 1986). However, the correlation between this layer and the  magnetic properties, particularly the anisotropy, are not well proved, although a number of papers have been devoted to this subject. The most frequently used methods are VSM  and TM, which is not quite suitable to this problem. This  insufficiency was noticed by Mitchell et al.(1985) and a series FMR  reports were  published which were mentioned briefly already in sec2.6.1.  They found that most of their samples had two resonances. One of them was attributed to the transition layer, which could occupy as much as half of the whole volume of the films. Two points are of interest in their study. They described that some films with as many represented in  as four unexplained  poor polycrystalline alignment could demonstrate resonances, and they believed that their  the first known confirmation by FMR  Co-Cr films. Before  that  time  there  were  reseach  of positive anisotropy(0.7 already  several  KOe)  reports which  claimed a large positive effective anisotropy (For example, 4 KOe by Fisher et al., 1984).  RESULTS AND  DISCUSSION / 61  This investigation also presents some surprising results. Primarily the large positive effective anisotropy of the thick (150nm out of 500nm) "transition" layer is contrary to previous results. Usually, it is believed that the transition layer should only have negative shape anisotropy and nm  for a film with  angular  500  nm  the thickness is less than  50  thickness. Also the failure of the fitting for the  dependence of linewidth under  the  independent grain model seems to  suggest a much more complicated microstructure associated with the film, one of which is phase segregation. Furthermore, the SEM  graphs have not shown any  thick transition layer.  Although  in reality there may  be  a number of phases, it is useful to  persue a simple model such as a two phase model. Both phases are supposed to be ferromagnetic rather than only one of them as before so that two resonances in FMR  should be detected. The scale of each magnetic unit may  to film, and  in some cases it can be much smaller than the grain size viewed  through SEM. phase) and  One  of the phases has positive effective anisotropy(named positive  the other, negative (negative phase). The  phases are realized from which may  vary from film  the compositional  not be universal. A  different anisotropics in two  fluctuation or the different shapes,  transition layer could develop but in most cases,  the influence of this layer is considered to be much weaker than that of these two phases.  This model can be applied to the discussion in last section and the results of Mitchell et al(1985) with the only substitution that the so-called "transition layer" actually now  is a new  phase. Moreover, this model allows the possibility  DISCUSSION / 62  RESULTS AND  that a number of phases could form, which of course could explain the several resonances different  reported by anthors  Mitchell. The  is understandable  reason now  for the variety  since any  phase  of results among  segregation  is quite  sensitive to the preparation conditions.  5.5.3. Substrate Temperature Dependence  The most important deposition parameter to influence the phase segregation is  surface  temperature  as  explained  in  sec2.4.2. The  variation  of surface  temperature should be intimately related to a) the substrate temperature,  and b)  the  surface  deposit  rate.  A  higher  deposit  rate  would  lead  to  a  higher  temperature.  The  surface temperature should be classified into three ranges according to  the phase segregation model: [1]  low  temperature:  the  phase  segregation  should  not  happen  and  the  temperature is too low to allow Co/Cr atoms to form hep structure with positive anisotropy; [2] medium temperature:  the film begins to form some hep structure and at the  same time phase segregation is starting to take place. As  the temperature goes  higher, more negative phase would be formed; [3]  high temperature:  if the temperature is too high, then Cr/Co diffuse out of  alloy causing a reduction in positive phase. Finally, even the atomic hep lattice structure could be destroyed and fee lattice would show up.  RESULTS AND DISCUSSION / 63 There is a suitable temperature range in which films with positive effective anisotropy could be made. In our case, T = 200°C seems to represent the transition from g  [2]  to  [3].  The K of one phase corrsponding to the "bulk" layer increases monotonically in u  Fig5.3,  and the resultant anisotropy goes from positive to negative when the  substrate temperature is increased. The tail in Fig3.4, which turns positive again, is believed to be due to some other causes and one possibility could be anhysteretic magnetization. The anomalies of K  2  and g factors may also be  attributed to this but not enough information is available to make any definite conclusions. Also this model predicts a lower substrate temperature would improve the properties of these kinds of films.  5.5.4.  Comparsion With Other Studies  The coercivity has a maximum in the study of Coughlin et  al.(1981)  within the range of 2 0 0 to 3 0 0 °C substrate temperature while the coercivity goes up as the T is increased from 0 to 2 0 0 °C as shown by Wieldingn and g  Lodder(1981).  The former is consistent with our result generally, although a  presice comparision is hardly possible. The first anisotropy constant spreads over a very large range while our K is quite close to the only available value at 2  the same composition obtained by Fisher et %,  O.44X10  5  al.(1984)  ergs/cc. Our K are distributed from 2  with TM: at Cr  -0.8  to  +0.8X10  5  20.5 at.  egs/cc.  CHAPTER 6. CONCLUSIONS AND SUGGESTIONS  The  Co-Cr films are quite inhomogeneous with possibly two  phases. FMR  can  potentially sort out  angular dependence investigation gives anisotropy and g factor. The quite  different constituents the  explained  in the  films.  The  possibility of determining the second  discrepancy between VSM  common, is qualitatively  ferromagnetic  with  the  and FMR, proposed  which may  two  be  ferromagnetic  phases. This model also is consistent with the magnetic properties of substrate temperature Moreover,  dependence, this  may  which  help  is  very  understand  sensitive the  to  diverse  preparation  properties  parameters.  occuring  under  different deposition conditions.  As  for  properties such layer. The  the as  future, the  Q  existence  band  FMR  could  of domain wall  be  employed  to investigate  resonance and  the transition  angular dependence of the linewidth could also be studied further.  64  BIBLIOGRAPHY Lutinger, J. M. and C. Kittel, Phys. Acta., 21, 480(1948).  Van Vleck, J. H., Phys. Rev., 78, 266(1950). Bozorth, R. M., Ferromagnetism, D. Van Nostrand Company, Inc., 1951.  Polder, D. and J. Smit, Revs. Mod. Phys., 25, 89(1953).  Smit, J. and H. G. Beljers, Phil. Res. Soc, A64, 968(1955).  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