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Experiments on paramagnetic resonance absorption in crystals at low temperatures Wesemeyer, Harald Kurt

Abstract

A paramagnetic resonance spectrometer operating at 3.2 cm has been built for use at liquid helium temperatures. This spectrometer employs a high Q transmission type cavity resonator which is equipped with windows on the broad faces to pass light through. The pole pieces of the electromagnet have axially centered holes for passing light through, too. This enables magneto-optical effects and paramagnetic resonances to be studied simultaneously. A high power klystron, and a high Q cavity resonator permit experiments with microwave power saturation. The influence of the paramagnetic resonance absorption saturation on the paramagnetic Faraday effect has been studied. The paramagnetic Faraday effect (i.e. the rotation of the plane of polarisation of light passing through a paramagnetic crystal in the direction of an external magnetic field) is proportional to the magnetic moment of the crystal. When the crystal is exposed to microwave radiation of the correct frequency to cause appreciable saturation of this resonance, the magnetic moment is reduced, and a reduction of the rotation angle should be noticed. This effect was predicted by A. Kastler 1951, and a theory of this effect has been developed by W. Opechowski 1953. This effect was found in Nd(C₂H₅SO₄)₃∙.9H₂O at 1.4°K. In this salt the magnetic moment is proportional to the difference of the populations of the two levels M(s)= ± ½ of the Nd⁺⁺⁺ ion, having an effective ground state of S= ½. Saturation levels of about 52%, 88%, and 100% were obtained and hence the rotation angle of the plane of polarisation reduced accordingly. Thus the effect predicted by A. Kastler has been established. From the known power levels of the applied microwave frequency radiation the electron spin-lattice relaxation time of the Nd⁺⁺⁺ ions, in the upper level was found to be about 1/20 s at 1.4°K. Guanidine aluminium sulphate hexahydrate, (CN₃H₆)Al(SO₄)₂∙6H₂O, a crystal with trigonal structure, having three Al⁺⁺⁺ ions in the unit cell, has recently been discovered to be ferroelectric. The ferroelectric direction is along the trigonal axis. The determination of the structure was not completed when this work was started. In order to complete the crystallographic data with respect to the crystalline field of the immediate neighbours of the Al⁺⁺⁺ ions, Cr⁺⁺⁺ ions were introduced into the lattice replacing Al by 2%, and then the resonance spectrum of the Cr⁺⁺⁺ ions investigated at 295°K, 195°K, 77°K, and 55°K. Two sets of axially symmetric spectra, a total of six lines were observed, set 1 being twice as intensive as set 2. The investigations show that the Cr⁺⁺⁺ ions are exposed to a trigonal field. All three ions lie on the three-fold axis, and since there are 3 ions per unit cell, two of them are magnetically equivalent. Both sets of spectra can be fitted to the spin-Hamiltonian [formula omitted] where S = 3/2. The exact Hamiltonian was diagonalized using a digital computer, and the predicted lines agree with the measured ones within 1%. It was found that the D₁ and D₂increase linearly with decreasing temperature, D₁ from (0.0576 ± 0.0005)cm⁻¹ at 295°K to (0.085 ± 0.003)cm⁻¹ at 35°K, and D₂ from (0.0730 ± 0.0003)cm⁻¹ at 295°K to (0.109 ± 0.005)cm⁻¹ at 35°K. g = 1.975 ± 0.005 and stays constant over the range of the mentioned temperatures. Further, ferroelectricity causes spontaneous distrotion of the lattice, and this way may influence the paramagnetic resonance spectrum through changes in the crystalline field. Since different parts of a (CN₃H₆)Al(SO₄)₂∙6H₂O crystal have different electrical properties, the spectrum of crystal fragments cleaved from different parts of a large crystal has been examined and no difference in the spectrum has been observed. The crystals were also polarised in a strong electric field along the trigonal axis and depolarised in a weakening alternating electric field with no effect on the spectrum. From this follows that the ferroelectric complex has very little effect on the immediate neighbours of the Cr⁺⁺⁺ ions. The paramagnetic resonance of Fe⁺⁺⁺ in orthorhombic amonium penta chlorindate hydrate, (NH₄)₂[(ln,Fe(10:1))Cl₅∙H₂O] has been investigated as this salt is interesting for adiabatic demagnetization. Noticeable signal strengths were obtained at about 25°K only. The positions of the lines as functions of the rotation angles of the crystal about all three crystalline axes separately with respect to the magnetic field have been measured. In general four sets of equally intensive spectra have been found for each rotation. From the maxima and minima of the curves of the spectra assuming the ions have axial symmetry it is possible to determine the directional cosines of the principle axes for the Fe⁺⁺⁺ ions. These are either ℓ = ± 0.1860, ℳ = ± 0.2212, ℳ = ± 0.9578 or ℓ = ± 0.7576, ℳ = ± 0.6357, ℳ = ± 0.1488 or ℓ = ± 0.6636, ℳ = ± 0.7123, ℳ = ± 0.22805 or ℓ = ± 0.3289, ℳ = ± 0.2762, ℳ = ± 0.9033 Because of the complexity of the spectra the spin-Hamiltonian has not yet been determined.

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