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Studies of magnetism using nuclear orientation and related NMR techniques Pond, James

Abstract

Nuclear Orientation and related NMR techniques have been used to study three magnetic insulators: Mn(C00CH₃)₂•4H₂0, MnCl₂•4H₂0 and CoCl₂•6H₂0. Continuous wave NMR thermally detected by Nuclear Orientation has been used to investigate the magnetic properties and spin dynamics of the quasi-2-dimensional ferromagnet ⁵⁴Mn-Mn(C00CH₃)₂•4H₂0. The system exhibits a frequency pulling effect due to the indirect Suhl-Nakamura interaction between nuclear spins and the electronic spin excitation spectrum is related to the coupling strength of the nuclear spins. The temperature dependence of the frequency pulling effect was measured for the two crystalline sublattices Mn1 and Mn2 in low magnetic field. The spectra show a structure not predicted theoretically. The current theory is valid only for I = 1/2 with uniaxial crystalline anisotropy fields. The theory of frequency pulling has been extended here to the case of I ≥ 1/2 and non-uniaxial crystalline anisotropy fields and the resonant frequencies and linewidths have been calculated as a function of temperature. The new theory and data agree well in terms of the magnitude and temperature dependence of the frequency pulling. Discrepancies are likely due to simplifying assumptions when calculating the electronic magnon spectrum. Classical and quantum numerical simulations confirm qualitatively the predictions of the model. The first Low Temperature Nuclear Orientation experiments on isotopes implanted into insulators is reported. Radioactive ⁵⁶Mn ions have been implanted into insulating, antiferromagnetic crystals of MnCl₂•4H₂0 and CoCl₂•6H₂0. In MnCl₂•4H₂0, comparison of the γ-ray anisotropy of the ⁵⁶Mn nuclei with that of ⁵⁴Mn, doped into the sample during growth, showed that both the ⁵⁶Mn and ⁵⁴Mn spins felt a very similar hyperfine field. The site occupancy factor in a simple, two site model was deduced to be 0.96 ⁺⁰‧⁰⁴₋₀․₀₇. In CoCl₂•6H₂0, the average hyperfine field for the implanted ⁵⁶Mn was significantly less than that for ⁵⁴Mn and corresponded to f = 0.53 ± 0.10. Radioactive and stable Co impurities, doped into MnCl₂•4H₂0 during growth, have been studied by Nuclear Orientation and NMR. The hyperfine fields of ⁵⁷Co and ⁶⁰Co, measured by Nuclear Orientation, were found to be significantly different. The NMR resonance of the Co impurities was not found, probably because the spin-lattice relaxation time is too short for the resonance to be observed with the radiofrequency modulation used. The effects of 3.96% stable Co on the host Mn spins were studied by NMR. The presence of the impurities broadens the NMR resonance and reduces the spin-lattice relaxation time by an order of magnitude. New aspects of the design of radiofrequency coils for use in continuous wave NMRON are reported, and the important effects of transmission lines are presented.

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