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Optical Faraday rotation studies of paramagnetic resonance in neodymium ethylsulfate Rieckhoff, Klaus Ekkehard

Abstract

Brief theoretical descriptions of the magneto-optical Faraday effect, the influence of paramagnetic resonance on the Faraday effect and the use of this effect for measurements of saturation spectra and spin-lattice relaxation times are presented. A detailed description of an apparatus and of experimental procedures is given, with the help of which the Faraday effect can be used to study paramagnetic resonance spectra and spin-lattice relaxation times at liquid helium temperatures. Measurements on concentrated neodymium ethylsulfate at liquid helium temperatures are reported. The paramagnetic resonance spectrum was resolved as a main triplet with an extensive hyperfine structure. The positions of the lines were found to be in agreement with a spin Hamiltonian derived earlier from conventional absorption spectra obtained at higher temperatures. The intensity distribution was found to become increasingly asymmetric with decreasing temperature below 4.2°K. This asymmetry is explained qualitatively for the main triplet in terms of existing theories. Measurements of saturation versus microwave power at constant magnetic field and bath temperature gave results not readily explainable in terms of existing theories. The existence of a spatial gradient in the spin-temperature was observed in the case of partial saturation of the resonance. Measurements on the spin-lattice relaxation time t gave the following results: t is independent of the magnetic field from 800 ∅ to 2600 ∅ at a temperature of 4.2°K, but strongly field-dependent at temperatures ≤ 2°K, indicating the importance of cross-relaxation processes at very low temperatures. Some experiments are suggested which may provide information necessary for the complete understanding of the complex processes involved in paramagnetic resonance phenomena at low temperatures.

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