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On the temperature dependence of the shape of magnetica resonance lines McMillan, Malcolm

Abstract

This thesis is devoted to a theoretical study of that temperature dependence of the shape of magnetic resonance lines in solids which remain when the direct effect of lattice vibrations can be neglected. This is the case at sufficiently low temperatures. To discuss the shape of resonance lines the "moment method" is used. This procedure was introduced by Van Vleck (1948) and was used also by Pryce and Stevens (1950) and Usui and Kambe (1952). A line shape function which describes the shape of the resonance lines is defined and the first and second moments of this function are calculated in various approximations. In particular, the question to what extent the standard formula of Van Vleck for the second moment is valid is discussed in great detail. The general formulae are applied to the case of a spherical sample of nickel fluosilicate crystal. From the general discussions and from this special case it follows that the temperature dependence of the characteristics of paramagnetic resonance lines becomes noticeable at liquid helium temperatures and that these characteristics are then also dependent on the shape of the sample.

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