Dispersion, a Most Useful Tool in Paramagnetic Resonance

Abstract

The authors present a theoretical and experimental study on the possibilities of paramagnetic-resonance dispersion for investigating the properties of paramagnetic systems. It is shown that the dispersion line contains at least as much (and "better") information about the magnetic relaxation as does the absorption line. The parameters of both resonance lines and their relation to parameters of the relaxation function are discussed. New parameters of the dispersion line are introduced, corresponding and equivalent to the area under and the moments of the absorption line. These dispersion parameters are related to the static susceptibility and the derivatives of the relaxation function in a much simpler way than the corresponding absorption parameters. The foregoing results, being derived essentially for the ideal case of a symmetric nonsaturated line in a high magnetic field, are generalized afterwards to some nonideal cases. Experimental methods, applying the theoretical principles introduced in the paper are described, amongst others a simple, accurate, and selective method for measuring the static susceptibility by means of resonance dispersion. In order to check those experimental principles, a series of test measurements of the static susceptibility of diphenylpicrylhydrazyl (DPPH) were performed in a 9-GHz ESR spectrometer. An excellent reproducibility was found, without using any reference substance, and even after complete deregulation of the spectrometer. Low-temperature measurements were included. The values of the static susceptibility found by the resonance-dispersion method are compared with those of other authors, obtained by means of static methods. The Curie temperatures are calculated and some deviations from the Curie-Weiss law are discussed. A possible influence of the air surrounding some of the samples is detected.