Formal approach to various properties of antiferromagnets

Abstract

The formal representation of thermal and magnetic properties of moderately anisotropic antiferromagnets in the presence and absence of a constant, uniform external magnetic field is discussed both in selective ordered configurations and in the paramagnetic phase. The mean-field-theoretical formalism is shown to give a satisfactory account of the magnetic properties of a class of two-sublattice antiferromagnets in those ordered configurations where the applied magnetic field is at a right angle to the direction of the spontaneously evolving sublattice magnetic moments. Predictions of mean-field theory on the linearity of the magnetic moment in the applied field strength and the attendant invariant molar susceptibility throughout the relevant portions of the configuration phase diagrams, phase-boundary lines included, are seen to be well satisfied. This is verified in detail on the typical antiferromagnet MnCl2 · 4H2O, where the Mn2+ ions in their6S5/2 ground state determine the magnetic properties and for which numerous magnetic and thermal measurements have been accumulated in both the ordered and paramagnetic phases. The experimentally explored low-temperature properties of this antiferromagnet refer to the liquid3He and4He and liquid H2 bath-temperature intervals 0.3≤T≤4.2 K and 15≤T≤21 K, respectively. In contrast with the magnetic properties, mean-field theory is less successful in accounting for the thermal properties of antiferromagnets, but its qualitative predictions agree well with the observations. In the paramagnetic phase, the isotropic nearest-neighbor exchange-coupled model system is used, wherein the data on the manganese compound are assigned approximately to an effective simple-cubic distribution of the Mn2+ ions, with an effective empirical exchange-energy parameter,J<0, derived from magnetic and heat capacity data at asymptotically high temperatures. Here the formal approach corresponds to a series expansion of the various properties in ascending powers of ‖J‖/kT, in the absence of magnetic field, with an additional magnetic-field-exchange double series in the presence of an external field. In those temperature and field-strength regions where convergence is satisfactory, the above approach is shown to give a good account of the observed thermal properties of the manganese compound in the absence of magnetic field, as well as of the low-field paramagnetic susceptibility. The hitherto observed field-dependent properties are confined to a much too narrow temperature interval outside the region of convergence of the series-expansion formalism. Experimental control of this approach requires acquisition of new data in the appropriate temperature range of convergence of the model formalism.