Specific heat of hydrazinium-transition metal-sulfate linear chain compounds

Abstract

Specific-heat data in the temperature range of 1.3 K to 80 K are presented on polymeric M(II)(N 2H 5) 2(SO 4) 2 compounds where M(II) = Cu, Ni, Co, Fe and Mn. Broad maxima in the magnetic specific heat, pointing to magnetic linear chain exchange interactions, are observed. For the Co, Fe and Mn compounds, λ-like anomalies have also been observed which are indicative of the onset of three-dimensional long-range spin ordering. From the specific-heat measurements information is obtained about the anisotropy of the intrachain exchange interaction, single-ion anisotropy and the magnitudes of the intrachain and interchain interaction parameters of these compounds. The results on the Cu and Mn compounds were fitted to the antiferromagnetic Heisenberg linear chain model with spin 1 2 and 5 2 , respectively. Although this model yields reasonable fits, deviations were observed near the maxima of the curves. For the Cu compound this is ascribed to anisotropy in the exchange interactions and for the Mn compound to interchain coupling effects. In order to fit the data on the Ni and Fe compounds it was found necessary to introduce single-ion anisotropy. The results for the Ni compound were fitted to the Ising spin 1 2 linear chain model with in addition a Schottky curve, and to recent calculations for the spin 1 antiferromagnetic Heisenberg linear chain model with in addition uniaxial single-ion anisotropy. The exchange parameter J/k is found to be much smaller than the zero-field splitting parameter D/k, thus giving rise to Ising properties of the spins at low temperatures. The Co compound was found to exhibit anisotropic exchange interactions which are, in contrast to what is generally found for Co 2+, of the planar type. The results were fitted to the spin 1 2 antiferromagnetic XY linear chain model, as calculated by Katsura, with an exchange parameter J/ k = −7.05 K.