Pressure Dependence of Ferromagnetic Phase Transitions of Chromium Chalcogenide Spinels

Abstract

The shift in the Curie temperature T_c has been measured as a function of hydrostatic pressure up to 10 kbars for the chromium chalcogenide spinels CdCr₂Se₄, HgCr₂Se₄, CdCr₂S₄, and HgCr₂S₄, by measuring the mutual inductance of an astatic coil assembly containing the sample. Also the compressibility of these compounds has been measured for the same pressure range at three temperatures. The measured values of T_c and dT_c/dP for CdCr₂Se₄, HgCr₂Se₄, CdCr₂S₄, and HgCr₂S₄ are 128°, 105.5°, 84°, and 36.1°K and −0.82, −0.95, −0.58, and +0.14°K/Kbar, respectively. Combining the compressibility measurements with dT_c/dP the values of dT_c/da=+66, +99, +46, and −10°K/Å are obtained for CdCr₂Se₄, HgCr₂Se₄, CdCr₂S₄, HgCr₂S₄, respectively. These values of dT_c/da for CdCr₂Se₄ and CdCr₂S₄ are in qualitative agreement with the dilution studies for these compounds. However, the value of dT_c/da for HgCr₂S₄ is of opposite sign from that predicted from the solid‐solution studies for this compound. High‐pressure measurements seem to provide a better picture of variation of magnetic exchange interaction than dilution studies, because for the latter, the covalency effects become different due to changes in the nature of atomic environments. Conclusions are also made about the volume dependence of exchange interactions in these materials, and again it is quite different for HgCr₂S₄ as compared to the volume dependence of exchange interactions for the other three compounds. This seems to be consistent with the recent experimental and theoretical studies for HgCr₂S₄.