Pressure Dependence of Ferromagnetic Phase Transitions of Chromium Chalcogenide Spinels

Abstract

The shift in the Curie temperature Tc 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 Tc and dTc/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 dTc/dP the values of dTc/da=+66, +99, +46, and −10°K/Å are obtained for CdCr2Se4, HgCr2Se4, CdCr2S4, HgCr2S4, respectively. These values of dTc/da for CdCr2Se4 and CdCr2S4 are in qualitative agreement with the dilution studies for these compounds. However, the value of dTc/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.