Anomalous Exchangestriction in Ferromagnetic Pyrite and Chromium Chalcogenide Spinel Compounds

Abstract

Results are presented of x‐ray measurements of lattice constant versus temperature in the range 4°–500°K of the ferromagnetic compounds CoS₂ and Co_0.75Fe_0.25S₂. Earlier measurements in the range 4°–300° on CdCr₂S₄ and CdCr₂Se₄ are extended to the range 4°–700°K. For purposes of comparison, x‐ray data are also presented on the nonmagnetic compounds FeS₂ and CdIn₂S₄. The results of measurements of dT_c/dP of CoS₂ and Co_0.75Fe_0.25S₂ are also reported. The magnitude of the volume exchangestriction of the ferromagnetic compounds is determined by using Grüneisen‐Debye theory to fit the high‐temperature lattice constant data, and extrapolating this fit into the temperature range below T_c. The volume exchangestriction is found to be anomalously small when compared to thermodynamic predictions based on values of dT_c/dP and the magnetic energy determined from the high‐temperature susceptibility. The proposed explanation involves atomic displacements internal to the unit cell which modify the magnetic exchange integral J. This phenomenon is related to the coupling between optical phonons and magnons.