Strongly correlated behavior in magnetoresistiveFe1−xCuxCr2S4spinels

Structural and thermodynamic properties of the spinel-type colossal magnetoresistance (CMR) system ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.3)$ have been investigated. Synchrotron x-ray powder diffraction measurements on $x=0.2$ reveal significant changes in the lattice at the Curie temperature ${T}_{c}$, which bear a close resemblance to those of perovskite manganites exhibiting CMR. The heat capacity ${C}_{p}$ shows that the stoichiometric $\mathrm{Fe}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ undergoes an orbital ordering at $9.6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ induced by the dynamic-to-static Jahn-Teller (JT) distortion, which results in the opening of a spin gap. For $x\ensuremath{\geqslant}0.2$ the JT distortion remains dynamic at all temperatures, and the low-temperature ${C}_{p}$ contains the ${T}^{3∕2}$ term due to ferrimagnetic spin waves. These results suggest that ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{Cr}}_{2}{\mathrm{S}}_{4}$ represents another class of strongly correlated CMR materials where charge, spin, and lattice degrees of freedom are intimately interrelated.