Anomalous Absorption-Edge Shift in the Metamagnetic Temperature Range of HgCr2S4

Abstract

The shift of the optical absorption edge in metamagnetic Hg${\mathrm{Cr}}_2$ ${\mathrm{S}}_4$ is studied as a function of temperature and magnetic field. The large red-shift of the band edge with decreasing temperature is due to short- and long-range magnetic order in the crystal. Its temperature dependence can be explained by the variation of the spiral spin structure with temperature. The magnetic field dependence of the band edge at temperatures below 20 K is interpreted in terms of the conical-spin structure. Its ferromagnetic component which is parallel to the external magnetic field is strongly temperature-dependent. Our results and a recent neutron diffraction study indicate that Hg${\mathrm{Cr}}_2$ ${\mathrm{S}}_4$ has a critical temperature of 60 K.