Phonon anomalies and possible local lattice distortions in giant magnetocapacitive CdCr2S4

Abstract

We investigate the origin of large magnetocapacitive effects in CdCr2S4 single crystals focusing on phonon anomalies in the fluctuation regime T < T* = 130 K and with the onset of ferromagnetic order T-c = 84.4 K. The observation of longitudinal optical and formerly infrared active modes for T < T-c is used to propose a loss of inversion symmetry and Cr off-centering. Further anomalies in intensity, frequency, and linewidth have an onset temperature T* that coincides with the onset of magnetocapacitive effects. The respective intensity anomalies are attributed in part to an enhanced electronic polarizability of displacements that modulate the Cr-S distance and respective hybridization. This is taken as a prerequisite to the proposed Cr off-centering. Anomalies due to the previously disregarded photodoping and resonance are analyzed comparing different excitation energies.