Phonon anomalies and possible local lattice distortions in giant magnetocapacitive CdCr${}_2$S${}_4$

We investigate the origin of large magnetocapacitive effects in CdCr${}_2$S${}_4$ 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.

Figure 1

RS spectra of CdCr${}_2$S${}_4$ as a function of temperature taken with λ $=$ 532.1 nm. The curves are shifted for clarity. The phonon lines are marked by dashed lines and assigned according to the high-temperature structure (Ref. 15).

Figure 2

RS phonon frequency of CdCr${}_2$S${}_4$ as a function of temperature. The fluctuation regime between $T_c$ $=$ 84.4 K and $T$* $=$ 130 K is marked by a dashed bar. Arrows mark the approximate frequency of LO phonons at $T$ $=$ 10 K from earlier IR absorption experiments (Ref. 17).

Figure 3

Integrated RS intensity of CdCr${}_2$S${}_4$ as a function of temperature for the high-energy and the low-energy modes in the right panel (an exciting laser wavelength of λ $=$ 532.1 nm). The open symbols in the left and right panels correspond to the low-temperature modes at 106, 352, and 390 cm${}^{-1}$, respectively. The inset in the middle panel shows the results of the RS experiment with λ $=$ 632.8 nm.

Figure 4

The Raman phonon widths as a function of temperature of CdCr${}_2$S${}_4$.

Figure 5

(a) Evolution of the low-frequency $T_{2g}$ mode with an excitation wavelength of λ $=$ 532.1 nm at $T$ $=$ 10, 25, 65, 80, 95, 110 K, from top to bottom. (b) and (c) RS experiments with λ $=$ 488 nm (top, dashed line), 532.1 nm (middle, dotted line), and 632.8 nm (bottom) at $T$ $=$ 10 K. Curves have been shifted for clarity.

Figure 6

Displacement patterns and excitation energies of different modes that lead to a large modulation of the Cr-S hybridization (Refs. 15 and 23).