Raman Scattering Studies of Magnons and Magnetodielectric Effects in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$*

The multiferroic spinel CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ exhibits magnetodielectric behavior whose origin is a subject of controversy. We present a Raman spectroscopic study of the magnon spectrum of CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ as functions of temperature, pressure, and magnetic field, with the aim of elucidating the microscopic origin of magnetodielectric behavior. Below T$_{\mathrm{C}}=$ 94 K, we observe a 16 cm$^{\mathrm{-1}}$ magnon mode with an anomalously large Raman intensity that reflects large magneto-optical coupling in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$. The strong magnetic-field-induced suppression of the magnon Raman intensity in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ suggests that the magnetodielectric behavior in CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$ arises from the field-dependent suppression of magnetic fluctuations that are strongly coupled to long-wavelength phonons. Applied pressure suppresses the Raman susceptibility and field-dependence of the magnon, demonstrating that strain can be used to sensitively tune the magnetodielectric and magneto-optical properties of CoCr$_{\mathrm{2}}$O$_{\mathrm{4}}$.