Large drop in dielectric constant at ferrimagnetic ordering of CoCr$_{2}$O$_{4}$ thin film as observed by terahertz time domain spectroscopy*

Multiferroic spinel CoCr$_{2}$O$_{4}$ shows ferrimagnetic transition at $\sim $95 K and, at further low temperatures, it shows development of magnetic spin spirals simultaneous to a ferroelectric transition. We have deposited epitaxial CoCr$_{2}$O$_{4}$ thin film on MgO$ (100) $substrate and studied the film using temperature dependent terahertz (THz) time-domain spectroscopy. There is a large drop in the dielectric constant (\textit{$\varepsilon $}$_{1})$ to nearly half of its initial value at ferrimagnetic transition of CoCr$_{2}$O$_{4}$ thin films in the THz frequency range. This is contrary to few earlier studies by capacitance measurements which showed no significant change in the dielectric constant (\textit{$\varepsilon $}$_{1})$ at ferrimagnetic transition [1]. At lower temperatures, two dielectric anomalies were observed in the temperature dependent \textit{$\varepsilon $}$_{1,}$ which correspond to the onset of short-range magnetic spin spirals ($\sim $50 K) and the long-range ordering of the spirals at lower temperature ($\sim $26 K). Our results indicate a magnetoelectric effect at the ferrimagnetic transition of CoCr$_{2}$O$_{4}$ thin film in THz frequency range. [1] G. Lawes \textit{et al.}, Phys. Rev. B 74, 24413 (2006).