Thermally assisted and magnetic field driven isostructural distortion of spinel structure and occurrence of polar order inCoCr2S4

We report appearance of polar order with a ordering temperature $({T}_{C})$ at 28 K, which is well below the ferrimagnetic order at 225 K $({T}_{N})$ for ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$. Intriguingly, the value of spontaneous electric polarization $(P)$ is $\ensuremath{\sim}122 \ensuremath{\mu}\mathrm{C}/{\mathrm{m}}^{2}$ at 15 K, which is the second largest value in Cr octahedra-based spinels after ${\mathrm{CdCr}}_{2}{\mathrm{S}}_{4}$. Incidentally, the $P$ value is $\ensuremath{\sim}60$ times larger than the value of $P$ for the oxide counterpart ${\mathrm{CoCr}}_{2}{\mathrm{O}}_{4}$. The significant magnetoelectric coupling is verified from the magnetodielectric response and magnetic field dependent enhancement of $P$. We note that the field-dependent dielectric permittivity scales linearly to the squared magnetization in the low field regime below $\ensuremath{\sim}10$ kOe as described by the Ginzburg-Landau theory. Synchrotron diffraction studies over a wide temperature range, $15\text{--}300$ K, illustrate strong magnetoelastic coupling at ${T}_{N}$ and isostructural distortion at ${T}_{C}$. Analyses of the diffraction patterns reveal that the occurrence of polar order involves expansion of Co tetrahedra and contraction of Cr octahedra of the spinel structure and these distortions are further enhanced driven by the magnetic field. The delicate interplay between magnetoelastic, magnetoelectric, and electroelastic couplings in ${\mathrm{CoCr}}_{2}{\mathrm{S}}_{4}$ proposes the system as a potential candidate in multiferroics.