Structural distortion driven enhancement of exchange bias effect in Cu1−xCoxCr2O4

We investigate exchange bias (EB) effect in two members with $x$ = 0.4 and 0.6 of multiferroic Cu$_1−x$Co$_x$Cr$_2$O$_4$ series, which are close to the interface between two structures at room temperature and represent with structures having I4$_1$/amd and Fd $\bar{3}$ m space groups, respectively. Both the compounds exhibit EB effect having EB field (H$_E$) of 244 and 870 Oe at 2 K for $x$ = 0.4 and 0.6, respectively with a cooling field of 10 kOe, which decrease with increasing temperature and disappear near their ferrimagnetic ordering temperatures ($T_N$) close to 100 K. We note that higher H$_E$ correlates the higher coercivity ($H_C$), which is found highest for $x$ = 0.6 in the entire series of compounds. Our low temperature synchrotron diffraction studies for $x$ = 0.6 confirm magnetoelastic as well as electroelastic coupling observed close to T$_N$ and ferroelectric ordering temperature ($T_FE$). A structural transition to a polar Ima2 structure from I4$_1$/amd is observed close to 154 K, which coincides with $T_FE$. Below $T_FE$, thermal variation of structural distortion defined by (a − b)/(a + b) demonstrates significant magnetoelastic coupling. More importantly, the distortion parameter is found significantly higher than the rest member of the series and provides a hint for the highest corecivity in the entire series and correlates the higher EB effect. Current study demonstrates a correlation between structural distortion and EB effect and provides a clue of improving EB effect through the crystal structural engineering.