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

Abstract

We investigate exchange bias (EB) effect in two members with x = 0.4 and 0.6 of multiferroic Cu1−xCoxCr2O4 series, which are close to the interface between two structures at room temperature and represent with structures having I41/amd and Fd3¯m space groups, respectively. Both the compounds exhibit EB effect having EB field (HE) 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 (TN) close to 100 K. We note that higher HE correlates the higher coercivity (HC), 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 TN and ferroelectric ordering temperature (TFE). A structural transition to a polar Ima2 structure from I41/amd is observed close to 154 K, which coincides with TFE. Below TFE, 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.