Inverse exchange bias effects and magnetoelectric coupling of the half-doped perovskite-type chromites Gd0.5Sr0.5CrO3 and Gd0.5Ca0.5CrO3

The ${\mathrm{Cr}}⁴⁺$ oxidation state with two electrons in the $\mathrm{Cr}\phantom{\rule{0.28em}{0ex}}3d$ shell is not often observed in perovskite-type oxides, as high pressures and temperatures are generally required to stabilize the octahedral coordination. Herein, we present a comparative study of the half-doped perovskite-type chromites ${\mathrm{Gd}}_0.5{\mathrm{Sr}}_0.5{\mathrm{CrO}}₃$ (GSCO) and ${\mathrm{Gd}}_0.5{\mathrm{Ca}}_0.5{\mathrm{CrO}}₃$ (GCCO). Fifty percent of the Cr occurs in the ${\mathrm{Cr}}⁴⁺$ oxidation state after high-pressure synthesis at 6 GPa and 1200 \ifmmode^\circ\else\textdegree\fi{}C. The materials were investigated using synchrotron x-ray diffraction, magnetization, heat capacity, and dielectric measurements. The diffraction patterns show that GSCO and GCCO crystallize in orthorhombic ($Pnma$) structures with different degrees of local lattice distortion. GSCO exhibits a long-range magnetic order at temperatures of $<98\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, accompanied by magnetization reversal, suggesting that the magnetic ground state is ferrimagnetic. In contrast, GCCO displays antiferromagnetic characters at temperatures $<\ensuremath{\sim}100\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. In addition, GSCO exhibits a crossover between conventional and inverse exchange bias effects at low temperatures ($<50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$). This is likely caused by asymmetric exchange Dzyaloshinskii-Moriya interactions between the Cr ions of different valences (+3 and +4). Furthermore, significant magnetoelectric coupling at the onset of the magnetic order is supported by temperature-dependent dielectric measurements.