Zero-field-cooled exchange bias effect in phase-segregated La 2 − x A x CoMnO 6 − δ ( A = Ba,Ca,Sr ; x = 0 , 0.5 )

In the zero-field-cooled exchange bias (ZEB) effect, the unidirectional magnetic anisotropy is set at low temperatures even when the system is cooled in the absence of an external magnetic field. ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_{6}$ stands out as presenting the largest ZEB reported so far, while for ${\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoMnO}}_{6}$ the exchange bias field (${H}_{\text{EB}}$) is one order of magnitude smaller. Here we show that ${\mathrm{La}}_{1.5}{\mathrm{Ba}}_{0.5}{\mathrm{CoMnO}}_{6}$ also exhibits a pronounced shift of its magnetic hysteresis loop, with an intermediate ${H}_{\text{EB}}$ value with respect to Ca- and Sr-doped samples. To figure out the microscopic mechanisms responsible for this phenomenon, these compounds were investigated by means of synchrotron x-ray powder diffraction, Raman spectroscopy, muon spin rotation and relaxation, ac and dc magnetization, x-ray absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD). The parent compound ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ was also studied for comparison as a reference of a non-ZEB material. Our results show that the Ba-, Ca-, and Sr-doped samples present a small amount of phase segregation, and that the ZEB effect is strongly correlated to the system's structure. We also observed that mixed valence states ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ and ${\mathrm{Mn}}^{4+}/{\mathrm{Mn}}^{3+}$ are already present at the ${\mathrm{La}}_{2}{\mathrm{CoMnO}}_{6}$ parent compound, and that ${\mathrm{Ba}}^{2+}/{\mathrm{Ca}}^{2+}/{\mathrm{Sr}}^{2+}$ partial substitution at the ${\mathrm{La}}^{3+}$ site leads to a large increase of Co average valence, with a subtle augmentation of Mn formal valence. Estimates of the Co and Mn valences from the $L$-edge XAS indicate the presence of oxygen vacancies in all samples ($0.05\ensuremath{\le}\ensuremath{\delta}\ensuremath{\le}0.1$). Our XMCD results show a great decrease of Co moment for the doped compounds, and they indicate that the shift of the hysteresis curves for these samples is related to uncompensated antiferromagnetic coupling between Co and Mn.