Small Co-doping induced magnetic and electrical transitions in single crystal CaF⁢e0.95⁢C⁢o0.05⁢O3

Abstract

CaFe⁢O3 and CaF⁢e0.95⁢C⁢o0.05⁢O3 single crystals were grown by combing floating-zone method with high oxygen pressure treatment. Both crystals show charge disproportionation of Fe (F⁡e4+→F⁡e3++F⁡e5+) accompanied by a metal-to-insulator transition as well as a crystal structural transformation from Pbnm to 𝑃⁢21/𝑛. However, the slight introduction of Co significantly suppresses the critical temperature of charge disproportionation from 290 K in CaFe⁢O3 to 260 K in CaF⁢e0.95⁢C⁢o0.05⁢O3. Different from the single antiferromagnetic phase transition as observed in the polycrystalline CaFe⁢O3, two sequential antiferromagnetic transitions are found to occur in these two single crystals with the Néel temperatures around 119 and 112 K for CaFe⁢O3 crystal and 109 and 98 K for CaF⁢e0.95⁢C⁢o0.05⁢O3 crystal, respectively. Neutron diffraction illustrates the formation of a spiral antiferromagnetic structure. Moreover, as the temperature further decreases to 65 K, a third magnetic transition accompanied by a second electrical transition is observed in the slightly Co-doped CaF⁢e0.95⁢C⁢o0.05⁢O3 crystal. First-principles calculations suggest that the introduction of a moderate amount of Co and the peculiar spiral spin texture play an important role in the presence of such new magnetic and electrical transitions.