Orbitally driven spin reorientation in Mn-doped YBaCuFeO5

Abstract

The oxygen-deficient layered perovskite YBaCuFeO5 (YBCFO) is a rare type-II multiferroic material where ferroelectricity, driven by incommensurate spiral magnetic order, is believed to be achievable up to temperatures higher than room temperature. A cycloidal spiral order rather than a helical spiral order is an essential ingredient for the existence of ferroelectricity in this material. Motivated by a recent experimental study on Mn-doped YBCFO where the spiral plane was observed to cant more towards the crystallographic c-axis upon Mn doping at Fe sites than in the parent compound, we performed a detailed theoretical investigation using density functional theory calculations to understand the mechanism behind such spin reorientation. Our total energy calculations, within the generalized gradient approximation plus U plus spin–orbit coupling approach, reveal that Fe/Cu spin moments indeed align more towards the c-axis in Mn-doped YBCFO than in the parent compound. The largest exchange interaction (Cu–Cu in the a−b plane) is observed to decrease systematically with Mn doping concentration, reflecting the lowering of the transition temperature seen in experiments. Further, the inter-bilayer Cu–Mn exchange interaction becomes ferromagnetic in Mn-doped YBCFO, whereas the corresponding Cu–Fe exchange is antiferromagnetic in the parent compound, giving rise to frustration in the commensurate magnetic order. Most importantly, our electronic structure calculations reveal that in Mn-doped YBCFO because of hybridization with the Mn dz2 orbital, the highest occupied Cu orbital becomes the dz2 orbital as opposed to the dx2−y2 orbital in the parent compound. Therefore, we believe that the occupancy of the out-of-plane-oriented Cu dz2 orbital in place of the planar dx2−y2 orbital along with the frustrating exchange interaction drives the spins to align along the c-axis in Mn-doped YBCFO.