| Popis: |
Oxygen-deficient layered perovskite YBaCuFeO$_{5}$ (YBCFO) is one 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 rather than helical spiral order is essential ingredient for the existence of ferroelectricity in this material. Motivated by a recent experimental work on Mn-doped YBCFO where the spiral plane is observed to cant more towards the crystallographic $c$ axis upon Mn doping at Fe sites compared to that in the parent compound, we performed a detailed theoretical investigation using the density functional theory calculations to understand the mechanism behind such spin reorientation. Our total energy calculations, within GGA+U+SO approximation, reveal that Fe/Cu spin moments indeed align more towards $c$ axis in the Mn-doped compounds than they were in parent compound YBCFO. Largest exchange interaction (Cu-Cu in the $ab$-plane) is observed to decrease systematically with Mn doping concentration reflecting the lowering of transition temperature seen in experiment. Further, the inter-bilayer Cu-Mn exchange interaction becomes ferromagnetic in the doped compound whereas the corresponding Cu-Fe exchange was antiferromagnetic in the parent compound giving rise to frustration in the commensurate magnetic order. Most importantly, our electronic structure calculations reveal that in the doped compounds because of hybridization with the d$_{z^2}$ orbital of Mn, the highest occupied Cu orbital becomes d$_{z^2}$ as opposed to the parent compound where the highest occupied Cu orbital is d$_{x^2-y^2}$. Therefore, we believe that the occupancy of out of plane oriented Cu d$_{z^2}$ orbital in place of planar d$_{x^2-y^2}$ orbital along with the frustrating exchange interaction drive the spins to align along $c$ in doped compound. |
