Mn掺杂YBaCuFeO$_{5}$中轨道驱动的自旋重取向
Orbitally driven spin reorientation in Mn doped YBaCuFeO$_{5}$
AI总结：
针对Mn掺杂YBaCuFeO₅中螺旋平面向c轴倾斜的实验现象，通过密度泛函理论计算揭示其自旋重取向由Cu最高占据轨道从dₓ²₋ᵧ²变为d_z²及阻挫交换作用共同驱动。
摘要：
缺氧层状钙钛矿YBaCuFeO$_{5}$（YBCFO）是一种罕见的第二类多铁材料，其铁电性由非公度螺旋磁有序驱动，据信可在高于室温的温度下实现。摆线螺旋而非螺旋螺旋有序是该材料中存在铁电性的关键要素。受近期一项关于Mn掺杂YBCFO的实验研究启发——该实验观察到，与母体化合物相比，在Fe位掺杂Mn后，螺旋平面更倾向于向晶体学$c$轴倾斜——我们利用密度泛函理论计算开展了详细的理论研究，以揭示这种自旋重取向背后的机制。我们在GGA+U+SO近似下的总能计算表明，Mn掺杂化合物中Fe/Cu自旋矩确实比母体化合物YBCFO中的更倾向于沿$c$轴排列。研究发现，最大的交换相互作用（$ab$平面内的Cu-Cu相互作用）随Mn掺杂浓度系统性降低，这与实验中观察到的转变温度下降现象一致。此外，掺杂化合物中双层间的Cu-Mn交换相互作用变为铁磁性，而母体化合物中对应的Cu-Fe交换为反铁磁性，这在公度磁有序中引发了阻挫。最重要的是，我们的电子结构计算显示，在掺杂化合物中，由于与Mn的d$_{z^2}$轨道发生杂化，Cu的最高占据轨道变为d$_{z^2}$；而在母体化合物中，Cu的最高占据轨道是d$_{x^2-y^2}$。因此我们认为，面外取向的Cu d$_{z^2}$轨道取代平面型d$_{x^2-y^2}$轨道被占据，再加上阻挫交换相互作用，共同驱动了掺杂化合物中自旋沿$c$轴排列。
英文摘要：
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.
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