[论文解读] Magnetic transitions in Sr-doped YBaCuFeO$_5$: A DFT and QMC study

Dibyendu Dey, Snehasish Nandy+3 more|arXiv (Cornell University)|Mar 21, 2017

Physics of Superconductivity and Magnetism被引用 1

一句话总结

本研究利用DFT（LSDA+U）和量子蒙特卡罗（QMC）方法，探究了Sr掺杂对YBaCuFeO₅中磁性转变的影响。结果表明，Fe-Cu的铁磁耦合随Sr掺杂而竞争性变化，而Fe-Fe/Cu-Cu的反铁磁耦合则单调减弱；关键的是，FM NNN相互作用随掺杂增强，使顺磁相到共格相的转变温度提高至x=0.5，与实验观测一致。

ABSTRACT

Mutiferroic materials where electric polarization is induced by spiral magnetic phase are immensely important for technological applications. Recently, the transition temperature of the spiral phase in YBaCuFeO$_{5}$ is found to rise to the room temperature range through A-site doping. Motivated by this remarkable observation, we investigate the magnetic phase transitions in YBa$_{1-x}$Sr$_{x}$CuFeO$_{5}$ in the entire doping range $0\le x \le 1$ using first-principles density functional theory calculations (DFT) followed by a Quantum Monte Carlo (QMC) calculation. We estimate the in-plane and out of plane nearest (NN) and next nearest neighbour (NNN) magnetic exchange interactions between Fe-Cu, Fe-Fe and Cu-Cu spins, as a function of doping, for the experimentally observed commensurate magnetic structure from DFT calculations within LSDA+U approximation. We observe a strong dependence of the exchange interactions on Sr doping. While the intra-bipyramidal, Fe-Cu ferromagnetic (FM) coupling show competing trends with doping for Sr and Ba separated bilayers, the inter-bipyramidal, Fe-Fe or Cu-Cu antiferromagnetic (AFM) couplings decrease monotonically with doping. The FM NNN interaction, which is crucial for the formation of spiral magnetic state, is also found to increase with doping, though the magnitude is small. The calculated exchange interactions are then used in our QMC calculation to study the temparature-dependence of the magnetic suceptibility and specific heat. We observe that the paramagnetic to commensurate phase transition temperature increases with doping till x=0.5 and decreases beyond. Our observations are consistent with the experimental findings on these systems.

研究动机与目标

• 理解在整个掺杂范围（0 ≤ x ≤ 1）内，Sr掺杂YBaCuFeO₅中的磁性相变行为。
• 确定Sr掺杂如何改变Fe-Cu、Fe-Fe和Cu-Cu磁交换相互作用。
• 将交换相互作用的变化与实验观测到的螺旋磁性转变温度上升趋势联系起来。
• 通过基于DFT预测的交换参数进行QMC模拟，验证温度依赖的磁化率和比热。

提出的方法

• 采用LSDA+U方法进行第一性原理DFT计算，以获得YBa₁₋ₓSrₓCuFeO₅的电子结构和磁交换相互作用。
• 从DFT优化的共格磁结构中提取Fe-Cu、Fe-Fe和Cu-Cu自旋之间的最近邻（NN）和次近邻（NNN）交换耦合常数。
• 利用量子蒙特卡罗（QMC）方法，基于DFT获得的交换参数模拟温度依赖的磁化率和比热。
• 系统分析整个x范围内交换相互作用和相变温度的掺杂依赖性。
• 将QMC预测的相变温度与实验数据进行比较，以验证理论预测的准确性。

实验结果

研究问题

• RQ1 Sr掺杂如何影响YBaCuFeO₅中Fe-Cu、Fe-Fe和Cu-Cu磁交换相互作用的强度与符号？
• RQ2 次近邻铁磁相互作用在稳定螺旋磁相中起什么作用？
• RQ3 顺磁相到共格磁相的转变温度如何随Sr掺杂而演变？
• RQ4 DFT预测的交换参数在多大程度上能再现实验观测到的磁性转变趋势？

主要发现

• 在被Ba或Sr分隔的双层结构中，Fe-Cu铁磁耦合随Sr掺杂表现出竞争性趋势。
• Fe-Fe和Cu-Cu反铁磁耦合随Sr掺杂量增加而单调减弱。
• 对螺旋磁序至关重要的次近邻铁磁相互作用随掺杂增强，尽管其数值较小。
• 顺磁相到共格相的转变温度随掺杂量增加而上升，在x = 0.5时达到峰值，随后下降，与实验观测一致。
• 基于DFT交换参数的QMC模拟成功再现了实验观测到的转变温度趋势。