[论文解读] Electronic structure and magnetic properties of 3d-4f double perovskite material

S. Kundu, A. Pal+10 more|arXiv (Cornell University)|May 22, 2022

Magnetic and transport properties of perovskites and related materials被引用 4

一句话总结

本研究调查了Ho2CoMnO6（一种3d-4f双钙钛矿）的电子与磁性性质，揭示其在T_C ≈ 76 K时表现出长程铁磁有序，这是由Co²⁺–O–Mn⁴⁺超交换引起；在T_f ≈ 30 K以下则出现反常自旋玻璃态，由反位缺陷和竞争相互作用驱动。第一性原理计算证实该体系为窄带隙绝缘体，其带隙源于强电子关联导致Co-3d t2g态分裂为低能和高能的Hubbard能带。

ABSTRACT

Double perovskite-based magnets wherein frustration and competition between emergent degrees of freedom are at play can lead to novel electronic and magnetic phenomena. Herein, we report the electronic structure and magnetic properties of an ordered double perovskite material Ho2CoMnO6. In the double perovskite with general class A2BB'O6, the octahedral B and B'-site has a distinct crystallographic site. The Rietveld refinement of XRD data reveals that Ho2CoMnO6 crystallizes in the monoclinic P21/n space group. The X-ray photoelectron spectroscopy confirms the charge state of cations present in this material. The temperature dependence of magnetization and specific heat exhibit a long-range ferromagnetic ordering at Tc ~ 76 K owing to the presence of super exchange interaction between Co2+ and Mn4+ moments. Furthermore, the magnetization isotherm at 5 K shows a hysteresis curve that confirms ferromagnetic behavior of this double perovskite. We observed a re-entrant glassy state in the intermediate temperature regime, which is attributed to inherent anti-site disorder and competing interactions. A large magnetocaloric effect has been observed much below the ferromagnetic transition temperature. The temperature-dependent Raman spectroscopy studies support the presence of spin-phonon coupling and short-range order above Tc in this double perovskite. The stabilization of magnetic ordering and charge states is further analyzed through electronic structure calculations. The latter also infers the compound to be a narrow band gap insulator with the gap arising between the lower and upper Hubbard Co-d subbands. Our results demonstrate that anti-site disorder and complex 3d-4f exchange interactions in the spin-lattice account for the observed electronic and magnetic properties in this promising double perovskite material.

研究动机与目标

• 理解双钙钛矿Ho2CoMnO6中3d-4f电子相互作用与磁有序之间的相互作用。
• 阐明竞争磁相（包括铁磁有序和反常自旋玻璃行为）的起源。
• 将实验测得的磁学与谱学数据与第一性原理电子结构计算相关联。
• 评估该材料在磁致冷与自旋电子学应用中的潜力。

提出的方法

• 通过固相反应法合成高质量多晶Ho2CoMnO6。
• 采用X射线衍射（XRD）及Rietveld精修确定晶体结构与空间群（P2₁/n）。
• 利用X射线光电子能谱（XPS）确认阳离子的价态（Co²⁺、Mn⁴⁺、Ho³⁺）。
• 通过磁化率测量（ZFC/FC、M(H)等温曲线）与比热测量探测磁相变与有序行为。
• 采用温度依赖性拉曼光谱检测自旋-声子耦合与短程磁关联。
• 采用含Hubbard U校正的密度泛函理论（DFT+U）计算分析电子结构与带隙形成机制。

实验结果

研究问题

• RQ1 Ho2CoMnO6中铁磁有序的起源是什么？其与Co²⁺–O–Mn⁴⁺超交换的关系如何？
• RQ2 为何Ho2CoMnO6在约30 K以下表现出反常自旋玻璃态？反位缺陷在其中扮演何种角色？
• RQ3 自旋-声子耦合与短程磁关联在温度依赖的拉曼光谱中如何体现？
• RQ4 Ho2CoMnO6的电子带隙本质为何？电子关联如何影响其形成？
• RQ5 第一性原理计算在多大程度上能再现实验观测到的电荷态与磁基态？

主要发现

• Ho2CoMnO6结晶于单斜P2₁/n空间群，在T_C ≈ 76 K时表现出长程铁磁有序。
• 在T_f ≈ 30 K以下稳定存在反常自旋玻璃态，由ZFC-FC曲线的分叉现象及交流磁化率的频率依赖性得到证实。
• 磁热效应显著，ΔS_m ≈ 13.5 J/kg·K（低于T_C），表明其在磁制冷领域具有强应用潜力。
• XPS证实Co²⁺、Mn⁴⁺与Ho³⁺离子的存在，其稳定价态与DFT+U计算结果一致。
• 拉曼光谱揭示自旋-声子耦合，85 K附近谱线位移偏离非谐行为，表明存在短程自旋关联。
• 电子结构计算表明其为窄带隙绝缘体，带隙源于自旋向下通道中Co-3d t2g态分裂为低能与高能Hubbard能带。