Ames National Laboratory, Ames, Iowa, 50011, USA.
Department of Physics and Astronomy, Iowa State University, Ames, IA, 50011, USA.
Nat Commun. 2023 May 9;14(1):2658. doi: 10.1038/s41467-023-38174-5.
Ferromagnetic (FM) order in a two-dimensional kagome layer is predicted to generate a topological Chern insulator without an applied magnetic field. The Chern gap is largest when spin moments point perpendicular to the kagome layer, enabling the capability to switch topological transport properties, such as the quantum anomalous Hall effect, by controlling the spin orientation. In TbMnSn, the uniaxial magnetic anisotropy of the Tb ion is effective at generating the Chern state within the FM Mn kagome layers while a spin-reorientation (SR) transition to easy-plane order above T = 310 K provides a mechanism for switching. Here, we use inelastic neutron scattering to provide key insights into the fundamental nature of the SR transition. The observation of two Tb excitations, which are split by the magnetic anisotropy energy, indicates an effective two-state orbital character for the Tb ion, with a uniaxial ground state and an isotropic excited state. The simultaneous observation of both modes below T confirms that orbital fluctuations are slow on magnetic and electronic time scales < ps and act as a spatially-random orbital alloy. A thermally-driven critical concentration of isotropic Tb ions triggers the SR transition.
在二维 kagome 层中，铁磁（FM）有序预计会在没有外加磁场的情况下产生拓扑 Chern 绝缘体。当自旋矩垂直于 kagome 层指向时，Chern 隙最大，从而能够通过控制自旋方向来切换拓扑输运性质，例如量子反常霍尔效应。在 TbMnSn 中，Tb 离子的各向异性磁矩有效地在 FM Mn kagome 层中产生 Chern 态，而在 310 K 以上的易面各向异性自旋重定向（SR）跃迁为切换提供了一种机制。在这里，我们使用非弹性中子散射来提供对 SR 跃迁基本性质的关键见解。两个 Tb 激发态的观察，它们被磁各向异性能分裂，表明 Tb 离子具有有效的两态轨道特性，具有各向同性的基态和各向同性的激发态。在 T 以下同时观察到这两种模式证实了轨道涨落在磁和电子时间尺度< ps 上很慢，并且起到了空间随机轨道合金的作用。各向同性 Tb 离子的热驱动临界浓度触发了 SR 跃迁。
