MnBi2Te4 has recently been established as an intrinsic antiferromagnetic (AFM) topological insulator and predicted to be an ideal platform to realize quantum anomalous Hall (QAH) insulator and axion insulator states. We performed comprehensive studies on the structure, nontrivial surface state and magnetotransport properties of this material. Our results reveal an intrinsic anomalous Hall effect arising from a non-collinear spin structure for the magnetic field parallel to the c-axis. We also observed remarkable negative magnetoresistance under arbitrary field orientation below and above the Neel temperature (TN), providing clear evidence for strong spin fluctuation-driven spin scattering in both the AFM and paramagnetic states. Further, we found that the nontrivial surface state opens a large gap (~85 meV) even at temperatures far above TN = 25K. These findings demonstrate that the bulk band structure of MnBi2Te4 is strongly coupled to the magnetic structure and that a net Berry curvature in momentum space can be created in a canted AFM state. In

Atomically flat thin films of topological insulator Bi

The quantum anomalous Hall (QAH) effect is a quintessential consequence of non-zero Berry curvature in momentum-space. The QAH insulator harbors dissipation-free chiral edge states in the absence of an external magnetic field. On the other hand, the topological Hall (TH) effect, a transport hallmark of the chiral spin textures, is a consequence of realspace Berry curvature. While both the QAH and TH effects have been reported separately, their coexistence, a manifestation of entangled chiral edge states and chiral spin textures, has not been reported. Here, by inserting a TI layer between two magnetic TI layers to form a sandwich heterostructure, we realized a concurrence of the TH effect and the QAH effect through electric field gating. The TH effect is probed by bulk carriers, while the QAH effect is characterized by chiral edge states. The appearance of TH effect in the QAH insulating regime is the consequence of chiral magnetic domain walls that result from the gate-induced Dzyaloshinskii-Moriya interaction and occur during the magnetization reversal process in the magnetic TI sandwich samples. The coexistence of chiral edge states and chiral spin textures potentially provides a unique platform for proof-of-concept dissipationless spintextured spintronic applications. Electronic band structures of nontrivial topology in momentum-space and magnetic chiral spin textures in real-space have attracted enormous attention in the past decade since they harbor elegant Berry curvature physics 1, 2, 3 . The intrinsic anomalous Hall (AH) effect is such an example: it is induced by the Berry curvature in momentum-space in ferromagnetic (FM) materials 4 and can even be quantized under certain circumstances, leading to the quantum anomalous Hall (QAH)effect. The QAH effect has been theoretically proposed 5, 6, 7, 8 and experimentally realized 9,10,11,12,13 in magnetically doped topological insulator (TI) films. On the other hand, chiral spin textures (e.g. skyrmions) provide another example of nontrivial topology, but in real-space. It has been shown that chiral spin textures can also induce a Hall current: this is known as the topological Hall (TH) effect and is generally regarded as the transport signature of non-zero spin chirality 3 . The TH effect has been experimentally observed in many metallic systems, such as MnSi 14, 15 , MnGe 16 , FeGe 17 , and SrIrO3/SrRuO3 interface 18,19 as well as magnetically doped TI films and heterostructures 20, 21 . The TH effect in these systems is usually observed accompanied by the AH effect. However, there is no conclusive evidence to date that the AH effect found in these metallic systems to be intrinsic, i.e., exclusively induced by the momentum-space Berry curvature 4 .The QAH and TH effects have been separately observed in magnetically doped TI 9, 10, 20, 21 , with distinctly different sample geometries. The QAH effect can be realized only in the insulating regime of a magnetic TI 9,10,11,12,13 while the TH effect is usually seen in metallic systems 20, 21 .In this Article, we re...

Non-coplanar spin textures with scalar spin chirality can generate effective magnetic field that deflects the motion of charge carriers, resulting in topological Hall effect (THE), a powerful probe of the ground state and low-energy excitations of correlated systems 1-3 . However, spin chirality fluctuation in two-dimensional ferromagnets with perpendicular anisotropy has not been considered in prior studies. Herein, we report direct evidence of universal spin chirality fluctuation by probing the THE above the transition temperatures in two different ferromagnetic ultra-thin films, SrRuO 3 and V doped Sb 2 Te 3 . The temperature, magnetic field, thickness, and carrier type dependences of the THE signal, along with our Monte-Carlo simulations, unambiguously demonstrate that the spin chirality fluctuation is a universal phenomenon in two-dimensional ferromagnets with perpendicular anisotropy. Our discovery opens a new paradigm of exploring the spin chirality with topological Hall transport in two-dimensional magnets and beyond 4-7 .