當自旋極化的載流子絕熱地經過特定的實空間拓撲自旋結構時，這些自旋結構能像外加的磁場一樣對載流子施加額外的作用，使載流子獲得一定的貝裡相位，系統因此産生類似于霍爾效應一樣的橫向電壓。但和霍爾電壓不同，這種電壓既不和外加磁場成正比，也不和系統的磁化強度成正比，這一效應被稱之為'幾何霍爾效應'或'拓撲霍爾效應'，是倒空間貝裡相位所産生的反常霍爾效應在實空間所對應的物理現象。産生該效應的主要原因是系統中存在的拓撲自旋結構，這種拓撲自旋結構的一個典型代表是自旋電子學領域中被廣泛研究的磁斯格明子。近年來在B20化合物和重金屬多層結構的研究中發現，磁斯格明子能夠存在于特定對稱破缺的材料或者界面，而'幾何霍爾效應'也成為探測磁斯格明子等拓撲自旋結構的一種實驗手段。
beat365量子材料科學中的何慶林研究員和合作者在三維拓撲絕緣體和反鐵磁體的界面處發現了拓撲自旋結構存在的實驗證據，即幾何霍爾效應。更重要的是，由于材料系統的巧妙設計，利用了拓撲絕緣體(Bi，Sb)2Te3強大的自旋軌道耦合作用，以及A型反鐵磁MnTe的界面交換耦合，拓撲絕緣體的表面态被磁化；同時，利用界面處反鐵磁體的尼爾序對所産生的拓撲自旋結構的釘軋效應，實驗上能實現控制拓撲自旋結構的産生和湮滅。如圖所示，當在尼爾溫度以上時，對系統外加一個垂直方向的磁場并将系統冷卻至低溫，由于拓撲絕緣體的表面态被磁化，産生了反常霍爾效應。這時候，反鐵磁體在界面處會有大量受這種場冷作用而被釘軋的自旋結構。在磁化強度接近飽和的磁場附近，界面處形成大量中心自旋方向和釘軋自旋方向平行的拓撲自旋結構，因此在該磁場附近所産生的幾何拓撲效應非常明顯；當外加磁場反向時，同樣在磁化強度接近飽和的磁場附近，這時候界面會形成反拓撲自旋結構，其中心自旋方向和反鐵磁中釘軋自旋結構反平行。因此，反拓撲自旋結構不容易在釘軋自旋結構附近形成，因此隻能産生少量的反拓撲自旋結構，幾何拓撲效應微弱。以上新的效應是基于傳統的交換偏置作用，但又從中提煉出新的物理現象：傳統的交換偏置作用利用反鐵磁有序實現對鐵磁體中的磁矩翻轉的調控，而在這個研究中，反鐵磁有序所調控的是拓撲自旋結構，也即調節了拓撲電荷的形成機制。
該工作于2018年7月17日發表在知名學術期刊《自然-通訊》上。論文鍊接：https://www.nature.com/articles/s41467-018-05166-9.
該項工作由量子中心的何慶林研究員、美國加州大學洛杉矶分校的王康隆教授團隊、美國國家标準與技術研究院的Alexander J. Grutter博士和Brian J. Kirby博士、美國先進光源實驗室的Padraic Shafer博士和Elke Arenholz博士、北京工業大學的韓曉東教授團隊、美國加州大學河濱分校的Roger K. Lake教授團隊合作完成。其中，何慶林研究員、Gen Yin博士、Alexander J. Grutter博士為文章第一作者，何慶林研究員和王康隆教授為文章共同通訊作者。該項工作得到了國家重點研發計劃（2018YFA0305601）和中組部“青年千人”計劃的支持。
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圖：拓撲電荷的交換偏置機制示意圖。在反鐵磁體表面的釘軋自旋能幫助正拓撲電荷的成核（紅色圓），但抑制負拓撲電荷的成核（綠色圓）。
Figure: A schematic demonstration of the exchange-biased topological charges. The anchoring spins in the AFM layer assist the nucleation of positive topological charges (red circles) while prohibit the negative ones (green circles). |
Nature Communications reports Prof Qing Lin He et al.’s study on exchange biasing the topological charge using antiferromagnetism
Spin-polarized carriers adiabatically moving through certain real-space topological spin textures can obtain a Berry’s phase as though they were in an applied magnetic field, resulting in a transverse carrier transport. Induced by this transverse transport, an extra Hall voltage can be observed, which is proportional to neither the applied external field nor the total magnetization. This spin-texture-induced extra Hall component is usually referred to as the 'topological' or 'geometric' Hall effect (GHE), which is a real-space counterpart of the k-space Berry phase in an intrinsic anomalous Hall effect (AHE). GHE is typically observed near magnetic reversal, within a window of the applied magnetic field and the temperature. Since the discovery of magnetic skyrmions in B20 compounds and heavy-metal multilayers, GHE has been considered as an experimental signature of topological spin textures (such as skyrmions) and, along with other real-space detection methods, it enables mapping of the magnetic topological phase diagram.
Recently, Prof Qing Lin He in ICQM – Peking University reported an experimental observation of the GHE modulated by uncompensated pinned spins in the antiferromagnetic (AFM) layer at the interface between an intrinsic TI thin film of (Bi,Sb)2Te3 and an AFM layer of MnTe. This suggests that a topologically nontrivial chiral spin texture is induced in the TI through interactions with the spin-polarized Mn planes of the MnTe. We find that the magnetic topological charge can be manipulated by a ‘seeding effect’ of pinned spins in the AFM layer. Systematic experimental results of the carrier magneto-transport, neutron scattering, and magnetic X-ray absorption spectroscopy support that the interfacial FM layer is induced in the TI through proximity interactions with the AFM layer.
Experimentally, it has been shown that special domain nucleation patterns can be induced by the spins in an adjacent AFM layer due to interfacial exchange coupling. In the following, it will be shown that this exchange coupling can result in a ‘seeding effect’ for the spin-texture topology. The microscopic picture is schematically shown by the four scenarios illustrated in the figure. After positive field cool (FC), some pinned spins can be frozen in the AFM layer along the FC direction (black arrows) due to thermoremanence. When the applied field sweeps to (i), positive topological charges [red circles with up central spins in (i)] are created through interactions with the pinned spins. After the saturation along a negative field (ii), these positive topological charges are annihilated. When the field scans to (iii), negative topological charges are prohibited by the pinning spins and therefore are more likely to nucleate outside the spin-pinned regions, but with a lower density [green circles with down central spins in (iii)]. These negative charges again vanish after magnetic saturation (iv). A similar modulation of topological charge occurs after a negative FC.
The above work was published on Nature Communications on July 17, 2018. The link to this paper is: https://www.nature.com/articles/s41467-018-05166-9.
This work was carried out by Prof Qing Lin He in ICQM, the group led by Prof Kang L Wang in UCLA, Dr. Alexander J. Grutter and Dr. Brian J. Kirby in National Institute of Standards and Technology (NIST), Dr. Padraic Shafer and Dr. Elke Arenholz in Advanced Light Source (ALS), the group led by Prof Xiaodong Han in Beijing University of Technology, and the group led by Prof Roger K. Lake in UC-Riverside. Prof Qing Lin He, Dr. Gen Yin, and Dr. Alexander J. Grutter are the first authors of the paper, while Prof Qing Lin He and Prof Kang L. Wang are the corresponding authors. This work is supported in part by the National Key R&D Program of China (Grant No. 2018YFA0305601) and the National Thousand-Young-Talents Program.
