Critical analysis of proximity-induced magnetism in MnTe/Bi₂Te₃ heterostructures

Physical Review Materials American Physical Society (APS) 6:5 (2022) 53402

Authors:

G Awana, R Fujita, A Frisk, P Chen, Q Yao, Aj Caruana, Cj Kinane, N-J Steinke, S Langridge, P Olalde-Velasco, Ss Dhesi, G van der Laan, Xf Kou, Sl Zhang, T Hesjedal, D Backes

Abstract:

An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in MnTe/Bi2Te3 heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the Bi2Te3 layer. Furthermore, the interface between MnTe and Bi2Te3 is not abrupt, but extending over ∼2.2 nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.

Robust kagome electronic structure in the topological quantum magnets XMn6Sn6 (X=Dy,Tb,Gd, Y)

Physical Review B 105:15 (2022)

Authors:

X Gu, C Chen, WS Wei, LL Gao, JY Liu, X Du, D Pei, JS Zhou, RZ Xu, ZX Yin, WX Zhao, YD Li, C Jozwiak, A Bostwick, E Rotenberg, D Backes, LSI Veiga, S Dhesi, T Hesjedal, G Van Der Laan, HF Du, WJ Jiang, YP Qi, G Li, WJ Shi, ZK Liu, YL Chen, LX Yang

Abstract:

Crystal geometry can greatly influence the emergent properties of quantum materials. As an example, the kagome lattice is an ideal platform to study the rich interplay between topology, magnetism, and electronic correlation. In this work, combining high-resolution angle-resolved photoemission spectroscopy and ab initio calculation, we systematically investigate the electronic structure of XMn6Sn6 (X=Dy,Tb,Gd,Y) family compounds. We observe the Dirac fermion and the flat band arising from the magnetic kagome lattice of Mn atoms. Interestingly, the flat band locates in the same energy region in all compounds studied, regardless of their different magnetic ground states and 4f electronic configurations. These observations suggest a robust Mn magnetic kagome lattice across the XMn6Sn6 family, thus providing an ideal platform for the search for, and investigation of, new emergent phenomena in magnetic topological materials.

Robust Kagome Electronic Structure in Topological Quantum Magnets XMn6Sn6 (X = Dy, Tb, Gd, Y)

ArXiv 2203.10542 (2022)

Authors:

X Gu, C Chen, WS Wei, JY Liu, X Du, D Pei, JS Zhou, RZ Xu, ZX Yin, WX Zhao, YD Li, C Jozwiak, A Bostwick, E Rotenberg, D Backes, LSI Veiga, S Dhesi, T Hesjedal, G van der Laan, HF Du, WJ Jiang, YP Qi, G Li, WJ Shi, ZK Liu, YL Chen, LX Yang

Covalent mixing in the two-dimensional ferromagnet CrSiTe₃ evidenced by magnetic x-ray circular dichroism

physica status solidi (RRL) - Rapid Research Letters Wiley 16:4 (2021) 2100566

Authors:

Barat Achinuq, Ryuji Fujita, Wei Xia, Yanfeng Guo, Peter Bencok, Gerrit van der Laan, Thorsten Hesjedal

Abstract:

The low-temperature electronic structure of the van der Waals ferromagnet CrSiTe3 has been investigated. This ferromagnetic semiconductor has a magnetic bulk transition temperature of 33 K, which can reach up to 80 K in single- and few-layer flakes. X-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr L2,3 and Te Mb edges on in vacuo cleaved single crystals, give strong evidence for hybridization-mediated super-exchange between the Cr atoms. The observed chemical shift in the XAS, as well as the comparison of the XMCD with the calculated Cr L2,3 multiplet spectra, confirm a strongly covalent bond between the Cr 3d(eg) and Te 5p states. Application of the XMCD sum rules gives a non-vanishing orbital moment, supporting a partial occupation of the eg states, apart from the t2g. Also, the presence of a non-zero XMCD signal at the Te Mb edge confirms a Te 5p spin polarization due to mixing with the Cr eg bonding states. The results strongly suggest that superexchange, instead of the previously suggested single ion anisotropy, is responsible for the low-temperature ferromagnetic ordering of 2D materials such as CrSiTe3 and CrGeTe3. This demonstrates the interplay between electron correlation and ferromagnetism in insulating two-dimensional materials.

Superposition of emergent monopole and antimonopole in CoTb thin films

Physical Review Letters American Physical Society 127 (2021) 217201

Authors:

Yao Guang, Kejing Ran, Junwei Zhang, Yizhou Liu, Senfu Zhang, Xuepeng Qiu, Yong Peng, Xixiang Zhang, Markus Weigand, Joachim Graefe, Gisela Schuetz, Gerrit van der Laan, Thorsten Hesjedal, Shilei Zhang, Guoqiang Yu, Xiufeng Han

Abstract:

A three-dimensional singular point that consists of two oppositely aligned emergent monopoles is identified in continuous CoTb thin films, as confirmed by complementary techniques of resonant elastic x-ray scattering, Lorentz transmission electron microscopy, and scanning transmission x-ray microscopy. This new type of topological defect can be regarded as a superposition of an emergent magnetic monopole and an antimonopole, around which the source and drain of the magnetic flux overlap in space. We experimentally prove that the observed spin twist seen in Lorentz transmission electron microscopy reveals the cross-section of the superimposed three-dimensional structure, providing a straightforward strategy for the observation of magnetic singularities. Such a quasi particle provides an excellent platform for studying the rich physics of emergent electromagnetism.