Unraveling Proximity and Topology at Interfaces with Next Generation Neutron Reflectometry

Alexander J. Grutter, NIST

Whether acting as a platform for quantum transport effects or ultra-efficient spintronics, heterostructures incorporating topologically nontrivial materials are among the most exciting playgrounds in condensed matter physics. Despite their promise, topological spintronic devices represent a difficult materials engineering challenge wherein the need to introduce magnetic order must be balanced with ensuring that topologically nontrivial conduction channels dominate the transport behavior. These competing requirements are highlighted by the first reported quantum anomalous Hall (QAH) insulator, Cr-doped (Bi,Sb)2Te3, where gap-inhomogeneity is thought to suppress the quantization temperature.[1,2] While this issue may be mitigated by increasing dopant density, defect channels rapidly come to dominate the conduction and prove equally detrimental to observing the physics of interest.
It is in this context that magnetic proximity effects have drawn considerable interest. By growing an ordered magnetic material in direct contact with the relevant electronic states, magnetic order may be induced through proximity without the introduction of additional defects. Despite the successful realization of a proximity-induced QAH effect, this approach has yielded no improvement in quantization temperature, highly inconsistent reports of ordering temperatures, and even disagreement over the existence of proximity effects in many systems.[3-5] A proper understanding of magnetic proximity effects at topologically nontrivial interfaces hinges critically on our ability to precisely isolate the properties of the interface from the bulk of the system. By decomposing the magnetic and electronic properties on a layer-by-layer and element-resolved basis, new quantum material systems may be robustly understood and designed. In this talk, we will examine approaches for accurately identifying magnetic proximity effects and other forms of magnetic interface coupling in systems such as (Bi,Sb)2Te3 and Cd3AS2, with a special emphasis on combining polarized neutron reflectometry with X-ray scattering, spectroscopy and electron microscopy.[6-8] We will conclude with a discussion on the future of ultra-sensitive probes of magnetic interfaces and the potential impact from highly multiplexing neutron instrumentation.

[1] C.-Z. Chang et al., Science 340, 167 (2013)
[2] E. O. Lachman et al., Science Advances 1, e150074 (2015)
[3] F. Katmis et al., Nature 533, 513 (2016)
[4] A. I. Figueroa et al., Physical Review Letters 125, 226801 (2020)
[5] R. Watanabe et al., Applied Physics Letters 115, 102403 (2019)
[6] Q. L. He et al., Nature Materials 16, 94 (2017)
[7] C.-Y. Yang et al., Science Advances 6, eaaaz8463 (2020)
[8] W. Yanez et al., Physical Review Applied 16, 054031 (2021)

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PDF file of the talk available here