Asymmetry of magnetic proximity interaction offers routes for control in semiconductors

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Single 2D monolayers of the semiconductor MoSe₂ were stacked atop thin ferromagnetic CrBr₃ via van der Waals assembly. Optical spectroscopy using right-circular and left-circularly polarized light revealed and quantified MPIs for both spin-up and spin-down excitons in the MoSe₂ monolayer (red and blue, respectively). [Image: Sarah Tasseff, Los Alamos National Laboratory]

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The ability to impart magnetic functionality into otherwise non-magnetic materials has exciting prospects for hybrid devices that combine, for example, the optical and electrical properties of semiconductors with additional tuning parameters that couple to magnetic and spin degrees of freedom. New research published in Nature Materials by a Los Alamos team explores these interesting “magnetic proximity interactions” (MPIs) in layered stacks comprising magnetic and non-magnetic 2D materials, suggesting routes to control electron spins in nominally non-magnetic semiconductors, without the use of applied magnetic fields.

As recent work has demonstrated, MPIs can be achieved by placing an atomically thin sheet of semiconductor crystal directly atop the clean crystalline surface of a ferromagnet. The intimate proximity of the two materials allows for the electrons in the two materials to “see” each other and interact quantum mechanically. The hybrid structure is held together only by weak van der Waals interactions between the two layers, rather than actual chemical bonds. These so-called “van der Waals heterostructures” are assembled layer-by-layer by mechanical stacking of individual 2D crystals of the constituent materials.  In sufficiently clean structures having pristine interfaces, MPIs originate in the nanometer-scale coupling between the spin-dependent electronic wavefunctions of the two materials. Historically, the influence of such MPIs has been regarded as an effective magnetic field acting on the nominally nonmagnetic semiconductor.

But the Los Alamos team’s work showed that the widely held picture of effective magnetic fields, while useful, is in fact fundamentally incomplete. Rather, the influence of MPIs is actually quite asymmetric. That is, electrons with spin-up and spin-down are not affected equally and oppositely, as would be the case for a real magnetic field. The team used circularly polarized optical spectroscopy of MoSe₂/CrBr₃ van der Waals structures to reveal strikingly different energy shifts for spin-up and spin-down excitons (electron-hole pairs) in the MoSe₂ semiconductor layer, due to MPIs from the ferromagnetic CrBr₃ layer. Importantly, spin-asymmetric MPIs were confirmed by density functional theory calculations, and shown to depend sensitively on the spin-dependent hybridization of overlapping electronic bands in the two materials. As such, asymmetric MPIs are likely a general feature of all magnetic/nonmagnetic hybrid van der Waals structures.

An important implication of the research is the possibility to selectively control specific spin states (and associated valley degrees of freedom) in 2D semiconductors through rational design of component materials and their stacking arrangement. Such combinations open up new possibilities for combining functionality such as information processing and nonvolatile storage.

See also News and Views feature, "Asymmetry in the Magnetic Neighborhood," in Nature Materials, authors T. Zhou & I. Zutic (University at Buffalo).

Reference

“Asymmetric magnetic proximity interactions in MoSe₂/CrBr₃ van der Waals heterostructures,” Nature Materials (2022); DOI: 10.1038/s41563-022-01424-w. Authors: Junho Choi, Christopher Lane, Jian-Xin Zhu and Scott A. Crooker (Los Alamos National Laboratory).

See also News and Views feature, "Asymmetry in the Magnetic Neighborhood," Nature Materials (2023); DOI: 10.1038/s41563-022-01466-0. Authors: T. Zhou and I. Zutic (University at Buffalo).

Funding and mission

Experimental studies at the National High Magnetic Field Laboratory were supported by the Los Alamos Laboratory Directed Research and Development program. The National High Magnetic Field Laboratory is supported by National Science Foundation (DMR-1644779), the state of Florida and the U.S. Department of Energy. Computational studies were supported in part by the Center for Integrated Nanotechnologies, a U.S. Department of Energy Basic Energy Sciences user facility, in partnership with the Los Alamos National Laboratory Institutional Computing Program for computational resources. Additional computations were performed at the National Energy Research Scientific Computing Center, a U.S. Department of Energy Office of Science user facility using National Energy Research Scientific Computing Center support. This work supports the Global Security mission area and the Materials for the Future capability pillar.

Technical contact: Scott Crooker (MPA-MAGLAB)