[Paper Review] Direct probe of ferromagnetic proximity effect at the interface in Fe/SnTe heterostructure by polarized neutron reflectometry

Ryota Akiyama, Ryo Ishikawa+8 more|arXiv (Cornell University)|Oct 23, 2019

Topological Materials and Phenomena1 references4 citations

TL;DR

This study directly probes the ferromagnetic proximity effect (MPE) at the Fe/SnTe heterointerface using polarized neutron reflectometry (PNR), demonstrating that magnetism penetrates ~3 nm into the topological crystalline insulator SnTe down to room temperature. The results confirm that interfacial magnetism is induced via MPE without disorder from magnetic doping, preserving topological surface states essential for quantum anomalous Hall effect and axion insulator states.

ABSTRACT

Introducing magnetic order into a topological insulator (TI) system has been attracting much attention with an expectation of realizing exotic phenomena such as quantum anomalous Hall effect (QAHE) or axion insulator states. The magnetic proximity effect (MPE) is one of the promising schemes to induce the magnetic order on the surface of TI without introducing disorder accompanied by doping magnetic impurities in TI. In this study, we investigate the MPE at the interface of a heterostructure consisting of a topological crystalline insulator (TCI) SnTe and Fe by employing polarized neutron reflectometry. The ferromagnetic order penetrates $\sim$ 3 nm deep into the SnTe layer from the interface with Fe, which persists up to room temperature. Our findings demonstrate that the interfacial magnetism is induced by the MPE on the surface of TCI preserving the coherent topological states, which is essential for the bulk-edge correspondence, without introducing disorder arising from a random distribution magnetic impurities. This opens up a way for realizing next generation electronics, spintronics, and quantum computational devices by making use of the characteristics of TCI.

Motivation & Objective

• To investigate the magnetic proximity effect (MPE) at the interface of a Fe/SnTe heterostructure, a system combining a ferromagnetic metal with a topological crystalline insulator (TCI).
• To determine the depth and persistence of induced magnetism in SnTe due to MPE, particularly whether it can preserve topological surface states.
• To avoid disorder from magnetic doping by using a heterostructure approach, enabling clean interfacial magnetism critical for topological quantum phenomena.
• To establish a platform for realizing dissipationless spintronic devices based on the quantum anomalous Hall effect (QAHE) or axion insulator states in TCI systems.

Proposed method

• Growth of a single-crystalline Fe/SnTe heterostructure on a CdTe/GaAs(001) substrate via molecular beam epitaxy (MBE) at optimized temperatures to ensure lattice matching and prevent interdiffusion.
• Use of reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD) to confirm epitaxial growth and structural quality of the heterostructure.
• Employment of cross-sectional transmission electron microscopy (TEM) and selected area electron diffraction (TED) to verify the epitaxial relationship and abrupt interface between Fe and SnTe layers.
• Polarized neutron reflectometry (PNR) at the J-PARC MLF BL17 SHARAKU beamline using a pulsed, highly polarized neutron beam (polarization >98.5%) to probe the depth-resolved magnetization profile.
• Data analysis via least-squares fitting using the Motofit program, combining PNR data with X-ray reflectivity (XRR) to decouple structural and magnetic profiles.
• Application of an external magnetic field (10 kOe) parallel to the sample plane to align the neutron spin and measure the magnetic scattering length density.

Experimental results

Research questions

• RQ1 To what depth does ferromagnetic order from Fe penetrate into the SnTe layer via the magnetic proximity effect?
• RQ2 Does the induced magnetism in SnTe persist up to room temperature, indicating robust interfacial coupling?
• RQ3 Can the magnetic proximity effect be established without introducing disorder from magnetic dopants, preserving the topological nature of SnTe?
• RQ4 Is the magnetization profile in SnTe consistent with a clean, abrupt interface without intermixing or secondary phases?
• RQ5 How does the MPE depth in the TCI SnTe compare to that observed in conventional Z2 topological insulators like Bi2Se3?

Key findings

• The magnetic proximity effect induces ferromagnetic order that penetrates approximately 3 nm into the SnTe layer from the Fe/SnTe interface, as directly measured by polarized neutron reflectometry.
• The induced magnetism persists up to room temperature, indicating strong interfacial coupling and potential for room-temperature topological spintronic applications.
• The interface between Fe and SnTe is abrupt with no observable intermixing or diffusion, confirmed by high-resolution TEM and TED analysis.
• The magnetization profile is well described by a superposition of two Gaussian peaks, indicating a well-defined interfacial magnetic layer.
• The proximity depth in SnTe (~3 nm) is comparable to or greater than that observed in Bi2Se3, a conventional Z2 topological insulator, suggesting enhanced MPE in TCIs.
• The results confirm that topological surface states in SnTe remain coherent and unperturbed by the proximity-induced magnetism, preserving the bulk-edge correspondence essential for QAHE and axion insulator states.