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Wednesday, 13 January 2021

Absence of magnetic proximity effect at the interface of a topological and a magnetic insulator

A study conducted by a team of researchers from the ICN2, in collaboration with colleagues from ALBA Synchrotron Light Source (Spain) and Diamond Light Source (United Kingdom), has found absence of magnetic proximity effects at the interface of heterojunctions made of a topological insulator and a magnetic insulator. These results are highly relevant to clear up the discrepancy between the experimental and theoretical analysis of these systems, as they were obtained using a direct experimental probe of locally induced magnetism, in contrast to previous experimental works.

The quantum Hall effect, the quantized version of the classical Hall effect, has been and still is largely studied in a variety of materials and conditions, due to its relevance for metrology applications. In order to produce this phenomenon, though, strong magnetic fields and low temperatures are required, which limit the possibilities of taking advantage of it. Quantisation without the need of an external magnetic field – and perhaps at high temperatures – can be actually observed by means of a phenomenon known as quantum anomalous Hall effect (QAHE).

This effect has been investigated in topological insulators (TIs) – materials that behave as insulators in their interior while current can flow along their surface – which are doped by introducing magnetic impurities in the lattice. Since these impurities can affect the electronic behaviour of the host material, an interesting alternative to achieve a robust QAHE would be inducing magnetism in the TI by means of proximity effects, without adding such dopants. To achieve this, heterostructures are produced, in which a magnetic insulator (MI) is in contact with the topological insulator. Several measurements conducted with a wide variety of experimental techniques on such TI-MI structures have yielded contradictory results. The most intriguing phenomena of induced magnetism in the TI have been reported in stacks incorporating the topological insulator Bi2Se3 in contact with the magnetic insulator EuS, by using indirect methods such as diffraction or neutron scattering. However, first-principle calculations do not support such induced magnetic moment in Bi2Se3, nor a significant enhancement of the local magnetic moment of Eu.

In order to shed light on those experimental results, a team of researchers from the ICN2 Physics and Engineering of Nanodevices group, led by ICREA Prof. Sergio O. Valenzuela, the ICN2 Atomic Manipulation and Spectroscopy group, headed by ICREA Prof. Aitor Mugarza, from ALBA Synchrotron Light Source (Barcelona, Spain) and Diamond Light Source (Didcot, United Kingdom), has carried out a systematic study, by applying advanced and powerful techniques to provide important information about microscopic interactions at the interface between the two materials. This work has been recently published in Physical Review Letters; the paper is signed by Dr Adriana I. Figueroa as first author, as well as corresponding together with group leader Prof. Valenzuela.

The authors have used techniques that are able to detect element-specific magnetic moments induced in the host TI lattice. In particular, they have employed X-ray absorption spectroscopy and X-ray magnetic circular dichroism methods to measure TI/EuS interfaces, with TIs of the (Bi, Sb)2(Se, Te)3 family, which have revealed no indication of proximity-induced magnetism in the topological insulator, in agreement with theoretical predictions. This study suggests that the aforementioned results about TI/EuS, previously reported, may originate not from proximity-induced magnetism but from a different mechanism, such as – the authors propose – magnetic doping by Eu diffusion into the TI.

 

Reference article:

A. I. Figueroa, F. Bonell, M. G. Cuxart, M. Valvidares, P. Gargiani, G. van der Laan, A. Mugarza, and S. O. Valenzuela, Absence of Magnetic Proximity Effect at the Interface of Bi2Se3 and (Bi,Sb)2Te3 with EuS. Phys. Rev. Lett. 125, 226801, November 2020. DOI: 10.1103/PhysRevLett.125.226801