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"The role of 3d impurity states and pd-exchange interactions in the onset of ferromagnetism in the magnetic topological insulators (V,Cr):(Bi,Sb)2Te3" - ALBA Synchrotron Seminar 
Monday, 25 February 2019,  3:00
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The role of 3d impurity states and pd-exchange interactions in the onset of ferromagnetism in the magnetic topological insulators (V,Cr):(Bi,Sb)2Te3

Thiago Peixoto (Würzburg University)

Abstract

The discovery of the quantum anomalous Hall (QAH) effect in the magnetic topological insulators Cr: (Bi,Sb)2Te3 [1] and V:(Bi,Sb)2Te3 [2] has raised a long-standing debate on the origins of their unexpectedly discrepant transport and magnetic properties [3]. While the QAH state in V-doped films is found to be significantly superior, the differences in the electronic structure and in the dominating mechanisms of magnetic ordering for V- and Cr-doping remain elusive. Particularly, the coupling mechanisms underlying the onset of ferromagnetism have been intensively disputed. Here we combine x-ray magnetic circular dichroism (XMCD), resonant photoemission spectroscopy (resPES) and density functional theory (DFT) to trace element-specific contributions to the electronic and magnetic properties of these systems. Our resPES experiments show that V-doping gives rise to a metallic impurity band localised at the Fermi level, whereas Cr-doping leads to a broad 3d band, strongly overlapping with the host valence band. Our XAS and XMCD results challenge previous claims in favour of the invoked van Vleck mechanism, and confirm the presence of double exchange and ferromagnetic superexchange interactions, stemming from the contrasting fingerprints of the V and Cr 3d density of states, as the source of the reported discrepancies. Furthermore, we elucidate the detrimental effect of electron-doping on ferromagnetism, as it fosters a higher covalency of the transition metal ions, thereby screening Hund’s rule via pd-exchange with the neighbouring Te atoms [4]. The competition between charge transfer and Hund’s exchange ultimately determines the magnetic ground state.

Keywords: magnetic topological insulators, quantum anomalous Hall systems, transition metal impurities, x-ray magnetic circular dichroism, resonant photoemission, magnetic interactions, pd-exchange, 3d impurity bands

NOTE: People from outside ALBA should contact Daimi Perez (This email address is being protected from spambots. You need JavaScript enabled to view it.) in advance, in order to arrange access to the Lab premises.

Location ALBA Synchrotron, Maxwell Auditorium
Contact This email address is being protected from spambots. You need JavaScript enabled to view it.
https://indico.cells.es/indico/event/197/

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