Phil King
Control over materials thickness down to the single-atom scale has emerged as a powerful tuning parameter for manipulating not only the single-particle band structures of solids, but increasingly also their interacting electronic states and phases. Recently, magnetism has emerged as the new frontier in the area of 2d materials. Here, I will show how direct measurement of the electronic structure using angle-resolved photoemission (ARPES) can lead to valuable insight not only on the question of whether a 2d material does in fact exhibit long-range magnetic order, but also on the microscopic mechanisms of the magnetic ordering when it occurs. First, I will consider the example of epitaxial monolayers of VSe2.1 Our ARPES measurements, combined with x-ray magnetic circular dichroism (XMCD), demonstrate that a putative magnetic order is prevented from occurring by the formation of a robust charge density wave,2 which gaps the complete Fermi surface thus removing a Stonor-like channel for ferromagnetism. The instability can be expected to be nearby in phase space, however, and we show how ferromagnetism can be induced via proximity coupling with a ferromagnetic Fe layer.3 Second, I will show ARPES results from bulk Cr2Ge2Te6, which is an established van der Waals ferromagnet,4 where long-range order has been shown to persist to the bilayer thickness.5 From ARPES, we identify atomic- and orbital-specific band shifts upon cooling through TC. From these, together with XMCD, we identify the states created by a covalent bond between the Te 5p and the Cr eg orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr t2g states that carry the majority of the spin moment. This reflects a rather direct observation of how 90° superexchange leads to ferromagnetism, and demonstrates how an experimental band-structure perspective can give important insight even in a “local moment" magnetic system.
This work was performed in close collaboration with M.D. Watson, A. Rajan, K. Underwood, J. Feng, D. Biswas, D. Burn, T. Hesjedal, G. van der Laan, M. Ciomaga Hatnean, G. Balakrishnan, G. Vinai, G. Panaccione and colleagues from the Universities of St Andrews, Oxford, Warwick, Diamond, and Elettra.
1 Rajan et al., Phys. Rev. Materials 4 (2020) 014003
2 Feng et al., Nano Lett. 18 (2018) 4493
3 Vinai et al., Phys. Rev. B 101 (2020) 035404
4 Carteaux et al., J. Phys. Condens. Mat. 7 (1995) 69
5 Gong et al., Nature 546 (2017) 265
6 Watson et al., Phys. Rev. B 101 (2020) 205125