Magnetic van der Waals (vdW) materials are subjects of intense interest due to their potential use in spintronic and optoelectronic devices (see [1] and references therein). While the two-dimensional long-range ferromagnetism is destroyed at finite temperatures by thermal fluctuations in an isotropic Heisenberg system, strong magnetocrystalline anisotropy promotes a stable ordering in a 2D-limit. Such long-range ferromagnetism has been experimentally confirmed in atomically thin layers of e.g. CrGeTe3 [2] or CrI3 [3]. As the number of known materials with these unique properties was limited, a renaissance of the vdW transition metal trihalides MX3 (M − d-metal, X-halogen atom) research began.
Most transition-metal trihalides are dimorphic. The chromium-based triad, CrCl3, CrBr3, CrI3, is characterized by the low-temperature trigonal phase while the structure of the high-temperature phase is monoclinic. The structural transition between the two crystallographic phases is of the first-order type with large thermal hysteresis. CrI3, the most intensively studied trihalide, is an Ising-type ferromagnetic semiconductor with the highest TC = 61 K. Also its Cr counterpart, CrBr3 exhibits ferromagnetism whereas CrCl3 becomes antiferromagnetic.
On the contrary, in the vanadium trihalides, the high-temperature phase has a higher symmetry. In VI3, rapid progress has led to the subsequent discovery of ferromagnetism in which the unusual magnetocrystalline anisotropy accompanied by two magnetic and structural transitions points to a strong magneto-elastic coupling in these materials [4]. The recent results on VBr3 prepared in a single-crystal form for the first time, show that the material orders antiferromagnetically with an unusual presence of tricritical point in its phase diagram.
Except transition metal trihalides, also first experimental results reveal the ground state properties of novel actinide trihalides, such as the UI3 antiferromagnet [5].
In the talk, I will briefly introduce some of the recent discoveries of these vdW materials studied by various methods including low-temperature XRD and spectroscopy techniques.
[1] X. Jiang et al., Appl. Phys. Rev. 8, 031305 (2021); K. S. Burch et al., Nature 563, 47 (2018).
[2] C. Gong et al., Nature 546, 265 (2017).
[3] B. Huang et al., Nature 546, 270 (2017).
[4] A. Koriki, M. Kratochvílová, et al., Phys. Rev. 103, 174401 (2021); P. Doležal, M. Kratochvílová et al., Phys. Rev. Matter. 3, 121401(R) (2019) ; J. Valenta, M. Kratochvílová et al., Phys. Rev. Matter. 103, 054424 (2021).
[5] D. Hovancik, M. Kratochvílová, et al., J. Sol. State Chem. 316, 123580 (2022).
The seminar will be chaired by Tim Verhagen, Department of Dielectrics.