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Group Seminar 2023/24

Group Seminar
Wednesday 18 October 2023, 14:00-15:00
William Henry Bragg LT (2.37)
Prof Paolo G. Radaelli
Department of Physics, Oxford University
Merons, bimerons and skyrmions in the natural antiferromagnet α-Fe2O3

Abstract: After more than a decade of intensive research on skyrmions in ferromagnetic metal as topologically protected information carriers, the idea of antiferromagnetic skyrmions is gaining significant traction. Antiferromagnets have obvious advantages, such as much-reduced sensitivity to magnetic fields and much faster dynamics, but, with few exceptions, antiferromagnetic topological textures are generally observed only in ‘syntetic systems’. An interesting alternative is represented by so-called merons and bimerons, which are topologically related to skyrmions but exist in the presence of easy-plane magnetic anisotropy. Natural antiferromagnetic oxides adopting the A-type magnetic structure, in which ferromagnetically aligned planes are stacked anti-parallel to each other, are particularly promising candidates. A famous example is hematite (a-Fe2O3), which orders at a high Néel temperature (TN ~ 960 K). At room temperature and above, the spins are aligned in the planes and perpendicular to the high-symmetry trigonal axis and are also slightly canted due to the Dzyaloshinskii-Moriya interaction, giving rise to a ‘weak’ but measurable net ferromagnetism. Below the Morin transition temperature (TM ~ 260 K), the spins flip out of plane and lose their canting, leading to perfect antiferromagnetism and the loss of the net magnetic signal. Recently [1,2], we showed that hematite supports a rich variety of topological textures (merons, antimerons, bimerons), which can be tuned in and out of existence simply by cycling temperature over a narrow range through TM or by application of biaxial/uniaxial strain [3]. I will illustrate this phenomenology in both epitaxial films and free-standing single-crystal membranes using real-space images, collected using X-ray spectral microscopy (X-PEEM, transmission X-ray microscopy) and N-V centre microscopy [4]. I will also demonstrate the subtle interplay between antiferromagnetic textures in hematite and corresponding ferromagnetic textures induced in a thin Co overlayer, and argue that the latter could provide a way to detect the former.



[1] F. P. Chmiel, N. Waterfield Price, R. D. Johnson, A. D. Lamirand, J. Schad, G. Van Der Laan, D. T. Harris, J. Irwin, M. S. Rzchowski, C. B. Eom, and P. G. Radaelli, Nat. Mater. 17, 581 (2018).

[2] H. Jani, J. C. Lin, J. Chen, J. Harrison, F. Maccherozzi, J. Schad, S. Prakash, C. B. Eom, A. Ariando, T. Venkatesan, and P. G. Radaelli, Nature 590, 74 (2021).

[3] H. Jani, J. Harrison, S. Hooda, S. Prakash, P. Nandi, J. Hu, Z. Zeng, J.-C. Lin, G. ji Omar, J. Raabe, S. Finizio, A. V.-Y. Thean, A. Ariando, and P. G. Radaelli, arXiv:2303.03217 [cond-mat.mtrl-sci] (2023).

[4] A. K. C. Tan, H. Jani, M. Högen, L. Stefan, C. Castelnovo, D. Braund, A. Geim, M. S. G. Feuer, H. S. Knowles, A. Ariando, P. G. Radaelli, and M. Atatüre, Nat. Mater., in press, arXiv:2303.12125 [cond-mat.mes-hall] (2023).