Prof Paolo Radaelli (University of Oxford): Merons, bimerons and skyrmions in α-Fe2O3: from cosmology to spintronics
Venue: Small Lecture Theatre
The field of quantum matter/quantum materials draws inspiration from a variety of theories, seeking materials in which they can be embodied and verified. In some cases, the materials in question end up being rather useful, though not necessarily in a way that is closely related to the original research motivation. A celebrated example [1] is the analogy, proposed by Wojciech H. Zurek, between cosmological strings [2] and vortex lines in the superfluid, which suggested cryogenic experiments to test cosmological string formation in 4He. The work I will describe in this talk started off as an attempt to seek the analogue of cosmological strings in easy-axis antiferromagnets with low in-plane anisotropy, leading to an approximate U(1) symmetry.
A well-known example of such materials is hematite (a-Fe2O3), which orders at a high Néel temperature (TN ~ 960 K). At room temperature and above, the spins are aligned 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. We reasoned that the above-Morin phase should be topologically rich and could in principle support vortices, while the easily accessible low-temperature phase would be topologically ‘trivial’. I will discuss our recent work [3,4], in which we demonstrated that, indeed, 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 [5]. Remarkably, these magnetic textures can be imaged in real space by X-ray spectral microscopy (X-PEEM, transmission X-ray microscopy and holography [6]). I will also discuss the recent collaboration with Cambridge colleagues, in which we imaged by N-V centre microscopy [7] the ‘emergent’ multipolar charges associated with topological textures. My final question is: can topological textures in hematite be at all useful? I will argue that the answer may be yes, particularly in the blooming field of antiferromagnetic spintronics/skyrmionics.
References
[1] Zurek, W. H. Cosmological experiments in superfluid helium? Nature 317, 505–508 (1985).
[2] Kibble, T. W. B. “Topology of cosmic domains and strings.” J. Phys. A: Math. Gen. 9, 1387–1398 (1976);
[3] 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).
[4] 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).
[5] 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, Nat. Mater https://doi.org/10.1038/s41563-024-01806-2 (2024 in press) arXiv:2303.03217 [cond-mat.mtrl-sci]
[6] J. Harrison, H. k. Jani, J. Hu, M. Lal, J-C Lin, H. Popescu, J. Brown, N. Jaouen, A. Ariando, and P. G. Radaelli, Optics Express 32, pp. 5885-5897 (2024) , https://doi.org/10.1364/OE.508005
[7] 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 23, 205-212 (2023), https://doi.org/10.1038/s41563-023-01737-4