Seminar : Room-temperature Half-skyrmions and Bimerons in an antiferromagnetic insulator via the Kibble-Zurek mechanism

Speaker : Prof. Paolo Radaelli from University of Oxford

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Abstract: IT technology experts agree that the next phase of Moore's law will be based on energy efficiency rather than transistor density.  Indeed, the energy efficiency of CMOS devices is 5-6 orders of magnitude worse that than of the human brain, which is at least 3 orders of magnitude away from the ultimate physical limits.  Quantum materials (QM) will be crucial to achieve the ambitious goal of 1-10 aJ/operation.  A number of promising concepts have been proposed, many involving the creation and manipulation of real-space magnetic topological structures such as skyrmions in metallic ferromagnets.  Antiferromagnetic analogues have recently come into intense focus, but their experimental realizations in natural systems were yet to emerge.  Previously, we have speculated that topological structures such as vortices might emerge at the Néel temperature in simple antiferromagnets via the so-called Kibble-Zurek mechanism [1], initially proposed in the context of cosmology.  Using linear and circular X-ray photoelectron emission microscopy, we indeed observed vortices/antivortices in antiferromagnetic haematite (a-Fe₂O₃) epitaxial films, in which the primary whirling parameter is the staggered magnetization [2].  Ferromagnetic topological objects with the same vorticity and winding number as the a-Fe₂O₃ vortices were ‘imprinted’ onto an ultra-thin Co ferromagnetic over-layer by interfacial exchange. In this talk, I will present our more recent results on a-Fe₂O₃ [3].  By replacing Co with Pt, we manage to activate the so-called Morin first-order transition, which occurs in bulk a-Fe₂O₃ just below room temperature, and to tune it precisely by small amounts of Rh doping.  Remarkably, the first-order analogue of the Kibble-Zurek mechanism produces a much richer variety of topological objects, such as merons (half skyrmions) and bimerons, which, unlike flat vortices, carry a true topological charge.  Driven by current-based spin torques from the Pt over-layer, some of these AFM textures could emerge as prime candidates for low-energy antiferromagnetic spintronics at room temperature

[1] Kibble, T. W. B. “Topology of cosmic domains and strings.” J. Phys. A: Math. Gen. 9, 1387–1398 (1976); Zurek, W. H. Cosmological experiments in superfluid helium? Nature 317, 505–508 (1985).

[2] Chmiel, F. P. et al. “Observation of magnetic vortex pairs at room temperature in a planar α- Fe₂O₃/Co heterostructure.” Nature Materials 17, 581–585 (2018).

[3] Jani H. et al., “Half-skyrmions and Bimerons in an antiferromagnetic insulator at room temperature”, arXiv:2006.12699 [cond-mat.mtrl-sci]