Simulating multiferroic skyrmions and their dynamics in multiferroic BiFeO3

Antiferromagnetic materials (AF) are of particular interest for the field of spintronics because of the robustness of their ordering against external field and their high resonance frequencies reaching the THz. In addition, when topology is introduced by asymmetric exchange (Dzyaloshinskii-Moriya interaction), non-collinear structures can be generated among which the bubble-like entities called skyrmions can benefit from an extra protection of topological nature. These are stabilized by their topology and can be used as bits of magnetic information. Although the exchange asymmetry is generally produced by interface effects in metallic multilayers, the relevant interaction can also be induced by coupling with an electrical polarization in some particular compounds called multiferroics. The archetype of such materials, Bismuth Ferrite BiFeO3 (BFO) is at ambient conditions a ferroelectric antiferromagnet, where the intrinsic magneto-electric interaction leads to a one-dimensional cycloidal winding of its antiparallel spin structure [1]. This property has the potential to be used to generate skyrmions.

Since the experimental measurement of these entities is very challenging, it is important to start with numerical simulations in order to pinpoint the required real life material parameters for their existence. Consequently, we have developed an atomistic code based on the Landau–Lifshitz–Gilbert (LLG) evolution equation where several important terms in the Hamiltonian can be considered. Due to the long wavelength (64 nm) of the cycloidal AF arrangement of BFO, simulating 2D magnetic textures requires massive computation and thus we wrote a home-made code to parallelize the computation on GPUs.

Using this tool, we will present the phase diagram for the stability of these entities, their internal magnetic and electrical structure as well as their expected motion using spin currents and electric fields.

[1] I. Gross, et al., Nature 549, 252 (2017).

[2] D. Sando, et al., Nature materials 12, 641 (2013).