SCALES · Multiferroic phase field models of the coupled dynamics of bismuth ferrite

Multiferroic phase field models of the coupled dynamics of bismuth ferrite

Multiferroics are an interesting class of materials. They display both electric and magnetic order. They also exhibit a non-trivial coupling between the two phenomena leading to the possibility of i.e. manipulating the magnetization with an electric field. This one example (among others) is advantageous in the context of next-generation device design because suitable properties can be obtained with low-power or with ultrafast stimulii. Therefore, multiferroics have been proposed as components in device prototypes ranging from beyond-CMOS logic gates, magnetoresistant spintronic valves, to electro-optic modulators among many others. Within the SCALES project, we developed a model of a particularly popular multiferroic BiFeO3 (BFO). This is a ferroelectric antiferromagnet with useful room temperature properties. To study these materials theoretically, atomistic methods can become prohibitive if too many atoms are in the simulation box. This leads to difficulties to predict and understand physical phenomena at the device-relevant scale (typically a few hundred nanometers). Therefore, we were motivated to develop a continuum model of this material which coarse-grains or averages the material behavior over longer distances. This involved coupling the ferroelectric phase field method to the micromagnetic simulation approach. This allows one to simulate both the electric and magnetic order in a single simulation with real-time scales. These types of simulations are important for the field because materials properties of BFO (and other multiferroics) can then be extracted from an arbitrary simulation geometry of a nanostructure. We aimed to benchmark and develop a useful model for not only the low energy ground states of the structure (which are static in time) but also understand the dynamical behavior of the order parameters (electric polarization or magnetic spin) under applied fields. Furthermore, we aimed to develop this model in an open-source framework (MOOSE) that is freely available and extendable to other physical phenomena in BFO (or other types of multiferroics) and is continuously-integrating with the underlying software libraries (thus preserving reproducibility).

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