Dynamics of Composite Domain Walls in Multiferroics in Magnetic Field and Their Instability

Abstract

We study theoretically the dynamics of composite domain walls (DW) in multiferroic material GdFeO3 driven by magnetic field H. Two antiferromagnetic orders of Fe and Gd spins and Gd-ion displacement interact with each other in this system with coupling c at low temperatures. We have numerically simulated the corresponding time-dependent Ginzburg–Landau equations in which magnetic field H couples to Fe-spin order parameter. We vary H and c systematically and calculate velocity v and the internal structure in a stationary state for magnetic DW and magneto-electric DW. DW mobility v/H increases with H and decreases with c for both DWs, but their characteristics differ between the two. We have also analytically studied the smooth characteristics of magneto-electric DW by a perturbation theory. Another finding is a splitting instability at a large H. A magneto-electric DW splits into a pair of magnetic DW and electric DW when c is large, while a magnetic DW splits when c is small. The internal structure of composite DW deforms with increasing c and H, and modulations in different order parameters separate in space. Their relative distances show a noticeable enhancement with approaching the splitting instability.