Local Exact Controllability of Landau-Lifshitz-Gilbert Equation

Abstract

We prove a local exact controllability result for controlled Landau--Lifshitz--Gilbert equations on $\mathbb T^2$: if the initial energy is sufficiently small, then for any terminal time $T>0$, there is a localised external magnetic field such that the system can be steered exactly to the terminal value of any nearby uncontrolled trajectory. We first transform the equation to a quasilinear parabolic system on $\mathbb R^2$ by a suitable stereographic chart. Then the Carleman estimate is established for the linearised system through a decomposition adapted to the self-adjoint and skew-adjoint structure of the conjugated adjoint operator. This yields observability and $L^\infty$-null controllability for the linearised system. The nonlinear projected equation is then recovered by a Kakutani fixed-point argument. We also obtain a semi-global controllability result under a hemisphere condition.