Flexoelectricity and Flexomagnetism

Abstract

Strain gradients, unlike homogeneous strain, are odd under space inversion and unlock couplings that are strictly forbidden in the parent crystal. At the nanoscale, and in curved low-dimensional systems in particular, such gradients are readily attainable and often unavoidable, so that curvature acts as a symmetry-breaking field written into the lattice itself. This chapter reviews the resulting couplings to electric polarization and magnetism. After establishing the geometric relation between strain gradients and curvature, we review flexoelectricity as the better-established reference case: the bulk, surface and dynamic contributions that complicate its definition, its microscopic mechanisms, its manifestation in two-dimensional materials, and the first-principles methodology now available for its calculation. Using the magnetoelectric effect as a conceptual bridge, we then develop flexomagnetism in a broad sense, covering any coupling phenomena between magnetism and strain gradients, and specifically discuss the linear magnetization response to curvature and curvature-induced Dzyaloshinskii-Moriya interactions, beyond the established curvilinear treatment. Finally, we briefly describe some additional effects fitting under this broad umbrella of flexomagnetism, including nonlinear ones, inhomogeneous magnetoelectric and Rashba-type phenomena, and connections to altermagnetism.