Quantitative tests of flexomagnetism via patterned strain gradients

Flexomagnetism is the coupling between magnetic ordering and strain gradients in a material. Despite its theoretical interest, our understanding of flexomagnetism is limited as it is challenging to control spatially varying strain gradients reproducibly and over large areas in bulk crystals and epitaxial films. In this work, we have developed a method to pattern lateral strain gradients using helium ion implantation through a lithographically defined mask on epitaxial thin films of the polar semimetal GdAuGe grown on graphene, which has an antiferromagnetic ground state at low temperature. This approach allows us to control transverse strain gradients of the form dϵ_zz/dx in GdAuGe and we observe a near-room-temperature ferrimagnetic phase in the antiferromagnetic GdAuGe, a change in magnetic ordering that is not achievable by applying homogeneous strain alone. This strain-gradient-induced transformation is significant because, unlike magnetostrictive coupling to homogeneous strain, flexomagnetism is an odd function of the strain gradient that enables bistable switching of the sign of magnetization. By combining magnetometry and nanobeam x-ray diffraction we were able to map the strain profile and have a better understanding of the couplings between strain gradient, magnetic anisotropy, and exchange interactions in this system. These results open new avenues for the precise control of magnetic phases in thin films of quantum materials via patterned strain gradient.