Flexoelectric nanodomains in rare-earth iron garnet thin films under strain gradient

Authors

Hiroyasu Yamahara*, Bin Feng, Munetoshi Seki, Masaki Adachi, Md Shamim Sarker, Takahito Takeda, Masaki Kobayashi, Ryo Ishikawa, Yuichi Ikuhara, Yasuo Cho, Hitoshi Tabata

Abstract

Flexoelectricity is a universal property associated with dielectric materials, wherein they exhibit remanent polarization induced by strain gradient. Rare-earth iron garnets, R₃Fe₅O₁₂, are ferrimagnetic insulators with useful magnetic properties. However, they are unlikely to show remanent dielectric polarization because of their centrosymmetric structure. Here, to induce flexoelectricity, we investigate various rare-earth iron-garnet thin films deposited on lattice-mismatched substrates. Atomic-resolution scanning transmission electron microscopy demonstrates the presence of 15 nm-thick strain gradients in Sm₃Fe₅O₁₂ films between epitaxially strained tetragonal and relaxed cubic structures. Furthermore, negatively polarized nanodomains are imaged by scanning nonlinear dielectric microscopy. It suggests a generation of flexoelectricity, where the polarization points down toward the substrate in the out-of-plane direction. X-ray magnetic circular dichroism demonstrates hysteresis with a large coercive field originating from the strain-gradient layer. We believe that our study will pave the way for achieving dielectric polarization even in nonpolar centrosymmetric materials by strain-gradient engineering.



Communications Materials : https://www.nature.com/articles/s43246-021-00199-y