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

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

doi:10.1038/s43246-021-00199-y

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.

Highlights

Among the various Rare-earth iron-garnet (RIG), SmIG films grown on garnet substrates of Gd3Ga5O12 (GGG) substrates, where the lattice mismatch between the film and substrate was 1.17%, were investigated in detail
scanning transmission electron microscopy (STEM) revealed an almost 15 nm-thick strain gradient around dislocations; a flexoelectric polarization of 0.02 μC cm−2 is expected with an order-of-magnitude approximation
Nanoscale spatial distribution of the dielectric domains was observed by Scanning nonlinear dielectric microscopy (SNDM), indicating that negatively polarized dielectric nanodomains with 30 nm diameters exist in the strain-gradient layer

Summary

Methods

Rare-earth iron-garnet (RIG) films were deposited on garnet substrates of Gd3Ga5O12 (GGG) and Y3Al5O12 (YAG) (001) through pulsedlaser deposition (PLD) using an ArF excimer laser with a wavelength of 193 nm. The PLD targets were prepared through conventional solid-state reaction. R2O3 and Fe2O3 powders were mixed stoichiometrically and sintered at 1200 °C for 12 h. During the PLD process, the substrate temperature and ambient oxygen pressure were maintained at 750 °C and 0.1 Pa, respectively. The ArF excimer laser was projected onto the prepared targets at a repetition rate of 5 Hz

Results

Discussion

Conclusion