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

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.

Scaling of the Thermally Induced Sign Inversion of Longitudinal Spin Seebeck Effect in a Compensated Ferrimagnet: Role of Magnetic Anisotropy

“…Recently, the peak splitting has been related to differently strained layers in samarium iron garnet (SmIG) thin films due to the relaxation of the film unit cell with increasing film thickness. [ 30 ] Additionally, for the thinner films, Laue oscillations are apparent. While these oscillations indicate single‐crystalline order with smooth interfaces, the Laue peaks move closer with increasing film thickness and are therefore hardly visible for the thicker films.…”

“…[ 29 ] Another interesting fact is that at a fixed temperature, H C for the GSGG/GdIG(31 nm) film is more than twice of that for the GGG/GdIG(31 nm) film (see Figure S5, Supporting Information), which is consistent with a recent report on the strain‐induced enhancement of H C in REIGs. [ 30 ]…”

“…[29] Another interesting fact is that at a fixed temperature, H C for the GSGG/GdIG(31 nm) film is more than twice of that for the GGG/GdIG(31 nm) film (see Figure S5, Supporting Information), which is consistent with a recent report on the strain-induced enhancement of H C in REIGs. [30] Radio frequency (RF) transverse susceptibility (TS) measurements were employed to determine the temperature evolution of the effective magnetic anisotropy for both IP and OOP configurations. This TS technique can precisely detect the dynamic magnetic response of a magnetic material to a small and fixed amplitude RF magnetic field (H RF ) applied transverse to a static magnetic field (H DC ).…”

Stress-Induced Magnetic Properties of Gadolinium Iron Garnet Nanoscale-Thin Films: Implications for Spintronic Devices

“…However, the intensity of the scattered beam decreases strongly with increasing collector angles. Instead, incoherent Rutherford scattering, whose intensity depends heavily on the atomic number Z of the scattering center, dominates the signal at higher collector angles. ,− Consequently, using an annular detector (AD) with a small collector angle (in our case 19–38 mrad) hints at a strain contrast, while using a large AD collector angle (110–440 mrad) mainly maps mass–thickness ( Z -) contrast. Note that the probe-forming aperture semiangle was 27 mrad in both modes.…”

“…Note that a small compressive out-of-plane strain is expected for GdIG/GYSGG films, but a small tensile strain was observed. For garnets grown at low oxygen pressure, oxygen vacancies can lead to changed lattice parameters, − and could therefore cause a reduced tensile strain for films grown on GSGG or even compressive strain in films grown on GYSGG substrates. However, while a recent report showed strong implications on the structural and magnetic parameters of YIG thin films with intentionally introduced oxygen vacancies, this effect is believed to be small for PLD grown films .…”