Bulk-Rashba Effect with Spin-Coherent Transport in a Novel Polar Phase of BiInO₃

The Rashba effect, which arises from spin-orbit coupling induced by broken inversion symmetry, has been sought for use in low-power spintronics. While polar materials have been reported to exhibit bulk-Rashba effect arising from the polar crystal symmetry, the exploration of this effect has been hindered by the scarcity of polar materials exhibiting the bulk-Rashba effect and rapid spin-relaxation effects dictated by the D’yakonov–Perel’ (DP) mechanism. Theoretical works suggest that persistent-spin texture (PST) can be realized via inducing unidirectional spin texture enforced by crystal symmetry, such that the spin-relaxation effect is suppressed. Experimental evidence for the bulk-Rashba effect with unidirectional-spin texture, however, remains elusive so far. BiInO₃, for example, has been theoretically predicted to exhibit PST in a polar <i>Pna</i>2₁ version but stabilizing that polar structure has proven to be challenging. Prior efforts had, in turn, produced a non-polar <i>Pnma</i> version of BiInO₃ (henceforth, <i>O-</i>BiInO₃) in thin-films.<br/><br/>Here, two key observations are made: a novel polar LiNbO₃-type <i>R</i>3<i>c</i> phase of BiInO₃ (henceforth, a <i>R-</i>BiInO₃) can be stabilized using epitaxial control and this phase exhibits a bulk-Rashba effect with suppressed spin relaxation as the result of unidirectional-spin texture. While non-polar <i>O-</i>BiInO₃ was stabilized when grown on MgO (001) substrates, <i>R-</i>BiInO₃ was grown with twinned domain structures on DyScO₃ (110)<i>_O</i> substrates (henceforth, (001)-oriented <i>R-</i>BiInO₃ film), and a monodomain structure on SrTiO₃ (111) substrates (henceforth, (111)-oriented <i>R</i> BiInO₃ film). The domain structures and polar symmetry of BiInO₃ films were confirmed using extensive structural characterization studies including X-ray diffraction, scanning transmission electron microscopy, second-harmonic generation spectroscopy. Compared to the <i>O-</i>BiInO₃ films, the <i>R-</i>BiInO₃ films showed higher electrical conductivity due to a slightly reduced optical band gap and enhanced dielectric and piezoelectric responses corresponding to its polar nature. First-principles density functional theory (DFT) calculations explore both the structural stability of the <i>R-</i>BiInO₃ phase and reveal Rashba-type spin splitting with unidirectional spin texture around the Fermi level (<i>E_F) </i>exceeding no more than 3 meV above the conduction band minimum energy (<i>E_CBM</i>). The <i>R-</i>BiInO₃ films were systematically annealed in slightly oxygen-deficient environments to create oxygen vacancies such that the carrier concentration was tuned to access the potential unidirectional spin texture (<i>i.e.</i>, <i>E_F</i>-<i>E_CBM </i>&lt; 3 meV) and so that the films were adequately conductive for transport measurements. Crystallographic orientation-dependent anisotropic transport behavior was observed, where weak antilocalization (WAL) with spin relaxation was observed in annealed (001)-oriented <i>R-</i>BiInO₃ films, while weak localization (WL) showing spin-coherent transport with long spin-relaxation times exceeding 3.36 ns was observed in annealed (111)-oriented <i>R-</i>BiInO₃ films; the latter effects are likely due to the removal of the multidomain structure and anisotropic spin diffusion. Additional transport measurements revealing planar-Hall effect, anisotropic magnetoresistance, and non-reciprocal charge transport on the annealed (111)-oriented <i>R-</i>BiInO₃ films confirm the bulk-Rashba effect with a Rashba coefficient of 760 meV●Å. All told, a previously unreported polar phase, <i>R-</i>BiInO₃, simultaneously exhibits the bulk-Rashba effect and spin-coherent electron transport due to unidirectional spin texture. These findings offer insights into spin-orbit coupling physics within a polar materials system and suggest potential applications in emerging spin-based applications.