Strong evidence for d-electron spin transport at room temperature at a LaAlO3/SrTiO3 interface

Abstract

3A two-dimensional electron gas (2DEG) generated between LaAlO 3 (LAO) and SrTiO 3 (STO) consists of itinerant d-orbital electrons in the t 2g orbitals of Ti in STO 1,2 . This anisotropic orbit induces ferromagnetism, superconductivity, and even their coexistence at LAO/STO interfaces [3][4][5][6][7][8] . The subbands of LAO/STO interfaces have a complicated structure and cross each other due to the strong orbital hybridization 9-12 , resulting in the enhancement of spin-orbit coupling and nonmonotonic gate tunability [13][14][15][16] . It is believed that long-rage spin transport in the 2DEGs at LAO/STO interfaces is difficult to achieve due to the Rashba were prepared by using a high-power sputtering process, resulting in the pseudo-Ohmic contact at the Py-LAO/STO interface. The pseudo-Ohmic contact allows injecting spins from the Py and also detecting spin transport signals in the detector electrode. We applied a microwave (f = 9.63 GHz) and a static magnetic field with angle ! ! to the +x-axis (see Fig. 1a) to the Py strip on the LAO layer in order to excite the ferromagnetic resonance (FMR) of the Py. Under condition of the FMR, spin angular momentum is transferred from the Py to the 2DEG, resulting in the generation of pure spin current in the 2DEG 26,27 . The pure spin current in the 2DEG was detected by using the independent, separate NM (=Pt or Ta) strip deposited on the LAO layer. The propagating pure spin current is absorbed in the NM and converted to a charge current by the inverse spin Hall effect (ISHE) of the NM 28 , generating an electromotive force Information for details); the symmetric component is described in Fig. 2d.The ISHE has a characteristic symmetry in a relationship between the pure spin current direction ! ! , the charge current direction ! ! and the spin polarization vector !, i.e.where ! !"# is the spin Hall angle, ! is the electronic charge and ℏ is the Dirac constant 28 .The symmetry in equation (1) indicates that the polarity of the EMF is reversed when the direction of the external magnetic field, i.e., the direction of the spin polarization vector of the pure spin current is reversed. Figs. 2a-c and 2d-e show the FMR spectrum and EMF at ! ! = 0°, 90° and 180°, respectively. The ! ! dependence of the EMFs is consistent with the symmetry of the ISHE shown in equation (1): the polarities of electromotive forces at ! ! = 0° and 180°were reversed, and the signal at ! ! = 90° disappeared. These results strongly suggest that the observed EMFs under the FMR conditions can be ascribed to the ISHE in the Pt, i.e., the propagation of pure spin current in the 2DEG at RT. Deconvolutions of the EMFs were performed for ! ! = 0° and 180° and we used ( Table I. Notably, the sign of ! !"#$ obtained using a Ta electrode was opposite to that of Pt, which strongly supports the notion that long-range spin transport in the 2DEG indeed has been successfully achieved. It is important to note that the Py and Pt or Ta strips are only connected via the 2DEG at the LAO/STO interface, since the STO and t...

Study of equilibrium carrier transfer in LaAlO3/SrTiO3 from an epitaxial La1−x Sr x MnO3 ferromagnetic layer

“…The small spintransfer capability of LSMO across LAO is ultimately motivated by the large LSMO/LAO energy barrier for tunneling (equal to 3.5 eV according to our calculations). This small and confined magnetization is compatible in magnitude with the nul result of XMCD measurements, considering that such tiny effect may be plausibly washed out by extrinsic aspects, such as structural disorder, doping and presence of a LSMO dead-layer, which explains the spread of results found in literature [18][19][20]22]. Overall, our results indicate that spin transfer from LSMO to the 2DEG is actually possible for the pristine heterostructure, although difficult to achieve in practice.…”

“…Indeed, the presence or the lack of a spin-resolved charge could be then crucially related to the very specific characteristics of the considered sample. According to this interpretation, spin-polarized transport in LAO/STO would be a substantially fragile occurrence, thus accounting for the apparent contrast in the existing literature between cases of substantial [18][19][20] or marginal [22,23] presence of spin-polarization at the LAO/STO interface as transferred from FM electrodes.…”

“…In the same reference [18], the authors suggest that interface defects may play a role in amplifying the spin accumulation signal through a resonant tunneling mechanism. Very recently, spin-polarized carrier injection in LAO/STO generated by spin pumping from a permalloy electrode and detected by inverse spin-Hall effect in Pt and Ta electrodes [20] indicated an in-plane spin diffusion lengths in the range of 200-350 nm at room temperature, corresponding to spin lifetimes of ∼2-6 ns. Predictions based on effective spin-orbit Hamiltonians for non-magnetic materials [21] suggest that long spin lifetimes in LAO/ STO can be actually achieved.…”

Room-Temperature Giant Charge-to-Spin Conversion at the SrTiO3–LaAlO3 Oxide Interface

“…Therefore, the STO/LAO heterostructure can enable potential spintronic innovations and now is attracting rising research interests. [8][9][10][11][12][13] Experimentally, Narayanapillai et al 8 have reported an extremely strong charge current induced Rashba field in the 2DEG layer, which verifies the presence of strong Rashba SOC predicted at the STO/LAO interface. Most recently, the inverse Edelstein effect at the STO/LAO interface has been demonstrated by using the spin pumping technique, [10][11][12] which shows a notable spin-to-charge conversion.…”

Nonreciprocal transport in gate-induced polar superconductor SrTiO 3

“…Two-dimensional electron system (2DES) realized at the oxide surface or interface is a promising candidate for emergent physical properties and functionalities (1). Since it naturally hosts the spatial inversion symmetry breaking due to the structural asymmetry or crystal symmetry, characteristic spin-split bands with the spin-momentum locking emerge due to spin-orbit interaction (SOI), offering rich spintronic functionalities (2,3). Although 2DES is also an important platform for the exotic quantum phases such as quantum Hall effect (4) and superconductivity (5,6), the effect of the symmetry breaking on such quantum phases has been elusive.…”