Ruthenium oxide (RuO2) has several intriguing properties such as electrically-conduction like metal, topological electronic band structure called Dirac nodal line, and room temperature collinear antiferromagnet (AFM) [T. Berlijn et al, Phys. Rev. Lett. 118, 077201 (2017)]. We have discovered a novel spin-orbit torque (SOT) generation in epitaxially grown RuO2 thin films originated from recently predicted magnetic spin Hall effect (MSHE) [R. Gonzalez-Hernandez et al, arXiv:2002.07073(2020)]. The detected both damping-like and field-like torques clearly follow the Néel vector directions against the applied current directions in the epitaxial RuO2(101) and RuO2(100) films which have different Néel vectors on the substrate plane. We further discuss the mechanism of the SOT and the related phenomena in the AFM RuO2 films on this conference.
Spin-orbit (SO) field generated via spin Hall effect (SHE) or Rashba-Edelstein effect (REE) is one of the most important topics in spintronics for both fundamental physics and practical applications. Therefore a lot of SO materials such as heavy metals or topological insulators have been intensively studied so far for high conversion efficiency or functionality. But the SO torque efficiency is a material specific value in any case. It is ideal for us to synthesize the SO materials which have variable efficiency on our purposes. Here using a combination of heavy metals (HMs), we demonstrate to create extra SO fields such as damping-like (DL) and field-like (FL) fields on a Ni80Fe20(Py) layer via the REE at a W/Pt interface away from the Py layer through spin diffusion in the heavy metal layer. This means that we can manipulate the effective SO fields via the interface generated by a combination of materials apart from SHE contribution, and synthesize a designable SO material. Moreover we found opposite sign of the extra FL field in an inverse stacking structure like Py/Pt/W compared with in the Py/W/Pt. This implies that the extra field is coming from the HMs interface which has Rashba effect due to the change of the effective field direction at the interfaces. This leads us to a new concept of “synthetic SO materials” which paves a way for seeking high efficiency or superior functionalities in SO torque.
Spin-momentum locking provides a basic concept to control electron’s spin and charge flow in variety of material systems such as topological insulators, semiconductors, and heavy metals. Induced helical spin texture at the Fermi surface allows us to efficiently generate and detect spin polarization without an external magnetic field or magnetic materials. While spin generation and spin detection using spin-momentum locking have been intensively explored at Rashba interfaces and topological surface states, spin manipulation has yet to be demonstrated: it remains the missing ingredient towards full set of spin control by spin-momentum locking. Here, we experimentally manifested spin manipulation by spin-momentum locking in a magnetic focusing device [1]. We employ an InGaAs/InGaAsP heterostructure which shows strong SO-induced effective magnetic field. Such a large SO field generates and detects in-plane spin-polarized electrons by combining with lateral quantum point contacts (QPCs). By employing the lateral magnetic focusing device, weak out-of-plane external magnetic field focuses ballistic electrons from emitter to collector QPC by Lorentz force. Two QPCs allow us to polarize and detect electron spin along in-plane orientation due to strong SO interaction. We observed the enhanced focusing signal in collector QPC under spin polarized magnetic focusing regime. This indicates that the spin orientation in focused electrons have taken the opposite direction from that of the emitted spins. Electron spin rotates in a circular orbital motion while spin orientation is locked towards the SO field due to the spin-momentum locking. The findings of this study highlight the significance of the electron trajectory in controlling the spin phase through spin-momentum locking. The ability to control the spin phase by the orbital motion opens the door to the development of new concept on spintronic devices as well as topological electronics.
[1] M. Kohda et al., Scientific Reports in press (2019).
Several types of spin-based logic devices have been proposed in recent years. Almost all of them are operated by employing the drift transport of electron spins under in-plane electric fields. However, it is still unclear how an in-plane electric field influences the spin dynamics of drifting electrons. Here, we demonstrated long-distance spin transport in a GaAs quantum well, where the spin-orbit interactions (SOI) were set at exactly the SU(2) symmetry generating a persistent spin helix (PSH) state. The spatial distribution and temporal development of optically-injected electron spins in a modulation-doped GaAs single quantum well were detected with Kerr rotation microscopy based on the pump-probe technique at T = 8 K. We found that the transport length of drifting spins in a PSH condition exceeds 100 micrometers due to the suppression of D’yakonov-Perel spin relaxations [1]. The spin precession period, spin decay length and transport path of drifting spins can be controlled by electrical means in a single device. We also observed a drift-induced change in the spin precession period for drifting electron spins by visualizing the spatial distribution of electron spins. This phenomenon results from the cubic term of the Dresselhaus SOI enhanced by the application of in-plane electric fields [2]. Our finding will be of great importance for a deeper understanding of spin transport dynamics as well as for relevant spintronics applications using an in-plane electric field.
[1] Y. Kunihashi et al., Nat. Commun. 7, 10722 (2016).
[2] Y. Kunihashi et al., Phys. Rev. Lett. 119, 187703 (2017).
In two-dimensional electron gas (2DEG), spatial gradient of effective magnetic field due to spin orbit interaction yields
spin dependent force. By taking this advantage, Stern-Gerlach spin filter in 2DEG has been proposed for generating spin
polarized currents without any external magnetic fields and ferromagnetic materials [Phys. Rev. B 72, 041308(R)
(2005)]. In order to demonstrate the spin filtering effect, detection of spin polarized electrons becomes crucial
importance. Here, we propose an electrical detection of spin filtering by introducing an in-plane magnetic field in
mesoscopic Stern-Gerlach spin filter. In-plane magnetic field induces spin polarized electrons due to Zeeman splitting,
generating the imbalance between up-spin and down-spin currents after the spin separation. Calculated spin separation
angle becomes 20º based on experimentally accessible parameters. Time evolution of wave packet shows the spin
separation as well as the charge imbalance under the in-plane magnetic field. By fabricating Y-branch shaped narrow
wire structure with two split gate electrodes at the junction, spin filtering effect can be detected as the magnitude
difference of each branch currents. Gate bias dependence of each branch current is measured in Bex= ±15 T at T=4.2 K.
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