Impurity-Assisted Tunneling Magnetoresistance under a Weak Magnetic Field

Abstract

Injection of spins into semiconductors is essential for the integration of the spin functionality into conventional electronics. Insulating layers are often inserted between ferromagnetic metals and semiconductors for obtaining an efficient spin injection, and it is therefore crucial to distinguish between signatures of electrical spin injection and impurity-driven effects in the tunnel barrier. Here we demonstrate an impurity-assisted tunneling magnetoresistance effect in nonmagnetic-insulator-nonmagnetic and ferromagnetic-insulator-nonmagnetic tunnel barriers. In both cases, the effect reflects on/off switching of the tunneling current through impurity channels by the external magnetic field. The reported effect is universal for any impurity-assisted tunneling process and provides an alternative interpretation to a widely used technique that employs the same ferromagnetic electrode to inject and detect spin accumulation.For the realization of semiconductor spintronic devices [1][2][3][4][5][6][7][8], the conductivity mismatch problem [9][10][11][12] and the difficulty of manipulating semiconductors at the nanoscale are the main issues delaying the progress of this research field. Employing the so-called three-terminal (3T) setup and making use of a single ferromagnetic/insulator contact for both injection and detection of spin-polarized currents was a big step towards this purpose [13]. Due to the simplicity of the micron-sized structures employed, this setup has gained popularity in semiconductor spintronics [13][14][15][16][17][18][19][20][21][22]. The Lorentzian-shaped magnetoresistance (MR) effect measured in 3T-semiconductor devices has been often attributed to spin injection on accounts of the resemblance to the celebrated Hanle effect in optical spin injection experiments [23]. However, it has been increasingly realized that the MR reported depends much on the tunneling process and too little on the semiconductor [13][14][15][16][17][18][19][20][21][22]. Furthermore, the typical junction working conditions employed for these measurements, with bias voltage settings much larger than the Zeeman energy, render the signal detection prone to subtle effects driven by impurities embedded in the tunnel barrier [14,24].In this Letter, we elucidate the physics behind such experiments by focusing on the tunnel barrier. Accordingly, our devices render a compact geometry with an aluminumoxide tunnel barrier created between metallic electrodes, M 1 /AlO x /M 2 , as sketched in Fig. 1(a). The M 1 /AlO x /M 2 devices were fabricated in-situ in a UHV electron-beam evaporation chamber with integrated shadow masks. The base pressure of the chamber is below 10 −9 mbar. The thickness of the top and bottom metallic electrodes, M 1 and M 2 , ranged between 10 nm and 15 nm. To decisively probe the role of impurities in the oxide, a series of devices were fabricated with 1) O 2 plasma exposure at 10 −1 mbar at a power ranging from around 24 to 40 W for 120 seconds to 210 seconds to minimize the impurity density, or 2) n−step (n...

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

“…[31][32][33] It is known that the defects, such as the oxygen vacancies (VO), can be present in the LAO layer during film growth and form localized states (LS) in the LAO band gap. [34][35][36][37] Hence, the IET process via these LS [31][32][33][38][39][40] via LS is the main mechanism of the spin transmission through the LAO layer, the SOT efficiency is expected to exhibit a very strong temperature dependence as indicated by the Glazman-Matveev (GM) theory, 31,33,38,41 , which describes the IET with a power law dependence on temperature.…”

Origin of the Magnetoresistance in Oxide Tunnel Junctions Determined through Electric Polarization Control of the Interface

“…This spin accumulation in the NM material generates a small spin-dependent voltage across the junction, which can be removed by Hanle dephasing under an applied magnetic field, producing junction MR with a Lorentzian line shape. Since the observation of such signals at room temperature in Sibased junctions [1], many groups have observed unexpectedly large Lorentzian MR in FM-I-NM heterojunctions, seemingly independent of many known materials parameters and spin-dephasing behavior, generating much debate over the origin of the observed phenomena [5,[7][8][9][10][11][12]. As an example, long spin-coherence times have been inferred in p-type semiconductors despite the known rapid dephasing in the valence band [13].…”

Experimental Demonstration of Room-Temperature Spin Transport inn-Type Germanium Epilayers

“…Unfortunately, these studies used a three-terminal method, the reliability of which is now being called into question. Many recent studies demonstrated that signals in the threeterminal geometry originate not from spin accumulation in nonmagnetic channels but from magnetic field-dependent tunneling through localized states [21][22][23][24][25][26][27]. Other studies revealed that NiFe itself generates electromotive forces by the inverse spin-Hall effect (ISHE) [28] and the planar Hall effect [29].…”