Negative tunneling magneto-resistance in quantum wires with strong spin–orbit coupling

Abstract

We consider a two-dimensional magnetic tunnel junction of the FM/I/QW(FM+SO)/I/N structure, where FM, I and QW(FM+SO) stand for a ferromagnet, an insulator and a quantum wire (QW) with both magnetic ordering and Rashba spin-orbit (SOC), respectively. The tunneling magneto-resistance (TMR) exhibits strong anisotropy and switches sign as the polarization direction varies relative to the QW axis, due to interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the QW. The results may provide a possible explanation for the sign-switching anisotropic TMR recently observed in the LaAlO3/SrTiO3 interface.The magnetic tunneling junction (MTJ) consisting of two ferromagnetic electrodes (FM) separated by a thin insulating barrier (I) is a prototype structure in the rapidly developing field of spintronics [1]. The tunneling magneto-resistance (TMR), depending on the relative magnetic polarization of the two FMs, is a key issue not only for the spintronic applications but also for the study of fundamental magnetic properties [2,3]. Due to the spin selection rule the TMR, if any, is typically positive. Two exceptional cases have been known. One involves magnetic impurities in the tunnel barriers and is not surprising. The other (more important) case is associated with the resonant tunneling and spin-dependent interfacial phase shift in double-barrier FM/I/N/I/FM structures, where N represents a non-magnetic normal metal [4][5][6][7][8].In this work we explore another non-trivial example of negative TMR in a two-dimensional (2D) doublebarrier MTJ of the FM/I/QW(FM+SO)/I/N structure [see Fig. 1 (a)], where QW(FM+SO) stands for a quantum wire (QW) with both magnetic ordering and Rashba spin-orbit coupling (SOC). Our MTJ structure should be distinguished from more common 1D MTJs of the FM/I/QW/I/FM structure such as in [5], where the QW is non-magnetic and the junction interface is perpendicular to the axis of the QW. In our case, the QW itself has a magnetic ordering and the junction interface is parallel to its axis. Thus, transport occurs across, not along the QW. We find that the TMR exhibits strong anisotropy and even changes sign as the polarization direction of the FMs varies relative to the QW axis. This sign-switching anisotropic TMR is attributed to the interplay among the one-dimensionality, the magnetic ordering, and the strong SOC of the QW. It is interesting to recall that anisotropic TMR was previously studied in the FM/I/FM structure where the insulating barrier (not the FMs) had SOC (see [10] and references therein), but the TMR remained positive without switching its sign.Our MTJ structure is peculiar in that nanoscale QWs with both strong SOC and magnetic ordering are rare. However, an important motivation is the recent experiment [11] on the transition metal oxide interface between LaAlO 3 (LAO) and SrTiO 3 (STO) [see Fig. 1 (b)], where the measured TMR is strongly anisotropic and switches sign as the magnetization direction varies in the interface plane. Since the LAO/STO interface...

Polarity-tunable magnetic tunnel junctions based on ferromagnetism at oxide heterointerfaces

“…To explain the sign-switching anisotropic TMR observed in the hybrid junctions of Co and LAO/STO heterostructure, here we propose a possible scenario based on a simplified model 38 considering a strong spin-orbit coupling in the LAO/STO heterostructure. Our model is summarized as follows.…”

“…However, in our hybrid MTJ model, TMR is determined by the momentum mismatch and the spin overlap across the tunnel junction, which are governed by the quantization of transversal momentum in the QW and the relative angle between the Zeeman and Rashba fields. Numerical calculation 38 using typical parameters of the LAO/STO heterostructure [42][43][44][45] , such as the Fermi energy E F = 40 meV, the Rashba spin-splitting energy Δ SO = E F /2, the in-plane Zeeman field Δ Zeeman = E F /16, and the thickness of the LAO/STO interface d = 1 nm, shows that the sign and magnitude of the TMR vary depending on the angle between the in-plane magnetic field and the Rashba field directions, revealing two-fold rotational symmetric cos(α/2) behaviour. This theoretical prediction is quite consistent to our observations of sign-switching and two-fold anisotropic TMR as shown in Fig.…”

Tunneling Conductance in Two-Dimensional Junctions between a Normal Metal and a Ferromagnetic Rashba Metal

“…1(a)] 12,13 . Current topic of spintronics is to study charge transport in the presence of spin-orbit coupling with magnetization or an applied magnetic field [14][15][16][17][18][19][20][21][22][23][24][25] .…”

Spin-Dependent Conductance in a Junction with Dresselhaus Spin-Orbit Coupling

“…Recently, various spin-dependent transports in the presence of spin-orbit couplings (SOCs) have been discussed. Among the spin-dependent transports due to the SOCs, Rashba and Dresselhaus spin-orbit coupling (RSOC and DSOC, respectively) have been intriguingly studied [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . RSOC and DSOC are caused by the structural and bulk inversion symmetry breaking, respectively [21][22][23][24][25] .…”

“…Hence, the spin polarization is generated by the applied electric field and its polarization is perpendicular (parallel) to the applied electric field in the presence of RSOC (DSOC). In the context of a ballistic regime, spin-dependent tunneling conductance has been studied in twodimensional systems 1-3,5,7,8, [10][11][12][13][14][15][16][17][18]20,[27][28][29][30][31][32][33][34] .…”