Observation of large planar Hall effect on the current induced spin-orbit effective fields in Ta/MgO/CoFeB/Ta

Current induced spin-orbit effective magnetic fields in metal/ferromagnet/ oxide trilayer provide an alternative switching mechanism that is more efficient than the conventional spin torque transfer. Perpendicularly magnetised MgO/CoFeB/Ta structures have been widely investigated for logic and non-volatile memories due to its perpendicular magnetic anisotropy and large current induced spin-orbit effective fields.1, 2 However, the understanding of the planar Hall effect of these stacks at low fields are limited. As such, this work evaluates the current dependence of the spin-orbit effective field by considering both the anomalous Hall and planar Hall effect. Harmonic measurements were performed in conjunction with micromagnetic simulations under an in-plane field configuration to affirm the presence of chiral dendritic-like domains, as shown in Figure 1, with and without the application of an in-plane sinusoidal current. Figure 1(c) shows the differential Kerr images of chiral domains, formed due to the presence of Dzyaloshinskii-Moriya interaction (DMI)in the CoFeB/MgO interface. An elongation of the chiral magnetic domains was observed in both measurement configuration such that the elongation direction is perpendicular to the direction of the applied magnetic field. Planar Hall effect contributes significantly to the alteration of the magnetisation state due to lowly tuned anisotropy of CoFeB in both configurations as shown in Figure 1(d). Furthermore, with opposite initial magnetisation states, the second harmonic signals were closely symmetrical about the horizontal axis. As the field was swept and the current was modulated, the second harmonic Hall signals reveal asymmetry and symmetry about the vertical axis for the longitudinal and transverse measurements, respectively. The relationship between the harmonic Hall voltage and the applied magnetic field was used to determine the effective fields through the total energy approach. The results suggest there was a large transverse spin-orbit effective field that arises together with the longitudinal field in the longitudinal measurement. On the other hand, there was larger transverse field than the longitudinal effective field in the transverse measurement. Strong correlations between the modulated current density and large effective fields indicate promising future applications in torque induced logic and memory devices.