Tailoring the Current Driven Domain Wall Motion by Engineering Two Heavy Metal Underlayers.

The ability to move magnetic domain walls by electrical current enlightens novel ways for developing non-volatile memory and logic devices [1], [2]. The realization of current driven domain wall motion (DWM) in heavy metal (HM)/ ferromagnet (FM)/ oxide heterogeneous structures can be attributed to the combined effect of spin Hall effect from HM and Dzyaloshinskii-Moriya interaction (DMI) at HM/FM interface [3], [4]. Different combinations of spin Hall angle (positive or negative) and effective DMI field (left or right) determine the direction of DWM with respect to the current direction [5]–[7]. Alternating the direction of DWM often requires the selection of different HM or FM materials. Here, we show that it’s able to control the direction as well as velocity of the current induced DWM by varying the relative thicknesses of two heavy metal underlayers. In our experiments, all films were deposited by magneton sputtering (DC for metals, RF for oxide) with base pressure $\le 2 \times 10 ^{-8}$ Torr. The film stacks were substrate/ Ta(0.5)/ Pt(3)/ W(0.6, 0.8, 1, 1.5)/ FeCoB (1.2)/ MgO(2)/Ta(1) with units in nanometers. Note that the bottom and top thin Ta layers are for film adhesion and capping purposes, and the effect of spin Hall torques from these layers on the FeCoB layer are negligible. The films were then annealed at 350 °C under 0.4 T magnetic field perpendicular to the film surface. Magnetization curves were measured by Alternating Gradient Field Magnetometer and all films exhibited perpendicular magnetic anisotropy. Magnetic bubble domain testing [8] was conducted to measure the domain wall chirality at the W/ FeCoB interface. Figure 1 shows the velocity of down-up and up-down domain walls as a function of in-plane magnetic field under μ 0 H z = 0.5 mT. The results show that the domain wall has right-handed chirality at the W/ FeCoB interface, which agrees with previous observations by other group [5]. The minima in Figure 1 indicate the effective DMI field is around 43 mT. Next, films were lithographically patterned into 4 μm-wide 26 μm-long wires with bowtie-like contact pads on each side. The domain wall motion under nanosecond current pulses was tracked by Kerr microscope, where the velocity of domain wall motion was determined with the sequences of Kerr images. The velocity of current driven domain wall is plotted in Figure 2 as a function of W layer thickness, where positive velocity indicates domain wall motion along the current direction. We observe that the domain wall moves in the same direction as the current when W layer thickness is above 0.8 nm. Meanwhile, the domain wall velocity is linearly dependent on W layer thickness. Once the thickness is reduced below 0.8 nm, however, the motion direction switch to that against the current direction. Such phenomenon arises from the injection of two spin currents into the FeCoB layer with opposite signs of spin polarization (Figure 2 inserted), which further causes the competition of two opposite spin orbit torques. The two spin currents are generated by the spin Hall effect of the two heavy metal underlayers, where they reach balance at 0.8 nm thick W layer, as indicated by the zero domain wall velocity. In summary, we have demonstrated that by changing the relative thicknesses of two heavy metal underlayers we are able to control the direction and velocity of current driven domain wall motion. It results from the competition of two pure spin currents with opposite signs of spin polarization generated by the spin Hall effect of the heavy metal underlayers. Such way of manipulating current driven DWM would spark novel designs in spintronic devices and applications.