Fig. 2: Current-driven DW motion in a FM–SAF lateral junction.
a, DW velocity v versus applied current density J in the FM region (orange triangles) and SAF region (blue triangles). DW motion is monitored using current pulses with a temporal length \(\tau _P^J = 5\,{{{\mathrm{ns}}}}\) within the FM and SAF regions. Error bars represent the standard deviation. b, Kerr microscope images of current-induced DW motion across the FM–SAF junction. Threshold current densities for DW motion (or injection) in each region—FM (green shaded), SAF (blue shaded), FM → SAF (red shaded) and SAF → FM (blue shaded)—are shown. The black dashed line shows the position of the junction in each panel. Here θJ = 0°. A single DW is moved along the current flow direction by five consecutive current pulses with J ≈ 1.5 × 108 A cm−2. DW positions in the SAF region are indicated by white vertical lines and black arrows. The bright and dark contrast corresponds to the domain configuration, down (↓) and up (↑), respectively. t, time. Light blue, red and green shaded regions correspond to the cases where the DW is located at the SAF, FM–SAF junction and FM region, respectively. The circled cross and dot symbols represent down and up domain configurations, respectively. c, \(J_{\mathrm{{th}}}^i(i = {\mathrm{FM}},{\mathrm{FM}} \to {\mathrm{SAF}} \, {\mathrm{or}}\,{\mathrm{SAF}})\) versus current pulse length \(\tau _P^J\). The flow and thermally activated regimes of DW motion are denoted by blue and orange shaded backgrounds, respectively. \(J_{\mathrm{{th,flow}}}^{\mathrm{{FM}}}\) (orange triangles) and \(J_{\mathrm{{th,flow}}}^{\mathrm{{SAF}}}\) (blue triangles) measured in the flow regime correspond to the current at which the DW velocity reaches ~5 m s−1. The \(J_{\mathrm{{th}}}^i\) values in the thermally activated regime correspond to DW depinning with a probability of \(P_{\mathrm{{dep}}}^i = 0.5\) (i = FM or SAF). For DW injection (FM → SAF), \(J_{\mathrm{{th}}}^{\mathrm{{FM \to SAF}}}\) (green circles) corresponds to an injection probability of \(P_{\mathrm{{inj}}}^{\mathrm{{FM \to SAF}}} = 0.5\) in both the flow and thermally activated regimes. The dashed and solid curves correspond to fits to the relation \(J_{{{{\mathrm{th}},{\mathrm{flow}}}}}^i - J_{{{{\mathrm{th}}_0,{\mathrm{flow}}}}}^i \propto 1/\tau _P^J\) where i = FM, FM → SAF or SAF, and \(J_{{{{\mathrm{th}},{\mathrm{therm}}}}}^i \propto 1 - (1/E_{{J}}^i){{{\mathrm{ln}}}}(\tau _P^J/\tau _0)\) in the flow and thermally activated regime, respectively. The error bars in the flow and thermally activated regimes correspond to the standard deviation and 25/75% probabilities, respectively. d,e, Schematic illustration of the current-driven DW injection (d) and corresponding energy landscape (e) as a function of DW position along the racetrack. Initially, the DW is located in the FM region next to the junction (i) and is injected into the SAF region by a current pulse with a magnitude that is greater than \(J_{\mathrm{{th}}}^{\mathrm{{FM \to SAF}}}\) (ii). Here, the energy barrier for DW depinning in the FM region is \(E_{{J}}^{\mathrm{{FM}}}\) and for DW injection is \(E_{{J}}^{\mathrm{{FM \to SAF}}} = E_{\mathrm{{nc}}} + E_{{J}}^{\mathrm{{SAF}}}\), where Enc is the nucleation energy for an additional DW in the upper FM layer of the SAF (black dashed box) and \(E_{{J}}^{\mathrm{{SAF}}}\) is the DW depinning energy barrier (iii). By contrast, DW injection from the SAF into the FM region does not require any additional energy (iv).
