Sign Reversal of Spin-Transfer Torques

Abstract

Spin-transfer torque (STT) and spin-orbit torque (SOT) form the core of spintronics, allowing for the control of magnetization through electric currents. While the sign of SOT can be manipulated through material and structural engineering, it is conventionally understood that STT lacks a degree of freedom in its sign. However, this study presents the first demonstration of manipulating the STT sign by engineering heavy metals adjacent to magnetic materials in magnetic heterostructures. Spin torques are quantified through magnetic domain-wall speed measurements, and subsequently, both STT and SOT are systematically extracted from these measurements. The results unequivocally show that the sign of STT can be either positive or negative, depending on the materials adjacent to the magnetic layers. Specifically, Pd/Co/Pd films exhibit positive STT, while Pt/Co/Pt films manifest negative STT. First-principle calculations further confirm that the sign reversal of STT originates from the sign reversal of spin polarization of conduction electrons.

Keywords: magnetic domain‐walls; spin polarization; spintronics; spin‐transfer torques.

Figure 1

Plots of v _DW as a function of H _x for the a) Pt/Co/Pt and b) Pd/Co/Pd films, respectively. The dashed vertical lines indicate H _x =   − H _DMI . Plots of ε _tot. as a function of H _x for the c) Pt/Co/Pt and d) Pd/Co/Pd films, respectively. Plots of ε _STT (colored) and ε _SOT as functions of H _x for the e) Pt/Co/Pt and f) Pd/Co/Pd films, respectively. The solid curves guide the symmetric and anti‐symmetric nature of STT and SOT, respectively. The horizontal dashed lines guide eyes to ${\mathit{\epsilon }}_{\mathbf{STT}}^{\mathbf{max}}$. Schematic diagrams of STT‐induced DW motion for g) Pt/Co/Pt and h) Pd/Co/Pd films. The red and the blue arrows represent the STT‐induced DW motion along current direction and electron‐flow direction, respectively.

Figure 2

Plots of ε _STT (colored symbol) and ε _SOT (black symbol), respectively, as functions of H _x for the Pd/Co/Pd and Pt/Co/Pt films with different t _Co . The dashed vertical lines indicate H _x =   − H _DMI . The solid lines show the best fittings of the symmetry and anti‐symmetry of STT and SOT, respectively. The horizontal dashed lines guide eyes to ${\mathit{\epsilon }}_{\mathbf{STT}}^{\mathbf{max}}$.

Figure 3

Plots of ${\mathit{\epsilon }}_{\mathbf{STT}}^{\mathbf{max}}$ as a function of t _Co for the Pt/Co/Pt films (red symbol) and Pd/Co/Pd (blue symbol).

Figure 4

The total P as function of E for the a) Pt/Co/Pt and b) Pd/Co/Pd structures, respectively, where E _F is set to zero. The layer‐resolved spin polarization contributions: (c) and (d) for the sum of the L2 − L7 layers, (e) and (f) for the L1 layers, and (g) and (h) for the Co‐layer, respectively, for the Pt/Co/Pt and Pd/Co/Pd structures. The insets show P in the energy window of E _F ± 0.02   eV for better visibility.

Figure 5

Spin‐dependent DOS of the L2 − L7 layers as function of E for the a) Pt/Co/Pt and b) Pd/Co/Pd structures, respectively, where E _F is set to zero. Plots of Δ N _lm for d orbitals with different m : (c) and (d) for m  =   0, (e) and (f) for m  =   ± 1, and (g) and (h) for m  =   ± 2, for the Pt/Co/Pt and Pd/Co/Pd structures, respectively.