Energy-efficient spintronic devices require the following two criteria: (1) a large spin-orbit torque (SOT) and (2) low damping to excite magnetic precession with low current input. Conventional ferromagnet/nonmagnetic-metal bilayers can obtain sizeable SOTs; however, this comes at the expense of drastically increasing the damping. Because the origin or the transmission of spin is interfacial in nature, the ferromagnetic layer must be restricted to ∼1nm in thickness to see substantial SOTs. Here, we present an alternative approach to producing sizeable SOTs that allows for a thicker ferromagnetic layer maintaining low damping. Instead of relying on a single interface, we continuously break the bulk inversion symmetry with a vertical compositional gradient of two ferromagnetic elements: Fe with low intrinsic damping and Ni with sizable spin-orbit coupling. We find low effective damping parameters of αeff < 5 × 10−3 in the FeNi alloy films, despite the steep compositional gradients. Moreover, we reveal a sizable anti-damping SOT efficiency of θAD ≈ 0.05, even without an intentional compositional gradient. Through depth-resolved x-ray diffraction, we identify a lattice strain gradient as crucial symmetry breaking that underpins the SOT. Our findings provide fresh insights into damping and SOTs in single-layer ferromagnets for power-efficient spintronic devices.
Spin-orbit torque is a current-induced transfer of angular momentum from an atomic lattice to magnetic order. It is a promising mechanism to write magnetic memories and drive spin torque oscillators. Since its inception, the list of spin-orbit torque mechanisms has grown beyond the conventional spin Hall and Rashba-Edelstein mechanisms to include “unconventional” mechanisms, arising from spin and orbital current generation in ferromagnetic layers, nonmagnetic layers, and their interfaces. In this talk, we use micromagnetic, semiclassical, and first principles calculations to show that unconventional spin-orbit torques are potentially important for devices, from causing nonlocal spin torques in ferromagnetic trilayers to enabling large amplitude, easy-plane spin-orbit torque oscillators.
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