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.
Magnetic damping impacts essential dynamics for spintronic device applications, but its fundamental mechanisms in various materials – including simple ferromagnetic metals – have yet to be understood. Here, we experimentally correlate damping with structural and transport properties of epitaxial thin films of Fe. At room temperature, the effective Gilbert damping parameter is independent of whether these films are coherently strained or partially relaxed. However, at low temperature, we find that coherently strained Fe films with higher crystalline quality and conductivity exhibit higher damping. The enhancement of low-temperature damping is greater than that from classical eddy current loss. Our observation of such conductivity-like damping, possibly governed by the intraband scattering mechanism [1], provides fundamental insight into the role of crystallinity in damping in ferromagnetic metals.
[1] K. Gilmore, Y. U. Idzerda, M. D. Stiles, Phys. Rev. Lett. 99, 027204 (2007); M. A. W. Schoen et al. Nat. Phys. 12, 839 (2016).
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