Giant Anisotropy of Gilbert Damping in Epitaxial CoFe Films

Abstract

Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co50Fe50 thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborate from the magnitude of the anisotropic magnetoresistance in Co50Fe50.In magnetization dynamics the energy relaxation rate is quantified by the phenomenological Gilbert damping in the Landau-Lifshits-Gilbert equation [1], which is a key parameter for emerging spintronics applications [2][3][4][5][6]. Being able to design and control the Gilbert damping on demand is crucial for versatile spintronic device engineering and optimization. For example, lower damping enables more energy-efficient excitations, while larger damping allows faster relaxation to equilibrium and more favorable latency. Nevertheless, despite abundant approaches including interfacial damping enhancement [7-9], size effect [10,11] and materials engineering [12][13][14], there hasn't been much progress on how to manipulate damping within the same magnetic device. The only well-studied damping manipulation is by spin torque [15][16][17][18], which can even fully compensate the intrinsic damping [19,20]. However the requirement of large current density narrows its applied potential.An alternative approach is to explore the intrinsic Gilbert damping anisotropy associated with the crystalline symmetry, where the damping can be continuously tuned via rotating the magnetization orientation. Although there are many theoretical predictions [21][22][23][24][25], most early studies of damping anisotropy are disguised by two-magnon scattering and linewidth broadening due to field-magnetization misalignment [26][27][28][29]. In addition, those reported effects are usually too weak to be considered in practical applications [30,31].In this work, we show that a metallic ferromagnet can exhibit a giant Gilbert damping variation by a factor of four along with low minimum damping. We investigated epitaxial cobalt-iron alloys, which have demonstrated new potentials in spintronics due to their ultralow dampings [32,33]. Using spin-torque-driven and inductive ferromagnetic resonance (FMR), we obtain a fourfold (cubic) damping anisotropy of 400% in Co 50 Fe 50 thin films between their easy and hard axes. For each angle, the full-range frequency dependence of FMR linewidths can be well reproduced by a single damping parameter α. Furthermore, from first-principle calculations and temperature-dependent measurements, we argue that this giant damping anisotropy in Co 50 Fe 50 is due to the variation of the spin-orbit coupling (SOC) in the cubic lattice, which differs from the anisotropic density of state found in ultrathin Fe film [30]. We support our conclu...

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