February 1, 2021 feature
Ultralow magnetic damping of a common metallic ferromagnetic film
Ultralow damping is of key importance for spintronic and spin-orbitronic applications in a range of magnetic materials. However, the number of materials that are suited for charge-based spintronic and spin-orbitronic applications are limited due to magnon-electron scattering. To quantitatively calculate the transition metallic ferromagnetic damping, researchers have proposed theoretical approaches including the breathing Fermi surface model (to describe dissipative magnetization dynamics), generalized torque correlation model, scattering theory, and the linear response damping model. In a new report now published on Science Advances, Yangping Wei and a team of scientists in science, magnetism and magnetic materials, and chemical engineering in China and Singapore experimentally detailed a damping parameter approaching 1.5 x 10-3 for traditional, fundamental iron aluminide (FeAl) soft ferromagnets. The results were comparable to those of 3-D transition metallic ferromagnets based on the principle of minimum electron density of states.
Ultralow magnetic damping
Ultralow magnetic damping can allow to meet the energy and speed requirements of devices for spintronic and spin-orbitronic applications. Ultralow damping can, however, contradict the charge current requirements for most applications since such charge currents can cause high damping due to magnon-electron scattering. Yttrium-iron-garnet (YIG) materials are ferromagnetic insulators with low damping and are good candidates to achieve properties of low-energy consumption and high speed, suited for spintronic devices. Compared to 3-D transition metal ferromagnets, research efforts on the magnetic damping of traditional, fundamental iron aluminide (FeAl) soft ferromagnets, which possess excellent mechanical and functional properties at a low cost, remain rare. The comparatively low magnetic damping achieved for an FeAl metallic system can make it a promising material for spintronic and spin-orbitronic applications. In this work, Wei et al. examined the electronic structure of Fe1−xAlx using density functional theory (DFT) calculations conducted with the Vienna Ab initio simulation package (VASP) and the generalized gradient approximation (GGA). The team also grew a high-quality single-crystalline Fe-Al alloy film with a thickness of 20 nm and a 3-nm-thick capping aluminum layer on magnesium oxide (MgO), using molecular beam epitaxy (MBE) and studied the compositional effect of damping on the alloys. The team then used in situ reflection high-energy electron diffraction (RHEED) and high-resolution X-ray diffraction (HRXRD) methods to demonstrate the single domain texture of the FeAl films. Using frequency sweeps with various mixed-magnetic field ferromagnetic resonance (FMR) measurements, Wei et al. found low magnetic damping effects.
Density functional theory calculations and the characterization of crystalline structures
During the study, Wei et al. used eight body-centered cubic (bcc) unit cells with 16 iron atoms to construct a supercell to calculate pure iron density of states (DOS). The Fe1−xAlx contained different concentrations of x, where iron on various sites were replaced by aluminum atoms. The team obtained several representative DOS for the FeAl alloy and found them to exhibit a minimum at the Fermi level at aluminum concentrations of 25%. The team then set the chamber pressure of the custom-designed molecular beam epitaxy for sample growth at a favorable rate to fabricate high quality, single-crystalline Fe1−xAlx alloy films under nonequilibrium conditions. The RHEED (reflection high-energy electron diffraction) patterns showed the attainment of a pure single-orientation relationship. The team assessed the dependence of the fine crystal structure of the Fe1−xAlx films on the concentration of aluminum using HRXRD (high-resolution X-ray diffraction). As the aluminum concentration increased, they noted the formation of a solid solution of aluminum in iron. The team then assessed the thickness and roughness of the films using an X-ray reflectometry scan.
Characterization of basic magnetization
To explain the easy and hard magnetizing directions of the iron-aluminum films, Wei et al. measured angle-remnant curves using a vibrating sample magnetometer (VSM). As the aluminum concentration varied from zero to 25%, the saturation magnetization of the sample changed. Meanwhile, in the hard magnetization direction, as the saturation field decreased with increasing aluminum concentration, the magnetocrystalline anisotropy became weaker. To determine the value of the magnetic anisotropy of the material, the team used angular-dependent ferromagnetic resonance measurements. The team then measured the damping torque, known as Gilbert damping in the setup, where its direction was given by the vector product of the magnetization and its time derivative. For instance, the resulting Gilbert damping parameter (α) for Fe75Al25 films was comparable to values described in previous studies.
In this way, Yangping Wei and colleagues observed ultralow magnetic damping of 1.5 x 10-3 in traditional FeAl crystalline ferromagnets at an aluminum concentration of 25%. The work offers a new opportunity to select low-cost materials not limited to 3-D transition metallic elements for spintronic and spin-orbitronic applications. The team obtained these novel results on the basis of the principle of minimum density of states proposed previously. The results further verified magnetic damping to be proportional to the density of states at the Fermi level in the same alloy. The work enables a new approach to screen materials for spintronic and spin-orbitronic applications and expand the method to a broader range of low-damping materials.
Schoen M. A. W. et al. Ultra-low magnetic damping of a metallic ferromagnet, Nature Physics, doi.org/10.1038/nphys3770
Gilmore K. et al. Identification of the dominant precession-damping mechanism in Fe, Co, and Ni by first-principles calculations. Physical Review Letters, doi.org/10.1103/PhysRevLett.99.027204
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