February 1, 2021 feature
Ultralow magnetic damping of a common metallic ferromagnetic film
Ultralow damping is of key importance for and 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 . To quantitatively calculate the transition metallic ferromagnetic damping, researchers have proposed theoretical approaches including the breathing model (to describe dissipative magnetization dynamics), , 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 (FeAl) soft ferromagnets. The results were comparable to those of based on the principle of minimum .
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. 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 (DFT) calculations conducted with the (VASP) and the (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 (MgO), using (MBE) and studied the compositional effect of damping on the alloys. The team then used in situ (RHEED) and (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 effects.
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 (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 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 .
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RHEED patterns of (a) (100)-oriented MgO, the electron beam is along the in-plane direction of [010] and (b) (100)-oriented MgO, the electron beam is along the in-plane direction of [011]. (c, d) RHEED patterns of Fe1-xAlx film grown on it, respectively. Credit: Science Advances, doi: 10.1126/sciadv.abc5053 -
The angular dependence of the remanent magnetization and ferromagnetic resonance (FMR) and the dependence of magnetocrystalline anisotropy on Al content. (A) 0° is the starting point along the MgO[010] direction in the measured angle-remanent curves showing second minimum Mr that indicates the hard magnetization direction corresponding to the Fe1−xAlx [011], and Mr reaches its maximum value at 45o corresponding to the easy magnetization direction along the Fe1−xAlx [010]. The dashed line is a guide for identifying the first and second minimum Mr. (B) Magnetic hysteresis loops along the easy and hard magnetization axes of the Fe1−xAlx showing the dependence on Al concentration. The saturation field along the easy magnetization direction labeled with 45o remains constant and the hard magnetization direction labeled with 0o decreases as the Al concentrations increases, indicating that the magnetocrystalline anisotropy of Fe1−xAlx becomes weaker with increasing Al content. (C) Derivative FMR absorption spectra for Fe3Al from 0o (corresponding to the MgO[010] direction) to 180o at a microwave frequency of 9.4 GHz. (D) Series of resonant fields fitted by the experimental data for the extraction of H2∥ and H4∥. Credit: Science Advances, doi: 10.1126/sciadv.abc5053
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 (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 measurements. The team then measured the damping torque, known as 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 .
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 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.
More information: Wei Y. et al. Ultralow magnetic damping of a common metallic ferromagnetic film, Science Advances,
Schoen M. A. W. et al. Ultra-low magnetic damping of a metallic ferromagnet, Nature Â鶹ÒùÔºics,
Gilmore K. et al. Identification of the dominant precession-damping mechanism in Fe, Co, and Ni by first-principles calculations. Â鶹ÒùÔºical Review Letters,
Journal information: Science Advances , Â鶹ÒùÔºical Review Letters , Nature Â鶹ÒùÔºics
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