Uniaxial anisotropy, intrinsic and extrinsic damping in Co₂FeSi Heusler alloy thin films

Abstract

Ferromagnetic resonance (FMR) technique has been used to study the magnetization relaxation processes and magnetic anisotropy in two different series of the Co 2 FeSi (CFS) Heusler alloy thin films, deposited on the Si(111) substrate by ultra high vacuum magnetron sputtering. While the CFS films of fixed (50 nm) thickness, deposited at different substrate temperatures (T S ) ranging from room temperature (RT) to 600 • C, constitute the series-I, the CFS films with thickness t varying from 12 nm to 100 nm and deposited at 550 • C make up the series-II. In series-I, the CFS films deposited at T S = RT and 200 • C are completely amorphous, the one at T S = 300 • C is partially crystalline, and those at T S = 450 • C, 550 • C and 600 • C are completely crystalline with B2 order. By contrast, all the CFS films in series-II are in the fully-developed B2 crystalline state. Irrespective of the strength of disorder and film thickness, angular variation of the resonance field in the film plane unambiguously establishes the presence of global 'in-plane' uniaxial anisotropy. The uniaxial anisotropy field decreases as the crystalline order in the films increases and goes through a minimum at t = 50 nm as a function of film thickness. Landau−Lifshitz−Gilbert damping and two-magnon scattering dominantly contribute to the line-broadening in both 'in-plane' and 'out-of-plane' configurations. The two-magnon scattering has larger magnitude in the amorphous films than in the crystalline ones. Angular variation of the linewidth in the film plane reveals that, in the CFS thin films of varying thickness, a crossover from the 'in-plane' local four-fold symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy) occurs as t exceeds 50 nm. Gilbert damping parameter α decreases monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing T S ) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film with t = 75 nm. Such variations of α with T S and t are understood in terms of the changes in the total (spin-up and spin-down) density of states at the Fermi level caused by the disorder and film thickness. Spin pumping across the Co 2 FeSi film/Ta cap-layer interface makes negligible contribution to α. We propose that disorder and/or the film thickness can be used as control parameters to tune α, whose value is decisive in choosing a ferromagnetic film for a given spintronics application.

Fabrication and Magneto-Structural Properties of Co2-Based Heusler Alloy Glass-Coated Microwires with High Curie Temperature

“…Figure 3 shows the magnetic properties (dependencies of magnetic moment, M, versus magnetic field, H) of Co 2 FeSi G-CMMWs at different temperatures, measured in an applied magnetic field between ±50 kOe, at a temperature range from 5 to 400 K. As plotted in Figure 3a,b, all M-H loops showed ferromagnetic behavior over the entire measured temperature range. The maximum values of the magnetic parameters, such as magnetic moment and magnetic remanence, M r , were detected at 5 K and the lowest values were observed at 400 K. Additionally, all the hysteresis loops showed rectangular M-H loop shapes, similar to the behavior observed in Co 2 FeSi alloys deposited by different techniques and in different forms [12,14,17,29,61]. For the temperature range from 200 K to 400 K the hysteresis loops showed low saturation field, H s , anisotropy field, H k , M r and H c values, as indicated in Figure 3b.…”

“…Co 2 -based full/half-Heusler compounds with high Curie temperature (T c > 1100 K), high magnetic moment (~6 µ B /f.u. ), unique electronic structure and low Gilbert damping constant (α = 0.004) are the most promising materials for multi-function applications [11][12][13]. Moreover, unique exotic transport properties and a large anomalous Hall effect have been reported on Co 2 -based Heusler alloys, due to the large Berry curvature linked with their band structure [1,2].…”

Control of Structural and Magnetic Properties of Polycrystalline Co2FeGe Films via Deposition and Annealing Temperatures

“…The in-plane anisotropy H ani extracted using Equation ( 2) was found to be vanishing (<10 Oe for all cases). It is in a good agreement with prediction that for fine polycrystalline films (like ours) H ani can be neglected due to the randomization of the magnetocrystalline anisotropy axis within sample volume [17]. The gyromagnetic ratio γ/2π shows no significant variation both with T S and T a .…”

“…Thus, FHA films are considered as attractive candidates for different multifunctional applications. It was shown in [ 5 , 17 , 19 , 22 , 23 ] that substrate temperature (T S ) and post-deposition annealing (T a ) are powerful tools to control the properties of FHA films. However, so far there is no universal law for thermal treatment conditions.…”

“…Thus, FHA with high spin polarization and moderate values of M S and A comparing to ferromagnetic metals like Fe and Co are attractive candidates for STT applications. On the other hand, a large value of α is needed to improve thermal stability in current-perpendicular-to-plane giant magnetoresistance (GMR) read sensors [17] Physical properties of FHA can be effectively tuned in a wide range by changing atomic ordering, microstructure and composition [1,2,4,[17][18][19][20][21]. Thus, FHA films are considered as attractive candidates for different multifunctional applications.…”