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

Abstract

Thin polycrystalline Co₂FeGe films with composition close to stoichiometry have been fabricated using magnetron co-sputtering technique. Effects of substrate temperature (T_S) and post-deposition annealing (T_a) on structure, static and dynamic magnetic properties were systematically studied. It is shown that elevated T_S (T_a) promote formation of ordered L2₁ crystal structure. Variation of T_S (T_a) allow modification of magnetic properties in a broad range. Saturation magnetization ~920 emu/cm³ and low magnetization damping parameter α ~ 0.004 were achieved for T_S = 573 K. This in combination with soft ferromagnetic properties (coercivity below 6 Oe) makes the films attractive candidates for spin-transfer torque and magnonic devices.

Keywords: Ferromagnetic resonance; Heusler alloys; magnetostatic properties; thin films.

Figure 1

X-ray reflectivity for Co₂FeGe films: (a) deposited at T_S = RT (S1), T_S = 573 K (S2), T_S = 773 K (S3); (b) deposited at RT and annealed for 1 h at T_a = 573 K (S4), T_a = 773 K (S5); (c) fit of the experimental XRR spectrum for the film deposited at T_S = 573 K. Fitting parameters are summarized in Table 1.

Figure 2

GIXRD patterns for Co₂FeGe films: (a) deposited at T_S = RT (S1), T_S = 573 K (S2), T_S = 773 K (S3); (b) deposited at RT and annealed for 1 h at T_a = 573 K (S4), T_a = 773 K (S5). Close-ups for (111) reflection area for films: (c) deposited at different T_S; (d) for films deposited at RT and annealed at different T_a for 1 h.

Figure 3

Magnetic hysteresis loops (M vs. H) for Co₂FeGe films: (a) deposited at T_S = RT (S1), T_S = 573 K (S2), T_S = 773 K (S3); (b) deposited at RT and annealed for 1 h at T_a = 573 K (S4), T_a = 773 K (S5). Close-ups for −30 Oe < H < 30 Oe region for films: (c) deposited at different T_S; (d) for films deposited at RT and annealed at different T_a for 1 h.

Figure 4

Real part of the U(f) function calculated from measured complex S₂₁ spectrum for the film deposited at T_S = 573 K at different applied fields (a); Example of fitting of permeability ReU(f) for the same sample at a static applied field H_ext = 2000 Oe (b). The open circles represent the ReU(f) values extracted from the experimental S₂₁ parameters. The two dashed lines show the best individual fitting functions ReU_fit(f) for each peak while the solid curve shows cumulative fit for the whole spectrum.

Figure 5

Dependencies of the FMR frequency and the first order PSSW on applied magnetic field for Co₂FeGe films deposited at various conditions: (a) T_S = RT; (b) T_S = 573 K; (c) T_S = 773 K; (d) T_S = RT+ annealing at T_a = 573 K for 1 h; (e) T_S = RT+ annealing at T_a = 773 K for 1 h. Solid red lines represent fit of the experimental data for FMR using Equation (2). Dashed lines represent fit for the first order PSSW using Equation (3). The values of effective magnetization $M_{eff}$ and the exchange stiffness constant $A$ extracted from the fitting are listed in Table 2.

Figure 6

Dependencies of FMR linewidth ΔH as a function of resonance frequency for Co₂FeGe films deposited at T_S = RT (S1), T_S = 573 K (S2), T_S = 773 K (S3) and deposited at RT and annealed for 1 h at T_a = 573 K (S4), T_a = 773 K (S5). Experimental data (points) are accompanied by fit (lines) according to Equation (3). Estimated values of damping parameter α are listed in Table 2.