Annealing temperature and thickness dependencies of structural and magnetic properties of ${\mathbf{Co}}_2\mathbf{FeAl}$ thin films

${\mathrm{Co}}_2\mathrm{FeAl}$ (CFA) thin films, of various thicknesses ($3\text{nm}\le t\le 50\mathrm{nm}$), have been grown by sputtering on (001) MgO single-crystal substrates and annealed at different temperatures ($\text{RT}\le T_a\le 600{}^{\circ }\mathrm{C}$, where RT is the room temperature). The influence of the CFA thickness ($t$), as well as ex situ annealing temperature ($T_a$), on the magnetic and structural properties has been investigated by x-ray diffraction (XRD), vibrating sample magnetometry, and broadband microstrip ferromagnetic resonance (MS-FMR). The XRD revealed an epitaxial growth of the films with the cubic [001] CFA axis normal to the substrate plane and that the chemical order varies from the $B2$ phase to the $A2$ phase when decreasing $t$ or $T_a$. The deduced lattice parameters showed an in-plane tetragonal distortion and in-plane and out-plane strains that increase with $T_a$ and $1/t$. For all $T_a$ values, the variation of the effective magnetization, deduced from the fit of MS-FMR measurements, shows two different regimes separated by a critical thickness, which is $T_a$ dependent. It decreases (increases) linearly with the inverse thickness ($1/t$) in the first (second) regime due to the contribution of the magnetoelastic anisotropy to surface (to volume) anisotropy. The observed behavior has been analyzed through a model allowing for the separation of the magnetocrystalline, magnetoelastic, and Néel-type interface anisotropy constants to the surface and the volume anisotropies. Similar behavior has been observed for the effective fourfold anisotropy field which governs the in-plane anisotropy present in all the samples. Finally, the MS-FMR data also allow one to conclude that the gyromagnetic factor remains constant and that the exchange stiffness constant increases with $T_a$.

Figure 1

(a) Typical example of $2\theta /\omega$ (out-of-plane) x-ray diffraction patterns for 50 nm thick CFA films annealed at different temperatures. The patterns have been shifted vertically for better visibility. (b) Evolution of the integral intensities of the (002) and (004) CFA peaks $\left[A\right(002)/A(004\left)\right]$ with respect to the annealing temperature for different film thicknesses $t$. (c) In-plane and out-of-plane lattice parameter variations as function of the annealing temperature for different film thicknesses. In (b) and (c), symbols refer to measurements while solid lines are guides to the eye.

Figure 2

Thickness dependence of (a) the unstrained cubic lattice parameter $a_0$, and (b) the in-plane ${\varepsilon }_{\parallel }$ and (c) the out-of-plane strains ${\varepsilon }_{\perp }$ with the annealing temperature. Symbols refer to measurements and solid lines are linear fits.

Figure 3

(a) Three-dimensional (3D) plot of the thickness dependencies of the saturation magnetic moment per unit area for ${\mathrm{Co}}_2\mathrm{FeAl}$ thin films annealed at different temperatures $T_a$. Symbols refer to measurements and solid lines are the linear fits. (b) Variations of the magnetization ($M_s$) at saturation and the magnetic dead layer ($t_d$) as function of the annealing temperature of ${\mathrm{Co}}_2\mathrm{FeAl}$ thin films. Symbols refer to measurements and solid lines are used as guides to the eye.

Figure 4

(a) Angular dependence of the resonance field at a 10 GHz driven frequency for ${\mathrm{Co}}_2\mathrm{FeAl}$ films of thickness $t$. (b) Easy axis field dependencies of the UPM and PSSW mode frequencies. In (a) and (b), solid lines refer to fits obtained using Eq. (1). (c) Variations of the exchange stiffness constant ($A_{\text{ex}}$), as a function of the annealing temperature of ${\mathrm{Co}}_2\mathrm{FeAl}$ thin films. Symbols refer to measurements and solid lines are used as guides to the eye.

Figure 5

3D plot of the thickness dependence of (a) the effective magnetization ($4\pi M_{\text{eff}}$) and (b) fourfold anisotropy field, extracted from the fit of FMR measurements, of ${\mathrm{Co}}_2\mathrm{FeAl}$ thin films annealed at different temperatures $T_a$. The solid lines are linear fits.

Figure 6

Annealing temperature dependence of (a) the volume and (b) surface effective perpendicular anisotropy constants in the two regimes below and above the critical thickness. Variation of the different contributions of the (c) magnetocrystalline ($K_{me\perp }$) and the volume ($K_{me,v\perp }^I$) magnetoelastic and (d) the Néel-type interface ($K_{N\perp }$) and the surface ($K_{me,s\perp }^{II}$) anisotropy constants to the perpendicular anisotropy. The data in (c) and (d) were obtained from the measurements presented in (a) and (b) using Eqs. (6) and (7). Symbols refer to measurements and solid lines are used as guides to the eye. Dashed lines refer to grid lines for zero.

Figure 7

Annealing temperature dependence of (a) the volume and (b) surface effective in-plane fourfold anisotropy constants in the two regimes below and above the critical thickness. Variation of the different contributions of the (c) magnetocrystalline ($K_{me4}$) and the volume ($K_{me,v4}^I$) magnetoelastic and (d) the Néel-type interface ($K_{N4}$) and the surface ($K_{me,s4}^{II}$) anisotropy constants to the in-plane fourfold anisotropy. The data in (c) and (d) were obtained from the measurements presented in (a) and (b) using Eqs. (6) and (7) by replacing $\perp$ with 4. Symbols refer to measurements and solid lines are used as guides to the eye. Dashed lines refer to grid lines for zero.