Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation

The effect of graded anisotropy on static and dynamic magnetic properties of ${\mathrm{Ar}}^{+}$-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by ${\mathrm{Ar}}^{+}$ irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing $\alpha$, a decreasing thickness of the hard $\mathrm{L}{1}_0$ phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (${\nu }_0\approx 6$ GHz), which could be reproduced assuming the presence of a nonzero “rotatable” anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.

Figure 1

In-plane (red line) and out-of-plane (black line) hysteresis loops of a 15-nm-thick $\mathrm{L}{1}_0$-FePt film measured by AGFM before (a) and after (b) irradiation performed at an incidence angle $\alpha ={45}^{\circ }$.

Figure 2

(a) In-plane (red line) and out-of-plane (black line) components of the magnetization of a 15-nm-thick $\mathrm{L}{1}_0$-FePt film irradiated at incidence angle $\alpha ={45}^{\circ }$, measured by vVSM applying an in-plane magnetic field. (b) Angular dependence of remanent magnetization measured in the film plane.

Figure 3

Magnetic force microscopy (MFM) images of a 15-nm-thick $\mathrm{L}{1}_0$-FePt film (a) in the as-grown state, and (b) after irradiation at incidence angle $\alpha ={45}^{\circ }$. In the latter case, the MFM image was taken at remanence, after in-plane saturation of the magnetization by a 20 kOe magnetic field.

Figure 4

Frequency of the thermally excited spin waves versus the intensity of the in-plane magnetic field, measured by Brillouin light scattering (BLS) in two 15-nm-thick $\mathrm{L}{1}_0$-FePt films, irradiated at incidence angle $\alpha ={85}^{\circ }$ (blue diamonds) and $\alpha ={45}^{\circ }$ (black circles). The continuous lines denote the corresponding calculated frequencies. For the sample irradiated at $\alpha ={45}^{\circ }$, a tentative fit, made assuming the in-plane anisotropy to be exactly zero, is shown as a dashed line. The dashed line and the continuous line are indistinguishable for $H\ge 2$ kOe. The insets show two BLS spectra obtained at 1 kOe, after irradiation at $\alpha ={85}^{\circ }$ (black lines) and ${45}^{\circ }$ (blue lines), respectively.

Figure 5

Canting angle with respect to the film normal versus plane index, calculated for two 15-nm-thick $\mathrm{L}{1}_0$-FePt films, irradiated at incidence angle (left panel) $\alpha ={85}^{\circ }$ and (right panel) $\alpha ={45}^{\circ }$, respectively. Different symbols denote different ground-state configurations, calculated for selected values of the magnetic field $H$ applied in plane. A canting angle equal to $0^{\circ }$ (${90}^{\circ }$) corresponds to the magnetization of a given plane being oriented completely out-of-plane (completely in-plane). High (low) plane indices correspond to outermost (innermost) soft (hard) planes. H, G, S indicate the hard, graded, and soft regions, respectively.

Figure 6

(a) Dependence of the spin-wave frequency on the in-plane rotation angle, as measured (symbols) at remanence by BLS, and calculated (lines) from Eq. (4) for the 15-nm-thick $\mathrm{L}{1}_0$-FePt film irradiated at ${45}^{\circ }$ incidence angle. The continuous line is obtained assuming a nonzero rotatable anisotropy field, $H_{\mathrm{rot}}=510$ Oe. The dashed line is obtained setting $H_{\mathrm{rot}}=0$. (b) Rotatable anisotropy field $H_{\mathrm{rot}}$ versus in-plane applied field $H$, as obtained from the fitting of the BLS frequency versus $H$ (the line is a guide to the eye).