Tailoring dynamic magnetic characteristics ofFe60Al40films through ion irradiation

Abstract

Magnetization dynamics in $\mathrm{F}{\mathrm{e}}_{60}\mathrm{A}{\mathrm{l}}_{40}$ thin films possessing depth-varying saturation magnetization (${M}_{\mathrm{S}}$) have been studied experimentally and theoretically. Variation in ${M}_{\mathrm{S}}$ was achieved by irradiation of 40 nm thick, chemically ordered (B2 phase) $\mathrm{F}{\mathrm{e}}_{60}\mathrm{A}{\mathrm{l}}_{40}$ films with $\mathrm{N}{\mathrm{e}}^{+}$ ions with energies between 0--30 keV. The initial B2 phase is paramagnetic, and as the penetrating ions cause chemical disordering, the ion-affected region transforms to the ferromagnetic A2 phase. The effective ferromagnetic thickness and the depth of the A2/B2 phase boundary depend on the ion energy ($E$); the effective thicknesses are 8.5 and 40 nm, respectively, for $E=2.5$ and 30 keV. Thermally excited spin waves in films with varying effective ferromagnetic thicknesses were analyzed by employing Brillouin light scattering and vector network analyzer ferromagnetic resonance spectroscopy. The analytical calculations are in good agreement with the experimental values and show that the observed spin-wave modes are directly related to the effective ferromagnetic thickness; films irradiated with $E\phantom{\rule{0.28em}{0ex}}<\phantom{\rule{0.28em}{0ex}}15\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$ only show the Damon-Eshbach mode, whereas for $15\phantom{\rule{0.28em}{0ex}}\ensuremath{\le} E\phantom{\rule{0.28em}{0ex}}<\phantom{\rule{0.28em}{0ex}}20\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$, an additional lower frequency standing spin-wave mode is observed. In films irradiated with $E \ensuremath{\ge}\phantom{\rule{0.28em}{0ex}}20\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$, the Damon-Eshbach mode is observed to lie between two standing spin-wave modes. Furthermore, the A2/B2 phase boundary can be shown to act as an asymmetric pinning site. Controlling the depth of the phase boundary by varying the ion energy can be a path to manipulate spin-wave propagation in materials displaying the phenomenon of disorder induced ferromagnetism.