Tailoring the magnetic damping and anisotropy of Permalloy deposited on GaSb nanocones.

The fundamental understanding how patterning on the nanometer length scale is affecting the magnetic properties is crucial to improve magnetic devices or recording media [1]. This is even more important when going from 2-dimensional patterns to 3-dimensional structures. Currently, there are still many open questions on this issue. Furthermore, the magnetic properties of these systems like damping and anisotropy need to be investigated. For 2-dimensional arrays this is quite easy. However the third dimension adds a lot of complexity to this issue, especially for micromagnetic simulations, due to the large cell number. Nowadays, there are several methods common practice like lithography and nanoimprinting to obtain patterned surfaces on the nanometer scale. Self-assembled nanostructures are a promising alternative to cover the third dimension.
Applying Ar⁺-ions with a broad beam ion source at normal incidence one obtains a self-assembled uniform pattern of nanocones on GaSb [2]. Due to the ion erosion the GaSb cones have an amorphous surface layer of a few nanometer. The size is adjustable (aspect ratio = 1) in the range of 10 to 100 nm. The size depends on the ion energy and thus can be selected. The relation between the ion energy and the characteristic length l_c is described in [3]. The higher the ion energy is the higher and broader the nanocones can be adjusted. The size distribution is homogeneous within 10% and a short range hexagonal ordering is achieved as well. For the magnetic layers the substrate template was coated by molecular beam epitaxy with a seed layer of 5 nm Cr, followed by a 20 nm Ni₈₀Fe₂₀ and a 3 nm Cr cap layer. In Fig. (1) a transmission electron microscope (TEM) of a magnetically coated sample prepared at an ion energy of 200 eV and a characteristic length l_c of about 30 nm. It depicts the film growth on the nanocones.
We investigated the magnetic properties by vector network analyzer ferromagnetic resonance (VNA-FMR). With VNA-FMR one can achieve information about the magneto-static properties like anisotropy and g-factor as well as information about dynamic properties like damping and inhomogeneous linewidth broadening. We measured resonance field and linewidth frequency dependent from 1 to 45 GHz in the polar geometry at θ_H = 0° [out-of-plane (oop)] and θ_H = 90° [in-plane (ip)]. In addition the angular-dependent measurements at 15 GHz from θ_H = - 30° to 195° were analyzed. The results have to be fitted by the FMR resonance equations. The dynamic behavior is shown in Fig. (2a) and (2b). Fig. (2a) shows the Gilbert damping constant α versus the characteristic length l_c determined from the frequency dependence. Data for external field directions, i.e. θ_H = 0° (blue lines / points) and θ_H = 90° (red lines / points) are shown. The frequency dependence (not shown) evidences that only Gilbert damping contributes to the dynamic properties. The α values for the θ_H = 90° starts to increase between l_c = 34 nm and 51 nm from about 0.010 to 0.034. This is due to the different growth type with increasing nanocone dimension. Whereas the damping for the θ_H = 0° measurement stays almost constant for all cone sizes with a fitted average value α = 0.006. These resulting damping constants for the measurements in both geometries are comparable to the Permalloy bulk value α_bulk = 0.013(4) in literature [4]. The g-factor has been determined to be g = 2.095(4) which is in agreement with the literature value [5]. Fig. (2b) illustrates the behavior of the inhomogeneous linewidth broadening ΔH₀ for the two external field directions (blue: θ_H = 0° / red: θ_H = 90°). For the oop–geometry ΔH₀ is comparable to the bulk value magnitude ΔH₀ = 8.6 Oe [4] and almost constant over l_c. Along the ip–geometry ΔH₀ is significantly higher than for θ_H = 0°. This can be explained by the superposition of several local resonances around the cone with slightly different resonance fields due to the different local field directions with respect to the cone’s normal. In the oop–geometry no such different resonance fields occur.
Financial support provided by the Deutsche Forschungsgemeinschaft (DFG) via projects DFG FA 314-7.1 and AL 618-6 is gratefully acknowledged.
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[1] B. D. Terris et al., J. Magn. Magn. Mater. 41, 10 (2009).
[2] S. Facsko et al., Science 285, 1551 (1999).
[3] S. Facsko et al., Phys. Rev. B 63, 16 (2001).
[4] B. K. Kuanr et al., J. Magn. Magn. Mater. 286, (2005).
[5] D. Markó et al., Appl. Phys. Lett. 96, 022503 (2010).