Morphology induced two-magnon scattering in thin NiFe films


Morphology induced two-magnon scattering in thin NiFe films

Körner, M.; Lenz, K.; Fritzsche, M.; Facsko, S.; Fassbender, J.

When studying magnetization dynamics of thin magnetic films, intrinsic as well as extrinsic spin relaxation processes have to be taken into account. While intrinsic processes, summarized as Gilbert damping, are well known and studied for the last decades, the focus now has shifted to extrinsic contributions. In this context the two-magnon scattering (TMS) is of particular interest. This type of scattering is induced within thin magnetic films by defects and inhomogeneities. It was shown that periodic magnetic patterns can serve as defect structure, e.g. by periodically varying the magnetization saturation using ion beam irradiation combined with periodic sample patterning by electron beam lithography. Due to irradiation of the material a local variation of the magnetic properties can be achieved [1], where the TMS strength is set by the periodicity of the modification. However, directly patterning the material is time consuming and not suitable for large scale manufacturing. Hence a self-organized nanoscale patterning is more favorable. Broad ion beam erosion is a well-established technique for structuring large surface areas. By varying the irradiation parameters, e.g. ion energy, fluence, and incident angle sinusoidally modulated surfaces (ripples) can be created with a periodicity tuneable over a wide range [2]. Growing magnetic materials on these ripples imprints the corrugation to the material and induces by dipolar effects a wavelength dependent uniaxial magnetic anisotropy (UMA). Furthermore the imprinted corrugation can serve as a spin wave scattering center, modifying the two-magnon damping contribution. Here we present the influence of the substrate surface corrugation on the magnetic damping properties of 30 nm thin Ni80Fe20 (Py) films grown by molecular beam epitaxy at room temperature on rippled Si substrates. Due to ion beam erosion of flat Si as well as natural oxidation of the substrate prior to film deposition, Py films grown on top exhibit a polycrystalline structure that suppresses the intrinsic magneto-crystalline anisotropy almost completely. The in-plane magnetostatic and dynamic properties of these samples were investigated by means of angular and frequency dependent vector network analyzer ferromagnetic resonance (VNA-FMR).
Starting with a planar reference sample the angular together with the frequency dependent linewidth measurements reveal a Gilbert dominated relaxation process, whereby no TMS can be observed. Due to the polycrystalline film structure, only a very weak magnetic anisotropy is observed. This uniaxial magnetic anisotropy (UMA) has a two-fold symmetry and is randomly aligned with respect to the sample edges. Changing to rippled substrates the grown Py film maintains its polycrystalline structure. Depending on the ripple wavelength λ, ranging from 25 nm to 230 nm, an UMA is induced with its easy axis always aligned parallel to the ripple ridges. The strength of the UMA decays with increasing wavelength and is strongest for λ=25 nm. In this case no influence of the corrugation on the damping is observed. This changes drastically for samples with a higher wavelength of λ=230 nm. While the UMA is reduced to the value of the planar reference sample the linewidth measurements now show clear indications for defect induced TMS. This is shown in Fig. 1a, where the peak-to-peak linewidth is plotted as a function of the in-plane magnetic field angle (open circles). Modeling the linewidth results in a Gilbert contribution that is constant for all in-plane field orientations. Additionally an angle dependent TMS contribution is found, which consists of a small four-fold and a dominating two-fold (uniaxial) part. Thereby the direction of minimal linewidth aligns parallel with the ripple ridges, which in turn defines the uniaxial symmetry of the damping. Fig. 1b depicts the frequency dependent measurements parallel (red squares) and perpendicular (green circles) to the ripple ridges. In parallel configuration the damping is purely Gilbert-like, as already observed in the reference measurement. The monotonous increase of the linewidth with applied microwave frequency is instead lost in case of the perpendicular geometry. Here, a preeminent peak is observed with its center at f=10 GHz. Following the description of Barsukov et al. [1] this excessive linewidth increase is a result of defect induced TMS, where the width and frequency position of the peak is determined by the scattering potential, created by the corrugation of the film. The origin and wavelength dependence of these morphology induced linewidth manipulation will be discussed in detail.
We thank I. Barsukov, J. Lindner, and P. Landeros for fruitful discussions. This work is supported
by DFG grant no. FA 314/6-1.
1) I. Barsukov et al., Phys. Rev. B 84, 140410(R) (2011)
2) J. Fassbender et al., New J. Phys. 11, 125002 (2009)

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