Frequency-Tunable Magnetic Relaxation in Periodic Nanostructures Tailored by Ion Beam Irradiation

Tailoring magnetization dynamics in nano structures is a very important field. Here we present, how magnetic hybrid materials can be used to increase the relaxation rate just within several small frequency ranges.

Various elements like Pd, Cr, Ta, as well as several rare-earth elements can be used to modify the magnetic properties of thin ferromagnetic films. They are incorporated either by co-sputtering or ion implantation and are well known to reduce the Curie temperature, saturation magnetization, anisotropy and damping [1,2]. In combination with lithographic masking this allows for magnetic property patterning at the nanoscale [3,4].
In thin ferromagnetic films, the magnetization dynamics are governed by intrinsic effects like Gilbert damping and spin-pumping but also by extrinsic effects like two-magnon scattering [5] due to inevitable defect structures. By lithographic nano patterning or by using ion-eroded, nanoscale periodically modulated substrates (ripples) as templates we are able to artificially create and thus control those defect structures necessary to induce two-magnon scattering.

This preparation procedure is sketched in Fig. 1. First a thin film sample is prepared by molecular beam epitaxy. In our case we use a 30 nm thin Ni80Fe20 (Permalloy=Py) film covered by a 3 nm Cr cap layer. In the second step, using a ~100 nm PMMA resist and electron beam lithography, the periodic stripe pattern is written into the mask over an area of 1x1 mm2 with periodicities of 250 and 400 nm. After development the sample was irradiated with Cr+ ions with a kinetic energy of 5 keV and a fluence of 5x1015 ions/cm2. The Cr ions either get absorbed by the PMMA or penetrate the topmost 8 nm of the sample as depicted in step (iii) of Fig. 1 [4]. This mixes the Cr coming from the ions and the cap layer into the Py layer, hence reducing the saturation magnetization in the irradiated stripe areas. Thus, the modified Py+Cr stripes act as magnetic defects respectively scattering centers.

Broadband ferromagnetic resonance is used to measure the resonance linewidth ΔH for different field directions. From the frequency and angular dependence of ΔH the damping contributions are disentangled like described in Ref. [5]. The frequency-dependent measurements with the external magnetic field aligned parallel to the stripes show a linear increase of ΔH. Therefore the magnetic relaxation is purely Gilbert-like (see Fig. 2a). With the magnetic field aligned perpendicular (Fig. 2b), the frequency dependence exhibits a non-monotonous increase due to two-magnon scattering. There are several distinct peaks (marked by arrows in Fig. 2b). Depending on the stripe periodicity the peak positions shift and the number of visible peaks changes as well.
The conventional model of two-magnon scattering in thin films [5] does not cover this effect. However, the stripe defects resemble a periodic scattering field, which couples the uniform with the final-state magnons in the two-magnon scattering process. The coupling strength and so the FMR linewidth scale with the square of the Fourier transform of the scattering field. Figure 2c shows the corresponding simulated two-magnon scattering strength as function of frequency and stripe periodicity. The black squares ("linescans"), correspond to the FMR linewidth measurements qualitatively very well. Note that a quantitative agreement depends very sensitively on the knowledge of the the static magnetic properties. For spintronic devices it could be very interesting to have a selectively higher damping at certain frequencies---a feature that could be even switched-off simply by changing the external field direction.

In summary, this magnetic hybrid material allows for designing samples where the spin relaxation rate can be easily switched between high and low damping just by slightly varying the frequencies. In contrast to that, with conventional materials only a monotonous increase of damping with frequency is achievable.

This work was supported by the DFG grants FA 314/6-1, FA314/3-2, and SFB491.