BESSY-measurements

In order to confirm the findings discussed above in section5.2.1and to perform a better matching between the simulations and the measurements, additional and more detailed STXM-FMR measurements were carried out at BESSY (see section 3.2.2). These are feasible due to the significantly shorter time that is needed for one scan (around 10 minutes compared to around 2 hours for one SLAC-measurement). The BESSY-measurements were performed at f_MW = 9.43GHz using a stripes microantenna (see Fig.3.7(a)). The sample used for the STXM-FMR measurements was produced under the same conditions as the one measured with the microantenna-based FMR described in the previous section 5.1.1.

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Fig. 5.8: (a,d) Simulated FMR spectra, overview of the (b,e) simulated using MuMax3 spin-wave profiles and (c,f) combined and normalized amplitude/phase data extracted from BESSY-measurements along the vertical strip of the Py (a,b,c) T-strips and (d,e,f) L-strips.

Spin-wave Overview

The amplitude and phase data were extracted from the measured data and processed as described in sections3.3.2and3.3.3, giving an overview of the combined and normalized

5.2. STXM-FMR Results 76

amplitude/phase data comparable to the simulations. The resulting overview plots of the STXM-FMR measurements along the vertical strip of the T-strips and L-strips are shown in Figs.5.8(c) and (f), respectively. The corresponding simulated FMR spectra of the T- and L-strips are shown in Figs.5.8(a) and (d), respectively, together with the simulated spin-wave overviews in (b) and (e). The vertical dashed lines indicate the correlation between the measurements and the simulations. For better visible matching in the figure the simulations in (b,e) are plotted with the same field step size as used for the measurements (c,f), i.e. 2 mT. The red rectangles in the insets of Figs.5.8(a) and (d) indicate the region of the Py strip that was used for averaging the data at each field for the overview plots (see sections 3.3.1 and 3.3.3).

In Fig.5.8 it is visible that the combined and normalized amplitude/phase data ex-tracted from STXM-FMR measurements fit very well the simulations. The color switch between the T-strips in Fig.5.8(c) and the L-strips in Fig.5.8(f) is due to the phase shift between the measurements. The phase shift can be a result of the different con-nection (wiring) of the samples. Comparison of the main FMR signal positions (for both samples) in the simulations and in the measurements, reveals a field mismatch.

That can be a result of the field calibration error of the measuring setup or/and a possible saturation magnetization difference between the samples measured with FMR and STXM-FMR. For T-strips the mismatch is approximately 16 mT and for the L-strips it is 12 mT. The possible reason for the varying mismatch values are a difference between the samples (placement of the strips, slight variation of the individual strips shape, defects), connections and recalibration of the electromagnet. The insignificant shape variation of the individual strips can be derived from the field gaps between the resonances measured with STXM-FMR. As it was shown for the single strip (see section4.1.2), the change in the sample shape also changes the relative positions of the resonances within one orientation. From the similarity of the field gaps between the resonances it is apparent that the shape and thickness of the T- and L-strips are very close. Further FMR modes will be referred to by using the simulated field values.

Spin-wave Dynamics

For the analysis of the dynamic magnetic contrast the phase shift in STXM-FMR results between the T- and L-strips is removed by adjusting the order of the scans.

Time-resolved simulations and STXM-FMR scans of the spatial distribution of the out-of-plane dynamic magnetization component m_z(t) (see Eq. (2.27)) within the vertical strip of the T- and L-strips are shown in Figs.5.9(a,c) and (b,d), respectively. The series of images are stacked in rows. The STXM-FMR scans and simulations depict 7 measured points of one magnetization precession period. In Fig.5.9(a,b) mz(t)spin dynamics corresponding to the FMR lines (at resonance) at 88 mT and 102 mT are shown (see Fig.5.8), and in (c,d) spin dynamics for the fields between the FMR lines (off resonance) at 96 mT and 98 mT are shown. The latter is analyzed, because in the SLAC-measurements the asymmetry of the magnetization dynamics in the vertical

5.2. STXM-FMR Results 77

Fig. 5.9: Simulated and measured with STXM-FMR time evolution ofm_z(t) across the vertical strip of the (a,c) T-strips and (b,d) L-strips demonstrated over one oscillation period.

strip of the L-strips was observed off resonance. The simulated images fit very well to the measured spin-wave configuration, and dynamics over the entire precession period.

Similar to the single strip sample discussed in section 4.2, both, measurements and simulations, reveal a nonstanding character of the spin waves at and off the resonance.

That can be concluded from tracking the position of the nodes of the waves, which in the images are the white borders between red and blue regions. In both, T- and L-strips, at the resonance fields (88 mT and 102 mT) the movement is symmetric with respect to the center of the strip, which is marked with the horizontal dashed line in the figure. In contrary, off the resonance (96 mT and 98 mT) the movement is asymmetric in the vertical strip of the L-strips while it is symmetric for the T-strips, confirming this way the SLAC-measurements (see section 5.2.1).

Directly observed spin-waves are the result of the interference of the waves in several directions [5, 9, 107]. It was reported earlier that edge roughness of the sample, edge surface profile [19], its shape [5] or micro variations of the external field applied during excitation [108] can cause significant changes in the spin-wave dispersion in one or several directions leading to different behavior of the resulting excitation pattern. Like

5.2. STXM-FMR Results 78

for the single strip, in T- and L-strips a variation of the nodes‘ position in space is observed, which can be a result of not perfectly hitting the resonance field value and/or a roughness of the sample edges. Another reason is the interference of the spin-wave eigenmodes in different directions (see Fig.4.2). This kind of dynamics is not detectable by conventional FMR technique but it is possible to image it directly with STXM-FMR. The direction of the observed movement can be altered by a very small shift of the internal field within the vertical strip introduced by the stray field of the horizontal strip. Additional influence of the second strip can lead to local disturbances, affecting the spin-wave dispersion in one or several directions along the strip and, hence, this leads to an observable asymmetry of the nodes’ movement of the resulting wave.

The spin waves seem to be more sensitive to the alteration off resonance, and more resistive to it at the resonance fields (where the conventional FMR line is observed).

In conclusion, time-resolved STXM-FMR experiments show that it is possible to di-rectly observe and map the quasi-uniform and spin-wave modes that can be detected with FMR technique in micron-sized Py microstructures, and dynamic magnetic exci-tations off resonance as well. More importantly, the possibility to alter the spin-wave behavior is demonstrated in the vertical strip, by placing the horizontal strip at differ-ent positions along its length. The sample design allows to modify the local magnetic field inhomogeneity within one of the strips, thus leading to the change of the spin-wave behavior.

6

Spin Pumping in a

Ferromagnet/Nonferromagnet Heterostructure

In this chapter the pumped ac spin polarization inside a Co0.5Zn0.5O (50% Co:ZnO)/

Py heterostructure is investigated using element-selective STXM-FMR imaging. First, results of multifrequency measurements are presented in order to evidence that angular momentum transfer via spin pumping is possible in 50% Co:ZnO/Py heterostructures.

Further, with preliminary microresonator-based FMR measurements it is confirmed that the R-shape resonator is sensitive enough to detect spin waves and that the 50% Co:ZnO film is sufficiently insulating for the microresonator to operate. In the following section results of the x-ray diffraction (XRD) and superconducting quantum interference device (SQUID) magnetometry show that the 50% Co:ZnO film growth is highly c-oriented not only on sapphire but on the SiN membranes as well and that there is no evidence of the formation of metallic Co clusters in 50% Co:ZnO. Finally, the STXM-FMR measurements allow for detection of the pumped ac spin polarization in the 50% Co:ZnO layer within and outside of the Py microstrip region.

Most of the results of this chapter were already published in [57].

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