Direct Imaging of the ac Component of Pumped Spin Polarization with Element Specificity

Abstract

Spin pumping in a ferromagnet-nonferromagnet heterostructure is directly imaged with spatial resolution as well as element selectivity. The time-resolved detection in scanning transmission x-ray microscopy allows us to directly probe the spatial extent of the ac spin polarization in Co-doped ZnO, which is generated by spin pumping from an adjacent permalloy microstrip. Comparing the relative phases of the dynamic magnetization component of the two constituents is possible and found to be close to antiphase. The correlation between the distribution of the magnetic excitation in the permalloy and the Co-doped ZnO reveals that laterally there is no one-to-one correlation. The observed distribution is rather complex, but integrating over larger areas demonstrates that the spin polarization in the nonferromagnet extends laterally beyond the region of the ferromagnetic microstrip. Therefore, the observations are better explained by a local spin pumping efficiency and a lateral propagation of the ac spin polarization in the nonferromagnet over the range of a few micrometers.

Spin pumping into a partially compensated antiferromagnetic/paramagnetic insulator

“…The transfer of angular momentum occurs into the paramagnetic fluctuations of the insulating Co:ZnO which corroborates previous works where spin pumping near magnetic phase transitions was theoretically predicted [13] which could recently also be verified experimentally for IrMn [14] or NiO [15] as a spin sink. We therefore can add further experimental evidence beyond [18,19] for the possibility of transfer of angular momentum from a FM into an insulating paramagnet which is maximum close to the para-to-antiferromagnetic phase transition.…”

“…The system of choice for the present study is a heterostructure consisting of Py in contact with Zn 1−x Co x O (Co:ZnO) with x = 0.3, 0.5 and 0.6 (30%, 50% and 60% Co:ZnO), a system where the presence of spin pumping at room temperature has been evidenced before via an increased Gilbert damping parameter derived from broad-band FMR [18]. In addition, the pumped ac spin-polarization of the Co:ZnO could be directly imaged using time-resolved scanning transmission x-ray microscopy in combination with FMR excitation [19]. These experiments were done using the x-ray magnetic circular dichroism and thus the non-compensated fraction of the Co magnetic moments inside the partially compensatred antiferromagnet Co:ZnO.…”

“…For the chosen x the Néel temperature T N ranges from 10 K to 25 K [20]. The net effective magnetic moment of the non-compensated Co moments is of the order of 0.10-0.15 µ B /Co atom and hardly depends on the actual Co concentration above x = 0.3 [20]; nonetheless, it was of sufficient magnitude to image the pumped ac spin-polarization in [19]. All Co:ZnO films are highly resistive and thus they represent a model-system for a paramagnetic insulator with a magnetic phase transition at low temperatures where the influence of increased spin fluctuations around the ordering temperature can be studied.…”

Quantifying the Spin-Wave Asymmetry in Single and Double Rectangular Ni80Fe20 Microstrips by TR-STXM, FMR, and Micromagnetic Simulations

“…In general, the raw data includes a static component corresponding to the chemical contrast of the scanned area and the dynamic component corresponding to the magnetic contrast. For each scanned point in space the magnetic part was extracted by dividing the counted x-ray photon signal at each time point by the time averaged value over all time points [14], [25], [28], [29]. Further, a background correction was performed as described in [30].…”

Element-specific visualization of dynamic magnetic coupling in a Co/Py bilayer microstructure

“…In this study Scanning Transmission X-ray Microscopy detected FMR (STXM-FMR) 16 has been used, offering temporal sampling down to 17 ps and nominal sub 50 nm lateral resolution in transverse XFMR geometry with a continuous wave excitation of the sample [16][17][18][19][20][21] . Uniform and non uniform resonant responses on the micro- 22,23 and sub 50 nm nanometer 24 scale have been monitored and analysed. Here we investigate resonant excitations of a bilayered microstructure consisting of a Cobalt (Co) stripe deposited on a Permalloy (Py) disk with element specificity.…”