Extracting the Dynamic Magnetic Contrast in Time-Resolved X-Ray Transmission Microscopy

Abstract

Using a time-resolved detection scheme in scanning transmission X-ray microscopy (STXM), we measured element resolved ferromagnetic resonance (FMR) at microwave frequencies up to 10 GHz and a spatial resolution down to 20 nm at two different synchrotrons. We present different methods to separate the contribution of the background from the dynamic magnetic contrast based on the X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the GHz microwave excitation and the X-ray pulses generated by the synchrotron, as well as the opening angle of the precession at FMR can be quantified. A detailed analysis for homogeneous and inhomogeneous magnetic excitations demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect. The dynamic magnetic contrast in time-resolved STXM has the potential be a powerful tool to study the linear and nonlinear, magnetic excitations in magnetic micro- and nano-structures with unique spatial-temporal resolution in combination with element selectivity.

Non-standing spin-waves in confined micrometer-sized ferromagnetic structures under uniform excitation

“…This recently reported STXM-FMR technique enables direct time-dependent imaging of the spatial distribution of the precessing magnetization over the sample during FMR excitation. [26][27][28][29] In most of the cases, spin-waves are investigated using a nonuniform excitation of the structure. 2,4,10,12 When the uniform excitation field is applied to the specimen, it is also possible to excite spin-waves, but only standing spin-waves are expected.…”

“…Thus, the maximum and minimum intensities on the STXM-FMR images (white and black contrast in the scans) mean the maximum deviation of the dynamic component of the magnetisation in opposite directions. 29 The STXM-FMR measurements were carried out at a Ni L 3 -edge at the microwave excitation frequency of f MW ¼ 9.446 GHz. An example of the STXM-FMR measurement of the stripe in hard orientation [marked with the red rectangle in Fig.…”

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

“…It is obvious that at the Ni L 3 edge the quasiuniform FMR excitation of the Py microstrip can be detected; see Ref. [27,28]. Outside the region of the strip, little dynamic magnetic contrast is visible.…”

“…The STXM-FMR measurements themselves have been described in detail in Refs. [14,23,27,28]. In brief, the microwave frequency is phase locked to a high harmonic of the internal time structure of the synchrotron to record the x-ray transmission at normal incidence using circular polarized light at six different relative phases between the microwave excitation and arrival of the x-ray light pulse.…”

“…For a more in-depth analysis of the observed dynamic magnetic contrast, the available magnetic information can be visualized differently. As done before [23,27], the magnetic contrast can be averaged over a region of interest and plotted versus (relative) time. For better clarity, one full precession period has been duplicated so that two identical consecutive cycles are shown.…”