The combination of micro-resonators with spatially resolved ferromagnetic resonance

Abstract

We present two new and complementary approaches to realize spatial resolution for ferromagnetic resonance (FMR) on the 100 nm-scale. Both experimental setups utilize lithographically fabricated micro-resonators. They offer a detection sensitivity that is increased by four orders of magnitude compared with resonator-based FMR. In the first setup, the magnetic properties are thermally modulated via the thermal near-field effect generated by the thermal probe of an atomic force microscope. In combination with lock-in detection of the absorbed microwave power in the micro-resonator, a spatial resolution of less than 100 nm is achieved. The second setup is a combination of a micro-resonator with a scanning transmission x-ray microscope (STXM). Here a conventional FMR is excited by the micro-resonator while focused x-rays are used for a time-resolved snap-shot detection of the FMR excitations via the x-ray magnetic circular dichroism effect. This technique allows a lateral resolution of nominally 35 nm given by the STXM. Both experimental setups combine the advantage of low-power FMR excitation in the linear regime with high spatial resolution to study single and coupled nanomagnets. As proof-of-principle experiments, two perpendicular magnetic micro-stripes (5 μm × 1 μm) were grown and their FMR excitations were investigated using both setups.

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

“…If the region surrounding the strip is averaged, only a very small signal is visible. It is important to note that the microwave excitation itself always creates a nonmagnetic effect also in the background [23] that is also seen here. This contrast modulation is directly and nonresonantly created by the microwave and, thus, serves as a reference phase of the driving microwave.…”

“…In a final step a strip-type resonant structure with a total Au-plating thickness of 600 nm is fabricated by optical lithography; see Ref. [23] for details.…”

“…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.…”

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.…”