Imaging of ferromagnetic-resonance excitations in Permalloy nanostructures on Si using scanning near-field thermal microscopy

The investigated structures were Permalloy (Py) stripes of 3×0.3 μm² and a thickness of 24 nm. Two samples with lattices of stripes of 2 μm distance between the stripes and a lattice constant of 5 μm are reported in this paper. One of the samples had crossed perpendicular lattices. The samples were prepared using standard lithography lift-off processes and comprised large arrays of at least 1000 stripes, such that enough material was available to perform conventional ferromagnetic-resonance (FMR) measurements. The overall anisotropy and resonance line behavior of the samples were determined by conventional FMR measurements. The origin of the FMR modes was then deduced locally resolved in single stripes using a scanning thermal microscope (SThM), which features a lateral resolution of 100 nm and a temperature resolution of the order of a few millikelvins, mounted on a conventional FMR setup. The technique is based on the detection of the dissipated heat due to microwave absorption while in FMR. This setup provides an exact correlation of the SThM-FMR image and the simultaneously taken topography. In the crossed lattice sample the two resonance lines in the conventional spectra were identified as the parallel and perpendicular FMR excitations in each sublattice, respectively. A homogenous heating of a single stripe is observed while in FMR. In the single Py stripe lattice with external field parallel to the long axis of the stripe an additional resonance was found with slightly higher resonance field than the uniform mode. SThM-FMR measurements prove that this additional excitation is located at the polar edges of one stripe and is due to the round edges of each stripe and stray field effects.