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

Abstract

A non-standing character of directly imaged spin-waves in confined micron-sized ultrathin permalloy (Ni 80 Fe 20 ) structures is reported along with evidence of the possibility to alter the observed state by modifications to the sample geometry. Using micromagnetic simulations the presence of the spin-wave modes excited in the permalloy stripes along with the quasi-uniform modes were calculated. The predicted spin-waves were imaged in direct space using time resolved scanning transmission X-ray microscopy, combined with a ferromagnetic resonance excitation scheme (STXM-FMR). STXM-FMR measurements revealed a non-standing character of the spin-waves. Also it was shown by micromagnetic simulations and confirmed with STXM-FMR results that the observed character of the spin-waves can be influenced by the local magnetic fields in different sample geometries.

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

“…At the Ni L 3 edge the amplitude is maximal (white) within the strip with decreasing amplitude towards the edges, while it is close to zero (black) outside. The phase has a uniform value across the strip (homogeneously red), corroborating a quasiuniform excitation characteristic of the main FMR mode in a microstrip, i.e., all spins are precessing in phase [25,28]. Note that outside the region of the strip the derived phase values are not significant because of the very small amplitude of the signal.…”

“…2(a). These inhomogeneous excitations have already been observed in STXM-FMR experiments [27]; however, it turned out that these types of excitation can also have a nonstanding character [28]. In Fig.…”

“…This can be caused by a slight variation of the strip thickness, the edge roughness, or influences of local stray fields of unknown origin. The fact that local variations of the effective field can alter the behavior of the spin-wave modes has already been demonstrated with STXM FMR [28].…”

“…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.…”