Microwave absorption of patterned arrays of nanosized magnetic stripes with different aspect ratios

Abstract

Arrays consisting of nanosized stripes of Permalloy with different length-to-width ratios have been fabricated using electron beam nanolithography, magnetron sputtering, and lift-off process. These stripes have a thickness of 100nm, a width of 300nm, and different lengths ranging from 300nmto100μm. The stripes are separated by a distance of 1μm. Magnetization hysteresis loops were measured using a superconducting quantum interference device susceptometer. Microwave absorption at 9.8GHz was determined by means of ferromagnetic resonance technique. The dependence of the resonant field on the angle between the nanostructure and the in-plane dc magnetic field indicates the presence of uniaxial magnetic anisotropy associated with the aspect ratio of the stripes. A maximum change of the resonant field of 1600Oe was observed in the longest stripes, yet it was only 200Oe for square shaped stripes. The linewidth of the resonant curve varied with the angle, in the range from 120to300Oe. Most of the ferromagnetic resonance spectra exhibited multiple resonant peaks due to dimensional confinement of spin waves in the nanosized stripes. The maximum squareness of the magnetization hysteresis loop was for the field applied along the stripes, but the coercivity did not have a monotonic angular dependence as expected from the Stoner-Wohlfarth model for coherent magnetization rotation of the systems with uniaxial anisotropy.