ISBN 978-3-8439-1768-1

978-3-8439-1768-1, Reihe Physik

Gesche Nahrwold
Sputter Deposition of Permalloy Nanostructures Optimized for Domain-Wall Depinning Experiments

130 Seiten, Dissertation Universität Hamburg (2014), Softcover, A5

Zusammenfassung / Abstract

Novel storage concepts are based on the current-induced motion of ferromagnetic domain walls in nanostructures, most prominently represented by the racetrack memory invented by S. Parkin et al., so that current-induced domain-wall motion is a subject of wide interest.

Permalloy (Ni80Fe20) is broadly used to prepare magnetic nanostructures for high-frequency experiments where the magnetization is either excited by electrical currents or magnetic fields. Detailed knowledge of the material properties is mandatory for a thorough understanding of its magnetization dynamics. In this work, thin Permalloy films are grown by DC-magnetron sputtering on heated substrates and by thermal evaporation, partially with subsequent annealing. The specific resistance is determined by the van der Pauw method. Point-contact Andreev reflection is employed to determine the spin polarization of the films. The topography is imaged by atomic force microscopy, and the magnetic microstructure by magnetic force microscopy. Transmission-electron microscopy and transmission-electron diffraction are performed to determine atomic composition, crystal structure, and morphology. From ferromagnetic resonance absorption spectra the saturation magnetization, the anisotropy, and the Gilbert damping parameter are determined. Coercive fields and anisotropy are measured by magneto-optical Kerr magnetometry. The sum of these findings enables optimization of Permalloy for spintronic experiments.

An evaporated curved Permalloy nanowire reveals the oscillatory behavior of the domain-wall depinning probability with current pulse length due to the walls' resonant motion in the restoring potential. For sample preparation with Permalloy sputtered on heated substrates a subtractive process is needed as due to the high temperatures during deposition lift-off processing is impossible. For that reason curved wires are fabricated by subtractive ion-milling and RF-sputtering of Permalloy films sputtered on substrates heated to 280 °C-320 °C. Current-induced domain-wall experiments with subtractively prepared samples yield low depinning fields below 1 mT and a reliable depinning process.