Jan M. Scholtyssek

Structural, magnetic, and transport properties of Permalloy for spintronic experiments

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 thorough understanding its magnetization dynamics. In this work, thin Permalloy films are grown by dc-magnetron sputtering on heated substrates and by thermal evaporation with subsequent annealing. The specific resistance is determined by van der Pauw methods. 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. Co...

Spin polarization of Ni2MnIn and Ni80Fe20 determined by point-contact Andreev spectroscopy

We present point-contact Andreev reflection (PCAR) spectroscopy on Permalloy (Ni80Fe20) and on the half-metallic Heusler alloy Ni2MnIn. The thin Permalloy films are deposited on Si, the Ni2MnIn films on Si as well as on in situ cleaved (110) surfaces of InAs. A highly conductive layer under the ferromagnetic film almost eliminates a series resistance and thus facilitates the determination of the spin polarization from the differential conductance curves. We obtain a spin polarization of Permalloy of 35%. The spin polarization of Ni2MnIn depends on the substrate, presumably due to the growth of different crystal structures. It is shown that the surface sensitive PCAR spectroscopy cannot determine the spin polarization of the bulk material of half-metals where the degree of spin polarization strongly depends on the crystal structure.

Current-induced domain-wall depinning in curved Permalloy nanowires

Domain walls in curved Permalloy (Ni80Fe20) nanowires are depinned by nanosecond current pulses and magnetic fields. A significant dependence of the depinning probability on the pulse length is observed. We measure an oscillatory dependence with a characteristic frequency of 270 MHz. The quality of the Permalloy used for the nanowires is crucial for efficient domain-wall depinning. By sputtering Permalloy on heated substrates we are able to considerably decrease the specific resistance that is found to directly correspond to the depinning field of the domain wall.

Point contact Andreev spectroscopy of epitaxial Co2FeSi Heusler alloys on GaAs (001)

The predicted half-metallicity of Co2FeSi in combination with its high Curie temperature of above 980 K makes this Heusler alloy interesting for spinelectronics. Thin Co2FeSi films are grown by molecular-beam epitaxy on GaAs (001) with a close lattice match. We present a study of point-contact measurements on different films, varying in thickness between 18 nm and 48 nm and in substrate temperature during deposition between 100 °C and 300 °C. Transport spin polarizations at the Fermi level are determined from differential conductance curves obtained by point-contact Andreev-reflection spectroscopy. A maximum transport spin polarization of about 60% is measured for a 18 nm thin Co2FeSi film grown at 200 °C.

Growth and Characterization of Ferromagnetic Alloys for Spin Injection

Spin electronics with semiconductor/ferromagnet hybrids is a topic of ongoing interest. We review developments in hybrid spintronics and give an overview of achievements in efficient spin injection from ferromagnetic metals into semiconductors. The focus of this work is on thin Heusler films grown on semiconductor substrates. Ni2MnIn films are deposited on a variety of substrates by coevaporation of nickel and the alloy MnIn. The almost perfect lattice match between Ni2MnIn and InAs qualifies this alloy for basic research in spintronics. Point-contact Andreev spectroscopy serves to quantify the spin polarization relevant to transport. Nanopatterning of Ni2MnIn electrodes with electron-beam lithography and lift-off processing is examined. In this context, the influence of post-growth annealing on the film’s morphology and crystal structure is studied in situ using transmission-electron microscopy. The electrodes are completed by a copper strip to form a lateral spin-valve. In first measurements in local geometry we have detected the spin-valve effect.