Uniformly sized and crystalline iron oxide nanoparticles (IONPs) with spinel structure and mean diameters of about 3, 6 and 9 nm were synthesized in high yield using the microemulsion route at room temperature. The nanoparticles (NPs) were stabilized in situ by organic surfactant molecules which acted both as a stabilizer of the microemulsion system and as a capping layer of the NP surface. NP size control was attained by careful adjustment of the preparation conditions. The structure, morphology and NP size distribution were investigated by x-ray diffraction, transmission electron microscopy and scanning electron microscopy. A particular effort was devoted in this work to study the effect of size and capping of these NPs on their magnetic structure by in-field Mössbauer spectroscopy at 4.2 K. The mean canting angle (relative to the applied field direction) of the Fe spins was observed to increase with decreasing NP size due to the enhanced surface-to-volume ratio. Comparing bare and capped NPs of the same diameter, we verified that the spin canting was not affected by the organic capping. This implied almost identical magnetic orientations of bare and capped NPs. Simultaneously, the capping material was capable of preventing agglomeration effects which can occur in case of direct particle contact. Using a core/shell model, we showed that spin canting originated from the surface shell of the NPs. Furthermore, the Mössbauer spectral parameters provided evidence for the existence of a high fraction of Fe3O4 (magnetite) in the IONP.

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We report experimental and theoretical studies of spin wave eigenmodes in transversely magnetized thin film Permalloy wires. Using broadband ferromagnetic resonance technique, we measure the spectrum of spin wave eigenmodes in individual wires as a function of magnetic field and wire width. Comparison of the experimental data to our analytical model and micromagnetic simulations shows that the intrinsic dipolar edge pinning of spin waves is negligible in transversely magnetized wires. Our data also quantify the degree of extrinsic edge pinning in Permalloy wires. This work establishes the boundary conditions for dynamic magnetization in transversely magnetized thin film wires for the range of wire widths and thicknesses studied, and provides a quantitative description of the spin wave eigenmode frequencies and spatial profiles in this system as a function of the wire width.

The effect of microwave irradiation on the spin-torque-driven magnetization dynamics is studied in (Co/Ni)based nanopillar spin valves with perpendicular magnetic anisotropy. For this purpose, a setup was developed to measure the ac as well as the dc resistance of the nanopillar under applied fields and injected polarized currents, while irradiating microwaves with varying frequency (6-18 GHz) and power. We find that the microwave irradiation amplifies and maintains the precessional state of the eigenresonance within a larger field range. The experiments are discussed in comparison to micromagnetic as well as macrospin simulations utilizing the nonlinearized Landau-Lifshitz-Gilbert equation.

We investigate the influence of the spin polarized current on magnetic damping in the ferromagnetic layers of pillar structures by means of ferromagnetic resonance (FMR). The pillars are prepared and electrically contacted by high-resolution electron beam lithography and electron beam evaporation in a multi-step process. FMR measurements are carried out on a pillar array, in which all pillars are connected in series, so that the current density reaches values high enough to observe the effect of the spin transfer torque on the FMR signal. The temperature rise due to the high current density is avoided by cooling. An influence of the spin transfer torque effect on the intrinsic damping in the ferromagnet is observed. The fabrication of the serial connection can be easily applied to arbitrary structures.