J.C. Lodder

Magnetization curling in perpendicular iron particle arrays (alumite media)

Alumite has been investigated by measuring the hysteresis loops for different field directions to determine the influence of the particle interactions in an perpendicular oriented particle medium on the angular dependence of the coercivity and the hsyteresis loss. Alumite is an ideal model material for this purpose because the particle size and particle distance are well defined and can be varied independently. It is shown that the coercivity of the particle array can be identified with the nucleation field of one single particle for perpendicular fields. It is clear from the radius dependence of the coercivity that the reversal mode is the curling mode. The angular dependence of the coercivity does not agree with the well-known theoretical curve for one particle, which is caused by the particle interactions. The same can be said about the angular dependence of the hsyteresis loss. Calculations of hysteresis loops of particle arrays with interacting particles show the influence of these interactions on the angular dependence of the coercivity.

Observation of magnetic domains using a reflection mode scanning near-field optical microscope

It is demonstrated that it is possible to image magnetic domains with a resolution of better than 60 nm with the Kerr effect in a reflection-mode scanning near-field optical microscope. Images taken of tracks of thermomagnetically prewritten bits in a Co/Pt multilayer structure magnetized out-of plane showed optical features in a track pattern whose appearance was determined by the position of an analyzer in front of the photomultiplier tube. These features were not apparent in the topography, showing this to be a purely magneto-optic effect.

Evidence of stress anisotropy and role of oxygen pressure in growth of pulsed-laser-deposited hexaferrite films

BaFe12O19 films have been grown by pulsed laser deposition (PLD) on (001) sapphire using two different conditions with and without oxygen. Layers grown in O2 atmosphere exhibit an epitaxial structure stressed at the interface with a perpendicular easy axis whereas magnetic anisotropy is randomly oriented in those deposited without O2 . The random structure is a consequence of the high energy of ions in the PLD plume which damage the smoothness of the substrate at an earlier stage of growth. Oxygen pressure can reduce the energy of arriving atoms and prevent deterioration of the interface sharpness, and lead to epitaxial growth. Stress induces in-plane anisotropy with the coercivity strongly dependent on the temperature (T 0.75 law). An estimate of the stress confirms that it is too low to compete with magnetocrystalline anisotropy.

Transfer ratio of the spin-valve transistor

We describe the factors that control the transfer ratio of the spin-valve transistor. An increase in transfer ratio is obtained by a systematic variation of the height of emitter and collector Schottky barrier, and of the nonmagnetic metals. Next, we found that in some cases, a thicker base leads to a higher transfer ratio. Finally, the thickness of the magnetic layers in the Ni 80 Fe 20 /Au/Co spin-valve base can be optimized for a maximum absolute change of collector current. An overall increase by a factor of 24 was achieved, without loss of the magnetocurrent.

Magnetic viscosity in perpendicular media

The remanent magnetization of fine particle media of perpendicular anisotropy is known to exhibit a time decay of pseudologarithmic form. The viscous properties are to a large extent determined by the magnetostatic particle interaction and the particle size. A meanfield model of a perfectly aligned ensemble of Stoner-Wohlfarth particles is presented that gives a qualitative description of the hysteretic and time-dependent properties of particulate perpendicular media. The time decay of the remanent magnetization of Alumite media after initial magnetic saturation was measured. The decay was found to be logarithmic within the measuring period, and a coefficient of magnetic viscosity was obtained. Although the reversal mechanism for the particles in Alumite media is known to be incoherent, a good qualitative agreement between the theoretical model and the measurements was found by introducing a reduced effective volume acting as a scaling factor that accounts for the discrepancy in reversal mechanisms.

Magneto-Optical Kerr Rotation Spectra in Ordered and Disordered Phases of Fe-Pt Alloy Films

The magneto-optical Kerr rotation (θK) spectra and the related properties have been measured in ordered and disordered phases of several kinds of Fe-Pt alloy films. The |θK| in as-deposited (disordered) films becomes gradually larger with increasing Fe content until about 50 at% Fe, but the shape of θK spectra does not change so much. After annealing, however, all the θK spectra were changed. A considerable change of the θK spectra near 300 nm, which would be expected by the anti-parallel alighment of the polarized Pt spins to Fe spins after annealing, was not observed in the films with approx. 50 at% Fe.

The influence of the packing density on the magnetic behaviour of alumite media

The influence of the pore size, the ratio of length to diameter, the microstructure of the initial growth part of the Fe cylinder and the packing density of the Fe needles on the magnetic behaviour of alumite media are reviewed. It is found that the magnetization reversal is controlled by curling rotation, if the applied field lies along the film normal. The Hc is mainly determined by the diameter of the needles, but it slightly decreases with increasing packing density. If the applied field deviates from the film normal, the magnetic behaviour of alumite media is strongly affected by the packing density. For alumite with a low packing density, the magnetization reversal is controlled by a cos-type of incoherent rotation and a demagnetizing field. For high packing densities, the reversal can be considered as the superposition of a cos-type of incoherent rotation with perpendicular anisotropy and in-plane domain-wall motion. The alumite media can exhibit both pe rpendicular as well as longitudinal anisotropy by an appropriately controlled aspect ratio and the morphology of initial growth part of the iron needles.

Low-field magnetocurrent above 200% in a spin-valve transistor at room temperature

A spin-valve transistor (SVT) that employs hot electrons is shown to exhibit a huge magnetotransport effect at room temperature in small magnetic fields. The SVT is a ferromagnet-semiconductor hybrid structure in which hot electrons are injected into a NiFe/Au/Co spin valve, and collected on the other side with energy and momentum selection. This makes the collector current extremely sensitive to spin-dependent scattering. The hot-electron current output of the device changes by more than a factor of three in magnetic fields of only a few Oe, corresponding to a magnetocurrent above 200% at room temperature.

Stripe domains in Fe-Zr-N nanocrystalline films

We report on the transition between a magnetic stripe domain structure and in-plane orientation of the spins, as a function of nitrogen content, for 500nm thick Fe-Zr-N films prepared by DC reactive sputtering on glass substrates. The saturation field decreases and the saturation magnetization increases with decreasing nitrogen content. For 4at% N, the magnetic behavior of the films becomes specific for a soft magnetic material. The magnetic spin distribution was investigated by transmission Mossbauer spectroscopy (TMS) to probe the entire sample and Magnetic Force Microscopy to image the surface.

The spin-valve transistor: Fabrication, characterization and physics

An overview is given of the fabrication, basic properties, and physics of the spin-valve transistor. We describe the layout of this three-terminal ferromagnet/semiconductor hybrid device, as well as the operating principle. Fabrication technologies are discussed, including vacuum metal bonding. We characterize properties of the device relevant for possible applications in magneto-electronics, such as relative magnetic response, output current, and noise behavior. Furthermore, we illustrate the unique possibilities of the spin-valve transistor for fundamental studies of the physics of hot-electron spin transport in magnetic thin film structures.