Ralph Pulwey

Magnetization pattern of ferromagnetic nanodisks

We explore the magnetization pattern of Co and permalloy disks with diameters between 80 nm and 1 μm by using two complementary experimental techniques: Lorentz microscopy and magnetic force microscopy (MFM). By means of Lorentz microscopy we show that the dominating magnetization pattern of the disks is a vortex structure with closed flux lines in the plane of the disks. Complementary MFM measurements demonstrate that the magnetization in the center of the disks is tilted out of the plane of the disk. The experimental findings closely agree with corresponding micromagnetic calculations.

Magnetic domains in epitaxial nanomagnets with uniaxial or fourfold crystal anisotropy

In order to prepare submicron sized particles with strong magnetocrystalline anisotropies high quality epitaxial bcc-Fe films were grown on GaAs(110) and GaAs(001) by molecular beam epitaxy. Whereas Fe(110) on GaAs(110) is a model system with uniaxial in-plane anisotropy, Fe(001) on GaAs(001) has a strong fourfold anisotropy for films thicker than

Electrodeposition of NiFe and Fe nanopillars

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Planar Hall sensors for micro-Hall magnetometry

5 nm. Various shapes like circular, square, or rectangular elements with sizes from 200 nm up to 6 µm were fabricated by electron beam lithography and ion beam etching. The remanent states after saturation along different directions or ac demagnetization along the easy axis were examined by using magnetic force microscopy. The experimental results clearly reflect the interplay of the different magnetocrystalline and shape anisotropies depending on the different magnetic histories.

Transition of magnetocrystalline anisotropy and domain structure in epitaxial Fe(001) nanomagnets

Electron beam lithography combined with electrodeposition was used to fabricate arrays of NiFe and Fe nanomagnets with high aspect ratios (height/diameter). MFM-measurements show the single domain behavior of the NiFe-pillars, which have an easy axis parallel to their long axis. AGM-measurements carried out on large NiFe-arrays clearly show the increasing importance of interaction effects with decreasing periodicity. On top of overgrown Fe-pillars a nanocrystalline structure can be observed.

Ferromagnet-semiconductor hybrid structures: Hall devices and tunnel junctions

In this work we present a new method to fabricate planar Hall sensors from GaAs� AlGaAs heterojunctions, which can be used to examine the local stray field at a specific section of a micron-sized magnet. Instead of mesa etching we implanted oxygen ions with an energy of 1.5 keV which deplete the two- dimensional electron gas underneath the exposed areas but leave the wafer flat. Planar double Hall cross devices were employed to investigate 30 nm thick electroplated Ni rings with outer and inner diameters ranging from 1.2 to 2 µm and from 0.3 to 1.6 µm, respectively. By comparing the signals from both Hall crosses of the sensor, we can distinguish between local stray field variations and changes of the global magnetization pattern. A hysteresis loop measured at a temperature of 110 K suggests that magnetization reversal occurs via a magnetic vortex structure.

Method of processing of an object

The magnetocrystalline anisotropy of epitaxially grown bcc-Fe(001) films on GaAs(001) shows a transition from a fourfold intrinsic anisotropy in thick films to an uniaxial one in ultrathin films (<3 nm) and hence can be tuned by varying the film thickness. Here we investigate the consequence of such an anisotropy tuning for the magnetization configurations of nanomagnets. The thickness was varied between 2.5 and 30 nm in steps of 2.5 nm. Disks with diameters between 200 nm and 2 μm were patterned with electron beam lithography and ion beam etching. The remanent and ac-demagnetized states as well as the switching behavior were examined by magnetic force microscopy. In addition, we employed micromagnetic simulations to compare with the measured results.

Method for producing a representation of an object by means of a particle beam, as well as a particle beam device for carrying out the method

For spintronics, both, the properties of small ferromagnetic particles as well as the properties of ferromagnet-semiconductor hybrid structures are of importance. Below we describe how micro-Hall magnetometry provides information about the magnetisation switching of small ferromagnetic particles. In the second part we demonstrate spin dependent transport through thin GaAs membranes. The latter experiments point to the important role of spin-flip scattering in ferromagnet-semiconductor hybrid structures.