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.

We investigate both experimentally and by means of micromagnetic calculations magnetic states preceding vortex formation in permalloy nanodisks. In experiment, we used micro-Hall sensors fabricated from GaAs/AlGaAs heterojunction material to measure stray field hysteresis loops of individual disks. Micromagnetic calculations involving different micromagnetic codes allowed us to interpret the experimental results. Both calculations and experiments suggest that vortex formation can be reached via different precursor states.

No abstract

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 ∼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.

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.