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.

We studied the interaction between magnetic vortices and artificial point defects by using micro-Hall magnetometry. Disk-shaped Permalloy particles with diameters between 300 and 800 nm and thicknesses from 20 to 60 nm, which contain a single lithographically defined defect, were examined. Magnetization reversal curves were measured for different in-plane directions of the applied field. The data indicate that the magnetic vortex structure can be pinned at the point defect.

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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.

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Micropatterned Hall-sensors are used to measure the magnetic switching of nanopatterned rhombic Ni-elements at temperatures below 4.2 K. The length of the short axis ("waist") was systematically varied between 700 and 100 nm while the length of 2 m of the long axis was kept fixed. The thickness of the Ni-rhombs was 70 nm. Whereas the hysteresis loops of the broader rhombs show many small steps, which seem to be connected to domain wall movement and pinning, the narrowest two only reveal one single step during magnetization reversal. The process of magnetization reversal is additionally studied by Lorentz transmission electron microscopy. Placing Hall-sensors on both ends of acicular Ni elements allows one to investigate the switching process on both sides simultaneously.