Michael Rahm

Vortex nucleation in submicrometer ferromagnetic disks

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.

Multistable switching due to magnetic vortices pinned at artificial pinning sites

Magnetic vortices form the ground state in micron and submicron ferromagnetic disks. By inserting artificial defects (antidots) into a submicron ferromagnetic disk, magnetic vortices can be pinned controllably thus enabling a different way for magnetic switching. We show that by inserting n antidots into a disk magnetization reversal takes place via n-1 jumps of the vortex core between neighboring antidots. This cannot only be used to establish stable two-state switching for n=2, but also to realize a multilevel remanent state with low switching fields.

Vortex pinning at individual defects in magnetic nanodisks

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.

Switching behavior of vortex structures in nanodisks

This work presents an experimental investigation of the switching behavior of single Co- and permalloy (Py) nanodisks employing magnetic force microscopy (MFM) and micro Hall magnetometry. We used both methods to study the magnetization reversal of individual disks with diameters between 200 nm and 2 /spl mu/m and thicknesses between 5 nm and 60 nm with a magnetic vortex phase. It is shown, that the helicity of the vortex (clockwise or counterclockwise) is independent of the direction of the normal component in the center, and vice versa. By an externally applied field it is possible to switch the direction of the central out of plane component selectively.

Influence of point defects on magnetic vortex structures

We employed micro-Hall magnetometry and micromagnetic simulations to investigate magnetic vortex pinning at single point defects in individual submicron-sized Permalloy disks. Small ferromagnetic particles containing artificial point defects can be fabricated by using an image reversal electron beam lithography process. Corresponding micromagnetic calculations, modeling the defects within the disks as holes, give reasonable agreement between experimental and simulated pinning and depinning field values.

Programmable logic elements based on ferromagnetic nanodisks containing two antidots

Magnetoresistive elements for data storage or logic operations require reliable bistable magnetic switching. Soft magnetic nanodisks containing two antidots, which serve as pinning sites for a magnetic vortex, provide an alternative route for bistable magnetic switching. Here we show by means of micromagnetic simulations that field pulses generated by two orthogonal metallic current lines can switch the magnetic vortex core between antidots on a subnanosecond time scale. Using a third strip line to enable switching of the element’s magnetically hard layer, the logic operations AND, OR, NAND, and NOR can be established.

Magnetization configurations and hysteresis loops of small permalloy ellipses

We investigated systematically the easy axis magnetization reversal of 20 nm thick permalloy ellipses with a fixed major axis of 1.47 µm and minor axes of 0.22–1.47 µm. Lorentz transmission electron microscopy was used to image the micromagnetic configurations during magnetization reversal. Hysteresis loops of single ellipses were recorded by means of micro-Hall magnetometry and could be traced back to certain reversal mechanisms observed by Lorentz microscopy. In most cases, the magnetization reversal is initiated by the evolution of a magnetization buckling, followed by the formation of a single, a double, or a trapped vortex configuration. For ellipses with high aspect ratio (length-to-width ratio), the magnetization switches in the reversed magnetic field without creation of a stable vortex configuration. Our experiments show that the characteristic field values for vortex creation, single vortex annihilation, and switching strongly depend on the shape anisotropy of the elements.

Planar Hall sensors for micro-Hall magnetometry

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.

Ferromagnet-semiconductor hybrid structures: Hall devices and tunnel junctions

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.