Observation of a new low-field minimum in magnetically modulated systems

The magnetoresistance of a magnetically modulated two-dimensional electron gas (2DEG) shows a positive magnetoresistance at low fields, which can be ascribed to �snake orbits\', traveling along contours of vanishing magnetic field. At slightly higher fields anomalies in the resistance result from the commensurability of the classical cyclotron orbit of the electrons at the Fermi energy and the period of the magnetic modulation. In our experiments we observed, besides the minimum at B=0, an additional minimum of the magnetoresistance at B=0

Commensurability effects in two-dimensional electron gases with periodically arranged Ni and NiFe nanopillars

We have investigated the magnetoresistance of high mobility two-dimensional electron gases (2DEG) under the influence of periodically arranged ferromagnetic nanopillars. A huge positive magnetoresistance and commensurability oscillations (COs) indicate that the 2DEG is subjected to a surprisingly strong modulation. Angular-dependent magnetoresistance measurements as well as the phase of the COs suggest that the modulation is dominated by electrostatic rather than by magnetic contributions

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

Anomalous temperature dependence of commensurabilit oscillations in one two dimensional lateral superlattices

Two-dimensional electron systems were laterally modulated using the method of in situ interferometric illumination. Magnetotransport measurements on 1D and 2D modulated systems revealed a phase change of the commensurability oscillations depending on temperature. This behaviour is surprising and cannot be explained by existing theories

Composite Fermions in a weakly density modulated 2DEG: Directive evidence of a periodic magnetic field around n=1/2

We investigate the magnetoresistance of a weakly density modulated high mobility two- dimensional electron system around filling factor . The density modulation with periods a between 300 nm and 500 nm was generated by in situ interferometric illumination. At low B we found commensurability oscillations of the magnetoresistance characteristic for transport in weak electrostatic potentials with minima positions on the magnetic field axis given by 2RC=(ë-1/4)a. Here 2RC is the electron cyclotron diameter at the Fermi- energy and ë an integer. In contrast, we find clear magnetic commensurability features around filling factor with a pronounced minimum for a composite fermion cyclotron radius RC,CF matching 1.25a. This gives direct evidence that the density modulation results in a modulated effective magnetic field for composite fermions. The experimental ñxx-traces around are well described by novel model calculations, based on a semiclassical solution of the Boltzmann equation, taking into account anisotropic scattering

Vortex nucleation in sub-micrometer 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