Anisotropic magnetoresistance contribution to measured domain wall resistances of in-plane magnetised (Ga,Mn)As

We demonstrate the presence of an important anisotropic magnetoresistance contribution to the domain wall resistance recently measured in thin-film (Ga,Mn)As with in-plane magnetic anisotropy. Analytic results for simple domain wall orientations supplemented by numerical results for more general cases show this previously omitted contribution can largely explain the observed negative resistance.Comment: 4 pages; submitted to Phys Rev

Muon-spin rotation measurements of the vortex state in Sr2_2RuO4_4: type-1.5 superconductivity, vortex clustering and a crossover from a triangular to a square vortex lattice

Muon-spin rotation has been used to probe vortex state in Sr$_2$RuO$_4$. At moderate fields and temperatures a lattice of triangular symmetry is observed, crossing over to a lattice of square symmetry with increasing field and temperature. At lower fields it is found that there are large regions of the sample that are completely free from vortices which grow in volume as the temperature falls. Importantly this is accompanied by {\it increasing} vortex density and increasing disorder within the vortex-cluster containing regions. Both effects are expected to result from the strongly temperature-dependent long-range vortex attractive forces arising from the multi-band chiral-order superconductivity.Comment: 13 pages, 4 figure

Angular dependence of domain wall resistivity in artificial magnetic domain structures

We exploit the ability to precisely control the magnetic domain structure of perpendicularly magnetized Pt/Co/Pt trilayers to fabricate artificial domain wall arrays and study their transport properties. The scaling behaviour of this model system confirms the intrinsic domain wall origin of the magnetoresistance, and systematic studies using domains patterned at various angles to the current flow are excellently described by an angular-dependent resistivity tensor containing perpendicular and parallel domain wall resistivities. We find that the latter are fully consistent with Levy-Zhang theory, which allows us to estimate the ratio of minority to majority spin carrier resistivities, rho-down/rho-up~5.5, in good agreement with thin film band structure calculations.Comment: 14 pages, 3 figure

Competing symmetries and broken bonds in superconducting vortex-antivortex molecular crystals

Hall probe microscopy has been used to image vortex-antivortex molecules induced in superconducting Pb films by the stray fields from square arrays of magnetic dots. We have directly observed spontaneous vortex-antivortex pairs and studied how they interact with added free (anti)fluxons in an applied magnetic field. We observe a variety of phenomena arising from competing symmetries which either drive added antivortices to join antivortex shells around dots or stabilize the translationally symmetric antivortex lattice between the dots. Added vortices annihilate antivortex shells, leading first to a stable “nulling state” with no free fluxons and then, at high densities, to vortex shells around the dots stabilized by the asymmetric antipinning potential. Our experimental findings are in good agreement with Ginzburg-Landau calculations

Room temperature magnetic imaging of magnetic storage media and garnet epilayers in the presence of external magnetic fields using a sub-micron GaAs SHPM

Cataloged from PDF version of article.A multipurpose room temperature scanning Hall probe microscope (RT-SHPM) system incorporating an ultra-high sensitive sub-micron GaAs Hall probe (active area of similar to0.8 x 0.8 mum(2) room temperature Hall coefficient of 0.3 Ohm /G) exhibiting extremely high magnetic field sensitivity (0.04G/root Hz) was used for the direct, non-invasive and quantitative imaging of magnetic field fluctuations in very close proximity to the surfaces of thin film magnetic storage media, Bi substituted iron garnet epilayers and demagnetized Sr-ferrite permanent magnets placed in external magnetic fields. A scanning tunnelling microscope tip integrated adjacent to the Hall probe was used for precise vertical positioning of the probe. RT-SHPM images of 1.4 MB written floppy disks clearly showed well-defined magnetic transitions to coalesce into small island-like structures under external perpendicular fields greater than 1300 Oe. The RT-SHPM is demonstrated to be a versatile means of quantitatively monitoring micron-sized magnetic domain structures in the presence of external magnetic fields at: room temperature. (C) 2001 Elsevier Science B.V. All rights reserved

The search for spontaneous edge currents in Sr2RuO4 mesa structures with controlled geometrical shapes

This work was supported by the Engineering and Physical Sciences Research Council (EPSRC) in the United Kingdom under Grant No. EP/J010626/1.Scanning Hall microscopy has been used to search for spontaneous edge fields in geometrically shaped mesa structures etched into the ab surface of Sr2RuO4 single crystals in order to test recent theories of the direction of edge current flow as a function of facet orientation and band filling. We find no evidence for spontaneous edge fields in any of our mesa structures above our experimental noise floor of ± 25 mG. We do, however, observe pronounced vortex clustering at low fields and temperatures, consistent with the established semi-Meissner scenario whereby a long range attractive component to the vortex-vortex interaction arises due, for example, to the multiband nature of the superconductivity. We also see clear evidence for the formation of a square vortex lattice inside square mesa structures above 1.3 K. Our results are discussed in terms of recent relevant experimental results and theoretical predictions.Publisher PDFPeer reviewe

Manipulation of magnetic-flux landscapes in superconducting Bi2Sr2CaCu2O8 + δ crystals

We demonstrate experimentally that the micromagnetic profile of the out-of-plane component of magnetic induction of layered superconductors, Bz, can be manipulated by varying the in-plane magnetic field, H∥. Moving Josephson vortices, confined between layers, drag pancake vortex stacks carrying out-of-plane flux, and the magnetic profile, Bz(x), can be controllably shaped across the entire sample. Depending on the magnetic history and temperature we can increase or decrease the out-of-plane flux density at the center and near the edges of the crystal by as much as 40%, realising both “convex and concave magnetic flux lenses”. Our experimental results are well described by molecular dynamics simulations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58115/2/epl_76_6_1151.pd