A. Gerber

Magnetic method of magnetocaloric effect determination in high pulsed magnetic fields

Abstract A contactless method of magnetocaloric effect measurements is described, based on the comparison of magnetisation curves obtained in adiabatic and isothermal processes. By this method the magnetocaloric effect of paramagnetic garnet Gd3Ga5O12 is studied in pulsed magnetic fields up to 400 kOe.

Extraordinary Hall effect in magnetic films

We review some of the recent developments in the studies of the extraordinary Hall effect in magnetic films. Three major topics are discussed: (1) physics and characterization of the effect in heterogeneous and geometrically confined magnetic systems, (2) use of the effect as a tool for the study of ultra-thin films and nanoscopic magnetic objects, and (3) potential technical applications for magnetic sensors and memory devices.

Correlation between the extraordinary Hall effect and resistivity

We propose to reconsider the correlation between the extraordinary Hall effect and resistivity by using the skew scattering model and Matthiesens rule to separate contributions of different scattering sources. The model has been experimentally tested for the cases of scattering by magnetic nanoparticles embedded in normal-metal matrix, insulating impurities in magnetic matrix, surface scattering, and temperature-dependent scattering.

Probing planar arrays of magnetic nano-particles by the extraordinary Hall effect

The extraordinary Hall effect has been used to determine the blocking temperature, magnetocrystalline anisotropy and magnetic moment of far-separated Co nano-particles arranged in single-layer arrays. The total mass of 3 nm diameter Co particles produced by the low energy clusters beam deposition (LECBD) technique corresponded to as little as 0.01 nm thick layers. The results have been confirmed by magnetization measurements of similar but much thicker samples. The technique is shown to be a simple and reliable information source in the limiting cases of diluted planar arrays of magnetic nano-particles.

Magnetization-driven metal-insulator transition in strongly disordered Ge:Mn magnetic semiconductors

We report on the temperature- and field-driven metal-insulator transition in disordered Ge:Mn magnetic semiconductors accompanied by magnetic ordering, magnetoresistance reaching thousands of percents, and suppression of the extraordinary Hall effect by a magnetic field. Magnetoresistance isotherms are shown to obey a universal scaling law with a single scaling parameter depending on temperature and fabrication. We argue that the strong magnetic disorder leads to localization of charge carriers and is the origin of the unusual properties of Ge:Mn alloys.

Origin of anomalous magnetite properties in crystallographic matched heterostructures: Fe3O4(111)/MgAl2O4(111)

Magnetite films grown on crystallographically matched substrates such as MgAl2O4 are not expected to show anomalous properties such as negative magnetoresistance and high saturation fields. By atomic resolution imaging using scanning transmission electron microscopy we show direct evidence of anti-phase domain boundaries (APB) present in these heterostructures. Experimentally identified 1/4<101> shifts determine the atomic structure of the observed APBs. The dominant non-bulk superexchange interactions are between 180° octahedral-Fe/O/octahedral-Fe sites which provide strong antiferromagnetic coupling across the defect interface resulting in non-bulk magnetic and magnetotransport properties.

Asymmetric field dependence of magnetoresistance in magnetic films

We study an asymmetric in field magnetoresistance that is frequently observed in magnetic films and, in particular, the odd longitudinal voltage peaks that appear during magnetization reversal in ferromagnetic films, with out-of-plane magnetic anisotropy. We argue that the anomalous signals result from small variation of magnetization and Hall resistivity along the sample. Experimental data can be well described by a simple circuit model, the latter being supported by analytic and numerical calculations of current and electric field distribution in films with a gradual variation of the magnetization and Hall resistance.

Magnetism and magnetotransport in symmetry matched spinels: Fe3O4/MgAl2O4

In this work, we show that Fe3O4 films grown by oxygen plasma assisted molecular beam epitaxy have anomalous magnetic properties such as negative magnetoresistance and high saturation magnetic fields. The film substrate mismatch of 3% is relieved by the formation of misfit dislocations at the interface. Transmission electron microscopy results show that misfit dislocations are not the cause of antiphase domain boundary (APB). This suggests that in this system APB formation is a property of the three dimensional Fe3O4 growth.

Proximity effect in granular superconductor-normal metal structures

We fabricated three-dimensional disordered Pb-Cu granular structures, with various metal compositions. The typical grain size of both metals is smaller than the superconductor and normal metal coherence lengths, thus satisfying the Cooper limit. The critical temperature of the samples was measured and compared with the critical temperature of bilayers. We show how the proximity effect theories, developed for bilayers, can be modified for random mixtures and we demonstrate that our experimental data fit well the de Gennes weak coupling limit theory in the Cooper limit. Our results indicate that, in granular structures, the Cooper limit can be satisfied over a wide range of concentrations.

Absence of the ordinary and extraordinary Hall effects scaling in granular ferromagnets at metal-insulator transition

Universality of the extraordinary Hall effect scaling was tested in granular three-dimensional