A. A. Fraerman

Antivortex state in crosslike nanomagnets

We report on results of computer micromodelling of anti-vortex states in asymmetrical cross-like ferromagnetic nanostructures and their practical realization. The arrays of cobalt crosses with 1 mkm branches, 100 nm widths of the branches and 40 nm thicknesses were fabricated using e-beam lithography and ion etching. Each branch of the cross was tapered at one end and bulbous at the other. The stable formation of anti-vortex magnetic states in these nanostructures during magnetization reversal was demonstrated experimentally using magnetic force microscopy.

Magnetic force microscope tip-induced remagnetization of CoPt nanodisks with perpendicular anisotropy

We report on the results of a magnetic force microscopy investigation of remagnetization processes in arrays of CoPt nanodisks with diameters of 35 and 200 nm and a thickness of 9.8 nm fabricated by e-beam lithography and ion etching. The controllable magnetization reversal of individual CoPt nanodisks by the magnetic force microscope (MFM) tip-induced magnetic field was demonstrated. We observed experimentally two essentially different processes of tip-induced remagnetization. Magnetization reversal of 200 nm disks was observed when the probe moved across the particle while in case of 35 nm nanodisks one-touch remagnetization was realized. Micromagnetic modeling based on the Landau–Lifshitz–Gilbert (LLG) equation demonstrated that the tip-induced magnetization reversal occurs through the essentially inhomogeneous states. Computer simulations confirmed that in case of 200 nm disks the mechanism of embryo nucleation with reversed magnetization and further dynamic propagation following the probe moving across the particle was realized. On the other hand one-touch remagnetization of 35 nm disks occurs through the inhomogeneous vortexlike state. Micromagnetic LLG simulations showed that magnetization reversal in an inhomogeneous MFM probe field has a lower energy barrier in comparison with the mechanism of coherent rotation, which takes place in a homogeneous external magnetic field.We report on the results of a magnetic force microscopy investigation of remagnetization processes in arrays of CoPt nanodisks with diameters of 35 and 200 nm and a thickness of 9.8 nm fabricated by e-beam lithography and ion etching. The controllable magnetization reversal of individual CoPt nanodisks by the magnetic force microscope (MFM) tip-induced magnetic field was demonstrated. We observed experimentally two essentially different processes of tip-induced remagnetization. Magnetization reversal of 200 nm disks was observed when the probe moved across the particle while in case of 35 nm nanodisks one-touch remagnetization was realized. Micromagnetic modeling based on the Landau–Lifshitz–Gilbert (LLG) equation demonstrated that the tip-induced magnetization reversal occurs through the essentially inhomogeneous states. Computer simulations confirmed that in case of 200 nm disks the mechanism of embryo nucleation with reversed magnetization and further dynamic propagation following the probe moving acro...

Rectangular lattices of permalloy nanoparticles: Interplay of single-particle magnetization distribution and interparticle interaction

Regular 2D rectangular lattices of permalloy nanoparticles (40 nm in diameter) were prepared by the method of the electron lithography. The magnetization curves were studied by Hall magnetometry with the compensation technique for different external field orientations at 4.2K and 77K. The shape of hysteresis curves indicates that there is magnetostatic interaction between the particles. The main peculiarity is the existence of remanent magnetization perpendicular to easy plain. By numerical simulation it is shown, that the character of the magnetization reversal is a result of the interplay of the interparticle interaction and the magnetization distribution within the particles (vortex or uniform).

Magnetic force microscopy of helical states in multilayer nanomagnets

We have used magnetic force microscopy (MFM) to investigate noncollinear helical states in multilayer nanomagnets, consisting of a stack of single domain ferromagnetic disks separated by insulating nonmagnetic spacers. The nanomagnets were fabricated from a [Co∕Si]×3 multilayer thin film structure by electron beam lithography and ion beam etching. The structural parameters (Co layer and spacer thicknesses) were optimized to obtain a clear spiral signature in the MFM contrast, taking into account the magnetostatic interaction between the layers. MFM contrast corresponding to the helical states with different helicities was observed for the optimized structure with Co layer thicknesses of 16, 11, and 8nm, and with 3nm Si spacer thickness.

Properties of Josephson junctions in the inhomogeneous magnetic field of a system of ferromagnetic particles

The effect of a system of ferromagnetic particles on the field-dependent critical current of a Josephson junction is experimentally studied for junctions of different geometries. For edge junctions, the effect of commensurability between the periodic magnetic field of the particles and the Josephson vortex lattice is observed. The effect manifests itself in additional maxima of the field-dependent critical current. For overlap junctions, giant (greater than sixfold) variations of the maximum critical current are observed depending on the magnetic state of the particles. The changes in the “Fraunhofer” pattern of the overlaped Josephson junctions are attributed to the formation of Abrikosov vortices due to the effect of uniformly magnetized particles. The effects revealed in the experiments can be used to analyze the inhomogeneous magnetic field of a system of submicron particles and to control the transport properties of Josephson junctions.

Skyrmion states in multilayer exchange coupled ferromagnetic nanostructures with distinct anisotropy directions

Abstract We report the experimental observation of magnetic skyrmion states in patterned ferromagnetic nanostructures consisting of perpendicular magnetized Co/Pt multilayer film exchange coupled with Co nanodisks in vortex state. The magnetic force microscopy and micromagnetic simulations show that depending on the magnitude of Co/Pt perpendicular anisotropy in these systems two different modes of skyrmion formation are realized.

Control of the magnetic state of arrays of ferromagnetic nanoparticles with the aid of the inhomogeneous field of a magnetic-force-microscope probe

The work presents a survey of the results of studies of the processes of magnetization reversal of ferromagnetic nanoparticles under the action of the field of a magnetic force microscope probe.

Comparative x-ray reflectometry and atomic force microscopy of surfaces with non-Gaussian roughness

Theoretical and experimental investigations have been carried out with the aim of comparing atomic force microscopy (AFM) and x-ray reflectometry (XRR) as methods for characterizing surface roughness. It is shown that AFM gives more appropriate information about the surface roughness in comparison to XRR. The method for estimating the parameters that characterize x-ray scattering on the basis of AFM data is developed.

Photogalvanic effect in ferromagnets with a noncoplanar magnetization distribution

The photogalvanic effect in ferromagnetic materials is predicted. This is the appearance of the direct electric current in a ferromagnet sample due to a high-frequency alternating electric field. The phenomenological theory of this photogalvanic effect in ferromagnets is developed and the simplest microscopic model explaining the occurrence of the photogalvanic effect in a medium with the helical magnetic structure is proposed. The photogalvanic effect arises owing to the breaking of the symmetry with respect to the reversal of the electron motion in the medium with a noncoplanar magnetization distribution.

Effect of ferromagnetic nanoparticles on the transport properties of a GaMnAs microbridge

A chain of Co nanoparticles was formed along a GaMnAs microbridge by electron beam lithography. The magnetic state of the particles was manipulated by a magnetic force microscope probe. It was found that resistance of the microbridge depended on the state of the particles and was different for the particles in the single-domain and vortex states. The resistance exhibited steplike behavior in an external magnetic field. This behavior was the result of the effect of the inhomogeneous stray fields of the particles on the microbridge resistance. The observed phenomenon can be used as an alternative way to control GaMnAs transport properties.