L.A. Fomin

MAGNETIC EPITAXIAL NANOSTRUCTURES FROM IRON AND NICKEL

Magnetoresistance of a planar single crystalline Ni nanostructures has been measured at room and liquid helium temperatures. Bending GMR effect increases from 4% under diffusive to more than 200% under ballistic electron transport in cross-type nanostructures. Magnetoresistance measured for longitudinal and transverse magnetic fields are qualitatively different and explained by a nonsymmetrical magnetic domain structure in the central part of the cross.

Complementary Analysis of Epitaxial Fe (001) Films with Improved Electronic Transport and Magnetic Properties

Complementary analysis of morphology, crystal structure, electronic transport, and magnetic properties is applied to optimize the epitaxial growth of Fe(001) films with the improved electronic transport and magnetic properties. It was found that the mean free path of residual electrons and coercive forces are correlated in the epitaxial single-crystal Fe(001) films grown under different temperatures and depend on the film morphology.

Geometric Effects in Current-Voltage Characteristics of a Cross-Shaped MDM Ni/NiO/Fe Structure

The current–voltage (I–U) characteristics of a cross-shaped metal-dielectric-metal (MDM) Ni/NiO/Fe structures with NiO layers of different thicknesses are investigated. The dependence of the sign of the differential resistance on the current flowing through the structure and on the thickness of the NiO in a four-contact measuring system of potentials is revealed. In a region 8–10 nm thick, the voltage on the structure changes sign thrice and has the form of an N-shaped curve. This dependence is explained by the geometric effect, which appears due to the competition between the vertical and lateral electron transport, when the ratio of the resistances of the dielectric interlayer and metallic leading electrodes changes under the influence of the current. The numerical calculation confirms the experimentally observed I–U dependences.

Application of Magnetic Force Microscopy for Investigation of Epitaxial Ferro- and Antiferromagnetic Structures

Growing of epitaxial Fe50Mn50/Fe/Mo/R-sapphire films was performed with a new configuration of two in-plane easy axes of Fe(001)-layer magnetization in which application of annealing in a magnetic field forms an unidirectional anisotropy. The microstructures made from these films exhibited an exchange bias 25–35 G along an exchange field generated at antiferromagnet/ferromagnet (AFM/FM) interface. Magnetic force microscopy (MFM) experiments supported by micromagnetic calculations and magneto-resistive measurements allowed interpretation of the magnetic states of the Fe layer in these microstructures. The magnetic states of the iron layer are influenced more by crystallographic anisotropy of the Fe-layer than by unidirectional exchange anisotropy.

Anisotropic magnetoresistance of the epitaxial single-crystal Fe(001) film of nanoscale thickness

The dependence of the anisotropic magnetoresistance (AMS) of the single-crystal Fe(001) film grown with the aid of the pulsed laser evaporation on the direction and magnitude of the external magnetic field is analyzed. Three ranges of magnetic fields with different types of variation in AMS with the direction of magnetic field are revealed. The discontinuities of resistance and the hysteresis are demonstrated in a certain range of magnetic fields. The anisotropy field is estimated using the experimental results.

Epitaxial Fe films and structures

Iron film growth conditions and films properties on A- and R- sapphire surfaces were investigated. The best growth conditions were achieved at temperatures about 250 - 300°C. A 10 nm Mo seed layer on the A-sapphire surface improves Fe film morphology and film roughness becomes less than 1 nm. As a result epitaxial Fe (011) films with high residual electron mean free paths (about 0.5 mkm) were grown on the A-sapphire surface. These films can be used for ballistic ferromagnetic planar nanostructures fabrication. The magnetic domain configuration of epitaxial iron nanostructures shaped as bridges and crosses depending on the orientation relative to the easy magnetic axis Fe [100] was examined. If the long side of the bridge is directed along the easy magnetic axis, the single domain structure state is easily reached up to maximum structure width about 2 mkm. A stripe domain structure can be observed when the easy magnetic axis is normal to the long rectangular structure axis. The structure orientation at some angle with the easy magnetic axis leads to a magnetic domain configuration along the easy axis is independent of the structure size down to a structure width ~0.5 mkm and depends only on the easy axis direction. The single domain state can be obtained in structures with a width less than 0.5 mkm. The cross-type structures may have only a two-fold symmetry magnetic configuration. Trapeziform domains were found in structures directed along the hard Fe magnetic axis.

Switching the direction of circumferential magnetization of square epitaxial Fe (001) microstructures by spin-polarized current

The change in the direction of the circumferential lattice magnetization by using the spin-polarized current is found by magnetic force microscopy to be possible for square epitaxial Fe (001) microstructures in the basic state. Such a change of the magnetization occurs when applying the magnetic saturation field to the structure with the subsequent switching off of the field accompanied by passing the current with the density above a certain threshold on the order 1012 A/m2 through the structure. In the basic state of the structure, the change of the magnetization depends not only on the current density but also on the spin polarization and the region of the current flow. The effect of the spin-polarized current on the circumferential magnetization of the structure, which is related to the injection of spins by the current, is discussed.

Magnetic structure and magnetoresistance of patterned epitaxial iron thin films: The influence of shape and magnetocrystalline anisotropy

A magnetic-force-microscopy investigation is conducted into the influence of the shape and magnetic anisotropy on the magnetic structure and the mode of magnetization reorientation in epitaxial iron thin films patterned by subtractive techniques. Magnetic anisotropy is found to have a strong influence on the magnetic structure. With an applied field directed along a specimen, the latter is in a single-domain state when oriented parallel to the easy axis of magnetization, and is in a multidomain state when oriented perpendicular to the easy axis, the domain magnetic moments being parallel to the easy axis in both cases. The magnetic structure appears as a streaky pattern. Magnetization reorientation proceeds by domain-wall motion in both cases. The tanks do not change the mechanism of magnetization reorientation in the rectangular part, yet they ensure a single-domain state at lower magnetic flux densities. The results should offer some scope for using epitaxial iron thin films of suitable shape and crystallographic orientation in spin-valve injectors or detectors.

Investigation of re-switching properties of ferromagnetic contacts in Py/Mo microstructures

By means of magnetoresistance and magnetic force microscopy (MFM) measurements the electrical and magnetic properties of ferromagnetic strips and crosses of hybrid nanostructures Py/Mo (Py - permalloy (Ni 80 Fe 20), Mo - molybdenum) have been investigated. Unusual behaviour of the cross magnetoresistance in these nanostructures was found. The dependences of the cross resistance against external magnetic field have the image of the curves with hysteresis typical for anisotropic magnetoresistance (AMR). However, they demonstrate anomalous magnetoresistance dependence on orientation relatively to the axis of the ferromagnetic arm of the cross. In the transverse magnetic field (perpendicular to the axis of the ferromagnetic arm) magnetoresistance has two minima as for the longitudinal AMR. In the longitudinal magnetic field (parallel to the axis of ferromagnetic arm) there are two maxima, typical for the transverse AMR. The value of the effect is about one percent. The AMR of the strip has the supplementary extremes, indicated on a more complex character of magnetization reversal, depended on the shape of nanostructures. The data of the magnetoresistance measurements are compared with MFM images.

Epitaxial metallic electrodes, quantum dots, and wires for application in solid state qubit technology

A road map for the fabrication of epitaxial metallic electrodes, quantum dots and wires is considered for potential application in solid-state qubit technology. The nanotechnology developed includes high quality low-roughness film epitaxy, probe lithography and subtractive techniques for the fabrication of epitaxial nanostructures with a lateral resolution down to 10 nm.