L. S. Uspenskaya

Superconducting MgB2 films obtained by magnetron sputtering

Superconducting MgB2 films with a superconducting transition temperature of 24 K were obtained by magnetron sputtering. The high homogeneity of the films was demonstrated by the magneto-optical imaging of the magnetic flux penetration.

Domain structure and magnetization of the permalloy/niobium bilayers

Domain structure and kinetics of the magnetization reversal in the Py/Nb bilayers grown with lift-off lithography have been studied using visualization by magneto-optical indicator film technique. Various types of domain walls and switching behavior are observed depending both on film and substrate thickness. The observed transformations of the domain structure are in agreement with the model, which takes into account effective surface anisotropy governed by surface roughness, while influence of the misfit-induced elastic strain is found to be negligible.

Magnetic patterns and flux pinning in Pd0.99Fe0.01-Nb hybrid structures

The magnetic properties of hybrid thin-film Pd0.99Fe0.01-Nb structures are studied by a magneto-optical technique. It is shown that, below 14 K, the samples exhibit the ferromagnetic ordering corresponding to the formation of weakly coupled ferromagnetic nanoclusters. In the clusters, the effective spin polarization of Fe ions is about 4μB, corresponding to that in the bulk Pd3Fe alloy. The proximity of the ferromagnetic layer does not suppress the superconductivity in niobium. It does not affect the superconducting transition temperature but leads to an enhanced pinning and results in an increase in the critical current by about 30%. This behavior agrees well with the existence of the nanocluster structure in the ferromagnetic film.

Magnetism of ultrathin Pd99Fe01 films grown on niobium

Magnetic properties of ultrathin Pd99Fe01 films grown on niobium films are investigated by magneto-optic visualization, SQUID magnetometry, and Hall-voltage measurements in the temperature range from 3 to 40 K. We show that the films are ferromagnetic at thickness larger than 10 nm. The Curie temperature varies from 2 to 40 K with increase of film thickness to 80 nm. The value of spontaneous magnetization of the Pd99Fe01 depends on the PdFe film thickness. The estimated spin polarization is about 4 per Fe ion, which corresponds to the polarization of the Pd3Fe compound. In contrast to the homogenous bulk material, Pd99Fe01 films consist of ferromagnetic nano-clusters in a paramagnetic host, which is confirmed by characteristic features of the magnetization loops and by the increase of critical current density in the adjacent Nb layer. The size of the clusters is estimated as 10 nm, which is in agreement with the 30% increase of the supercurrent observed in the Nb.

Two-component magnetization in Pd99Fe01 thin films

Magnetometry data on 5- to 40-nm-thick PdFe films are presented for the first time. It is found that the decrease in the film thickness leads not only to the lowering of the temperature of the transition from the paramagnetic to ferromagnetic state and to the decrease in the saturation magnetization but also to the drastic changes in the film properties. In particular, the type of the temperature dependence of magnetization changes, the Curie temperature becomes strongly dependent on the applied magnetic field, and two transition temperatures are observed in zero magnetic field. The observed effects are attributed to the transition from the three-dimensional distribution of ferromagnetic clusters arising around impurity iron atoms to the two-dimensional one and can be treated within the model of impurity magnetism suggested by Korenblit and Shender.

Transformation of the Domain-Wall Structure in Permalloy Nanotape Under Electrical Current Pulses

Motion of the magnetic domain walls under the pulses of magnetic field or spin-polarized electric current is studied experimentally by direct visualization of the walls. The experiments are performed in the temperature range from 300 down to 6 K. Coercivity, mobility of the walls, and their variations with temperature are investigated. Wall velocity is observed to increase by orders of magnitude due to cooling from room to helium temperature in the case of current stimulated motion. The velocity becomes as high as the Walker velocity limit, about 1000 m/s. The walls are found to expand and tilt under the current pulses, which leads to their transformation into the domains with transverse magnetization.

Transformations of head-to-head domain walls in (La,Sr)MnO3 thin films

Kinetics of the in-plane magnetization reversal in a (La,Sr)MnO3 thin film has been studied at different temperatures using the magneto-optic visualization technique. In addition to changes of the domain wall coercivity and mobility, dramatic transformations of the domain wall structure have been observed. Motion of a single zig-zag domain wall is typical for high temperature magnetization reversal, whereas complicated diffuse transient structure between the two principal domains is observed at low temperature. Generation of secondary bubble domains in front of a moving wall has been observed for the first time in a quasistatic regime, contrary to previous superfast experiments.

On the possibility of operation with the 180 degree domain wall lattice using AC fields

Changes of the domain structure period and continuous generation and unidirectional motion of domains in ferrimagnetic garnets under ac fields are studied. Effects of bias dc fields and temperature on the dynamical domain structure behaviour are investigated. The domain structure changes are considered to be due to excitation of nonlinear magnetic modes in domain walls and domains.

A Simple Model for Investigation of the Pair Breaking Effect on the Parameters of HTS/FM Thin Films

A simple model was proposed for description of Cooper pair breaking effect (which is caused by transfer of spin-polarized carriers from the ferromagnetic (FM) manganite into the high temperature superconductor (HTS)) on the critical temperature and the microwave surface impedance of HTS/FM heterostructures. The model is based on the assumption on an exponential dependence of the Cooper pairs concentration (as well as number of the holes in CuO2 planes, responsible for the superconductivity) on the distance from HTS/FM interface. The model fits well the thickness dependence of HTS film parameters and is used for evaluation of the penetration depth of spin-polarized particles into HTS in the HTS/FM structures.

Modeling of Magnetization Distribution Near Shaped Boundary of Garnet Film Core in Fluxgate Magnetometer

Micromagnetic modeling of magnetization in-plane distribution near shaped boundary of garnet film was made to analyze the possibility to prevent chaotic formation of magnetic flux distortions in a fluxgate disc-like core excited by rotating magnetic field. Calculations made in the limit of zero anisotropy for sinusoidal, rectangular, and cogged (with right angle) boundaries show regular macroscopic vortex formation and annihilation in opposite phases of external magnetic field inside every apex of garnet film shaped boundary. Vortices are formed due to magnetostatic conservation of magnetization initial local direction along smooth or straight boundaries of the film during the whole period of rotation. Vortices near smooth curved boundary exhibit a stability range in the external field that exceeds that near rectangular and cogged shapes by an order of magnitude. Transformations of vortex states and low field saturation near broken line boundary are explained by peculiarities of exchange energy caused by mutually perpendicular spin orientations along adjacent straight sides of right angles. Calculation results show the way for control of magnetization distribution distortions in fluxgate garnet disc core by film boundary shape.