V. I. Nikitenko

Spring magnet films

The properties of exchange-spring-coupled bilayer and superlattice films are highlighted for Sm-Co hard magnet (nominally Sm/sub 2/Co/sub 7/) and Fe or Co soft magnet layers. The hexagonal Sm-Co is grown via magnetron sputtering in a- and b-axis epitaxial orientations. In both cases the c-axis, in the film plane, is the easy axis of magnetization. Trends in coercivity with film thickness are established and related to the respective microstructures of the two orientations. The magnetization reversal process for the bilayers is examined by magnetometry and magneto-optical imaging, as well as by simulations that utilize a one-dimensional model to provide the spin configuration for each atomic layer. The Fe magnetization is pinned to that of the Sm-Co at the interface, and reversal proceeds via a progressive twisting of the Fe magnetization. The Fe demagnetization curves are reversible as expected for a spring magnet. Comparison of experiment and simulations indicates that the spring magnet behavior can be understood from the intrinsic properties of the hard and soft layers. Estimates are made of the ultimate gain in performance that can potentially be realized in this system.

Study of flux distributions in high-Tc single crystals and thin films using magneto-optic techniques

Abstract The current progress of magneto-optical techniques for direct investigations of magnetic flux structures in type-II superconductors with high spatial resolution is presented. Wide possibilities are offered by the high-resolution Faraday effect technique requiring a thin layer of EuSe as an indicator and the method using ferrimagnetic garnet films with in-plane anisotropy. The advantages of both methods are combined to investigate flux structures of an YBa 2 Cu 3 O 7 (YBCO) thin film partly irradiated with 25 MeV oxygen ions and for inhomogeneously twinned YBCO single crystals.

Macroturbulence in high-Tc superconductors

Abstract The first observation of a new physical phenomenon - appearance of macroturbulence in type-II superconductors - is reported. It is found that during the remagnetization of high- T c single crystals in some temperature range the flux lines are condensed into “drops” of increased density which then move as hydrodynamical vortices. The observed structures are treated as the development of current instability resulting in the formation of turbulent current structures.

Direct experimental study of the exchange spring formation process

The remagnetization of a soft ferromagnetic film exchange coupled with a high-coercivity ferromagnetic film is studied by a magneto-optic imaging technique. If the magnetic field is antiparallel to the macroscopic unidirectional in-plane anisotropy, the soft layer reverses via the formation of exchange springs consisting of subdomains with opposite spin twistings. However, if the field is instead rotated in-plane, remagnetization initially proceeds via formation of a single uniform exchange spring. Then, at a critical angle, the spring incoherently untwists, leading again to subdomains with opposite chirality. These phenomena are attributed to the influence of inhomogeneity in the unidirectional magnetic anisotropy.

Mechanisms of energy dissipation during displacement- sensitive indentation in Ge single crystals at elevated temperatures

Abstract Load-displacement indentation tests have been performed in Ge single crystals on a newly designed displacement-sensitive hardness tester in the temperature range 20–450°C. The deformation substructure in the vicinity of the indentation impressions was investigated using selective etching. The energy dissipated during the loading-unloading indentation cycle has been measured and compared with the extent and the structure of the deformation zone. Mechanisms of plastic flow and fracture during indentation are discussed.

Influence of twins on the critical current in YBaCuO single crystals

Abstract Induction profiles in an inhomogeneously twinned YBa 2 Cu 3 O 7-δ single crystal were measured using a recently developed magneto-optic technique. The critical current value J c was derived from these profiles and temperature dependences of J c for twinned and untwinned regions were built. Twin boundaries are shown to contribute essentially to pinning at temperatures above 45 K, while at lower temperatures other defects dominate. A model is suggested which explains the exponential character of J c ( T ) obtained for the untwinned region.

Real-time observation of the effect of grain boundaries on magnetization of YBa2Cu3O7−x polycrystals

Abstract A high-resolution, magneto-optical method has been used to study the effect of grain boundaries on the magnetization dynamics of YBa 2 Cu 3 O 7− x polycrystals as a function of temperature (7–50 K) and applied magnetic field (0 to ± 65 mT). At low temperatures (7–20 K), with increasing field, flux penetrated first along high-angle grain boundaries (misorientation angle θ>10°), then along low-angle grain boundaries (LAGBs, θ

Magneto-optical study of twin boundary pinning in YBa2Cu3O7-δ

Abstract Using a high-resolution magneto-optical technique we studied the flux penetration and pinning in YBa 2 Cu 3 O 7-δ crystals in the presence of isolated twin boundaries. The most important factor determining the behavior of the twin boundaries is the direction of flux motion with respect to the boundary. Up to temperatures close to T c twin boundaries are effective barriers for flux motion transverse to the boundaries. For flux motion parallel to the boundary the characteristic “flame”-shaped flux patterns indicative of weak link behavior (i. e. low J c ) of the boundary are observed.

Chirality of a forming spin spring and remagnetization features of a bilayer ferromagnetic system

Distribution of a magnetic moment in an exchange-coupled bilayer Fe/SmCo epitaxial structure grown on a (110) MgO substrate is visualized by the magnetooptic indicator film technique. The direction and the magnitude of the effective magnetization in this structure are determined both under external magnetic fields of variable magnitude and direction and after the removal of these fields. It is shown that such a heterostructure is remagnetized by a nonuniform rotation of a magnetic moment both along the thickness of a sample and in its plane. A field antiparallel to the axis of unidirectional anisotropy gives rise to spin springs with opposite chiralities in different regions of the magnetically soft ferromagnetic layer. The contributions of these springs to the net magnetization cancel out, thus decreasing the averaged magnetic moment and the remanent magnetization without their rotation. When the external field deviates from the easy axis, the balance is violated and the sample exhibits a quasi-uniform rotation of the magnetic moment. Asymmetry in the rotation of the magnetic moment is observed under the reversal of the field as well as under repeated remagnetization cycles. It is established that a monochiral spin spring is also formed in a rotating in-plane magnetic field when the magnitude of the field exceeds the critical value. Possible mechanisms of remagnetization in this system are discussed with regard to the original disordered orientation of magnetization of the magnetically soft layer with respect to the easy axis, which is defined by the variance of unidirectional anisotropy axes of this layer on the interface.

Visualization of magnetization processes in HTSCs

A novel method for visualizing the magnetic flux distribution in superconductors using iron-garnet films operating in a wide temperature range is developed. Processes of magnetization in Y, Bi, and Tl cuprate single crystals and YBaCu films and ceramics were studied using this method. Screening patterns and local susceptibility data revealed inhomogeneity of superconducting properties of the samples associated with weak links due to defects. Magnetization of Y, Bi, and Tl cuprates appears to proceed in two stages. At smaller fields (but above a critical value) the magnetization is reversible and apparently determined by entry of the magnetic flux into samples along channels of weak links. At higher fields the flux begins to penetrate into regions of stronger superconductivity and is trapped there. With increasing temperature the flux pinning decreases and a considerable creep takes place. At T>0.6 T/sub c/ noticeable flux trapping is revealed only after field cooling in most of samples. High-temperature superconducting films demonstrate essentially stronger pinning in comparison with single crystals, indicating that anchoring at micrograin boundaries is perhaps the most effective pinning mechanism. >