Instability of the magnetization reversal front in superconductors with a nonlinear anisotropic current-voltage characteristic

The instability of a magnetic flux flow in a system of vortices and antivortices in superconductors with a power (exponent m) anisotropic current-voltage characteristic was studied theoretically. It was shown that instability arose even at a comparatively weak anisotropy of the current-carrying properties of a superconductor if m?1. The dispersion equation determining the dependence of the increment of instability growth on the wave number was derived and analyzed.     

Collapse of static magnetization of type II anisotropic superconductors

The effect of suppression of static magnetization (magnetization collapse) of an anisotropic hard superconductor by an alternating magnetic field is studied experimentally and theoretically. The measurements are performed in constant and alternating magnetic fields H and h(t) which are parallel to the sample surface and are mutually orthogonal. Superconducting samples of the Y-123 system are cut from a grain-oriented piece in such a way that the c axis lies in the plane of the plate. It has been established that the amplitude of the alternating field for which the complete suppression of the magnetization occurs significantly depends on the field orientation relative to the crystallographic axes of the sample. The dynamics of the magnetization collapse is studied, and it is established that, in some cases, the magnetic moment varies nonmonotonically with the growth in amplitude h₀ of the alternating field. A peculiar effect of transition (induced by field h(t)) of the superconductor from the paramagnetic into the diamagnetic state is discovered. The results of observations are explained in the framework of the model of critical state generalized to the anisotropic case.     

A new type of peak effect in the magnetization of anisotropic superconductors

Experimental and theoretical studies of the magnetization curves M(_H) obtained for plates of an anisotropic hard superconductor are performed. The samples are YBCO single-domain textured crystals with the c axis lying in the sample plane. It is shown that, for some orientations of the magnetic field in the sample plane, the magnetization curves contain additional extrema; at the same time, the virgin magnetization curve M(H) exhibits features characteristic of the peak effect. This proves that the anisotropy of the current-carrying ability can give rise to a new type of peak effect.     

The structure of the vortex lattice in anisotropic superconductors

The structure of the vortex lattice in anisotropic superconductors at an arbitrary temperature in magnetic fields close to critical was studied. Generally, a rhombohedral structure with a vertex angle depending on temperature, magnetic field, and material constants is formed. An important factor is the small (2%) difference of the free energies of the triangular and square lattices in the Ginzburg-Landau approximation. This factor also persists in anisotropic superconductors.     

Magnetic phase diagram of anisotropic layered superconductors via magnetization measurements forH‖c in Bi2212

The Bi₂Sr₂CaCu₂O₈ system is viewed as an archetypal of superconductors modelled as Josephson coupled CuO₂ bilayers. The isothermal and temperature dependent DC and AC magnetization measurements forH‖c in a single crystal of Bi2212 have been performed. Qualitative changes are observed to occur over a narrow range of temperature values before reaching the superconducting-normal transition. The observed behaviour can be ascribed to the rapid variation in the strength of the coupling between the superconducting CuO₂ planes (i.e., bilayers in the case of Bi2212). Strongly coupled planes behave like a 3D superconductor, whereas weakly coupled planes have a two component response attributable to 2D planes and interplanar couplings. We believe that this paper is a plethora of new findings. Our observations imply that resistivity across the planes becomes zero earlier than that within the planes. A new line (designated asH _2D(T)) above which the change in the electromagnetic response is dominated by quasi 2D-planes has been determined for the first time. This paper also contains the first observation of Differential Diamagnetic Effect (DDE) in the In-phase AC susceptibility data which signals the onset (atT _2D(H)) of dominance of response from 2D-planes. In addition to a host of interesting thermomagnetic history effects which are a consequence of interplay between the diamagnetic responses from the two components, a comparison of irreversibility lines (of the 3D state) determined by different methods on the same specimen of a HTSC is also being presented for the first time. We have come across Paramagnetic Meissner Effect (PME), first recognized in ceramic samples of Bi2212, in the temperature region of dimensional crossover in our single crystal sample, whichinter-alia confirms our labelling of the two component behaviour. A schematic phase diagram summarizing the various transformations that can occur nearT _c in the electromagnetic response of an anisotropic layered system has been drawn.     

Magnetic instability regions in patterned structures: influence of element shape on magnetization reversal dynamics

We report a time-resolved imaging study of the influence of shape on magnetic instabilities in patterned magnetic structures. We find that in rectangular structures magnetization reversal initiates at the ends and interior simultaneously, while in structures with tapered ends the reversal begins in the middle of the structures and spreads out to the ends. The degree of tapering is important for both the switching field and the time required for full reversal. A model based on the concept of local instability regions yields good agreement with the observed location of the reversal onsets.     

R2Fe14B型永磁材料中第二磁晶各向异性常数对反磁化过程的影响

针对低温下各向同性Pr2Fe14B永磁材料的最小形核场问题,用数值计算法和近似解析解研究了第二磁晶各向异性常数K2对最小形核场的影响.研究发现,尽管对于Nd2Fe14B永磁材料一级近似的解析解与数值计算结果很接近,但是对于低温下各向同性Pr2Fe14B永磁材料则至少要用二级近似下的解析解才能与数值计算结果相接近.用有关最小形核场的计算结果很好地解释了低温时各向同性Pr2Fe14B永磁材料的矫顽力与最小形核场的关系.     

Stimulation effect in anisotropic superconductors

The stimulation of superconductivity in anisotropic superconductors by electromagnetic and acoustic pumping as well as by the injection of a tunnel current at temperatures close to the superconducting transition temperature is studied. The features distinguishing the stimulation effect from the isotropic case are indicated. Zh. éksp. Teor. Fiz. 111, 2134–2146 (June 1997)     

Dynamic magnetization reversal in dipole systems

The possibility of magnetization reversal in two different systems of magnetic dipoles by preliminary excitation of certain oscillatory regimes is demonstrated using numerical analysis of bistable states of these systems. Cyclic magnetization reversal of the systems is executed by sequential alternation of the frequency of the applied ac field. The interaction of two ring-shaped dipole systems is analyzed and peculiarities of the change in the total magnetic moments induced by this interaction are revealed.     

Flux front penetration in disordered superconductors

We investigate flux front penetration in a disordered type-II superconductor by molecular dynamics simulations of interacting vortices and find scaling laws for the front position and the density profile. The scaling can be understood by performing a coarse graining of the system and writing a disordered nonlinear diffusion equation. Integrating numerically the equation, we observe a crossover from flat to fractal front penetration as the system parameters are varied. The value of the fractal dimension indicates that the invasion process is described by gradient percolation.     

Features of magnetization reversal in trilayer nanostructures

The process of magnetization reversal in an ultrathin magnetic trilayer is analyzed. It is shown that the shape of magnetization hysteresis loops and the giant magnetoresistance essentially depend on the relative magnitudes of magnetic parameters of the top and bottom layers. Hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis is performed of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange. A phase diagram is constructed, which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J ₁.