The scaling and relaxation behavior around the fishtail minimum is studied in detail in a wide temperature range ͑3-70 K͒ on DyBa 2 Cu 3 O 7Ϫ␦ single crystals exhibiting a pronounced fishtail effect. Magnetic hysteresis loops ͑MHL's͒ normalized with respect to the height and position of the fishtail maximum fall on a universal curve which form can be derived from the phenomenological model of a thermally activated flux creep proposed by Perkins et al. ͓Phys. Rev. B 51, 8513 ͑1995͔͒. This universal curve tends at low fields towards zero. At low temperatures, the drop of j s at low fields is usually masked by a wide central peak. By subtracting the universal curve from the experimental j s (B) data we separate the contribution of the central peak. It has a simple, exponentially decaying field dependence. This implies that the fishtail minimum at low fields might be understood as a result of an overlapping of two contributions originating from separate pinning mechanisms: one active mainly at high fields and dying away with B going to zero and another one ͑responsible for the central peak of the MHL͒ vanishing rapidly with increasing field. This concept is also supported by relaxation experiments. These experiments confirm that the shape of MHL's is given by a dynamic equilibrium between the induction, pinning, and relaxation processes. ͓S0163-1829͑97͒05305-8͔
Exact analytical results are obtained for the magnetization of a superconducting thin strip with a general behavior Jc(B) of the critical current density. We show that within the critical-state model the magnetization as function of applied field, Ba, has an extremum located exactly at Ba = 0. This result is in excellent agreement with presented experimental data for a YBa2Cu3O 7−δ thin film. After introducing granularity by patterning the film, the central peak becomes shifted to positive fields, Bcp > 0, on the descending field branch of the loop. Our results show that a positive Bcp is a definite signature of granularity in superconductors.
The superconducting transition region in a Nb disk showing the paramagnetic Meissner effect (PME) has been investigated in detail. From the field-cooled magnetization behavior, two well-defined temperatures can be associated with the appearance of the PME: T 1 (< T c ) indicates the characteristic temperature where the paramagnetic moment first appears and a lower temperature T p (< T 1 ) defines the temperature where the positive moment no longer increases. During the subsequent warming, the paramagnetic moment begins to decrease at T p and then vanishes at T 1 with the magnitude of the magnetization change between these two temperatures being nearly the same as that during cooling. This indicates that the nature of the PME is reversible and not associated with flux motion. Furthermore, the appearance of this paramagnetic moment is even observable in fields as large as 0.2 T even though the magnetization does not remain positive to the lowest temperatures. Magnetic hysteresis loops in the temperature range between T 1 and T p also exhibit a distinct shape that is different from the archetypal shape of a bulk type-II superconductor. These behaviors are discussed in terms of the so-called 'giant vortex state'.
An analytic expression is given for the ballistic demagnetizing factors, D′, of the general rectangular prism, in which the field is averaged over the middle cross section of the prism, and for the “side” demagnetizing factor, D″, in which the averaging is over the edges of the prism. The demagnetizing fields are compared with the applied field values when a 250×95×2.35 μm magnetic shield transfers into the saturated state. This shield was made of a Ni80 Fe20 film, with a small uniaxial magnetic anisotropy induced during the electroplating process. Domain structure and the process of magnetic saturation were imaged using wide-field Kerr microscopy in fields up to 400 Oe, both along the easy and the hard axes. The ballistic demagnetizing field is found to be close to the external field necessary to align magnetically the central part of the shield, while the full shield saturation takes place at a field above the recently published magnetometric demagnetizing field. Saturation fields along the hard uniaxial anisotropy axis are larger, due to effective anisotropy fields.
A YBa 2 Cu 3 O 7Ϫ␦ thin film is patterned into a hexagonal close-packed lattice of disks (2rϭ50 m) which are touching each other at the circumferences in order to enable the flow of an intergranular current. Such a sample was suggested by Koblischka et al. ͓Appl. Phys. Lett. 70, 514 ͑1997͔͒ as a model for a layered granular structure like in a (Pb,Bi) 2 Sr 2 Ca 2 Cu 3 O 10ϩ␦ ͑Bi-2223͒ tape. The magnetization measurements reveal an anomalous position of the low-field peak ͑central peak͒ similar to Bi-2223 tapes. Magneto-optic imaging is employed to visualize the local-field distributions. At low magnification, the flux patterns in the intergranular area between the disks are observed. The observations at high magnification reveal the flux penetration and pinning within the disks. It is shown that such samples may serve as model samples for granular high-T c superconductors.
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