A. Nabialek

Influence of the real shape of a sample on the pinning induced magnetostriction

The pinning induced magnetostriction in an isotropic superconductor was calculated for different sample shapes. We analyzed some special shapes for which the solution can be found analytically. The magnetostriction of a finite slab was considered. In order to determine the influence of demagnetization effects, the pinning induced magnetostriction of an infinitely long and thin strip was calculated. A simple, approximate formula can be used in particular cases to analyze the magnetostriction induced by pinning forces. In this formula the magnetostriction of a sample is connected directly with its magnetization. We also present some experimental results on high temperature superconductors which are analyzed in frames of the developed theory.

Giant magnetostriction and magnetostriction jumps in superconducting single crystalline

We have studied magnetostriction in single crystalline using the strain gauge technique in magnetic fields up to 12 T at temperatures 4.2 K < T < 35 K. We have observed giant transverse magnetostriction (of the order of at 4.2 K) for magnetic field and longitudinal magnetostriction (of the order of at 4.2 K) for orientation. Pronounced jumps in transverse magnetostriction have been observed in the temperature range between 4.2 K and 8.5 K. Experimental magnetostriction results are compared with magnetization measurements using a vibrating sample magnetometer and gradientometric Hall sensors measurements. The calculations were performed on the basis of a pinning-induced mechanism of magnetostriction. The simple adiabatic theory seems to be insufficient to describe the observed flux instabilities.

Synthesis and characterization of Ln(B0.5Mn0.5)O3 (Ln-lanthanoid; B = Ni, Co) perovskites

Abstract The Ln(B0.5Mn0.5)O3 (Ln-lanthanoid; B = Ni, Co) compounds with the orthorhombic perovskite structure have been prepared by a solid state reaction. They are ferromagnets with relatively high Curie temperatures. The Curie temperature decreases with decreasing average radii of the Ln-ion. It is shown that the Ln(Co0.5Mn0.5)O3 (Ln = Gd, Tb, Dy, Y, Ho) are metamagnets.

Flux Jumps and H-T Diagram of Instability for MgB2

We present a study of magneto-thermal instabilities in polycrystalline MgB2 superconductor, by magnetic hysteresis loop measurements and by investigations of magnetic flux dynamics with a miniature Hall probe. Temperature and magnetic field ranges where the flux jumps may be observed have been determined. On the basis of measurements of the magnetic flux dynamics, an average magnetic diffusivity describing the process of the flux jump is estimated. This parameter is compared with the thermal and magnetic diffusivities calculated on the basis of available data for thermal conductivity, heat capacity and resistivity. It is shown that the estimated value of the field of the first flux jump is influenced significantly by the field dependence of specific heat. In order to explain the observed phenomenon, the temperature reached by the sample during the flux jump at different magnetic fields is calculated.

Magnetic and transport properties of EuMnO3+x substituted by Ca, Sr and Cr ions

The magnetic properties of EuMnO 3+x , Eu(Mn 1-x Cr x )O 3 , Eu 1-x Ca x MnO 3 and Eu 1-x Sr x MnO 3 were investigated. It is established that EuMnO 3 is a weak ferromagnet with Neel temperature T N = 49 K. The increasing oxygen content leads to decreasing T N . The substitution of Eu by Ca and Sr causes a strong increase in magnetization and temperature of magnetic ordering, however, a truly ferromagnetic state does not occur in these systems. The insulating Eu 0.7 Sr 0.3 MnO 3 exhibits a colossal magnetoresistance effect which increases strongly with decreasing temperature. It is assumed that Eu(Mn 1-x Cr x )O 3 (0.2 ≤ x ≤ 0.5) solid solutions are mictomagnets with poorly defined temperatures of magnetic ordering. The results are explained in terms of superexchange via anions.

Magnetic ordering in perovskites containing manganese and cobalt

The magnetization measurement of systems with the nominal formula (R = Eu, Gd, Tb, Y) has been made. It has been found that and are weak ferromagnets of Dzialoshinsky - Moriya type whereas is an antiferromagnet. The substitution of manganese ions by cobalt ones leads to the appearance of different magnetic states: cluster-spin-glass-like , inhomogeneous ferromagnets with well defined Curie temperatures and again a spin glass state . Spontaneous magnetizations and Curie temperatures reach maxima for compounds with x = 0.5. External magnetic field induces a metamagnetic transition in (R = Gd, Tb, Y). The transition becomes irreversible at low temperature in the Tb- and Y-based compounds. The annealing in vacuum of leads to the increase of the Curie temperature and to the appearance of a metamagnetic transition for the compound. The metamagnetic behaviour is interpreted taking into account the ionic ordering of and ions and possible 3d-orbital ordering in the sublattice.

Magnetothermal instabilities in type II superconductors: The influence of magnetic irreversibility

Complete magnetization and magnetostriction loops for type II superconductors were calculated in terms of the following pinned critical state models which incorporate the flux jump instability criterion: the Kim–Anderson model, the exponential model, the linear model, and the model with peak effect. These results were used for constructing magnetic field–temperature (H–T) diagrams of instability of these critical state models. Magnetothermal instabilities of superconductors with peak effect were investigated experimentally and theoretically. Regions in the H–T diagram where flux jumps occur were built and compared with experimental M(H) hysteresis loops. We have explained the phenomenon of “island” jumps. The influence of flux trapping on the flux jump regions of magnetic fields was studied.

Pinning induced magnetostriction in ceramic high temperature superconductors

Abstract Transverse and longitudinal magnetostriction was measured in ceramic YBa2Cu3O7−δ and Hg0.8Pb0.2Ba2Ca2Cu3O8+δ superconductors by the strain gauge technique. The measurements were performed in an external magnetic field up to 12 T. The observed magnetostriction was shown to consist of two components. The first one is connected with the intragrain — and the second one with the intergrain screening currents. Both components of the magnetostriction were analyzed independently in frames of an isotropic theory of the pinning induced magnetostriction.

H-T PHASE DIAGRAM FOR THE GIANT MAGNETIC FLUX JUMPS IN LOW TEMPERATURE SUPERCONDUCTORS AND HIGH TEMPERATURE SUPERCONDUCTORS

We have studied magnetothermal instabilities both theoretically and experimentally. Magnetostriction and magnetization hysteresis loops with flux jumps were calculated over a wide range of experimental parameters employing three critical state models: the original Bean model, the Kim–Anderson model, and the exponential model. The influence of the magnetic prehistory on the flux jumps, magnetostriction, and the magnetization was investigated for the LaSrCuO crystal. H–T diagrams of flux instabilities were constructed from calculation and from experimental results. One can see good qualitative agreement between these two diagrams.

Two-and three-dimensional incommensurate modulation in optimally-doped Bi2Sr2CaCu2O8+δ

X-ray scattering measurements on optimally-doped single crystal samples of the high temperature superconductor Bi