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Local magnetic measurements in a highly anisotropic Nd-Ce-Cu-O crystal reveal a sharp onset of an anomalous magnetization peak at a temperature-dependent field B on . The same field marks a change in the field profiles across the sample, from profiles dominated by geometrical barriers below B on to Bean-like profiles above it. The temperature dependence of B on and the flux distribution above and below B on imply a disorder-induced transition at B on from a relatively ordered vortex lattice to a highly disordered, entangled vortex solid. Local magnetic relaxation measurements above B on show evidence for plastic vortex creep associated with the motion of dislocations in the entangled vortex structure.[S0031-9007 (97)04113-6]

A novel Hall probe array technique is used to measure the spatial distribution and time dependence of the magnetic induction in YBa 2 Cu 3 O 72d crystals. Analysis of the data based on the flux diffusion equation allows a direct, model-independent determination of the local activation energy U and the logarithmic time scale t 0 for flux creep. The results indicate that the spatial variations of U are small (6kT) and that U increases logarithmically with time. The time t 0 is inversely proportional to the field and it exhibits a nonmonotonic temperature dependence. These results confirm theoretical predictions based on the logarithmic solution of the flux diffusion equation.Thermally activated flux creep in high-temperature superconductors is a subject of intensive study. This phenomenon is commonly investigated by measuring the time dependence of the magnetic moment M averaged over the sample volume. Among the most significant parameters extracted from such data are the effective activation energy U and the logarithmic time scale t 0 for flux creep [1]. Recent models emphasize the nonlinear dependence of U on the current density j [2,3] and the macroscopic nature of the time scale t 0 [1-6]. While it is not possible to derive U͑j͒ directly from the experimental data, each of the above models gives a specific relaxation behavior that can be compared with experimental results. Such an approach for evaluating U͑j͒ is model dependent and involves fitting several parameters [7].Maley et al. [8] proposed a method to determine U͑j͒ avoiding the a priori assumption of a model for the dependence of U on the current density and field. Their method analyzes global magnetic relaxation data, utilizing an integrated form of the flux diffusion equation over the sample volume. It is important to realize that the activation energy determined by this method is actually the activation energy at the surface of the sample, while the current density j is averaged over the sample volume [9]. Although in the limit U͞kT ¿ 1 the activation energy should be almost constant over the sample volume [1,4], in the presence of surface barriers [10-12] the values of U at the surface and in the bulk may be different.In this work we propose a method to determine the local U and j in the bulk, utilizing the recent development of a miniature Hall probe array [12] to measure the local induction B at different locations simultaneously as a function of time. In contrast with the conventional techniques where only the time evolution of the total magnetization is recorded, we measure the time evolution of the spatial distribution of B, and thus are able to determine both the time and the spatial derivatives of B. This new information enables direct analysis of the local relaxation data using the basic diffusion equation governing the flux motion [4,13]:where D Bv is the flux current density and y y 0 exp͑2U͞kT ͒ is the effective vortex velocity. The preexponential factor y 0 Ajf 0 ͞ch, where f 0 is the unit flux, c is the light velocity, j is the current ...

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Sonochemically synthesized Fe 2 O 3 nanoparticles were annealed in air or in vacuum while their magnetization was continuously recorded. Annealing in vacuum at temperatures T a between 240 and 450°C produced nanophases of ␥-Fe 2 O 3 with average particle size ranging from 4 to 14 nm, depending on T a. Phase transformation into ␣-Fe 2 O 3 occurred directly by annealing in air, or via an intermediate Fe 3 O 4 phase by annealing in vacuum at temperatures higher than 450°C. Mapping the correlation between the magnetic properties and the annealing conditions, enables control of the annealing process to obtain nanocrystals of ␥-Fe 2 O 3 , ␣-Fe 2 O 3 , or Fe 3 O 4 with different particle size and magnetic properties.