Abstract
An analysis of the field dependence of the pinning force in different, high density sintered samples of MgB2 is presented. The samples were chosen to be representative for pure MgB2, MgB2 with additives, and partially oriented massive samples. In some cases, the curves of pinning force versus magnetic field of the selected samples present peculiar profiles and application of the typical scaling procedures fails. Based on the percolation model, we show that most features of the field dependence of the critical force that generate dissipation comply with the Dew-Hughes scaling law predictions within the grain boundary pinning mechanism if a connecting factor related to the superconducting connection of the grains is used. The field dependence of the connecting function, which is dependent on the superconducting anisotropy, is the main factor that controls the boundary between dissipative and non-dissipative current transport in high magnetic field. Experimental data indicate that the connecting function is also dependent on the particular properties (e.g., the presence of slightly non-stoichiometric phases, defects, homogeneity, and others) of each sample and it has the form of a single or double peaked function in all investigated samples.
Highlights
Magnesium diboride, MgB2, is one of the most exciting superconductors discovered in the last two decades due to a series of advantages that makes it attractive for applications
Considering that the pinning force is related to the critical current density, the effort was driven to find hints for the field dependence of Jc using different combinations of H, Jc and different derivatives of Jc leading to a linear dependence
We have shown that the reduced pinning force fp dependence on the reduced field h can be described in the case of polycrystalline bulk samples by the model of pinning on grain boundaries
Summary
Magnesium diboride, MgB2, is one of the most exciting superconductors discovered in the last two decades due to a series of advantages that makes it attractive for applications. One of the most important properties is the capacity to transport a high super current in an applied magnetic field The analysis of this process showed that grain boundaries act as the main pinning structure though other mechanisms could not be neglected. Considering that the pinning force is related to the critical current density, the effort was driven to find hints for the field dependence of Jc using different combinations of H, Jc and different derivatives of Jc leading to a linear dependence. These combinations seemed to work only in a limited field range, introducing two or three crossover fields. If different field-related regimes can be valid in superconducting cuprates, where the interplay between weak pinning, short coherence length, and long penetration depths generate different regimes of the collective pinning[15], it would raise difficulties regarding their interpretation in the case of MgB2 with a much longer coherence length and stronger pinning
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