Flux motion and isotropic effects in MgB<Subscript>2</Subscript> near the critical temperature

The anisotropic behaviour of polycrystalline MgB<Subscript>2</Subscript> was studied in details by means of angle dependent magnetovoltage measurements (V-θ curves) at the temperatures near the critical temperature T <Subscript>c</Subscript>, where θ is the angle between the external magnetic field H and the transport current I. The measurement of V-θ curves was carried out at fixed temperatures below T <Subscript>c</Subscript> for different values of H and I in a range of θ =  −20° −200°. Magnetovoltage measurements (V-H curves) were performed for perpendicular and parallel orientations of H with respect to I, i.e., the V <Subscript>⊥</Subscript>-H curves for<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$$ \vec H \bot \vec I $$</EquationSource> </InlineEquation> and V <Subscript>∥</Subscript>-H curves for <InlineEquation ID="IEq2"> <EquationSource Format="TEX">$$ \vec H\parallel \vec I $$</EquationSource> </InlineEquation>. The analysis of V-θ curves does not give any agreement with V(θ) ~ sin<Superscript>2</Superscript>(θ) or any modified angular relationship, so that the measured dissipation is nearly independent of the angle between H and I. The isotropic effects observed in MgB<Subscript>2</Subscript> were discussed in the frame of the continuum approximation, the discrete case and the flux line cutting associated with the double critical state model derived in the literature. Finally, it is concluded that structural disorder induced isotropic effects can explain the isotropic angular dissipation observed in V-θ, I-V and V-H curves. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2012