Eur. Phys. J. B (2012) 85: 382
https://doi.org/10.1140/epjb/e2012-30509-0

Regular Article

Flux motion and isotropic effects in MgB₂ near the critical temperature

Abant Izzet Baysal University, Department of Physics, Turgut Gulez Research Laboratory, 14280 Bolu, Turkey

^a e-mail: kilica@ibu.edu.tr

Received: 26 June 2012
Published online: 22 November 2012

Abstract

The anisotropic behaviour of polycrystalline MgB₂ was studied in details by means of angle dependent magnetovoltage measurements (V-θ curves) at the temperatures near the critical temperature T_c, 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_c 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_⊥-H curves for $\begin{array}{}{}_{\mathit{􏿻}}\\ \mathit{H}\end{array}\mathrm{\perp }\hspace{0.17em}\begin{array}{}{}_{\mathit{􏿻}}\\ \mathit{I}\end{array}$and V_∥-H curves for $\begin{array}{}{}_{\mathit{􏿻}}\\ \mathit{H}\end{array}\mathrm{\parallel }\begin{array}{}{}_{\mathit{􏿻}}\\ \mathit{I}\end{array}$ . The analysis of V-θ curves does not give any agreement with V(θ) ~ sin²(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 MgB₂ 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.