Disorder induced asymmetric magnetization in Bi 2Sr 2Ca 2Cu 3O 10

Abstract

Magnetization measurements as a function of external magnetic field H e, temperature T and time t were performed in a c-axis-oriented bulk Bi 2Sr 2Ca 2Cu 3O 10 compound sample with H e applied along the preferential c-axis orientation. It is found that hysteresis loops, M( H e), are asymmetric with respect to the equilibrium magnetization M eq, such that an additional magnetization ΔM m = M m − M eq arises for the flux entrance, where M m (H e ) = 1 2 [M +(H e ) + M −(H e )] is the mean magnetization, and M +, M −, the magnetization corresponding to the ascending and descending branches of the hysteresis loop, respectively. The time relaxation measurements made in H e = 1 T indicate that both M +( t) and M −( t) dependencies could be best described by the power law ( M − M ∞) ∼ t − β , where M ∞ is obtained from the extrapolation of the relaxation law to t = ∞. Above a ‘depinning’ temperature T d ∼ 20 K, ΔM m tends to zero, i.e. M ∞ + ∼ M eq, whereas at T < T d, ΔM m tends to the finite value M g 2 , and M ∞ + = M eq + M g . At the same t M ∞ − ∼ M eq for all temperatures. Furthermore, it is shown that both ΔM m and M g correlate well with the bulk vortex pinning, and that the found asymmetry can hardly be understood within frameworks of the classical Bean-Livingston surface barrier model. A possible physical picture for the disorder (pinning) induced asymmetry of both the hysteresis loops and the time relaxation is presented.