Nanoscale physics
Influence of the sample processing method on the roughness of the magnetic flux front in hard type II superconductors
V. F. Rusakov1, O.I. Kuchuk2, I. Abaloszewa3, O. M. Chumak2,3 , V. V. Chabanenko2, O. Abaloszew3, A. Nabiałek3, and R. Puz´niak3
1Vasyl’ Stus Donetsk National University, MOE, Ul. 600 Richcha 21, Vinnytsia, 21021 Ukraine, 
E-mail: v.rusakov@donnu.edu.ua
2O. Galkin Donetsk Institute for Physics and Engineering, NAS of Ukraine, Pr. Nauki 46, Kyiv, 03028 Ukraine
3Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland

The influence of the sample processing method and temperature on the roughness of the magnetic induction flux front penetrated into the hard type II superconductor sample was analyzed by means of magneto-optical visualization. In the study NbTi-50% disk having 0.1 mm thickness and 12 mm diameter was investigated. The sample was cut off from the cylindrical hydro extruded rod and subsequently annealed. The structure of the magnetic induction in the sample was examined in both shielding and flux trapping modes. The experiment was done in temperature range T = 4.5 - 8K. The studies were carried out at different external magnetic field values up to 600G both at the boundary between the Meissner and critical states, and at different levels of induction in the sample, at a fixed value of the external magnetic field. The corresponding levels were determined by the degree of grayscale. The roughness exponent was determined by the fast Fourier transform algorithm. The carried-out investigations revealed a very rough magnetic flux front in both described above cases: the boundary of the region of the magnetic field penetration into the sample and the different levels of induction in the region of the mixed state. It was shown that as a result of heat treatment, both the roughness exponent and the critical current density values increase. The experiment temperature has a weak effect on the roughness exponent. The influence of both the sample boundary shape [1] and its prehistory on the roughness of the magnetic flux front was analyzed. It was shown that the roughness exponent is a characteristic of the pinning centers of the sample. The obtained results enable to assert the existence of the fractal nature of the penetrating magnetic flux front structure. The Hausdorff parameter was calculated.

1. V. K. Vlasko-Vlasov, U. Welp, V. Metlushko, and G. W. Crabtree. Experimental test of the self-organized criticality of vortices in superconductors // Phys. Rev. B.- 2004.-69.-P. 140504(R).