Ac susceptibility studies of a superconducting gallium nanocomposite: Crossover in the upper critical field line and activation barriers

Ac magnetization measurements were carried out for a gallium nanocomposite consisted of a metal loaded porous glass with 3.5 nm pore size to study dynamics in the vortex system and H-T diagrams. Variations of ac magnetization with temperature and bias magnetic field were obtained at different frequencies and amplitudes of ac field. Double peaks in the imaginary part of ac susceptibility associated with doubly structured real parts with distinct dependences on the amplitude of ac field were observed. Activation barriers were evaluated from variations with frequency of the upper-temperature peak in χ″ at different bias fields. The field dependence of the activation barrier followed the power law with exponents equal to 0.12 and 1.0 below and above 20 kOe, respectively. A pronounced positive curvature was observed at low magnetic fields for the H_c2(T) line with the crossover to a common negative curvature near 20 kOe, also. Both the change of the exponent in the power law for activation barriers and the crossover to the negative curvature in the upper critical field line were suggested to occur due to transition in the vortex system from the geometry restricted to space between gallium particles into more homogeneous distribution with increasing field.