Magnetic relaxation near a fishtail peak of isothermal magnetization in B1 NbC(y)) encapsulated in multiwall carbon nanocages

Isothermal magnetization near a fishtail peak in nanocrystalline B1 NbC(y)) encapsulated in multiwall carbon nanocages is studied within the time window of 100 < t < 4000 s. The current density J exhibits a linear logarithmic time decay. The effective activation energy U(eff) increases linearly with temperature T and is independent of applied magnetic field H. The results of J(t) and U(eff) (T, H) are consistent with the Anderson-Kim flux-creep model for thermally activated motion of uncorrelated vortices or vortex bundles over a net potential barrier U(eff). U(eff) at a fishtail peak field H(fp) evolves quickly above a fishtail peak temperature T(fp), but slowly below that temperature. The result suggests that a decrease of flux viscosity coefficient above T(fp) at H(fp) is the origin of the fishtail peak in nanocrystalline B1 NbC(y)) encapsulated in multiwall carbon nanocages.