Irreversibility field and anisotropic δl-pinning in type II superconductors

The article presents a brief generalization of our recent experiments confirming that "irreversibility field" in Nb-Ti superconducting tapes is determined by the inhomogeneity of superconducting properties. It is shown that the effect is a direct consequence of the so-called δl-pinning, i.e. pinning caused by local reduction of the electrons mean free path. Based on the understanding of the irreversibility field nature, an assessment of the individual pinning force is performed. Good agreement is obtained with the estimations performed in the framework of the Larkin-Ovchinnikov pinning model (LO), which is valid for magnetic field less than the irreversibility field. The competing model is considered which is also valid for field range below the irreversibility field - the anisotropic pinning model (APM). The comparison of the fundamental length scales introduced in the LO and APM models is given.