Observation of magnetic flux pinning in a thin Nb film with a square lattice of nickel dots

Abstract

The pinning of magnetic flux by a square lattice of ferromagnetic Ni dots embedded in a superconducting Nb film was investigated. In the case of demagnetized Ni dots, both AC susceptibility and DC magnetization data exhibit clear anomalies at the first and second matching fields, H 1≈13 Oe and H 2=2 H 1, respectively. The second matching anomaly is clearly observed in both AC and DC data only when a remanent magnetization of Ni dots is induced along the external magnetic field, which is oriented perpendicular to the Nb film plane. The observed behavior supports theoretical predictions of a substantial contribution to the flux pinning energy by anisotropic interactions between the superconducting condensate of Nb and the Ni dot magnetization.