Periodic magnetization instabilities in a superconducting Nb film with a square lattice of Ni dots

Isothermal magnetization curves of a superconducting Nb film perforated by a square lattice of Ni dots exhibit quasiperiodic instabilities below simil ar to 4 K, with a field-dependent period equal to the first, second, or thi rd matching fields. The instabilities are found in a range of applied field s well above the saturation matching value, and where a Nb film with the sa me dimensions and magnetization remained stable. The recovery of matching e ffects well below T-c similar to 8.8 K is unexpected, because strong flux g radients generated by the geometric barrier and random pinning suppress the formation of large domains of matched flux lines and dots. The results sug gest that terraces of matched flux density may exist at the border of a flu x-free zone near the film edge. (C) 2000 Elsevier Science B.V. All rights r eserved.