EVIDENCE FOR DOMAIN FORMATION IN ULTRATHIN NI CU(001) FILMS NEAR THE CURIE-TEMPERATURE/

Perpendicular magnetized Ni/Cu(0 0 1) films 8-10 monolayer (ML) thick are studied via the polar magneto-optic Kerr effect (MOKE) near the Cu rie temperature T-c in ultrahigh vacuum. An unusual sharp drop of the remanent magnetization M-r at a temperature T-r is measured. A large d ifference of about 10 K is found between T-r and the temperature T-H, at which the magnetization M(H) in a static field of 20 G vanishes. Be tween T-r and T-H saturation fields are only a few G large. These effe cts cannot originate from superparamagnetic or two-dimensional behavio ur because of the morphological homogeneity and the large thickness of the samples. It is demonstrated that a power-law fit to M-r(T) would yield an erroneous T-c and a wrong critical exponent beta. We propose that at T-r the films break up into domains and that T-c has to be pla ced at a higher temperature close to T-H. (C) 1998 Elsevier Science B. V. All rights reserved.