Low-field magnetization study of CoFe–Al2O3 multilayers

Abstract Zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were recorded for a series of ion-beam-deposited metal–insulator CoFe( t )/Al 2 O 3 (30 A) multilayers, ranging from discontinuous insulating ( t =9 A) to continuous metallic ( t =25 A) behavior, for temperatures T from 8 to 300 K and in applied magnetic fields H from 10 to 100 Oe. Data allow the analysis of the different magnetic phases coexisting in this system. The superferromagnetic (SFM) fraction is characterized by the Curie temperature, T C , and ZFC remanence at T →0, M SFM , both being functions of the CoFe nominal thickness t . The remaining superparamagnetic (SPM) fraction starts getting blocked below a certain blocking temperature T b , which depends both on t and H . The SFM phase develops above some critical thickness t cr ≈11 A, so that M SFM ( t ) is almost linear: M SFM ∝( t − t cr ), at t cr t t p , and attains the maximum (bulk) value at t → t p ≈18 A, which is the metallic percolation threshold. In the SFM–SPM coexistence range t cr t t p , the blocking temperature T b shows a monotonic increase with thickness t but a pronounced decrease with field H . This latter effect is attributed to field-induced suppression of the energy barriers for alignment of pre-formed domains in the SFM fraction.