Low-field magnetization study of CoFe–Al 2O 3 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 2O 3(30 Å) multilayers, ranging from discontinuous insulating ( t=9 Å) to continuous metallic ( t=25 Å) 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 Å, 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 Å, 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.