Structural and magnetic characterization of ion-beam deposited nickel-iron/nickel-iron-oxygen composite films

Composite films of Ni₈₀Fe₂₀/Ni_xFe _1−xO were prepared by a dual ion-beam deposition technique. The structural, magnetic properties and transport properties of these composite films were studied. The pure Permalloy (Ni₈₀Fe₂₀) films consist of f.c.c. nanocrystallites while filrns prepared with 55% O2 in the assist beam consist of rock-salt structure Ni_xFe_1−x O solid solution. The rock-salt phase undergoes an unusual dissociation reaction from Ni_xFe_1−xO into Ni, NiO, and Ni ferrite after annealing. The pure Permalloy films exhibit soft magnetic properties while a magnetic hardening with increased coercivity was observed in the Ni ₈₀Fe₂₀/Ni_xFe_1−xO composite films. The enhanced He reveals the characteristic exchange coupling between metal and oxide. It has been demonstrated that this exchange-bias effect is dominant at low temperature as well as in films with higher oxygen content. The suppression of Curie temperature (Tc) was observed in both pure Permalloy and Ni ₈₀Fe₂₀/Ni_xFe_1−xO composite films. The Tc depression was reduced by annealing.; Electrical resistivity results suggest the existence of a percolation conduction mechanism for these composite films. Properties typical of anisotropic magnetoresistance (AMR) were observed in as-deposited and annealed samples. One important result is that for annealed samples, the shape of the MR curve is sharp and symmetric and the total MR ratio increases with oxygen content in the films, in contrast to as-deposited samples where the MR ratio decreases. Further, the total MR ratio of these annealed samples reaches a maximum (∼4.5% and 14% at 298 K and 77 K, respectively) in filrns prepared with 44% O ₂ in the assist beam. This enhanced AMR is attributed to the strong anisotropic scattering at interfaces between ferromagnetic metal and antiferromagnetic or ferrimagnetic oxide.