Room temperature superparamagnetism in ternary (Fe50Pt50)0.42Cu0.58 phase at interfaces on annealing of Fe50Pt50/Cu multilayer

The face- centered- tetragonal (FCT) or L10 phase of Fe50Pt50 (FePt) film is obtained on annealing FePt films at high temperature (∼600 °C), that usually grows as face-centered- cubic (FCC) phase. Cu as an additive up to ∼20 atomic%, which form ternary alloy FePtCu on mixing, is used to reduce the ordering temperature. We studied the formation of FePtCu ternary alloy: its structure, magnetic properties and grain growth at the interfaces of a FePt/Cu multilayer, as a function of annealing temperature (100 °C-600 °C), which is very less as compared to their melting temperatures. The study indicates the growth of a ternary alloy with composition of (FePt)0.42Cu0.58 in FCT phase on annealing the FePt/Cu multilayer at a moderate annealing temperature (∼400 °C). With a large addition of Cu at 56 atomic% in FePt, the ordering temperature reduces by ∼200 °C and form a ternary alloy with distinct magnetic properties. We observed rapid growth of ordered FCT phase with increase in crystallite size from 50 Å to 150 Å on annealing the multilayer in the range of 400 °C-600 °C. Interestingly, from macroscopic magnetization measurements we could identify the existence of a super-paramagnetic (SP) phase at room temperature with a typical particle size of ∼12 Å of ternary alloy on annealing the multilayer at 400 °C, which is much smaller than the crystallite size (∼50 Å). Annealing at 600 °C reduces the multilayer to a disordered magnetic phase. The room temperature SP nanoparticles of FePtCu alloy phase formed on annealing the multilayer at 400 °C are potential candidates in biomedical applications.