Effects of annealing on the magnetic properties, size and strain of gold-coated Permalloy nanoparticles

Fe20Ni80 @ Au core-shell nanoparticles have been prepared by a multi-step microemulsion technique and the influence of annealing from 200 to 500 degrees C on the particles' size, morphology and magnetic properties is reported. After annealing under dilute H-2 at 300 degrees C and above, FeNi3-structured Permalloy nanoparticles were observed in the XRD patterns with a lattice parameter of a = 3.560(2) angstrom. The sizes of the nanocomposites increased non-linearly with increasing annealing temperature, accompanied by a decrease in the expansive strain in the shells and cores. The samples were found to exhibit superparamagnetic behaviour with a ferromagnetic component that became dominant at high annealing temperatures due to an increase in the average particle diameter over the critical radius required for superparamagnetic behaviour. The saturation magnetization (M-S) of the samples taken at 300 K increased from 19 emu g(-1) for the as-prepared sample to 100 emu g(-1) for the sample annealed at 500 degrees C.