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

Abstract

Fe20Ni80@Au core–shell nanoparticles have been prepared by a multi-step microemulsion technique and the influence of annealing from 200 to 500 °C on the particles’ size, morphology and magnetic properties is reported. After annealing under dilute H2 at 300 °C and above, FeNi3-structured Permalloy nanoparticles were observed in the XRD patterns with a lattice parameter of a = 3.560(2) Å. 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 (MS) 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 °C.

Synthesis and characterization of permalloy nanostructured films by deposition of laser ablated nanoparticles

“…The saturation magnetization at 30 kOe (M s ) is 62.3 emu/g for the nanostructured film whereas the feedstock MP has a M s of 94 emu/g. This suggests that about 66% of the mass of the film contributes to M s at 300 K. A similar result showing a decrease in M s with decreasing particle size was observed in Au-coated permalloy NPs [10], in Ni-ferrite NPs [11], and was explained using a size-dependent cohesive energy model [12].…”

“…The zero field cooling curve (ZFC) exhibits a broad peak centered at the blocking temperature, T b , of $250 K. Above this temperature, the nanostructured films exhibit evidence of superparamagnetic behavior [2,9]. The broad peak and high blocking temperature are consistent with the broad size distribution observed in TEM since the blocking temperature is known to increase with particle size [10].…”

“…Instead, we attribute the increase in coercivity to the observed broad size distribution of NPs since it is known that the coercivity for single domain ferromagnetic NP that are too large to be superparamagnetic increases as the particle size increases whereas the coercivity decreases for larger multidomain NPs and MPs. The result is maximum in coercivity for permalloy for particles of about 10-30 nm [10]. Thus, the presence of crystallites within our films in this size range likely led to the increased coercivity we measured relative to the feedstock particles.…”

“…Higher coercivities in films than in powders of the same composition can arise from anisotropy [13,14] that results from residual stress or from substrate orientation effects [15]. However, previous measurements of residual stress in LAMA-produced films did not show evidence of significant residual stress or crystallographic texture [16] and measurements in Au-encapsulated permalloy NP showed virtually no dependence of the coercivity on residual stress [10]. Thus, it is unlikely that these were the causes of the increased coercivity.…”

Effect of Aspect Ratio and Temperature on Magnetic Properties of Permalloy Nanowires

“…12 13 Since last decade many magnetic properties studies have been done on the permalloy nanostructures (nanowire, nanotube, nanoparticle etc.). [14][15][16][17][18][19][20][21] Still aspect ratio and temperature dependent magnetic properties of permalloy NWs has not done yet. It is still not studied that how magnetic easy axis of permalloy nanowires depends upon the temperature and the aspect ratio.…”