Martensitic transformation in Fe–Ni nanoparticles prepared by a sol–gel process

Abstract

The Fe–29.5wt.%Ni, Fe–33.0wt.%Ni, Fe–35.9wt.%Ni nanoparticles with narrow size distribution were prepared via a sol–gel process followed by a reduction treatment. The MT (martensitic transformation) of Fe–Ni nanoparticles was studied by Magnetic Property Measurement System (MPMS) and X-ray Diffraction (XRD). It was found that a considerable amount of low temperature martensite in Fe–29.5wt.%Ni and a small amount of martensite in Fe–33.0wt.%Ni nanoparticles were detected by XRD at room temperature, while martensite was not detected in the Fe–35.9wt.%Ni nanoparticles, showing apparently compositional dependence of MT in Fe–Ni nanoparticles. Moreover, the Ms (defined as turning point of the increasing rate of magnetic susceptibility [1]) of Fe–33wt.%Ni nanoparticles with an average particle size of 19.4nm was tested as high as 246K by both MPMS and XRD, which was nearly 100K higher than the Ms of counterpart Fe–33wt.%Ni bulk specimen. The possible reasons for high Ms of nanoparticles were discussed.