Investigating the origin of exchange bias effect in ferromagnetic FeNi nanoparticles prepared via controlled synthesis

Abstract

In the present work, the FeNi nanoparticles have been synthesized using NaBH 4 reduced co-precipitation technique followed by annealing at 210 °C for 4 h in a simple box furnace. The nanoparticles were characterized using X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), Energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and dc magnetization. The XRD analysis revealed the formation of dual phases (FeNi 3 ; FeNi) with face-centred cubic symmetry along with the additional phases of oxonium Ni oxide (NiOOH), hexagonal-Fe and iron peroxide (FeOOH). The FE-SEM micrographs demonstrated the evolution of dumb-bell shape morphology due to the heterogeneous growth of the nuclei resulting into agglomeration of particles. The XRD and FE-SEM collectively confirmed the nanometre (nm) dimensions of the particles. The mapping performed using EDS spectra validates the elemental compositions of the Nanoparticles. The Raman spectra reiterated the presence of impurity phases in addition to the trivial amount of NiO. The M-H (field dependent magnetization) hysteresis curves, plotted under the conditions of field cooled (FC) and zero field cooled (ZFC), shifted horizontally by ~ 14Oe indicating presence of exchange bias. Alongside, FC-ZFC MT (temperature-dependent magnetization) curves also showed bifurcation at temperatures below 300 K that is also an indicative to the exchange bias. In brief, the exchange bias has been confirmed in ferromagnetic FeNi nanoparticles and the origin may be attributed to the exchange interactions at the particle interfaces having canted spins which emerge due to the presence of impurities and dual phases of FeNi.