Exchange bias in finite sized NiO nanoparticles with Ni clusters

Structural and magnetic properties of finite sized NiO nanoparticles are investigated with synchrotron X-ray diffraction (XRD), transmission electron microscopy, magnetometer and ferromagnetic resonance (FMR) spectroscopy. A minor Ni phase is detected with synchrotron XRD, attributed to the oxygen defects in the NiO core. A considerable exchange bias of ~100Oe is observed at 50K and it drops abruptly and vanishes above 150K, in association with the reduction of frozen spins. FMR data indicate a strong interaction between ferromagnetic (FM) and antiferromagnetic (AFM) phases below 150K, consistent with the picture of isolated FM clusters in AFM matrix. •Structural and magnetic properties of finite sized NiO nanoparticles are systematically investigated with several advanced techniques.•A strong interaction between ferromagnetic and antiferromagnetic phases is found below 150K.•Exchange bias field in finite sized NiO nanoparticles is due to anisotropy energy of Ni clusters over riding the domain wall energy of NiO.