Extraordinary Ferromagnetic Coupling and Magnetodielectric Phenomena in NiO Nanoparticles

We intend to understand the ferromagnetism and magnetodielectric (MD) phenomenon in NiO nanoparticles (NPs) with average diameter ~23 nm synthesized by a chemical route. Magnetic studies revealed that NiO NPs are weakly ferromagnetic (FM). Notably, temperature-dependent magnetization studies show a bifurcation of the FC-ZFC curves, suggesting a competition between FM and antiferromagnetic (AFM) interactions with the blocking temperature (TB) at ~210 K. In addition, we observed field-dependent exchange bias effect in the NPs. Furthermore, MD studies revealed that the changes in the dielectric constant and loss induced by the magnetic field are strongly frequency dependent, which originates from the combined effect of extrinsic Maxwell-Wagner polarization along with magnetoresistance. It was found that the magnetoimpedance (MI) changes sign from negative at low frequencies to positive at higher frequencies of an excitation signal. Cole-Cole studies showed that the negative MI at low frequencies arises from FM surface/grain boundary, and positive MI at higher frequencies originates from AFM core of NiO NPs.