Size-dependent scaling of exchange bias in NiFe2O4/NiO nanogranular systems synthesized by a phase separation method

Abstract

Exchange bias (EB) effect has been studied in a series of nanogranular systems of ferrimagnetic (FiM) NiFe2O4 nanoparticles embedded into antiferromagnetic (AFM) NiO matrix, synthesized by a phase pprecipitation method from diluted Ni(1−x)FexO3 (x = 0.09) oxides. For these systems, the crystalline size (DNFO) of NiFe2O4 ranging from ∼3 nm to ∼55 nm has been obtained with thermal treated at different temperatures from 550 °C to 1000 °C. Magnetization measurement shows that both exchange bias field (HEB) and vertical magnetization shifts (MShift) can be exhibited below 250 K after field cooling procedure. The HEB and MShift decrease monotonically with crystalline size, and their behavior strongly depend on the crystalline size of NiFe2O4 nanoparticles. Linear relationship between HEB and MShift is observed for systems with smaller sizes (DNFO ≤ 8 nm), reveals a straightforward correlation between them. This phenomenon is ascribed to the interfacial exchange coupling between FiM NiFe2O4 clusters and spin-glass-like (SGL) phases, where the frozen uncompensated spins in SGL phases play critical role of inducing EB effect. As DNFO is above 12 nm, the dependence of HEB on MShift deviates from the linear relationship, which is discussed in terms of the superimposed contribution from the exchange coupling between FiM NiFe2O4 core with the SGL phase, and the exchange coupling between FiM NiFe2O4 core and AFM NiO phases at the interfaces.