Magnetic hysteresis loop shift in NiFe2O4 nanocrystalline powder with large grain boundary fraction

Magnetic properties of nanocrystalline NiFe2O4 spinel mechanically processed for 350 h have been studied using temperature dependent from both zero-field and in-field 57Fe Mössbauer spectrometry and magnetization measurements. The hyperfine structure allows us to distinguish two main magnetic contributions: one attributed to the crystalline grain core, which has magnetic properties similar to the NiFe2O4 spinel-like structure (n-NiFe2O4) and the other one due to the disordered grain boundary region, which presents topological and chemical disorder features (d-NiFe2O4). Mössbauer spectrometry determines a large fraction for the d-NiFe2O4 region (62% of total area) and also suggests a speromagnet-like structure for it. Under applied magnetic field, the n-NiFe2O4 spins are canted with angle dependent on the applied field magnitude. Mossbauer data also show that even under 120 kOe no magnetic saturation is observed for the two magnetic phases. In addition, the hysteresis loops, recorded for scan field of 50 kOe, are shifted in both field and magnetization axes, for temperatures below about 50 K. The hysteresis loop shifts may be due to two main contributions: the exchange bias field at the d-NiFe2O4/n-NiFe2O4 interfaces and the minor loop effect caused by a high magnetic anisotropy of the d-NiFe2O4 phase. It has also been shown that the spin configuration of the spin-glass like phase is modified by the consecutive field cycles, consequently the n-NiFe2O4/d-NiFe2O4 magnetic interaction is also affected in this process.