Novel positive exchange bias effect in compacted Ni3Al/NiO core/shell nanoparticles

We have observed an unusual type of positive exchange bias (EB) in polycrystalline Ni3Al-NiO core-shell nanoparticle system in that the M-H loop center shifts in the same direction as that of the cooling magnetic field, H-cool, regardless of the sign and magnitude of H-cool. However, for weak (less than or similar to 50 Oe) negative cooling fields, as the temperature is raised, a crossover from positive to negative EB occurs at a temperature, which depends on the cooling temperature and H-cool. This kind of positive EB is not accompanied by the training effect but by the vertical shift in the hysteresis loops, M-E. A simple model of single-domain ferromagnetic particles embedded in a disordered antiferromagnetic matrix is shown to provide a straightforward explanation for the direct relation observed between the EB field, H-E, and -M-E. (C) 2016 Elsevier B.V. All rights reserved.