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

• Positive exchange bias in Ni3Al–NiO core-shell nanoparticle system.
• M–H loop center shifts in the direction of the cooling magnetic field.
• Exchange bias has the same sign as that of the cooling magnetic field.
• This type of exchange bias is not accompanied by the training effect.
• A direct relation exists between the exchange bias field and vertical loop shift.

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, Hcool, regardless of the sign and magnitude of Hcool. However, for weak (≲ 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 Hcool. This kind of positive EB is not accompanied by the training effect but by the vertical shift in the hysteresis loops, ME. 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, HE, and −ME.