Influence of microstructure on exchange bias in Ni–NiO composites

Ferromagnetic–antiferromagnetic bulk composites, which represent an interesting soft/brittle composite system, have been synthesized by high-pressure torsion (HPT) of powder blends consisting of Ni and NiO with and without prior ball milling. The deformation causes a strong reduction of the Ni and NiO phase sizes in the ultra-fine-grained and nanocrystalline regime. Magnetic coupling between Ni and NiO and a shift in the magnetic hysteresis (exchange bias) has been observed for all deformed composites. Depending on the composition and the deformation parameters, different final microstructures are obtained. The morphology of the Ni and NiO phase has an influence on the magnetic properties and the magnitude of the exchange bias. The highest values of exchange bias are observed in Ni–NiO composite with a homogeneous microstructure.