Tiny Ni–NiO nanocrystals with exchange bias induced room temperature ferromagnetism

• Water dispersible Ni/NiO nanocrystals of different diameters are obtained using Pd seeds mediated reduction.
• Exchange bias effects are seen in these particulates with amorphous NiO shells.
• Annealing to generate crystalline shells leads to strong exchange bias induced ferromagnetism at room temperature.

Ni nanocrystals coated with a thin layer of NiO with a diameter of 5.0 nm show exchange bias induced ferromagnetism at room temperature. These particulates are freely dispersible in water and were obtained by annealing Ni nanoparticles coated with a thin amorphous layer of NiO. Particulates with diameters between 5.0 and 16.8 nm are studied. Detailed magnetic measurements reveal signs consistent with strong exchange bias including elevated blocking temperatures and tangible loop shifts. The structure of the particulates are characterized by high resolution transmission electron microscopy, energy dispersive x-ray analysis and x-ray diffraction.