Interplay between microstructure and magnetism in NiO nanoparticles: breakdown of the antiferromagnetic order

3d metal oxides NPs with different morphologies are good candidates for a number of applications due to the size-dependent magnetic behaviours and the possibility of tuning the magnetic response and/or the coating with a functional layer. To study the relationship between microstructure and magnetic properties at the nanoscale, three samples with controlled sizes (2.5, 4 and 9 nm) of NiO NPs were prepared. Their crystal structure and microstructure were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) techniques. On the other hand, the magnetic properties were studied through the temperature and magnetic field dependences of the magnetization.

The EXAFS (extended X-ray absorption fine structure) spectra allowed proving that surface effects became more relevant as the particle size was reduced, even if crystallinity still remained.

 

In addition, the results provided by all the different techniques reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order,  typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni²⁺  magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, wheres larger NPs are formed by an uncompensated AFM core with a net magnetic mo

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