Synthesis and anomalous magnetic properties of hexagonal CoO nanoparticles

CoO nanoparticles in the 38–93 nm range have been prepared by thermal decomposition. The particles were characterized to be pyramid shape with a hexagonal close-packed structure. Their anomalous magnetic behavior includes: (i) vanishing of antiferromagnetic transition around 300 K; (ii) creation of hysteresis below a blocking temperature of 6–11 K; (iii) presence of relatively large moments and coercivities accompany with specific loop shifts at 5 K; and (iv) appearance of an additional small peak located in low field in the electron spin resonance spectrum. Further, the present results provide evidence for the existence of uncompensated surface spins. The coercivity and exchange bias decrease with increasing particle size, indicating a distinct size effect. These observations can be explained by the multisublattice model, in which the reduced coordination of surface spins causes a fundamental change in the magnetic order throughout the total CoO particle.

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► The as-synthesized CoO nanoparticles are of pyramid configuration with hcp structure.
► The hexagonal CoO particles do not exhibit antiferromagnetic transition around 300 K.
► The CoO particles have relative large saturation magnetization and coercivity at 5 K.
► The shift of hysteresis loops is consistent with the result of multisublattice model.
► The particles contain intrinsic antiferromagnetic structure and uncompensated spins.