Magnetic enhancement in antiferromagnetic nanoparticle of CoRh₂O₄

Abstract

Our magnetic investigation of CoRh₂O₄ nanoparticles show the signature of antiferromagnetic ordering at T_N≈ 27K, same as that of the bulk sample, for all particle sizes down to ≈16nm. However, we observe a systematic magnetic enhancement below T_N, being larger for a smaller particle size. We propose a core-shell model for the magnetization of nanoparticles, with the core retaining its antiferromagnetic order and the enhancement of magnetization below T_N attributed to the increasing number of frustrated shell (surface) spins. The scaling analysis of low temperature magnetization shows that the system approaches the limit of superparamagnetism for the particle size ~8nm. Our model also predicts an alternation of exchange interactions along the inter-particle distance, very similar to RKKY type interactions in the metallic spin glass systems.