Large coercivity and exchange bias in Mn3O4 nanoparticles prepared by laser ablation method

A facile and easily reproducible laser ablation method has been developed to synthesize high quality Mn₃O₄ nanoparticles from Mn pieces in ambient air. By changing the laser ablation air pressure to ~20 Pa, a mixture of α-Mn and Mn₃O₄ composite nanoparticles were obtained. Most of the Mn₃O₄ nanoparticles are spherical in shape size ranging from 20 to 90 nm. Besides spherical shape, a considerable amount of α-Mn/Mn₃O₄ nanoparticles show irregular shape and aggregation. The oxide nanoparticles exhibit large coercivity up to 1.05 T, which is almost four times to that of the Mn₃O₄ single crystals, owing to size effect. The coercivity was further enhanced to 1.17 T in α-Mn/Mn₃O₄ nanoparticles, owing to the additional interfacial effect between α-Mn and Mn₃O₄ besides size effect. The presence of the antiferromagnetic/ferrimagnetic α-Mn/Mn₃O₄ interface results in an exchange bias field up to ~65 mT in the nanoparticles.