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

•A facile laser ablation method was developed to synthesize Mn3O4 and Mn3O4/α-Mn nanoparticles.•A large coercivity up to 1.17 T was observed in the Mn3O4/α-Mn nanoparticles.•An exchange bias effect was observed in the Mn3O4 and the Mn3O4/α-Mn nanoparticles. 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.