Journal of Magnetism and Magnetic Materials 280 (2004) 281–286 Magnetic properties in BaFe 12 O 19 nanoparticles prepared under a magnetic field Jun Wang a,b , Qianwang Chen a,b, *, Shan Che a,b a Structure Research Laboratory, University of Science and Technology of China, Hefei 230026, China b Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China Received 8 June 2003 Available online 24 April 2004 Abstract It was observed that the nanocrystallites of BaFe 12 O 19 formed at 140  C under a 0.25 T magnetic field exhibited a higher saturation magnetization (6.1 emu/g at room temperature) than that of the sample (1.1 emu/g) obtained under zero magnetic field. Both of the two approaches yielded plain-like particles with an average particle size of 12 nm. However, the Curie temperature (T c ), a direct measuring of the strength of superexchange interaction of Fe 3+ –O 2 –Fe 3+ , increased from 410  C for the nanoparticles prepared without an external field applied to 452  C for the particles formed under a 0.25 T magnetic field, which indicates that external magnetic fields can improve the occupancy of magnetic ions and then increase the superexchange interaction. This was confirmed by electron paramagnetic resonance and M . ossbauer spectrum analysis. The results present in this paper suggest that in addition to oxygen defects, surface non-magnetic layer and a fraction of finer particles in the superparamagnetic range, cation vacancies should be responsible for the decreasing of saturation magnetization in magnetic nanoparticles. r 2004 Elsevier B.V. All rights reserved. Keywords: Magnetic materials; Nanomaterials; Magnetic fields 1. Introduction In the recent decades, considerable attention has been drawn to nanoscale magnetic materials, because of both their interesting physical proper- ties and the wide range of their potential applica- tions [1–4]. It is reported that the saturation magnetization of nanoparticles was lower than that of the value of correspondent bulk sample and decreased along with the reduction of the particle size [4–8]. These would hinder some applications of nanosized magnetic materials in some technique fields, such as high-density perpendicular magnetic and mag- neto-optic recording media. Nanoparticles exhibit a large surface to volume ratio and experimental evidence for surface spin disorder has been ARTICLE IN PRESS *Corresponding author. Department of Materials Science and Engineering, University of Science and Technology of China, Anhui, Hefei 230026, China. Tel.: +86-551-3607292; fax: +86-551-3631760. E-mail address: cqw@ustc.edu.cn (Q. Chen). 0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.03.045