Surface and exchange anisotropy fields in MnFe2O4 nanoparticles: Size and temperature effects

Angular measurements of magnetic resonance are used to investigate the surf ace anisotropy field as well as the exchange anisotropy field in spherical MnFe2O4 magnetic nanoparticles as a function of temperature and particle di ameter (D). The resonance field is a combination of angular dependent and a ngular independent fields, both affected by the surface anisotropy field, w hich in turn follows a D-alpha power law, with alpha very close to unity. T he angular dependent component probes the surface anisotropy field while th e angular independent component probes the exchange anisotropy field. In th e temperature range from 100 to 250 K a negative surface anisotropy field i s found, which increases as the particle size is reduced, indicating a radi al orientation of the spins at the MnFe2O4 nanoparticle surface. (C) 1999 A merican Institute of Physics. [S0021-8979(99)04510-7].