Noncollinear magnetic structure of CoFe2O4 small particles

The magnetic structure of small CoFe2O4 particles has been investigated as a function of the particle size. Samples (in the 10–100 nm size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mössbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic applied field, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect of the crystallites that make up a particle is the origin of this phenomenon. A model is proposed in which the CoFe2O4 crystallites that make up a particle consist of a core with the usual spin arrangement and a boundary surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependency of this structure is also examined.