Magnetic Properties of Fe3O4 Nanoparticles: Effects of Morphology and Dipolar Interparticle Interaction.

ENIO LIMA JR.; H R RECHENBERG; J M VARGAS; R COHEN; R D ZYSLER

Rio de Janeiro, Brasil

Workshop; VIII Latin Workshop on Magnetism, Magnetic Materials and Their Applications; 2007

We report changes in the internal magnetic order of Fe_3O_4 nanoparticles induced by thermal treatment, which increases the crystallinity degree of the particles, and by the dipolar interparticle interaction. Monodispersive Fe_3O_4 nanoparticles were prepared by high-temperature solution phase reaction of Fe(acac)_3 with 1,2 dodecanodiol in the presence of oleic acid and oleylamine. According to transmission electron micrographs, as-prepared sample is composed by nanoparticles with a narrow grain size distribution with mean diameter of = 9.3 nm. Moreover, as-prepared particles are constituted by a ordered core and a structurally disordered shell (~ 50 % vol.). M(H,T) measurements and In-field M\"ossbauer (up to 120 kOe) analysis clearly indicates that surface effects play the fundamental rule in the magnetic properties of as-prepared sample. In another way, annealed sample (30 min./573 K at Ar) are composed by well crystalline nanoparticles ( = 6.3 nm) that exhibit a magnetic behavior dominated by magnetocrystalline anisotropy, reflecting the very high crystallinity degree induced by the thermal treatment. Therefore, our results show that Fe_3O_4 nanoparticles about the same diameter but with distinct morphology present very different magnetic behavior. Regarding the effects of dipolar interparticle interactions, both samples were studied in two very distinct concentrations: dispersed (weakly-interacting) and powder (interacting) samples. Our results indicate that for as-prepared sample, dipolar interparticle interactions induce a magnetic short range order in the disordered surface region at the same time that the shell region reduces the effects of the dipolar interaction on the energy barrier of the particles. In another way, for annealed interacting sample, interactions promote an increase of the energy barrier of annealed particles as predicted by models presented in literature.