Surface effects in cobalt ferrite (CoFe2O4) nanoparticles as a function of size

B. PIANCIOLA; E. LIMA JR.; R.D. ZYSLER

Parma

Congreso; Joint European Magnetic Symposia JEMS2012; 2012

JEMS Society

We have systematically studied the magnetic properties of cobalt ferrite, CoFe2O4, nanoparticles in the size range 2?7 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe(acac)3 and Co(acac)2 in the presence of surfactants giving nanoparticles covered by oleic acid. TEM images and XRD diffraction patterns confirmsthat all samples are composed by crystalline nanoparticles with the spinel structure expected for the cobalt ferrite. Magnetic properties have been determined from the temperature dependence of magnetization and magnetization isotherms measurements. The samples exhibit characteristic of asuperparamagnetic system showing an increase in the blocking temperatures with the particle size, and evidence small size distribution confi rming the TEM measurements. It is observed a size dependence of the ferrimagnetic order of the ferrite. Larger particles have a bulk-type magnetic ordering while the smaller ones have broken the ferrimagnetic order behaving as paramagnetic and/ or superparamagnetic of small clusters, depending on temperature. The nanoparticles of 4.5 nm show a mixed behavior indicating that this particle size is the crossover between the system perfectly ordered and magnetically ordered. In this case the competition between exchange energy (which tends to magnetically ordered magnetic moments) and the surface anisotropy (which tends to cause magnetic disorder) are of the same order. For smaller sizes preponderance making thesurface anisotropy (magnetically disordered system) and larger sizes of energy exchange is the dominant (ferrimagnéticamente ordered system).