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

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

Buenos Aires

Workshop; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications (X LAW3M); 2013

LAW3M

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 difraction patterns confirms that 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 a superparamagnetic system showing an increase in the blocking temperatures with the particle size, and evidence small size distribution confirming the TEM measurements. It is observed a size dependence of the ferrimagnetic order of the ferrite by magnetic measurements and Mössbauer spectroscopy. 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 the magnetically disorder. 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 the surface anisotropy (magnetically disordered system), which is clearly observed in in-field Mössbauer spectroscopy, and larger sizes of energy exchange is the dominant (ferrimagnetically ordered system). In-field Mössbauer spectroscopy confirms the ferrimagnetic order and the fact that the Fe ions are preferentially substituted by Co ones in site B.