Size and surface effects in the magnetic order of CoFe2O4 nanoparticles

B. PIANCIOLA; E. LIMA JR.; H.E. TROIANI; LUIZ C. C. M. NAGAMINE; R. COHEN; R.D. ZYSLER

JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2015 vol. 377 p. 44 - 51

0304-8853

In this work, we have focused on the size dependence of the magnetic properties and the surface effects of CoFe2O4 nanoparticles synthesized by high-temperature chemical method with diameter d ~ 2, 4.5, and 7 nm, with narrow size distribution. transmission electron microscopy (TEM) images and X-ray diffraction (XRD) profiles indicates that samples with 7 and 4.5 nm present a high crystallinity while the 2 nm sample has a poor one. We have investigated by magnetization measurements (as function of applied field and temperature) and in-field Mössbauer spectroscopy the influence of the surface in the internal magnetic order of the particles. Particles with d = 7 nm have almost single domain behavior and the monodomain occupies approximately the whole particle. In the sample with d = 4.5 nm the surface anisotropy is large enough to alter the ferrimagnetic order in the shell of the particle. Then, a surface/volume ratio of ~60% is the crossover between a single domain nanoparticle and a frustrate order in a magnetic core-shell structure, due to the competition between surface anisotropy and exchange interaction + crystalline anisotropy in cobalt ferrite. In the sample with d = 2 nm, the poor crystallinity and the large surface/volume ratio avoid the ferrimagnetic order in the particle down to T = 5 K.