Size and surface effects in the magnetic order of CoFe2O4 nanoparticles

Author(s)

Pianciola, Betiana N. ^{[1, 2]} ; Lima, Jr., Enio ^[2] ; Troiani, Horacio E. ^{[1, 2]} ; Nagamine, Luiz C. C. M. ^[3] ; Cohen, R. ^[3] ; Zysler, Roberto D. ^{[1, 2]}

Affiliation

^[1] Inst Balseiro SC, RA-8400 San Carlos De Bariloche, Rio Negro - Argentina

^[2] CONICET SC, CNEA, Ctr Atom Bariloche, RA-8400 San Carlos De Bariloche, Rio Negro - Argentina

^[3] Univ Sao Paulo, Inst Fis, BR-05508090 Sao Paulo - Brazil

• Document type: Journal article
• Source: Journal of Magnetism and Magnetic Materials; v. 377, p. 44-51, MAR 1 2015.
• Web of Science Citations: 17

Abstract

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 similar to 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 and in held Mossbauer 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 particle shell. Then, a surface/volume ratio of similar to 60% is the crossover between a single domain nanoparticle and a frustrated order in a magnetic core -shell structure, due to the competition between surface anisotropy and exchange interaction + crystalline anisotropy in cobalt ferrite. In the d=2 nm sample the poor crystallinity and the large surface/volume ratio avoid the ferrimagnetic order in the particle down to T=5 K. (C) 2014 Elsevier B.V. All rights reserved. (AU)