Size and surface effects in the magnetic order of CoFe₂O₄ nanoparticles

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 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 particle shell. Then, a surface/volume ratio of ∼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. - Highlights: • This article analyzes the results of magnetization and Mössbauer spectroscopy measurements of cobalt ferrite (CoFe₂O₄) nanoparticles with different sizes. • Three samples with different sizes (and narrow size distribution) from 2 nm to 7 nm have been studied. • A clear evidence of the existence of magnetic disorder at the particle surface in 2 nm and 4.5 nm nanoparticles is reported. • The surface effect in this magnetic disorder as a function of nanoparticle size is discussed