Surface spin-glass freezing and interparticle interactions in ferrite nanoparticles
The magnetic phases of surface and core spins of small-sized nanomagnets are in the focus of current intense research. The magnetic order of the surface spins of a nanoparticle influences substantially the magnetic properties of the entire particle. The surface to volume ratio is very large in small sized nanoparticles and more atoms reside on the surface of the nanoparticles. The surface atoms have coordination bonds on the inner side (core) of the particle but no bonds on the outer side, therefore making dangling bonds on the surface of the nanoparticle. The giant core spin gets spontaneously ordered at rather high temperatures as compared to the freezing of disordered surface spins. At low temperatures, the interaction between the surface spins and core spins at the interface leads to frustration and disorder on the nanoparticle surface. In ferrite nanoparticles, surface disorder is dominated by bond frustration of the exchange interaction between ferrimagnetically coupled spins of different sublattices near the surface. As the particle size is reduced, the phenomenon of disorder and frustration at the nanoparticle’s surface becomes progressively dominant with a tendency to form the spin-glass phase. In addition, nanoparticle magnetism is also strongly influenced by interparticle dipolar and – in the case of “dense” nanomagnets – by exchange interactions across grain boundaries.