Reversible aggregation and magnetic coupling of α-Fe2O3 nanoparticles

The magnetic properties of nanoparticles, including their superparamagnetic relaxation and spin orientation, have been found to depend on the aggregation state due to magnetic exchange coupling being established between surface atoms of neighbouring particles. We show that for samples of alpha-Fe2O3 nanoparticles the agglomeration and the interparticle exchange coupling are reversible processes. Grinding or ultrasonic treatment of aggregated particles can disperse the particles and reduce the magnetic coupling, while drying aqueous suspensions of dispersed particles leads to aggregation and re-establishment of magnetic coupling. The establishment of exchange coupling between neighbouring particles gives evidence for overlapping electronic orbitals of surface atoms of neighbouring particles. This is important not only for understanding the magnetic properties but also for understanding other physical properties e. g. the mechanical properties of dried aggregates. The reversibility (and the decoupling of particles) provides information on the strength of the coupling.