Reversible aggregation and magnetic coupling ofα-Fe2O3 nanoparticles

Abstract

The magnetic properties of nanoparticles, including their superparamagnetic relaxation and spinorientation, have been found to depend on the aggregation state due to magnetic exchange couplingbeing established between surface atoms of neighbouring particles. We show that for samples ofα-Fe2O3 nanoparticles the agglomeration and the interparticle exchange coupling are reversibleprocesses. Grinding or ultrasonic treatment of aggregated particles can disperse theparticles and reduce the magnetic coupling, while drying aqueous suspensions of dispersedparticles leads to aggregation and re-establishment of magnetic coupling. Theestablishment of exchange coupling between neighbouring particles gives evidence foroverlapping electronic orbitals of surface atoms of neighbouring particles. This is importantnot only for understanding the magnetic properties but also for understanding otherphysical properties e.g. the mechanical properties of dried aggregates. The reversibility(and the decoupling of particles) provides information on the strength of thecoupling.