Ferromagnetism in assemblies of superparamagnetic nanoparticles: theory and applications

We develop a theoretical framework to create an effectual ferromagnet from aggregates of superparamagnetic nanoparticles. In the limit of strong anisotropy, the super moments can be treated as a collection of two-state Ising spins. When adequate in number, they interact via dipole-dipole coupling to produce a dipolar field and subsequently a permanent dipole moment. As a result, these assemblies exhibit hysteresis on the application of an oscillating magnetic field. Their important physical application is in remedial procedures for destroying tumor and cancer cells. Our framework provides procedures for heat manipulation in them and imparts a basis to the often used ad-hoc methodologies in this subject. Further, it explains experimental observations reporting manifestations of ferromagnetism in aggregates of superparamagnetic nanoparticles which have puzzled experimentalists in the recent years.