Supermagnetism

An ensemble of nanoparticles in which the inter-particle magnetic interactions are sufficiently weak shows superparamagnetic behaviour as described by the Neel-Brown model. On the contrary, when inter-particle interactions are non-negligible, the system eventually shows collective behaviour, which overcomes the individual anisotropy properties of the particles. At sufficiently strong interactions a magnetic nanoparticle ensemble can show superspin glass (SSG) properties similar to those of atomic spin glass systems in bulk. With a further increase in concentration, but still below physical percolation, sufficiently strong interactions can be experienced to form a superferromagnetic (SFM) state. SFM domains in a non-percolated nanoparticle assembly are expected to be similar to conventional ferromagnetic domains in a continuous film, with the decisive difference that the atomic spins are replaced by the superspins of the single-domain nanoparticles. In this review we highlight the most important developments in the field of supermagnetism, which comprises three fascinating subjects: superparamagnetism, SSG and superferromagnetism.