Strongly exchange coupled inverse ferrimagnetic soft/hard, Mn <inf>x</inf>Fe <inf>3-x</inf>O <inf>4</inf>/Fe <inf>x</inf>Mn <inf>3-x</inf>O <inf>4</inf>, core/shell heterostructured nanoparticles

Inverted soft/hard, in contrast to conventional hard/soft, bi-magnetic core/shell nanoparticles of Mn xFe 3-xO 4/Fe xMn 3-xO 4 with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (∼0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft/hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft/hard core/shell nanoparticles can be fine tuned to match specific applications. © 2012 The Royal Society of Chemistry.