ABSTRACT: Magnetic nanoparticles show great promise for many applications including hyperthermic treatment of cancer, MRI contrast agents, directed drug delivery and imaging, and data storage. Magnetite, in particular, has been extensively utilized because of its biocompatible nature and resistance to oxidation, although it exhibits a well-known reduction in magnetic saturation moment with decreasing particle size. Using small-angle neutron scattering (SANS) with analysis of neutron spin rotation upon scattering, it was recently shown that the missing moments within a densely packed system of 9 nm magnetite nanospheres were not due to either surface disordering or anisotropy-induced radial canting, but rather arose from the formation of unidirectional magnetic shells canted perpendicular to their ferromagnetic cores*. At 1.25 Tesla these shells could be varied in thickness from 1.0 to 1.5 nm at 160 K and 320 K, respectively. However, the shells disappeared altogether when both the external field was removed and the inter-particle correlations were minimized (300 K), confirming their magnetic, rather than structural, origin. Additionally the shells displayed inter-particle “cross-talk”, selecting a common orientation over clusters of tens of nanoparticles. I will discuss these results and propose a possible inter-particle coupling mechanism that could give rise to this unusual morphology within densely packed arrays.
*Physical Review Letters 104, 207203 (2010)