Local magnetism in granular iron/iron oxide nanostructures by phase- and site-selective x-ray magnetic circular dichroism

摘要

We present a study of the local magnetic properties of iron/iron oxide granular nanostructures by x-ray magnetic circular dichroism (XMCD). Metallic iron (α-Fe) nanoparticles, with average sizes ranging from 5 to 13 nm, are embedded in a nanocrystalline oxide matrix composed of both magnetite (Fe_3O_4) and maghemite (γ-Fe_2O_3). These granular samples were synthesized by cold compacting core-shell nanoparticles, in which a 2-3 nm-thick oxide layer surrounds the iron particles, synthesized by inert gas condensation. By exploiting the chemical selectivity and site sensitivity of XMCD, we were able to separate the magnetic contributions of the metallic core and of the two oxide phases present in the matrix and to study their behavior as a function of iron particle size and applied magnetic induction field. We detected the presence of a significant spin canting, predominantly affecting the octahedral sites of Fe in the oxide phase, and studied its dependence on the degree of structural disorder and applied magnetic induction field.