Bi-Magnetic Iron(III) Oxide Nanocrystals Embedded in MCM-41 Mesoporous Silica

This is a study on the magnetism of a Fe 2 O 3 @MCM-41 composite made up of iron(III) oxide nanocrystals of average length size of 4 nm embedded in the channels of a MCM-41 mesoporous matrix. Through Mössbauer spectroscopy, X-ray absorption experiments and static and dynamic magnetic measurements, we identify the guest iron oxide entities as composed by exchange-coupled hematite (weak ferromagnetic WF) and maghemite (ferrimagnetic FiM) arranged in a string of beads configuration. The WF/FiM nanocrystals behave as independent particle moments that block as a whole below T B = 17 K. The log-normal energy barrier distribution function determined from relaxation measurements shows a maximum at an energy E = 200 K. We explore to what extent the contact between domains of the two coexisting oxides controls the magnetism of this biphasic nanosystem.