Magnetic properties of nanocrystalline iron oxide/amorphous manganese oxide nanocomposite thin films prepared via photochemical metal-organic deposition

Maghemite (gamma-Fe2O3) nanocrystals have been prepared by the thermal decomposition of metal-organic precursors in the presence of oleic acid. These nanocrystals were characterized by transmission electron microscopy, SQUID magnetometry, and Mossbauer spectroscopy. Samples of maghemite nanocrystals with a median diameter of 4 nm were found to be superparamagnetic, with a blocking temperature of ca. 36 K. These nanocrystals were encapsulated in an amorphous manganese oxide matrix by means of photochemical metal-organic deposition. In this process, solutions containing alpha-Fe2O3 nanocrystals and manganese(II) 2-ethylhexanoate were spin coated on silicon substrates. Upon irradiation, the photosensitive manganese precursor undergoes decomposition, leaving a manganese oxide thin film containing dispersed nanocrystals. Photoresist-free lithographic deposition of micron-sized features composed of the alpha-Fe2O3 nanocrystals/amorphous manganese oxide nanocomposite was carried out. The magnetic properties of the nanocomposite thin films were measured, and shown to bear differences with the bulk nanocrystals. In particular, the blocking temperature is lowered with increased nanocrystal dilution. The magnetic properties of the system are discussed in terms of clusters of strongly interacting superparamagnetic nanocrystals.