Controlling the magnetic properties of spinel ferrite nanoparticles by chemical tuning

Magnetic nanoparticles, MNP, are expected to have a tremendous impact on several technological fields, ranging from electronics, to spintronics and clinical applications, one of the most prominent reasons relying on the possibility of fine tuning their physical properties to match the required optimal values. The feasibility of such a control requires the exact knowledge of the effect of the size, morphology, structure and chemical composition on the magnetic properties of the final product. However, large piece of information is still missing, mostly because of the difficulty in controlling independently each of the above parameters. However, the recent development of wet-chemistry syntheses, allowing for a tighter control on the MNP's characteristic, has boosted a renewed interest in the field. In this contribution we present a deep investigation on the magnetic properties of a family of doped ferrite MNPs of formula CoxNi1-xFe2O4 whose magnetic anisotropy was systematically varied by controlling the relative amount of divalent ions (0?x?1). TEM and XRD analysis showed all the samples comprise highly crystalline, monodisperse, spherical particles with average size of 6 nm. Temperature and field dependent magnetic measurements displayed the strong dependence of the magnetic properties on the chemical composition; indeed, on rising x a high increase of both magnetic anisotropy and magnetization saturation is observed. Magneto-optical spectra, XMCD Fe, Co and Ni L-edge and high-field temperature variable Mossbauer spectra allowed to explain the observed behavior, not only in terms of the intrinsic magneto-crystalline anisotropy of the cationic species involved, but also with their distribution between Td and Oh cavities in the spinel structure. In addition, a spin canted structure which becomes less prominent with increasing the Ni content was evidenced. Altogether these results underline the large tunability of the physical properties of this appealing class of nanomaterials. Research funded by project MAGNANO (INSTM-Regione Lombardia).