Magnetic and structural properties of ferrihydrite/hematite nanocomposites

A rich variety of ferrihydrite/hematite nanocomposites (NCs) with specific size, composition and properties were obtained in transformation reactions of 2-line ferrihydrite. Transmission electron microscopy (TEM) observations showed that the NCs consist of clusters of strongly aggregated nanoparticles (NPs) similarly to a “plum pudding”, where hematite NPs “raisins” are surrounded by ferrihydrite “pudding”. Magnetic measurements of the NCs correlate very well with TEM results; i.e., higher coercive fields correspond to greater hematite crystallite size. First order reversal curve (FORC) measurements were used for the characterization of the magnetic components of the NCs. FORC diagrams revealed that the NCs prepared at short times are composed by single domains with low coercivity, and NCs prepared at times larger than 60min exhibited elongated distribution along the Hc axis. It suggested that these samples consist of mixtures of different kinds of hematite particles, ones with low coercivity and others with coercivity greater than 600Oe. For NCs prepared at times larger than 60min, Mossbauer spectroscopy revealed the presence of two sextets, which one was assigned to fine hematite particles and other to hematite particles with hyperfine parameters near to bulk hematite. The correlation of the structural and magnetic properties of the ferrihydrite/hematite NCs revealed important characteristics of these materials which have not been reported elsewhere. •Ferrihydrite/hematite nanocomposites were prepared.•The “plum pudding” morphology of the ferrihydrite/hematite nanocomposites was found.•The FORC diagrams of ferrihydrite/hematite nanocomposites have been measured.