G. Balaji

Magnetic properties of interacting single domain Fe3O4 particles

Abstract The present study investigates magnetic properties of interacting 12.4 nm Fe3O4 spinel ferrite particles. Fe3O4 crystallizes in cubic structure with lattice parameters, a=0.839(1) nm. Ultrafine nature of the materials were ascertained by the X-ray diffraction (XRD) line broadening and field dependent magnetic analysis. The 57Fe Mossbauer spectrum is deconvoluted to two sextets indicating two different Fe sites and a central doublet indicating superparamagnetic fractions present. The saturation magnetization (at 298 K), σs=67.8 emu/g, is less than that of the bulk magnetic particles, i.e. σs (bulk)=92 emu/g. The reduction of σs in Fe3O4 particles is attributed to the presence of non-magnetic layer at the particle surface, cation distribution, superparamagnetic relaxation and spin canting because of the ultrafine nature of the material. The low field temperature dependence of magnetization below the Curie temperature, Tc, shows two distinct peaks, i.e. at 615 and 800 K. The maximization in magnetization near Tc and the decrease in Tc are attributed to a large degree of inversion of the Fe3O4 particles. A peak at 615 K in low field thermomagnetic studies on the zero field cooled (ZFC) samples confirms the dipolar interactions to be dominant over superparamagnetic blocking.

Magnetic Properties of Nanostructured MnFe2O4 Synthesized by Precursor Technique

Nanostructured MnFe 2 O 4 particles in the size range 7-30 nm are prepared by the thermal decomposition of manganese iron citrate precursor at different temperatures (623-973 K) in an inert atmosphere. The Curie temperature (T C ) of the as-prepared MnFe 2 O 4 nanosized particles is found to be higher than the bulk material by 70 K. This is explained on the basis of non-equilibrium cation distribution in our samples. The heat treatment of the particles at 673 K changes the cation distribution to an equilibrium state and correspondingly T C is modified. We observe that saturation magnetization decreases with decrease in the size of the particles and this is explained by a magnetic dead layer present on the surface of the nanostructured particles.

Enhancement of hyperfine fields for iron atoms in γ'-Fe4-xNixN (0.2 ≤ x ≤ 0.8) compounds

The effect of Ni substitution on hyperfine fields of the iron atoms in the Fe4N is investigated. Nanocrystalline nature of γ′-Fe4−xNixN materials was examined by XRD and SEM micrographs. The Mössbauer spectra were modeled for hyperfine field distributions. Hyperfine fields for the iron atoms, (Fec) situated at the corner positions of the fcc lattice, increase dramatically with the Ni atom substitutions and the results are explained by electronic structure and the presence of local inhomogeneities.

Mössbauer and magnetic studies of MFe 2 O 4 (M=Co, Ni) nanoparticles

Nanocrystalline MFe2O4 (M=Co, Ni) particles are synthesized by citrate precursor technique. Mossbauer and magnetic studies are carried out with the CoFe2O4 samples having particle sizes of 9, 14 and 30 nm and the NiFe2O4 samples having particle sizes of 9, 21 and 30 nm. The intrinsic magnetic parameters are found to vary with the particle size. The magnetic interactions and cation distribution present in these systems influence the room temperature Mossbauer parameters. Ferrimagnetic sextets are observed for all the different particle sizes. The observed reduction of the magnetic hyperfine field values with the decrease in the size of MFe2O4 particles are attributed to the intrinsic size effect and the canted spin structure at the surface of the nanoparticles.