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Nanostructured ZnFe 2 O 4 ferrites with different grain sizes were prepared by high energy ball milling for various milling times. Both the average grain size and the root mean square strain were estimated from the x-ray diffraction line broadening. The lattice parameter initially decreases slightly with milling and it increases with further milling. The magnetization is found to increase as the grain size decreases and its large value is attributed to the cation inversion associated with grain size reduction. The 57 Fe Mössbauer spectra were recorded at 300 K and 77 K for the samples with grain sizes of 22 and 11 nm. There is no evidence for the presence of the Fe 2+ charge state. At 77 K the Mössbauer spectra consist of a magnetically ordered component along with a doublet due to the superparamagnetic behaviour of small crystalline grains with the superparamagnetic component decreasing with grain size reduction. At 4.2 K the sample with 11 nm grain size displays a magnetically blocked state as revealed by the Mössbauer spectrum. The Mössbauer spectrum of this sample recorded at 10 K in an external magnetic field of 6 T applied parallel to the direction of gamma rays clearly shows ferrimagnetic ordering of the sample. Also, the sample exhibits spin canting with a large canting angle, maybe due to a spin-glass-like surface layer or grain boundary anisotropies in the material.

Nearly monodispersed CoFe2O4 nanoparticles with average sizes between 8 and 100 nm were synthesized by using seed-mediated growth dominant coprecipitation and modified oxidation methods. X-ray diffraction and Mössbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O4 for powders with different particle diameters. Rotational hysteresis loss (Wr) analysis showed an average switching field (Hp) of 17 kOe and a magnetic anisotropy field (Hk) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×106 erg/cc. The Hc and Hp results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (Hc) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe.

The possibility for direct synthesis of fct-FePt nanoparticles of the order of 3–4 nm in diameter through the coreduction of iron and platinum ions in a polyol has been explored. We have succeeded in the synthesis of face-centered cubic structured 3–4 nm diameter FePt particles whose composition was very close to Fe50Pt50. The Fe:Pt ratio was influenced little by the molar ratios of Fe and Pt acetylacetonate dissolved in ethylene glycol. However, depending on the polyol/Pt ratio, the as-prepared samples were either superparamagnetic or ferromagnetic. The transition temperature (Tt) and magnetic properties of the as-prepared FePt were very sensitive to the reaction conditions, and the Tt varied between 593 and 893 K and the particles were ferromagnetic. The as-prepared FePt under the optimum condition had a Tt as low as 593 K and Hc as high as 1.11 kOe at an applied field of 1 T at room temperature. Furthermore, when the as-prepared FePt nanoparticles with Tt around 593 K were annealed at 673 K in H2/N2 atmosphere for an hour they transformed to the ordered fct (L10) structure with coercivity as high as 4.2 kOe at 300 K. This confirmed the lowering of Tt by the manipulation of the reaction condition alone.

Synthesis strategies for the size and structure controlled Ni, Co, and FePt nanoparticles by manipulating the reaction kinetics of the polyol process are reported. In the case of Ni, particle size varied from a few micron to a few tens of nanometer, and the structure from fcc to a mixture of fcc and hcp was realized. The Co particle structure has been changed from a mixture of fcc and hcp at micron size range, to the fcc dominant mixture of fcc and hcp in the submicron size range, then to ε- and hcp-Co and finally to hcp-Co in the nanosize range. In the case of FePt, particles had fcc and fct phases with 5–10 nm in diameter. The magnetic properties of these particles are also reported.