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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.

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.

The Ni nanoparticles with fcc or hcp phases have been synthesized in tetraethylene glycol by using the modified polyol process. The crystal structure has been controlled by changing the polyol/Ni mole ratio and the reaction temperature. The saturation magnetization of the as-prepared particles depends on the relative volume fraction of the hcp phase. The x-ray diffraction and extended x-ray absorption fine structure studies suggest the formation of pure fcc and hcp Ni phases. The hcp Ni particles show nonmagnetic behavior and thermally stable below 673K.

Nanocrystalline cobalt ferrite particles with the grain size of 8 nm were synthesized by using the co-precipitation technique and subsequently heat treated to obtain larger grain sizes. The effect of grain size, cation distribution, frequency and temperature on their dielectric properties has been studied. The dielectric constant (ε′) of 8 nm grains is found to be an order of magnitude higher and the dielectric loss (tanδ) is an order of magnitude smaller compared to those of the micron-size particles. The dielectric relaxation is found to be non-Debye in nature with the activation energy decreasing with thermal annealing due to structural perfection.

The Néel temperature of Ni0.5Zn0.5Fe2O4 spinel ferrite increases significantly from 538K in the bulk state to 592K when the grain size is reduced to 16nm by milling in a high-energy ball mill. This has been attributed to an increase in the AB superexchange interaction strength due to a possible enhancement in the magnetic ion concentration in the A-site on milling, as is evident from extended x-ray absorption fine structure and in-field Mössbauer measurements.

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