Nd2Fe14B hard magnetic nanoparticles were synthesized by chemical synthesis techniques. Nd−Fe−B gel was prepared using NdCl3·6H2O, FeCl3·6H2O, H3BO3, citric acid, and ethylene glycol (EG) by a Pechini type sol−gel method. This gel was subsequently annealed to produce mixed oxide powders. Nd2Fe14B nanoparticles were prepared from these oxides by a reduction−diffusion process. The phase analysis, structure, and magnetic properties were determined by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), and vibrating sample magnetometer (VSM) techniques. The mechanism of Nd2Fe14B formation was investigated by differential scanning calorimetry (DSC), XRD, and thermodynamic free energy change data. Our experimental and modeling results showed that the reduction−diffusion of the Nd−Fe−B mixed oxide was a three step process. The reduction of Fe2O3 to Fe and B2O3 to B occurred at 300 °C. NdH2 and Fe was formed from Nd2O3 and NdFeO3 at 620 °C. The Nd2Fe14B phase was formed from NdH2, Fe, and B at 692 °C. The coercivity of as-synthesized powder was 6.1 kOe. The Henkel plot showed that this powder was exchange coupled; removal of CaO by washing led to dipolar interactions and a decrease in coercivity.

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A facile route for the synthesis of nickel nanoparticles in stable aqueous foams is reported. The Ni nanoparticles were roughly 12-15 nm in size and were stable as aqueous suspensions or powders when oleic acid was used as a capping agent. These Ni nanoparticles were subsequently coated with a silver shell in view of the extra stability and the enhanced manipulative ability afforded by the silver nanocoating. This was accomplished by a simple transmetallation reaction wherein the nanoparticle surface nickel atoms act as localized reducing agents for the silver ions in solution. As the silver shell is formed through the surface reaction a reduction in the average size of the Ni core occurs. After the core-shell structure formation, the Ni core has an average diameter of 10-20 nm while the Ag shell has a thickness of 2-4 nm. The pristine oleic acid coated Ni and Ni core Ag shell nanoparticles were probed for their magnetic characteristics by a vibrating sample magnetometer. The nascent, oleic acid coated Ni nanoparticles display a low superparamagnetic blocking temperature, T B , of 20 K. The field dependent magnetic behaviour above and below T B displays the standard features corresponding to superparamagnetism, as expected for very small Ni crystallites suggesting again that each 12 nm particle is polycrystalline. The magnetic contribution in the Ni core Ag shell system comes from only the Ni core and predictably, the blocking temperature of this system is below 12 K due to the smaller size of the Ni core.

The magnetic and magnetostrictive properties of polycrystalline Co1−xMnxFe2O4 (0 ⩽ x ⩽ 0.4) have been studied. Although the Curie temperature decreases continuously with increasing concentration of Mn, the magnetization remains high up to x = 0.3 and unexpectedly low coercivity is observed for this composition showing an unusual magnetostrictive behaviour. This composition shows a relatively larger magnetostriction at low fields. Moreover, the strain derivative which is the slope of the magnetostriction curve at low magnetic fields is almost doubled and the field at which maximum magnetostriction is observed is reduced to almost half for 30% of Mn substitution. The results show that x ≈ 0.3 in Co1−xMnxFe2O4 is an optimum composition with superior magnetostrictive properties for many applications.

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