The role of dipolar interactions among Ni nanoparticles ͑NPs͒ embedded in an amorphous SiO 2 / C matrix with different concentrations has been studied performing ac magnetic susceptibility ac measurements. For very diluted samples, with Ni concentrations Ͻ4 wt % Ni or very weak dipolar interactions, the data are well described by the Néel-Arrhenius law. Increasing Ni concentration to values up to 12.8 wt % Ni results in changes in the Néel-Arrhenius behavior, the dipolar interactions become important, and need to be considered to describe the magnetic response of the NPs system. We have found no evidence of a spin-glasslike behavior in our Ni NP systems even when dipolar interactions are clearly present.
Magnetic susceptibility and heat capacity measurements are used to infer information about the short-range magnetic order above the Néel temperature T N and the antiferromagnetically ordered states below T N of quasione-dimensional (quasi-1D) CuSb 2 O 6 , NiTa 2 O 6 , and CoSb 2 O 6 . It is shown that two antiferromagnetic sublattices, oriented at 90 • to one another, are likely present in NiTa 2 O 6 and CoSb 2 O 6 . Application of magnet field parallel to the quasi-1D chains of one sublattice is perpendicular to the chains of the other sublattice. This results in two antiferromagnetic transitions when the magnetic field H 2 T (∼0.2 meV). The anisotropic influence of magnetic field on the antiferromagnetic state leads to a magnetocaloric effect that is fully investigated in this work. The effect is associated with competition among Néel, dimer, and spin-liquid states that are all present at T N .
The thermal decomposition (TD) methods are among the most successful in obtaining magnetic nanoparticles with a high degree of control of size and narrow particle size distribution. Here we investigated the TD of iron(III) acetylacetonate in the presence of oleic acid, oleylamine, and a series of alcohols in order to disclose their role and also investigate economically attractive alternatives for the synthesis of iron oxide nanoparticles without compromising their size and shape control. We have found that some affordable and reasonably less priced alcohols, such as 1,2-octanediol and cyclohexanol, may replace the commonly used and expensive 1,2-hexadecanediol, providing an economically attractive route for the synthesis of high quality magnetic nanoparticles. The relative cost for the preparation of FeO NPs is reduced to only 21% and 9% of the original cost when using 1,2-octanediol and cyclohexanol, respectively.
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