Magnetic properties of Fe3O4 nanoparticles coated with oleic and dodecanoic acids

Magnetic nanoparticles (NP) of magnetite (Fe3O4) coated with oleic acid (OA) and dodecanoic acid (DA) were synthesized and investigated through Transmission Electron Microscopy (TEM),magnetization M, and ac magnetic susceptibility measurements. The OA coated samples were produced with different magnetic concentrations (78, 76, and 65%) and the DA sample with 63% of Fe3O4. Images from TEM indicate that the NP have a nearly spherical geometry and mean diameter ~ 5.5 nm. Magnetization measurements, performed in zero field cooled (ZFC) and field cooled (FC) processes under different external magnetic fields H, exhibited a maximum at a given temperature TB in the ZFC curves, which depends on the NP coating (OA or DA), magnetite concentration, and H. The temperature TB decreases monotonically with increasing H and, for a given H, the increase in the magnetite concentration results in an increase of TB. The observed behavior is related to the dipolar interaction (DI) between NP which seems to be an important mechanism in all samples studied. This is supported by the results of the ac magnetic susceptibility Xac measurements, where the temperature in which X' peaks for different frequencies follows the Vogel-Fulcher model, a feature commonly found in systems with dipolar interactions. Curves of H vs. TB/TB(H=0) for samples with different coatings and magnetite concentrations collapse into a universal curve, indicating that the qualitative magnetic behavior of the samples may be described by the NP themselves, instead of the coating or the strength of the dipolar interaction. Below TB, M vs. H curves show a coercive field (HC) that increases monotonically with decreasing temperature. The saturation magnetization (MS) follows the Bloch's law and values of MS at room temperature as high as 78 emu/g were estimated, a result corresponding to ~80% of the bulk value. The overlap of M/MS vs. H/T curves for a given sample and the low HC at high temperatures suggest superparamagnetic behavior in all samples studied. The overlap of M/MS vs. H curves at constant temperature for different samples indicates that the NP magnetization behavior is preserved, independently of the coating and magnetite concentration.Comment: 8 pages and 9 figure

Metal-insulator transition in Nd1−x_{1-x}Eux_{x}NiO3_{3} compounds

Polycrystalline Nd$_{1-x}$Eu$_{x}$NiO$_3$ ($0 \leq x \leq 0.5$) compounds were synthesized in order to investigate the character of the metal-insulator (MI) phase transition in this series. Samples were prepared through the sol-gel route and subjected to heat treatments at $\sim$1000 $^\circ$C under oxygen pressures as high as 80 bar. X-ray Diffraction (XRD) and Neutron Powder Diffraction (NPD), electrical resistivity $\rho(T)$, and Magnetization $M(T)$ measurements were performed on these compounds. The results of NPD and XRD indicated that the samples crystallize in an orthorhombic distorted perovskite structure, space group $Pbnm$. The analysis of the structural parameters revealed a sudden and small expansion of $\sim$0.2% of the unit cell volume when electronic localization occurs. This expansion was attributed to a small increase of $\sim$0.003 \AA{} of the average Ni-O distance and a simultaneous decrease of $\sim$$- 0.5^\circ$ of the Ni-O-Ni superexchange angle. The $\rho(T)$ measurements revealed a MI transition occurring at temperatures ranging from $T_{\rm MI}\sim 193$ to 336 K for samples with $x = 0$ and 0.50, respectively. These measurements also show a large thermal hysteresis in NdNiO$_{3}$ during heating and cooling processes suggesting a first-order character of the phase transition at $T_{\rm MI}$. The width of this thermal hysteresis was found to decrease appreciably for the sample Nd$_{0.7}$Eu$_{0.3}$NiO$_{3}$. The results indicate that cation disorder associated with increasing substitution of Nd by Eu is responsible for changing the first order character of the transition in NdNiO$_{3}$.Comment: 19 pages, 9 figure

Anelastic spectroscopy study of the metal-insulator transition of Nd(1-x)EuxNiO3

Measurements are presented of the complex dynamic Young's modulus of NdNiO3 and Nd0.65Eu0.35NiO3 through the Metal-Insulator Transition (MIT). On cooling, the modulus presents a narrow dip at the MIT followed by an abrupt stiffening of ~6%. The anomaly is reproducible between cooling and heating in Nd0:65Eu0:35NiO3 but only appears as a slow stiffening during cooling in undoped NdNiO3, conformingly with the fact that the MIT in RNiO3 changes from strongly first order to second order when the mean R size is decreased. The elastic anomaly seems not to be associated with the antiferromagnetic transition, which is distinct from the MIT in Nd0.65Eu0.35NiO3. It is concluded that the steplike stiffening is due to the disappearance or freezing of dynamic Jahn- Teller (JT) distortions through the MIT, where the JT active Ni3+ is disproportionated into alternating Ni3+d and Ni3-d. The fluctuating octahedral JT distortion necessary to justify the observed jump in the elastic modulus is estimated as ~3%, but does not have a role in determining the MIT, since the otherwise expected precursor softening is not observed.Comment: 11 pages, accepted by Phys. Rev.

Low-field remanent magnetization in Rb2FeCl5.H2O and in its site-diluted solid solutions Rb2Fe1-xInxCl5.H2O (x=0.04, 0.08, 0.15, and 0.35)

The observation of remanent magnetization M-r in single crystals of the easy-axis low-anisotropy antiferromagnet Rb2FeCl5 . H2O and in its site-diluted solid solutions Rb2Fe1-xInxCl5 . H2O (x=0.04, 0.08, 0.15, and 0.35) is reported. the anomaly is observed when the easy axis of the antiferromagnetic alignment in the crystal is oriented parallel to the applied magnetic field. the values of M, depend on the field in which the sample has been cooled and saturates at values as low as 1 Oe. the observed behavior is compared with anomalies previously reported for Mn1-xZnxF2 and K2Fe1-xInxCl5 . H2O. the saturation values of M-r for the Rb2Fe1-xInxCl5 . H2O solid solutions are found to be up to two and five orders of magnitude higher than for, respectively, the K2Fe1-xInxCl5 . H2O and Mn1-xZnxF2 ones. for most of the Rb-based samples as well as for all of the other ones the values of M-r can be scaled to collapse into a single universal curve. Such a curve is independent of the applied magnetic filed, of the value of x, and of the compound. the scaling of some samples of the Rb-based system, however, deviates from the universal curve. These samples exhibit M, values that are higher by one order of magnitude than those that follow the scaling and yet possess the same concentration of nonmagnetic ions. An explanation based on the pinning of antiferromagnetic domain walls by nonmagnetic impurities and lattice defects is given.Universidade Federal de São Paulo,INST FIS,São Paulo,BRAZILUniversidade Federal de São Paulo,INST FIS,São Paulo,BRAZILWeb of Scienc