We have performed a detailed characterization of the magnetic properties of Ni nanoparticles embedded in a ${\mathrm{SiO}}_2$ amorphous matrix. A modified sol-gel method was employed which resulted in Ni particles with average radius $\sim 3\mathrm{nm},$ as inferred by TEM analysis. Above the blocking temperature $T_B\approx 20\mathrm{K}$ for the most diluted sample, magnetization data show the expected scaling of the ${M/M}_S$ vs $H/T$ curves for superparamagnetic particles. The hysteresis loops were found to be symmetric about zero field axis with no shift via exchange bias, suggesting that Ni particles are free from an oxide layer. For $T<\mathrm{}{T}_B$ the magnetic behavior of these Ni nanoparticles is in excellent agreement with the predictions of randomly oriented and noninteracting magnetic particles, as suggested by the temperature dependence of the coercivity field that obeys the relation $H_C{\mathrm{}\left(T\right)=H}_{C0\mathrm{}}\left[1-{(T/T}_B{)}^{1/2}\right]$ below $T_B$ with $H_{C0\mathrm{}}\sim 780\mathrm{Oe}.$ The obtained values of $H_{C0\mathrm{}},$ considering the temperature dependence of the magnetic anisotropy constant, are discussed within the scenario of isolated randomly oriented and noninteracting single-domain particles.

The role of dipolar interactions among Ni nanoparticles (NPs) embedded in an amorphous ${\text{SiO}}_2/\text{C}$ matrix with different concentrations has been studied performing ac magnetic susceptibility ${\chi }_{\text{ac}}$ measurements. For very diluted samples, with Ni concentrations $<4\text{wt}\%$ Ni or very weak dipolar interactions, the data are well described by the Néel-Arrhenius law. Increasing Ni concentration to values up to $12.8\text{wt}\%$ 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.

We performed measurements of magnetic susceptibility, electrical resistivity, and magnetoresistance in magnetic fields of up to 18 T in the magnetic superconductor ${\mathrm{RuGd}}_{1.4}{\mathrm{Ce}}_{0.6}{\mathrm{Sr}}_2{\mathrm{Cu}}_2{\mathrm{O}}_{10-\delta }$ synthesized in oxygen pressures up to 95 atm. The magnetic-susceptibility data show the occurrence of an antiferromagnetic state below $T_N\sim 175\mathrm{K},$ followed by the development of a weak ferromagnetic state near $T_M\sim 100\mathrm{K},$ and followed further by the onset of superconductivity (SC) at $T_c\sim 42\mathrm{K}.$ The electrical resistivity as a function of temperature shows an evolution from nonmetal-to-SC behavior in samples prepared in a flux of ${\mathrm{O}}_2$ to a well defined metal-like behavior in samples prepared under 95 atm pressure of ${\mathrm{O}}_2.$ The electron-phonon coupling constant was calculated from transport data to be ${\lambda }_{\mathrm{tr}}\sim 0.17,$ a value comparable with other cuprates, indicating weak electron-phonon coupling in these ruthenates. The values of the upper critical field $H_{c2\mathrm{}}$ for the ${\mathrm{O}}_2$ high-pressure treated samples were obtained from the magnetoresistivity data yielding $H_{c2\mathrm{}}^{\mathrm{ab}}\mathrm{}\left(0\right)\mathrm{}\sim 39\mathrm{T},$ and the out-of-plane superconducting coherence length ${\xi }_c\mathrm{}\left(0\right)\mathrm{}\sim 28\AA .$ Based on the similarities between these ruthenates and the superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta },$ we estimated $H_{c2\mathrm{}}^c\mathrm{}\left(0\right)\mathrm{}\sim 8\mathrm{T}$ and ${\xi }_{\mathrm{ab}}\mathrm{}\left(0\right)\mathrm{}\sim 140\AA .$ We used these parameters to discuss the coexistence of long-range magnetic order and superconductivity on a microscopic scale on these materials.

The effects of dc excitation current on the current-voltage curves of polycrystalline samples of ${\mathrm{Nd}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{Mn}}_{0.96}{\mathrm{Cr}}_{0.04}{\mathrm{O}}_3$ were investigated. The experimental results show that an abrupt jump of the voltage is concomitant with a huge increase in the temperature of the sample. A simple model and estimates for Joule self-heating effects support the experimental data. Moreover, the data strongly suggest that both the current localization in the metallic paths and local Joule self-heating effects are essential ingredients to understand the current-induced phase transition in phase-separated manganites.

We have performed a detailed characterization of both magnetic and transport properties of ${\mathrm{La}}_{0.7-x}{\mathrm{Y}}_x{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ ($x=0$, 0.05, 0.10, and 0.15) compounds. These measurements provide clear evidence for three temperature intervals in which the magnetic and transport properties are altered. The most notable is an intermediate regime called the clustered state, $T_C<T<T^{*}$, of phase coexistence involving the presence of both ferromagnetic (FM) and antiferromagnetic (AF) clusters and remnants of the high-temperature paramagnetic (PM) phase. The results further indicate a strong connection between spin cluster formation and the colossal magnetoresistivity effect. This magnetically clustered regime was observed to be altered by both magnetic field and disorder. The metal-insulator transition temperature $T_{\mathrm{MI}}$ and the width of the peak of $\rho \left(T\right)$ were found to change linearly with increasing disorder (Y content) and application of magnetic field with negative and positive derivatives, respectively. This indicates that disorder and application of magnetic fields $\left(H\right)$ would cause the same effect but in an opposite way. We argue that correlated disorder effects influence both the shape and the magnitude of $\rho$ versus $T$ curves, as predicted theoretically. We have also inferred, from the computed excess of conductivity (or resistivity decrease) $\Delta \rho$ under decreasing $T$ and application of $H$, that the electronic conduction process in the clustered state is described by a hopping mechanism. This result suggests two contributions to the electrical conductivity in the clustered state: (1) a semiconductinglike contribution and (2) hopping of carriers between FM metallic clusters.