Structural, electrical and magnetic properties of carbon-nickel composite thin films

Carbon-nickel composite thin films (600 nm thick) were prepared by dc magnetron sputtering of Ni and C at several temperatures (25-800 °C) on oxidized silicon substrates. By transmission electron microscopy it was found that the composite consisted of Ni (or Ni₃C) nanoparticles embedded in a carbon matrix. The metallic nanoparticles were shaped in the form of globular grains or nanowires (of the aspect ratio as high as 1:60 in the sample prepared at 200 °C). The carbon matrix was amorphous, or graphite-like depending on deposition temperature. At low deposition temperatures T_S (25-400 °C) the Ni₃C nanoparticles were of hcp phase. Samples prepared at T_S ? 600 °C contained ferromagnetic fcc Ni nanoparticles. A correlation was found between the structural, electrical and magnetic properties of the composites. To characterise the films, dependences, such as resistivity vs. temperature, current vs. voltage, differential conductivity vs. bias voltage, and magnetoresistivity, were determined. For example, the tunneling effect was found in samples in which the metallic nanoparticles were separated by 2-3 nm thick amorphous carbon. When the metallic nanoparticles were connected by graphite-like carbon regions (having a metallic conductivity, in contrast to a-C), the temperature coefficient of the resistivity became slightly positive. An anisotropic magnetoresistivity of ∼0.1% was found in the sample that contained ferromagnetic columnar fcc Ni. Zero magnetoresistivity was found in the sample in which the metallic nanoparticles were of non-magnetic hcp phase.     

Anisotropic magnetoresistivity and magnetic entropy change in HoAl2

We report a correlation between magnetoresistivity Δρ_mag and magnetic entropy change ΔS_mag for specified crystallographic directions of a HoAl₂ single crystal. This ferromagnetic compound shows differentiated behavior among the members of the RAl₂ series due to the presence of spontaneous spin reorientation. We discussed this effect in order to establish a relationship between the two quantities studied. The calculations were performed using a self-consistent model based on a Hamiltonian containing exchange, Zeeman and crystal field terms. We found that the normalized relation {Δρ_mag} = (T/T_C)^m{ΔS_mag}, with m = 0.1 for the [100] and [110] directions matches satisfactorily the two quantities.     

Bi-based superconductors prepared with addition of CoFe2O4 for the design of a magnetic probe

We have investigated the microstructure and the magnetoresistivity of bismuth based superconductor bulks added with nano-sized CoFe₂O₄ particles (10 nm in diameter). Samples were prepared through the solid state reaction (SSR) technique by addition of CoFe₂O₄ nanoparticles during the last step of heat treatment. Phase examination using X-ray diffraction (XRD), morphology investigation by scanning electron microscope (SEM), microstructure and local chemical composition analyses using transmission electron microscope (TEM) coupled with energy dispersive X-ray spectroscope (EDXS), electrical resistance versus temperature ρ(T) under applied magnetic fields (B) and electrical resistance versus B at 77 K, ρ(B), were carried out. The CoFe₂O₄ added sample shows a great magnetoresistance to weak magnetic field at the temperature of liquid nitrogen (77 K). This result is attractive for practical, because CoFe₂O₄ added samples can be utilized as active elements in magnetic fields sensor devices.     

The effects of thermal treatment on the physical properties of Sr2FeMo1−xMxO6 perovskite with M = W, Ta and x ≤ 0.3

The Sr₂FeMo_1−xM_xO₆ double perovskites with M = W and Ta and x ≤ 0.3, were obtained by sintering at 1300 °C, during 4 and 8 h, respectively. The perovskites crystallize in a tetragonal structure having I4/mmm space group. The grains are more homogeneous when sintering time is increased. The samples sintered longer time show higher values of saturation magnetizations, resistivities and magnetoresistivities than the samples sintered during 4 h. The intergrain tunnelling magnetoresistance as well as the intragrain contributions, respectively were analysed as function of temperature and external field. The changes in the physical properties, when the sintering time is increased, have been correlated with the number of antisite defects as well as the nature of grain boundaries.     

Dirac fermion heating,current scaling,and direct insulator-quantum Hall transition in multilayer epitaxial graphene

We have performed magnetotransport measurements on multilayer epitaxial graphene. By increasing the driving current I through our graphene devices while keeping the bath temperature fixed, we are able to study Dirac fermion heating and current scaling in such devices. Using zero-field resistivity as a self thermometer, we are able to determine the effective Dirac fermion temperature (T_DF) at various driving currents. At zero field, it is found that T_DF ∝ I^≈1/2. Such results are consistent with electron heating in conventional two-dimensional systems in the plateau-plateau transition regime. With increasing magnetic field B, we observe an I-independent point in the measured longitudinal resistivity ρ_xx which is equivalent to the direct insulator-quantum Hall (I-QH) transition characterized by a temperature-independent point in ρ_xx. Together with recent experimental evidence for direct I-QH transition, our new data suggest that such a transition is a universal effect in graphene, albeit further studies are required to obtain a thorough understanding of such an effect.