Shifting the Superparamagnetic Limit of Nanosized Copper Iron Spinel

In this paper we analyze trend of EFG values measured at Cd impurity in a group of semiconducting delafossites with chemical formula CuBO₂ (B = Al, Fe, Cr, Nd). We conclude that this trend reveals one of the most subtle details in electronic spectrum of the compounds: if impurity states are formed within or out of the band gap. In CuAlO₂ and CuFeO₂ the Cd EFG exhibits larger value than in CuCrO₂ and CuNdO₂, when Cd substitutes the Cu atom. This occurs because in the first two compounds the Cd forms shallow band within the gap, and in the second two compounds does not. When Cd occupies the B position it exhibits almost the same EFG in all delafossites. In this case, Cd does not form its states within the gap in none of the compounds. To arrive to these conclusions we analyzed and calculated various systems (Cd-doped CuAlO₂ and CuCrO₂ compounds, fictitious molecules), using the FP-LAPW method.     

Magnetic properties and electron spin resonance of Ecuadorian obsidians. Application to provenance research of archeological samples

Obsidians from major Ecuadorian sources were analyzed by SQUID magnetometry and electron spin resonance (ESR). The association of these two techniques permits to differentiate obsidians from the sources of Cotopaxi volcano and from the Quiscatola and Mullumica–Callejones sources of the Chacana caldera, taking into account various parameters arising from the M vs. H cycles and the X-band ESR spectra. The analysis of 27 archeological samples coming from the prehispanic site of La Maná allowed us to infer that most of them come from the Mullumica–Callejones source.     

Magnetic moment oscillation in ammonium perchlorate in a DC SQUID-based magnetic resonance experiment

In this work we describe experimental results in which a DC SQUID (superconducting quantum interference device) is used as free induction decay detector. Measurements of a solid ammonium perchlorate (NH₄ClO₄) sample were performed, in zero field, at 4.2 K. Unexpected magnetic moment oscillations were detected at 1.5 kHz. The computation of the magnetic fields suggests that the proton nuclear magnetic resonance may explain the measured resonance, considering reorientation of the ammonium group by quantum tunneling of protons and a magnetic proton dipole–dipole intermolecular interaction model.     

Magnetic susceptibility effects on 13C MAS NMR spectra of carbon materials and graphite

13C high-resolution solid-state nuclear magnetic resonance (NMR) was employed to study carbon materials prepared through the thermal decomposition of four different organic precursors (rice hulls, endocarp of babassu coconut, peat, and PVC). For heat treatment temperatures (HTTs) above about 600 C, all materials presented 13C NMR spectra composed of a unique resonance line associated with carbon atoms in aromatic planes. With increasing HTT a continuous broadening of this resonance and a diamagnetic shift in its central frequency were verified for all samples. The evolution of the magnitude and anisotropy of the magnetic susceptibility of the heat-treated carbon samples with HTT explains well these findings. It is shown that these results are better understood when a comparison is made with the features of the 13C NMR spectrum of polycrystalline graphite, for which the magnetic susceptibility effect is also present and is much more pronounced.     

MARTA: a high-energy cosmic-ray detector concept for high-accuracy muon measurement

A new concept for the direct measurement of muons in air showers is presented. The concept is based on resistive plate chambers (RPCs), which can directly measure muons with very good space and time resolution. The muon detector is shielded by placing it under another detector able to absorb and measure the electromagnetic component of the showers such as a water-Cherenkov detector, commonly used in air shower arrays. The combination of the two detectors in a single, compact detector unit provides a unique measurement that opens rich possibilities in the study of air showers.     

Structural organization of polar-group-containing polymers in the molten state

Model compounds in which flexible polymethylenic sequences of variable length alternate with -NH₂-ZnX₂- groups were characterized both in the crystalline and in the molten state, in order to evaluate the effect of regularly distributed dipoles on the melting behavior of linear flexible chains. Microphase segregation of the polar groups was observed resulting in the formation of smectic mesophases or of smectic-like fluids. Possible analogies with the structural organization of (fx224-1) dipole-containing polymers and ionomers are briefly discussed.