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SPETTROSCOPIA MUONICA: UNA SONDA MICROSCOPICA PER IL MAGNETISMO
No full textThe muon spectroscopy is an experimental technique which employs
muon beams to study the properties of condensed matter. In particular, it allows us to
study static and dynamic processes in magnetic materials (e.g. ferromagnets/antiferromagnets,
spin glasses, frustrated, etc) and superconductors, as well as chemical reactions
and diffusion phenomena of charged and neutral particles.
Here we will describe at the elementary level the physical principles of this technique,
highlighting especially the use in the microscopic study of the magnetic properties of
materials
Magnetic susceptibility of the cluster compounds Mo6Se8 and Mo6Te8
No full textThe magnetic susceptibility of the cluster compounds Mo6Se8 andMo6Te8 has been measured at temperatures between 82 and 330 K, using a Faraday balance. The paramagnetic and diamagnetic components of the susceptibility have been analyzed. The Pauli component was evaluated using the density of states at the Fermi level obtained by band structure calculations,while the core component was evaluated using the calculated atomic core diamagnetism. The paramagnetic susceptibility of Mo6Se8 is due mainly to the Pauli contribution, while the Van Vleck contribution is small, in agreement with the metallic feature of the compound. The paramagnetic susceptibility of Mo6Te8 is due mainly to the Van Vleck term,while the Pauli contribution of the conduction electrons is very small. The result points out that the Mo6 clusters in the telluride retain their molecular character, with small intercluster interactions
Orthorhombic low-temperature superstructures in YBa2Cu3O6+x
No full textThe emergence of the cell-doubled (OII) and both cell-tripled (OIII and OIII*) phases of YBa2Cu3O6+x are observed on oxygen-chain-equalized and order-stabilized pair samples at an extended range of oxygen concentrations (0.28<x<0.83), including the interval of x, which is characteristic of the onset of superconductivity. The phase diagram for the asymmetric next-nearest-neighbor interaction model, extended to include an additional longer-range interaction potential between oxygen atoms, is calculated and compared to two experimental phase diagrams, obtained for intercalated as well as for deintercalated members of the pair samples. A fairly good agreement with experimental data on locations of the OII, OIII, and OIII* phases in the phase diagram and on the average length of the O-Cu-O chains is obtained
Structure and magnetic properties of Fe-Co alloy nanoparticles synthesized by pulsed-laser inert gas condensation
Full text linkFe-Co alloy nanoparticles of different compositions (Fe content of 76, 51, and 30 at%), along with pure Fe and Co
nanoparticles, were prepared by pulsed-laser inert gas condensation, consisting in laser ablation of Fe-Co alloy
targets under helium atmosphere. From the morphological point of view, the obtained nanoparticles have nearly
spherical shape, follow a lognormal size distribution and exhibit little aggregation. X-ray diffraction and highresolution
electron microscopy coupled with electron energy loss spectroscopy show that the Fe-Co nanoparticles
are single crystals with body-centered cubic structure. Furthermore, in the majority of nanoparticles the
composition is highly uniform across the whole diameter and there is little variation in composition from one
nanoparticle to another. Exposure to non-inert atmosphere leads to the formation of a core@shell metal@oxide
morphology characterized by a spinel oxide shell of 2–3 nm around the metallic alloy core. All samples display a
ferromagnetic behavior, characterized by a hysteretic magnetization loop. The saturation magnetization attains
a maximum value of 2.43 Bohr magnetons per atom for Fe content of 76 at%, in agreement with the Slater-
Pauling curve for alloys of 3d elements. Instead, the coercive field, ranging from 29 to 60 kA m−1, is much larger
than the reported values for polycrystalline bulk Fe-Co compounds and monotonically increases from pure Fe to
pure Co. These results demonstrate that pulsed-laser inert gas condensation allows to prepare high-quality
nanoalloys with tailorable magnetic properties, overcoming the limitations of thermal evaporation methods with
respect to compositional control
Magnetic susceptibility of the Cluster Compounds Mo6Se8 and Mo6Te8
No full textThe magnetic susceptibility of the cluster compounds Mo6Se8 andMo6Te8 has been measured at temperatures between 82 and 330 K, using a Faraday balance. The paramagnetic and diamagnetic components of the susceptibility have been analyzed. The Pauli component was evaluated using the density of states at the Fermi level obtained by band structure calculations,while the core component was evaluated using the calculated atomic core diamagnetism. The paramagnetic susceptibility of Mo6Se8 is due mainly to the Pauli contribution, while the Van Vleck contribution is small, in agreement with the metallic feature of the compound. The paramagnetic susceptibility of Mo6Te8 is due mainly to the Van Vleck term,while the Pauli contribution of the conduction electrons is very small. The result points out that the Mo6 clusters in the telluride retain their molecular character, with small intercluster interactions
Coexistence of magnetism and superconductivity in YBa2Cu3O6+x studied by muons; ISIS 2004 SCIENCE HIGHLIGHTS; ISIS, Chilton, UK, 2004.
No full textSensitivity of angle-resolved photoemission to short-range antiferromagnetic correlations
No full textAngle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and, in recent decades, it has lead to significant progress in the understanding of the band structures of cuprates, pnictides, and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron-scattering experiments. Sr2Cu1-xZnxO2Cl2, a prototype of a diluted spin S = 1/2 antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin-correlation length. estimated from Cl-35 NMR was established. It was found that even for correlation lengths of a few lattice constants, a significant spectral weight in the backbended band is present which depends markedly on xi. Moreover, the temperature dependence of that spectral weight is found to scale with the x-dependent spin stiffness. These findings prove that the ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed
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