C. Christides

Crystal structure, bonding, and phase transition of the superconducting Na2CsC60 fulleride

The crystal structure of superconducting Na2CsC60 was studied by high-resolution powder neutron diffraction between 1.6 and 425 K. Contrary to the literature, the structure at low temperatures is primitive cubic [See equation in the PDF file], isostructural with pristine C60. Anticlockwise rotation of the C60 units by 98� about [111] allows simultaneous optimization of C60-C60 and alkali-fulleride interactions. Optimal Na+-C603- coordination is achieved with each sodium ion located above one hexagon face and three hexagon-hexagon fusions of neighboring fulleride ions (coordination number 12). Reduction of the C60 molecule lengthens the hexagon-hexagon fusions and shortens the pentagon-hexagon fusions (to ∼1.43 angstroms). On heating, Na2CsC60 undergoes a phase transition to a face-centered-cubic [See equation in the PDF file] phase, best modeled as containing quasi-spherical C603- ions. The modified structure and intermolecular potential provide an additional dimension to the behavior of superconducting fullerides and should sensitively affect their electronic and conducting properties.

Preparation and characterization of the NdFe10T2Nx (T = Mo,V) compounds with the ThMn12 tetragonal-type structure

The dramatic changes in the magnetic properties observed in the R2Fe17 alloys upon nitrogen absorption have also been observed in the NdFe10T2Nx (T=Mo,V). The nitrides, prepared at T=773 K and 21/2 atm pressure of N2, showed an increased anisotropy HA ≳ 5 T at RT compared to the HA = 0 for the non‐nitrogenated materials. The Curie temperatures increase about 44% and the observed volume expansion is 3.6% for Mo and 2.8% for the V compound although the structure remains the same. A coercive field of 0.97 and 0.45 T has been observed at 4.2 K for NdFe10Mo2Nx and NdFe10V2Nx compounds, respectively.

Neutron Spectroscopy and Electron-Phonon Coupling in Alkali-Metal-Doped Fullerides

The vibrational spectrum of saturation-doped insulating Rb6C60 has been measured by inelastic neutron scattering at nonzero momentum transfer in the energy range (5 ÷ 200) meV at 20 K. The two lowest-energy fivefold degenerate Hg modes sharpen substantially in comparison with superconducting K3C60, acquiring comparable widths to those found in pristine C60. This is interpreted as evidence for the importance of these modes in the pairing interaction for superconductivity. We estimate that the radial Hg modes make a substantial contribution to the total electron-phonon coupling strength in the superconducting fullerides.

Neutron powder diffraction study ( T = 4.2-300 K) and polarization analysis of

The crystal and magnetic structures of the perovskite have been studied in the temperature range T = 4.2-300 K by powder neutron diffraction. In addition to the antiferromagnetic ordering transition at , a commensurate-incommensurate magnetic transition is detected at . Below this temperature, two sets of satellite peaks surround the (1/2,1/2,1/2) magnetic peak at ?, collapsing into a single set of satellites below 155 K. Polarization analysis has been performed to confirm the magnetic nature of the satellites.

Exchange biasing in La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 multilayers

A series of [La2/3Ca1/3MnO3/La1/3Ca2/3MnO3]15 multilayers, with bilayer thicknesses between 2 and 32 nm, has been prepared by pulsed laser deposition. The study of their magnetic and magnetotransport properties reveals, for the first time in this category of materials, the presence of an exchange biasing mechanism at low temperatures. Zero-field-cooling and field-cooling magnetic measurements reveal a blocking temperature around 70 K that is independent of the bilayer thickness, whereas the average film magnetization becomes zero at 250 K.

Microstructural modification in Co/Cu giant-magnetoresistance multilayers

Three different classes of [Co/1.1 nm/Cu/2.1 nm]30 multilayers were grown by magnetron sputtering deposition. The effect of magnetostatic interactions on the giant-magnetoresistance (GMR) and magnetic properties are examined in relation to the induced changes in the film microstructure as it is varied by: (i) the substrate surface roughness and (ii) the effect of thermal isolation of the Si(100) substrate from the cooling plate during deposition. A remarkable variation in shape and magnitude of GMR, and in the magnetic (M-H) loops, is observed for the three classes of films. It is found that there are three characteristic features in every sample that vary systematically: (i) The (ΔR/R)max ratio. (ii) The magnetic field range where a GMR loop reaches its minimum value. (iii) The (M-H) loops that vary from the characteristic antiferromagnetic to a typical ferromagnetic loop shape. Two well-separated grain size distributions below and above 12 nm were found from transmission electron microscopy. The smaller...

Magneto-transport and exchange biasing in La–Ca–Mn–O compositionally modulated ferromagnetic/antiferromagnetic multilayers

Compositionally modulated structures consisting of La2/3Ca1/3MnO3 ferromagnetic (FM) layers and La1/3Ca2/3MnO3 antiferromagnetic (AF) layers, with layer thickness 1.5 nm ⩽tFM,tAF⩽6 nm, were grown on (100)LaAlO3 by pulsed laser deposition. Thermomagnetic and isothermal magnetic measurements reveal an exchange-biasing mechanism with the same blocking temperature TB≈70 K for all the examined combinations of tFM and tAF. The ratio of colossal magnetoresistance increases in multilayers with larger tFM, whereas the exchange-biasing field increases in samples with larger tAF. The independence of TB on tFM and tAF shows that the artificially induced La1−xCaxMnO3 (x≈0.5) compositional modulation at the interfaces is of critical importance for exchange biasing in La–Ca–Mn–O based multilayers.

Magnetic anisotropy and magnetic transitions in RFe10Mo2

Compounds of the composition RFe10Mo2 with R = Y, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu were studied by means of the ac-initial susceptibility (chi) and the singular point detection (SPD) technique in the temperature range from 4.2 to 300 K. Spin reorientation transitions were detected for the compounds RFe10Mo2 with R = Nd and Dy. For the compounds RFe10Mo2 with R = Y, Ho, Er, Tm and Lu the ac-initial susceptibility data indicate a magnetic transition of unidentified nature. For SmFe10Mo2 a first-order magnetization process (FOMP) is observed below 170 K.

Magnetization and 11 B NMR study of Mg 1 − x Al x B 2 superconductors

We demonstrate the magnetic-field distribution of the pure vortex state in lightly doped

The Structure of Buckminsterfullerene Compounds

{\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}{\mathrm{B}}_{2} (xl~0.025)