Heloisa Nunes Bordallo

Glass transition in the polaron dynamics of colossal magnetoresistive manganites.

Neutron scattering measurements on a bilayer manganite near optimal doping show that the short-range polaron correlations are completely dynamic at high T, but then freeze upon cooling to a temperature T(*) approximately equal 310 K. This glass transition suggests that the paramagnetic/insulating state arises from an inherent orbital frustration that inhibits the formation of a long-range orbital- and charge-ordered state. Upon further cooling into the ferromagnetic-metallic state (T(C) = 114 K), where the polarons melt, the diffuse scattering quickly develops into a propagating, transverse optic phonon.

Structural and magnetic behavior of a quasi-1D antiferromagnetic chain compound Cu(NCS)2(pyz)

Abstract Synchrotron X-ray diffraction (XRD) and neutron powder diffraction (NPD) were used to determine the structure of Cu(NCS)2(pyz) (pyz=pyrazine=C4N2H4), which consists of a stacking of Cu pyz Cu chains. While NPD measurements showed no evidence of long-range magnetic ordering, the temperature dependence of the magnetic susceptibility and magnetization suggests that the system can be adequately described on the local scale as a spin-1/2 antiferromagnet (AFM) chain with an intrachain exchange interaction J/kB=−8 K (∼0.7 meV). Comparison of isothermal magnetization data acquired up to 30 T at 1.6 K to a linear chain model shows excellent agreement, making this material a nearly ideal example of an isotropic Heisenberg AFM chain.

Structure determination and a vibrational study for the hexagonal elpasolite Cs2NaGaF6:Cr3+

Single crystals of 0.5% Cr3+-doped Cs2NaGaF6 were studied by means of x-ray diffraction and polarized Raman scattering. The crystal exhibits a unique stacking interaction, with a hexagonal structure with Rm symmetry. Polarized Raman spectra recorded at 16 and 300 K reveal sidebands that are ascribed to the local vibrational of the [CrF6] coordination unit, which differ markedly from those of the Cs2NaGaF6 host lattice. The results are compared to findings for other elpasolite lattices, and the room temperature luminescence quantum yields are discussed in terms of a delicate balance between electron–phonon strength and lattice distortion.

Spin-driven phase transitions in ZnCr 2 Se 4 and ZnCr 2 S 4 probed by high-resolution synchrotron x-ray and neutron powder diffraction

The crystal and magnetic structures of the spinel compounds

Structural and vibrational study of chromium doped elpasolite crystals Cs2NaAlF6

{\text{ZnCr}}_{2}{\text{S}}_{4}

Vibrational Analysis of the Elpasolites Cs2NaAlF6 and Cs2NaGaF6 Doped with Cr3+ Ions by Fluorescence Spectroscopy

and

LATTICE DISPLACEMENTS ABOVE TC IN THE LAYERED MANGANITE LA1.2SR1.8MN2O7

{\text{ZnCr}}_{2}{\text{Se}}_{4}

Lattice displacements aboveTCin the layered manganiteLa1.2Sr1.8Mn2O7

were investigated by high-resolution powder synchrotron and neutron diffraction.

Interplay of spin and orbital ordering in the layered colossal magnetoresistance manganite La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7}(0.5{<=}x{le}1.0)

{\text{ZnCr}}_{2}{\text{Se}}_{4}

Spin ordering in the mixed-ligand antiferromagnet Mn(dca)2(pyrazine)

exhibits a first-order phase transition at