G. Dhalenne

COEXISTENCE OF DIMERIZATION AND ANTIFERROMAGNETISM IN SI-DOPED CUGEO3

We have performed elastic- and inelastic-neutron-scattering experiments on a single crystal of the quasi-one-dimensional system CuGe0.993Si0.007O3. Our experimental data show unambiguously that both quasi-gapless antiferromagnetic fluctuations and gapped magnetic fluctuations, associated with a dimerized phase, coexist in this material below a spin-Peierls transition temperature TSP ≈ 9.2 K. When the temperature is lowered below TN ≈ 4 K, part of the system undergoes a new phase transition towards long-range antiferromagnetism, coexisting with the dimerized non-magnetic phase. We will describe the temperature and wave vector dependences of magnetic responses in these two phases, which can be understood as the superposition of two independent spectra.

Work function changes in the double layered manganite La1.2Sr1.8Mn2O7

We have investigated the behaviour of the work function of La1.2Sr1.8Mn2O7 as a function of temperature by means of photoemission. We found a decrease of 55 +/- 10 meV in going from 60 K to just above the Curie temperature (125 K) of the sample. Above T_C the work function appears to be roughly constant. Our results are exactly opposite to the work function changes calculated from the double-exchange model by Furukawa, but are consistent with other measurements. The disagreement with double-exchange can be explained using a general thermodynamic relation valid for second order transitions and including the extra processes involved in the manganites besides double-exchange interaction.

Competition between Spin-Peierls Phase and Three-Dimensional Antiferromagnetic Order in CuGe1-xSixO3

We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies on single crystals of the quasi-one-dimensional antiferromagnet CuGe1-xSixO3, with 0 ≤ x ≤ 0.1. The Si doping has a drastic effect on the spin-Peierls temperature TSP which decreases at TSP (x)/TSP (0) 1-50x at low Si concentrations. For x ≥ 0.005, three-dimensional antiferromagnetic order with an easy axis along the Cu chains takes place below a well-defined Neel temperature TN. A qualitative interpretation of the phase diagram (T, x) is given.

Phonon and crystal field excitations in geometrically frustrated rare earth titanates

The phonon and crystal field excitations in several rare earth titanate pyrochlores are investigated. Magnetic measurements on single crystals of Gd(2)Ti(2)O(7), Tb(2)Ti(2)O(7), Dy(2)Ti(2)O(7), and Ho(2)Ti(2)O(7) are used for characterization, while Raman spectroscopy and terahertz time domain spectroscopy are employed to probe the excitations in the materials. The lattice excitations are found to be analogous across the compounds over the whole temperature range investigated (295-4 K). The resulting full phononic characterization of the R(2)Ti(2)O(7) pyrochlore structure is then used to identify crystal field excitations observed in the materials. Several crystal field excitations have been observed in Tb(2)Ti(2)O(7) in Raman spectroscopy, among which all of the previously reported excitations. The presence of additional crystal field excitations, however, suggests the presence of two inequivalent Tb(3+) sites in the low-temperature structure. Furthermore, the crystal field level at approximately 13 cm(-1) is found to be both Raman and dipole active, indicating broken inversion symmetry in the system and thus undermining its current symmetry interpretation. In addition, evidence is found for a significant crystal field-phonon coupling in Tb(2)Ti(2)O(7). The additional crystal field information on Tb(2)Ti(2)O(7) adds to the recent discussion on the low temperature symmetry of this system and may serve to improve its theoretical understanding.

The crystal field and exchange interactions in Yb2Ti2O7

In the pyrochlore-structure compounds R2Ti2O7, the rare-earth (R) sublattice forms a network of corner-sharing tetrahedra such that the magnetic interactions may be geometrically frustrated. The low-temperature magnetic properties of these compounds are fashioned both by the frustration and by the intrinsic properties of the rare earth, that is, by the degeneracy and anisotropy of the rare-earth crystal-field ground state and by the nature, size and strength of the inter-ionic magnetic coupling. For Yb2Ti2O7, we combine 170Yb Mossbauer spectroscopy, 172Yb perturbed angular correlation, magnetization and susceptibility measurements to establish the Yb3+ crystal-field level scheme and to show that the crystal-field ground state is a well isolated Kramers doublet having a planar anisotropy. The main contribution to the Yb3+-Yb3+ coupling is the exchange interaction which is ferromagnetic. We describe the frustration-related low temperature (<1 K) properties of Yb2Ti2O7 in a separate publication.

SPIN DYNAMICS IN THE MAGNETIC CHAIN ARRAYS OF SR14CU24O41 : A NEUTRON INELASTIC SCATTERING INVESTIGATION

Below

Reexamination of the microscopic couplings of the quasi-one-dimensional antiferromagnet CuGeO 3

T\ensuremath{\approx}150 \mathrm{K},

Magnetotransport properties of a single grain boundary in a bulk La–Ca–Mn–O material

the spin arrangement in the chain arrays of

Directional solidification in the NiO-ZrO2 system

{\mathrm{Sr}}_{14}{\mathrm{C}}_{24}{\mathrm{O}}_{41}

ac magnetic behavior of large-grain magnetoresistive La0.78Ca0.22Mn0.90Ox materials

is shown to develop in two dimensions. Both the correlations and the dispersion of the observed elementary excitations agree well with a model of interacting antiferromagnetic dimers. Along the chains, the intra- and interdimer distances are equal to 2 and