Long range magnetic ordering in the quasi-one-dimensional random-bond antiferromagnet BaCu$_2$(Si$_{1-x}$Ge$_{x}$)$_2$O$_7$ is studied in $\mu$SR experiments as a function of disorder strength. Compared to the disorder-free parent materials, the saturation ordered moment is found to be considerably reduced. Moreover, even in weakly disordered species, the magnetically ordered state is shown to be highly inhomogeneous. The results are interpreted in terms of weakly coupled random spin chains, governed by the ``infinite randomness`` fixed point.

‘Multiferroic’ materials possessing simultaneously magnetic and ferroelectric orders are scarce and most of them order at low temperatures. So far, bismuth ferrite, BiFeO3, is the only reliable room-temperature multiferroic: it is ferroelectric and antiferromagnetic. The absence of a net magnetisation in this compound is a problem when one wants to use the magneto-electric effect to address magnetic information with an electric field for potential applications in spintronic devices. We show here that β-NaFeO2 is also a multiferroic material at room-temperature but with the most interesting extra property of showing weak ferromagnetism. This makes it a potentially very promising material for applications and a model compound for fundamental studies of the interaction between ferroelectricity and magnetism.

Low-dimensional spin systems, consisting of arrays of spins arranged in chains or ladders, have been investigated intensively in recent years using both exactly solvable theoretical models as well as a variety of experimental techniques. In case of random variations of the exchange couplings the renormalization group theory predicts the existence of a random-singlet (RS) state, corresponding to spins coupled at all possible distances and energy scales. The scarce availability of suitable random systems, however, has so far prevented the experimental identification of this peculiar magnetic ground state. In a combined effort using nuclear magnetic resonance (NMR), dc magnetometry and numerical simulations, we now find compelling evidence for the formation of a random-singlet state in this class of materials. Randomness seems to generate a broad distribution of local magnetic fields, in turn reflected in stretched exponential NMR relaxations. This distribution exhibits a progressive broadening with decreasing temperature, caused by the growing inequivalence of magnetic sites, as expected from the RS theory. Our findings suggest that NMR is the tool of choice for probing the low-energy physics also in other disordered magnets, where extended-scale excitations are dominant.

Heisenberg spin-1/2 chain materials are known to substantially alter their static and dynamic properties when experiencing an effective transverse staggered field originating from the varying local environment of the individual spins. We present a temperature-, angular- and field-dependent Si-29 NMR study of the model compound BaCu2Si2O7. The experimental data are interpreted in terms of the divergent low-temperature transverse susceptibility, predicted by theory for spin chains in coexisting longitudinal and transverse staggered fields. First, our analysis employs a finite-temperature density matrix renormalization group (DMRG) study of the relevant one-dimensional Hamiltonian. Next, we compare our numerical with the presently known analytical results. With an analysis based on crystal symmetries, we show how the anisotropic contribution to the sample magnetization is experimentally accessible even below the ordering temperature, in spite of its competition with the collinear order parameter of the antiferromagnetic phase. The modification of static and dynamic properties of the system due to the presence of a local transverse staggered field (LTSF) acting on the one-dimensional spin array are argued to cause the unusual spin reorientation transitions observed in BaCu2Si2O7. On the basis of a Ginzburg-Landau type analysis, we discuss aspects of competing spin structures in the presence of magnetic order and of the enhanced transverse susceptibility.

The quasi-one-dimensional compound BaCu2Si2O7 demonstrates numerous spin-reorientation transitions both for a magnetic field applied along the easy axis of magnetization and a magnetic field applied perpendicular to it. The magnetic phase diagram for all three principal orientations is obtained by magnetization and specific heat measurements. Values of all critical fields and low-temperature values of magnetization jumps are determined for all transitions.

The experimental temperature dependence (T = 2-300 K) of single crystal bulk and site susceptibilities of rare earth titanate pyrochlores R2Ti2O7 (R = Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb) is analyzed in the framework of crystal field theory and a mean field approximation. Analytical expressions for the site and bulk susceptibilities of the pyrochlore lattice are derived taking into account long range dipole-dipole interactions and anisotropic exchange interactions between the nearest neighbor rare earth ions. The sets of crystal field parameters and anisotropic exchange coupling constants have been determined and their variations along the lanthanide series are discussed.

Time-and-frequency resolved pump-probe optical spectroscopy is used to investigate the effect of the impulsive injection of delocalized excitations through a charge-transfer process in insulating CuGeO3. A large broadening of the charge-transfer edge is observed on the sub-ps timescale. The modification of this spectral feature can not be attributed to the local increase of the effective temperature, as a consequence of the energy absorbed by the pump pulse. The measured modifications of the optical properties of the system are consistent with the creation of a non-thermal state, metastable on the ps timescale, after the pump-induced impulsive modification of the electron interactions.

Monocrystals of Cu(Ge,Si)O3 with self-organized SiO2–GeO2 vitreous fibers running along the growth axis of the crystal, is a promising class of composites. This new material is achieved by floating zone technique using a ceramics with a composition located in a two-phase domain of the stability diagram of Cu(Ge,Si)O3. We report our results on the relation between the various crystal growth parameters and the periodicity and the size of the fibers. We reached 1.2 and 0.6μm respectively, and the refractive index contrast is around 0.35. These figures open the door towards telecom applications. These ones are important information for the realization of this composite from these oxides. After observation of the transverse section of this material, we report long fibers (≈750μm) not published so far as second phase structure. After chemical analyses, we showed that physico-chemical conditions are met for the appearance of Turing like structures, explaining in such a way that the composite self-organizes during the solidification and turns into an ordered structure that can be useful as metamaterials or photonic crystal. This can help for suggesting other oxide systems and to define a general method for self-organization starting in the neighbourhood of an eutectic mixture.

We have carried out temperature- and pressure-dependent Raman and x-ray measurements on single crystals of Tb2Ti2O7. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Gruneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high-pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at similar to 9 GPa. We discuss possible implications of our results on the spin-liquid behavior of Tb2Ti2O7.

We studied the field induced magnetic order in R(2)Ti(2)O(7) pyrochlore compounds with either uniaxial (R=Ho, Tb) or planar (R=Er, Yb) anisotropy, by polarized neutron diffraction. The determination of the local susceptibility tensor {chi(parallel to),chi(perpendicular)} provides a universal description of the field induced structures in the paramagnetic phase (2-270 K), whatever the field value (1-7 T) and direction. Comparison of the thermal variations of chi(parallel to) and chi(perpendicular) with calculations using the rare earth crystal field shows that exchange and dipolar interactions must be taken into account. We determine the molecular field tensor in each case and show that it can be strongly anisotropic.

Absorption spectra of the mixed chain nickelates (NdxY1−x)2BaNiO5 were measured for different x. Experimentally two evidences for the presence of non-equivalent rare-earth centers (NEREC) were found. First, the linewidths show a very strong broadening for x when the calculated distribution over NEREC is wide. Second, a complicated lineshape of the lowest-frequency line in the region of the 4I9/2→4I11/2 electronic transition is in good agreement with statistics of NEREC. A simple modeling for the line broadening due to NEREC is suggested.

We have studied the field-induced magnetic structures in Tb2Ti2O7, in a wide temperature (0.3 < T < 270 K) and field (0 < H < 7 T) range, by single crystal polarized and unpolarized neutron diffraction, with H parallel[110] axis. A ferromagneticlike structure with k = 0 propagation vector is induced, whose local order at low field and low temperature is akin to spin ice. The four Tb ions separate in alpha and beta chains having different values of the magnetic moments, which is quantitatively explained by taking the crystal field anisotropy into account. Above 2 T and below 2 K, an antiferromagneticlike structure with k = (0,0,1) is induced besides the k = 0 structure. It shows a reentrant behavior and extends over a finite length scale. It occurs together with a broadening of the nuclear peaks, which suggests a field-induced distortion and magnetostriction effect.

An ac photopyroelectric calorimeter has been used to simultaneously measure the specific heat (c(p)), thermal conductivity (K), and thermal diffusivity (D) around the antiferromagnetic to paramagnetic phase transition in CoO and NiO single crystals. Up to now, no agreement on the critical behavior of both oxides has been obtained. The results for seven samples grown in different laboratories have been compared. We have found that, irrespective of the origin of the samples, the critical exponent and amplitude ratio of c(p), D, and K in NiO agree with the predictions of the three-dimensional Heisenberg model for isotropic antiferromagnets, which contradicts previous results. On the other hand, the sharpness of the peak of specific heat and of the dip of thermal diffusivity, together with the value of the critical exponent alpha in CoO, highly increase as crystal quality improves. For the best sample, a value alpha=0.81 has been obtained, which is not only higher than previously reported results, but far away from any universality class.

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 pre-edge structure of Cu 1s excitations of high T(c) cuprates reflects the lowest-lying unoccupied electronic states in these materials, and thus it is directly relevant to the interesting low-energy excitations and their nonlocal characters. We have performed theoretical calculations of the Cu K alpha resonant x-ray emission spectroscopy (RXES) in the pre-edge region of La(2)CuO(4) with a Cu(5)O(16) cluster model in order to analyze recent experimental data. The Cu 1s to on-site Cu 3d excitation by the electric quadrupole transition and the Cu 1s to off-site Cu 3d excitation by the electric dipole transition are taken into account, confirming that the off-site transition is indeed visible but that it is split due to the effect of both the local and,the nonlocal components of screening. The calculated results of RXES spectra, as well as of x-ray absorption spectra by partial fluorescence yield method, are in reasonable agreement with the experimental data. The influence of the low-energy tail of Cu 4p states and the effects of doping electrons and holes; are also calculated and discussed. Calculations are made for Cu K alpha RXES of one-dimensional corner and edge sharing cuprates and compared to new experimental results for Sr(2)CuO(3) and GeCuO(3).

We present here temperature-dependent Raman, x-ray diffraction, and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound Dy2Ti2O7 and its nonmagnetic analog Lu2Ti2O7. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in Dy2Ti2O7, appearing below a temperature of T-c=110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low-temperature Raman experiments on O-18-isotope substituted Dy2Ti2O7 confirm the phonon origin of the new mode. These findings, absent in Lu2Ti2O7, suggest that the room-temperature cubic lattice of the pyrochlore Dy2Ti2O7 undergoes a "subtle" structural transformation near T-c. We find anomalous redshift of some of the phonon modes in both the Dy2Ti2O7 and the Lu2Ti2O7 as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.

The magnetic dynamic structure factor of the one-dimensional $S=1∕2$ chain system $\mathrm{Ba}{\mathrm{Cu}}_{2}{({\mathrm{Si}}_{0.5}{\mathrm{Ge}}_{0.5})}_{2}{\mathrm{O}}_{7}$ is studied in a wide range of energy transfers and temperatures. Contrary to previous erroneous reports [T. Masuda et al., Phys. Rev. Lett. 93, 077206 (2004)], the scaling properties observed in the range $0.5--25\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ are found to be fully consistent with expectations for a Luttinger spin liquid. At higher energies, a breakdown of scaling laws is observed and attributed to lattice effects. The results are complementary to those found in literature for other $S=1∕2$ chain compounds, such as $\mathrm{K}\mathrm{Cu}{\mathrm{F}}_{3}$ and Cu benzoate.

We have successfully grown cm3-size single crystals of the metallic-ferromagnet Sm2Mo2O7 by the floating-zone method using an infrared image furnace. The growth difficulties and the remedies found using a 2-mirror image furnace are discussed. Magnetization studies along the three crystalline axes of the compound are presented and discussed based on our recent proposal of an ordered spin-ice ground state for this compound.