Search

International audience; In the multiferroic hexagonal manganite HoMnO3, inelastic neutron scattering and synchrotron based THz spectroscopy have been used to investigate the spin waves associated to the Mn order together with Ho crystal field excitations. While the Mn order sets in first below 80 K, a spin reorientation occurs below 37 K, a rare feature in the rare earth manganites. We show that several Ho crystal field excitations are present in the same energy range as the magnons, and that they are all affected by the spin reorientation. Moreover, several anomalous features are observed in the excitations at low temperature. Our analysis and calculations for the Mn spin waves and Ho crystal field excitations support Mn-Ho coupling mechanisms as well as coupling to the lattice affecting the dynamics.

The low energy magnetic excitations spectra of the pristine and doped quasi-one-dimensional spin chains cuprates $\mathrm{SrCu}{\mathrm{O}}_{2}$ have been investigated by inelastic neutron scattering. The momentum-integrated magnetic dynamical structure factor yields a constant integrated intensity with regard to energy in the pure compound, while it shows a strong decay, at low energies, in the compounds doped with nonmagnetic impurities, namely, $\mathrm{SrC}{\mathrm{u}}_{0.99}{\mathrm{M}}_{0.01}{\mathrm{O}}_{2}$ (with $M=\mathrm{Zn}$ or Mg) and $\mathrm{S}{\mathrm{r}}_{0.99}\mathrm{L}{\mathrm{a}}_{0.01}\mathrm{Cu}{\mathrm{O}}_{2}$ ($\mathrm{C}{\mathrm{u}}^{+}$ carrying $S=0$ created within the chains). These results evidence the opening of a spin pseudogap in the two-spinon continuum of $\mathrm{SrCu}{\mathrm{O}}_{2}$ upon doping, stemming from disruptions of the spin chains by quantum impurities.

We report on the structural and magnetic properties of In2Ga2Fe2O9 and Sc2Ga2Fe2O9. X-ray diffraction measurements confirmed the presence of a single orthorhombic phase, corresponding to the expected structure for M2Ga2Fe2O9 (M=In, Sc). Neutron diffraction measurements show considerable cation disorder at different lattice sites, and suggest that there is no long range magnetic ordering down to T=10 K. Temperature dependent magnetization measurements show a separation between field-cooled and zero-field-cooled curves near Tf≈19 K. Below this temperature the M(H) curves also exhibit history dependent behavior. The temperature dependent ac susceptibility shows a peak in the magnetic dissipation, with the temperature of the peak varying systematically with frequency suggesting that this feature may be associated with a glassy relaxation. Zero-field specific heat measurements down to 4 K do not reveal any long range magnetic ordering of these compounds, nor is there any transition to an ordered state under the application of a 7 T field.

We present a neutron scattering study carried out on single-crystals of the magnetoelectric compound GaFeO3, prepared by the floating-zone (FZ) method. The refinement of the crystal structure indicates the existence of a large amount of iron on the gallium site, corresponding to a strong internal substitution disorder. We show that the Ga/Fe disorder and the magnetic transition temperature are weakly dependent on the growth conditions. The disorder also affects the magnetic excitations; our measurements evidence damped spin waves, well defined only close to the Brillouin zone centre.

We present the synthesis and the NMR characterization of a 17O enriched LaMnO3 crystalline sample. We checked that it is single phase and, more important, stoichiometric in oxygen. Its 17O enrichment estimated by NMR is about 5.5%. These first 17O NMR results obtained at T = 415 K in an undoped parent LaMnO3 manganite demonstrate that the two oxygen sites of the structure probe very different Mn spin correlations in the paramagnetic orbital ordered phase. This work opens the way to study experimentally the interactions responsible for the orbital order.

Centimetre-size single crystals of LiCoO2 were successfully grown using a floating-zone method associated with an image furnace. A high-pressure oxygen atmosphere and a relatively small growth speed were found necessary to prevent the formation of impurity phases in the grown crystal. A single crystal with a reduced amount of lithium was obtained by means of the chemical de-lithiation process using as grown LiCoO2 single crystal as the starting material. Growth conditions together with magnetic and structural properties of these crystals are discussed here.

Magnetic properties of bulk SrTi 1− x Co x O 3− δ solid solutions (for x from 0 to 0.05), prepared in air as those partially reduced, were studied by following the influence of the dopant concentration and valence. A strong paramagnetic and/or diamagnetic contribution and an extremely weak ferromagnetic contribution maintained up to room temperature is observed for all the studied compositions, including the undoped samples. While the paramagnetic contribution shows a classical evolution with cobalt concentration and valence, the ferromagnetic part of the magnetization seems to be independent of the doping process. While some of the observations can support the assumption of an intrinsic property of the SrTiO 3 matrix, the hypothesis of a ferromagnetism associated to some contamination with external magnetic impurities cannot be completely discarded and will be retained here.

The EXAFS analysis of LiV2O4, under pressure, is discussed. Subtle changes observable above 13 GPa suggest that a charge-ordering state is induced at high pressure. This state is probably similar to the one recently discussed by Katsufuji and coworkers [T. Katsufuji et al., Phys. Rev. Lett. 96 086046 (2006).] for a similar compound, which proposed the formation of “vanadium molecules” in a geometrically frustrated crystal.

We have studied the ferromagnetic and antiferromagnetic fluctuations in the charge-ordered Pr0.65Ca0.35MnO3 antiferromagnet by triple-axis neutron spectrometry. Whereas ferromagnetic fluctuations are observed above and below the charge-ordering transition (TCO), the antiferromagnetic fluctuations develop only below TCO .T he dynamical exponent z of both ferromagnetic and antiferromagnetic fluctuations are determined. The ferromagnetic fluctuations are not completely suppressed below TCO and their correlation lengths are short-ranged at all temperatures. The results are discussed with respect to the Zener polaron model recently introduced to describe the charge-ordered state of Pr0.6Ca0.40MnO3.

In the R 1− x D x MnO 3 ( x ∼0.5) manganites, the structural phase transition at T CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn 3+ and Mn 4+ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T CO on a Pr 0.6 Ca 0.4 MnO 3 single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO 6 octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 A, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn 3+ and Mn 4+ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T CO . High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.

We report on the electrical transport properties of the Mn-doped LaFe 1− x Mn x AsO oxypnictide system. We show that the substitution of iron by manganese in the FeAs layer leads to a gradual expansion of the lattice parameters and of the Fe–As and Fe–Fe bond lengths, as well as a change in the angles of the FeAs 4 tetrahedron which becomes more regular. The structural transition from tetragonal to orthorhombic coupled to the emergence of a spin density wave is rapidly suppressed by Mn doping. Interestingly, Mn doping does not lead to p-type behaviour. Moreover, no superconducting transition is observed, and the system evidences a metal–semiconductor transition around x = 0.03. Thus, there is apparently no symmetric behaviour of the manganese doping and cobalt doping in the FeAs layer of the LaFeAsO system.

The double-perovskite oxides Ca2FeMoO6, Sr2FeMoO6 and Ba2FeMoO6 show evidence of ferromagnetic metallic behaviour, with Curie temperatures of 345-426 K and saturation magnetization of 3.5-3.9 µB/formula unit. Ceramics show little anisotropic magnetoresistance, but the low-field isotropic negative magnetoresistance of Sr2FeMoO6 is 7% in 1 T at room temperature and reaches 15% at 77 K. An effect of about 1% is found in 20 mT at room temperature in both ceramics and pressed powders of this compound.

In2Ga2Fe2O9 and Sc2Ga2Fe2O9 are prepared by solid state reaction of stoichiometric mixtures of M2O3 (M: In, Sc), Ga2O3, and Fe2O3 (air, 11375—1400 °C, 24—30 h).

The Mn spin correlations were studied near the O'-O phase transition at T{JT} = 750 K, up to 950 K with 17O and 139La NMR in a stoichiometric LaMnO3 crystalline sample. The measured local hyperfine fields originate from the electron density transfered from the eg- and t2g-orbitals to the 2s(O) and 6s(La) orbits, respectively. By probing the oxygen nuclei, we show that the correlations of the Mn spins are ferromagnetic in the ab-plane and robust up to T{JT} whereas along the c-axis, they are antiferromagnetic and start to melt below T{JT}, at about 550 K. Above T{JT} the ferromagnetic Mn-Mn exchange interaction is found isotropic. The room temperature orbital mixing angle, {\phi}nmr = 109+-1.5{\deg}, of the eg ground state is close to the reported value which was deduced from structural data on Jahn-Teller distorted MnO6 octahedra. For T > T{JT} LaMnO3 can be described in terms of non-polarized eg-orbitals since both eg-orbitals are equally occupied., Comment: 7 pages, 7 figures

We report a systematic study of the $c$-lattice parameter in the Na${}_{x}$CoO${}_{2}$ phases versus Na content $xg0.5$, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an antiferromagnetic order at a N\'eel temperature ${T}_{N}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.28em}{0ex}}$22 K, which is found to occur for $x\ensuremath{\approx}0.77(1)$. Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical ${}^{23}$Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display $T$ dependencies of their NMR shifts, which scale with each other. This allows us to establish that the $T$ variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3${}^{+}$. The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of ${}^{23}$Na lines permits us to reveal that the two-dimensional structure of the Na order corresponds to ten Na sites on top of a thirteen-Co-sites unit cell, that is with $x=10/13\ensuremath{\approx}0.77$. This structure fits with that determined from local density calculations and involves triangles of three Na sites located on top of Co sites [so-called Na1 sites]. The associated ordering of the Na vacancies is quite distinct from that found for $xl0.75$.

The Pr1-xCaxMnO3 (x≈0.5) manganites display an electronic localization together with a modulated distortion of the structure below a transition temperature TCO. It is commonly attributed to charge and orbital ordering (CO/OO), which are purely electronic processes driven by the cooperative Jahn–Teller effect and the Coulomb interaction. However, an unconstrained structure refinement from a neutron-diffraction experiment below TCO on a Pr0.60Ca0.40MnO3 single crystal, gives us a structure whose distortions invite us to reconsider the microscopic mechanism of the transition. The result strongly suggests that the transition has a magneto-structural origin, associated with the formation of ferromagnetic pairs below TCO. The electronic localization and the magnetic properties of these types of compounds are re-interpreted in this context.

Layered cobaltates Na${}_{\mathrm{x}}$CoO${}_{2}$ (with $x=0.75$ and 0.79) have been studied by x-ray absorption spectroscopy at the Co-$K$ edge under pressures up to 19 GPa at room temperature. Changes in the absorption spectra and interatomic distances as well as Debye-Waller factors indicate a structural phase transition at a pressure between 6 and 8 GPa. This phase transition can be attributed to Na ion displacements.

We have investigated the magnetoelastic coupling in HoMnO${}_{3}$ by measuring the temperature and magnetic field dependence of two elastic moduli ${C}_{11}$ and ${C}_{33}$ with an ultrasonic velocity technique. In comparison with similar data in YMnO${}_{3}$, we have identified an enhanced elastic softening, predominantly on ${C}_{11}$, that scales with the spin susceptibility over a wide temperature range. Spin fluctuations induced by Ho-Mn interactions are responsible for this softening: when the softening is quenched by an in-plane magnetic field with a concomitant stabilization of the Mn order, it is rather enhanced for a field oriented along the $c$ axis. The H-T phase diagrams deduced from elastic anomalies appearing at the different phase transitions are consistent with previous microwave data. The origin of the Ho-Mn coupling is discussed in terms of a fourth-order trigonal anisotropy term in the spin Hamiltonian.

We report on the electrical resistivity, thermoelectric power, and electronic phase diagram of rhodium-doped NdFeAsO. Rhodium doping suppresses the structural phase transition and spin-density wave observed in the undoped material, and superconductivity emerges at $x$ close to 0.05, despite the distortion of ${\text{FeAs}}_{4}$ tetrahedra induced by the large size difference between Rh and Fe elements. The ${T}_{\text{c}}(x)$ curve is domelike, and the highest ${T}_{\text{c}}$ is reached at $x=0.1$, with ${T}_{\text{c}}^{\text{onset}}=18\text{ }\text{K}$. An upturn of the electrical resistivity above ${T}_{\text{c}}$ has been observed, with a Kondo-type behavior above ${T}_{\text{c}}$ and a Fermi-liquid behavior close to room temperature.

The double perovskite Ca2FeMoO6 is a poor metal which orders magnetically at 345 K. The 57Fe Mossbauer hyperfine field and isomer shift at 4.2 K (48 T and 0.70 mm/s relative to α-Fe) indicate an Fe3+ configuration 3d5↑ which is strongly hybridized with the Mo 4d↓t2g orbitals. The saturation magnetization of 3.6μB/formula is consistent with neutron diffraction measurements which indicate iron and molybdenum moments of 4.0(1) and −0.2(3)μB per iron or molybdenum, respectively. The sample exhibits approximately 6% cation disorder relative to the ideal NaCl-type order of Fe and Mo in the double perovskite structure.

We report an ultrasonic investigation of the elastic moduli on a single crystal of hexagonal $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$ as a function of temperature. Stiffening anomalies in the antiferromagnetic N\'eel state below ${T}_{N}=72.4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ are observed on all the four elastic moduli ${C}_{ii}$. The anomalies are the most important on ${C}_{11}$ and ${C}_{66}$ for in-plane elastic deformations; this is consistent with a strong coupling of the lattice with the in-plane exchange interactions. We use a Landau free energy model to account for these elastic anomalies. We derive an expression which relates the temperature profile of the anomaly to the order parameter; the critical exponent associated with this parameter $\ensuremath{\beta}=0.42$ is not consistent with a chiral $XY$ or three-dimensional Heisenberg universality class, but more in agreement with a conventional antiferromagnetic long-range order. A tiny softening anomaly on ${C}_{11}$ for which hysteresis effects are observed could be indicative of an interaction between ferroelectric and magnetic domains at ${T}_{N}$. Moreover, magnetic fluctuation effects both above and below ${T}_{N}$ are identified through abnormal temperature and magnetic field effects.

We have studied the magnetism of the half-doped charge ordered manganite YBaMn2O6. A formation of ferromagnetic plaquettes of four Mn atoms in the charge ordered phase below T_{CO} approximately 480 K is inferred from high temperature magnetic susceptibility data and the magnetic structure, as determined by neutron powder diffraction at T=1.5 K. The results indicate that new fourfold Mn paramagnetic units form between T_{N}

Extended x-ray absorption fine structure analysis of $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ at the vanadium $K$ edge under pressure is discussed. Changes in the absorption spectra at room temperature show that a structural phase transition is induced at high pressure. This phase transition can be attributed to a charge ordering, as it was observed in the isostructural compound $\mathrm{Al}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. The absorption spectra have been analyzed by considering the two models previously proposed for $\mathrm{Al}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$. Our results indicate that the high pressure structure of $\mathrm{Li}{\mathrm{V}}_{2}{\mathrm{O}}_{4}$ is probably implying the formation of ``vanadium molecules'' in a geometrically frustrated crystal.

Inelastic neutron scattering measurements have been performed on $Y{\mathrm{MnO}}_{3}$, aiming to study the interplay between spin and lattice degrees of freedom in this hexagonal multiferroic material. Our study provides evidence for a strong coupling between magnons and phonons, evidenced by the opening of a gap below ${T}_{N}$ in the dispersion of the transverse acoustic phonon mode polarized along the ferroelectric axis. The resulting upper phonon branch is found to lock on one of the spin-wave modes. These findings are discussed in terms of a possible hybridization between the two types of elementary excitations.

We report on the pressure effects on the orbital polaron lattice in the lightly doped manganites $\mathrm{La_{1-x}Sr_xMnO_{3}}$, with $x\sim 1/8$. The dependence of the orbital polaron lattice on $negative$ chemical pressure is studied by substituting Pr for La in $\mathrm{(La_{1-y}Pr_y)_{7/8}Sr_{1/8}MnO_{3}}$. In addition, we have studied its hydrostatic pressure dependence in $\mathrm{(La_{0.9}Pr_{0.1})_{7/8}Sr_{1/8}MnO_{3}}$. Our results strongly indicate that the hopping $t$ significantly contributes to the stabilization of the orbital polaron lattice and that the orbital polarons are ferromagnetic objects which get stabilized by local double exchange processes. The analysis of short range orbital correlations and the verification of the Grueneisen scaling by hard x-ray, specific heat and thermal expansion data reinforces our conclusions.

We report on Raman scattering measurements of single crystalline ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($x=0$, 0.06, 0.09, and 0.125), focusing on the high-frequency regime. We observe multiphonon scattering processes up to fourth order which show distinct features: (i) anomalies in peak energy and its relative intensity and (ii) a pronounced temperature, polarization, and doping dependence. These features suggest a mixed orbiton-phonon nature of the observed multiphonon Raman spectra.

We present an inelastic light scattering study of single crystalline ${({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y})}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3}$ ($0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14$, $y=0$ and $x=1∕8$, $0\ensuremath{\leqslant}y\ensuremath{\leqslant}0.5$). A giant softening up to $20\char21{}30\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ of the $\mathrm{Mn}\ensuremath{-}\mathrm{O}$ breathing mode has been observed only for the ferromagnetic insulating (FMI) samples $(0.11\ensuremath{\leqslant}x\ensuremath{\leqslant}0.14)$ upon cooling below the Curie temperature $({T}_{C})$. With increasing Pr-doping the giant softening is strongly suppressed. This is attributed to a coupling of the breathing mode to orbital polarons, which are present in the FMI state. Moreover, the softening that persists to lowest temperature without saturation suggests a continuous, temperature-dependent evolution of orbital polarons below ${T}_{C}$.

We study the spin dynamics of the manganites ${\mathrm{La}}_{0.9}{\mathrm{Ca}}_{0.1}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.91}{\mathrm{Sr}}_{0.09}\mathrm{Mn}{\mathrm{O}}_{3}$ in magnetic field. Previous studies in zero field have shown that the magnetic ground state is inhomogeneous with ferromagnetic $(F)$ droplets embedded in a canted antiferromagnetic (CAF) matrix. The spin dynamics consists of two spin wave branches in a high and a low-energy range, respectively. However, the assignment of these two branches either to a mean CAF medium or to $F$ droplets embedded in an AF matrix, was uncertain. The present study shows that these two branches do not follow the mean-field calculation for a homogeneous CAF state and do not correspond either to a phase separation between AF and $F$ states. The progressive spin-flop transitions observed for $H\ensuremath{\Vert}\mathbf{b}$ and $H\ensuremath{\Vert}\mathbf{a}+\mathbf{b}$, give rise to strong changes in the dispersion of the spin waves. The low-energy branch, only visible around $F$ zone centers at zero field, appears at any $q$-value in magnetic field. Moreover, as soon as the AF spin component deviates from its easy axis, leading to a decrease of the magnetic anisotropy, the high-energy branch is no more defined near the zone boundary. Its intensity is transferred to a higher flat level, likely related to defects in the CAF matrix. These new results strongly support a picture where these magnetic excitations arise from a modulated canted antiferromagnetic ground state consisting of two coupled $F$ and AF components.

We have studied the magnetic properties of a single crystal of the mineral paramelaconite, ${\mathrm{Cu}}_{4}{\mathrm{O}}_{3},$ showing a tetragonally distorted (space group $I{4}_{1}/amd)$ magnetic pyrochlore sublattice of $S=1/2.$ The magnetic susceptibility shows a drop around 40 K that could be interpreted as an antiferromagnetic transition or the appearance of a nonmagnetic state at lower temperatures. Neutron diffraction measurements at low temperature show unambiguously the transition to an ordered state of propagation vector $\mathbf{k}=(1/2,1/2,1/2)$ with respect to the reciprocal basis of the conventional body centered cell. Referred to the pseudocubic $\mathrm{Fd}3m$ setting it is ${\mathbf{k}}_{C}=(0,1,1/2).$ To the best of our knowledge, this is an unprecedented magnetic ordering in pyrochlore lattices. We argue that the observed magnetic ordering cannot be explained within models limited to isotropic superexchange interactions.

The magnetic phase separation in single crystals of the ${\mathrm{P}\mathrm{r}}_{0.67}{\mathrm{C}\mathrm{a}}_{0.33}{\mathrm{M}\mathrm{n}\mathrm{O}}_{3}$ manganites is studied using polarized small angle neutron scattering. The measured spectra give a fractal dimension consistent with a configuration in ferromagnetic filaments of nanometric diameter. We argue here that localized charge carriers hop in a random walk fashion mediating a ferromagnetic "hopping exchange" which coexists with superexchange to create the filamentary phase separation. The arguments for this physical picture are validated by Monte Carlo simulations, where magnetism and transport are treated in a self-consistent manner.

We have refined the crystal structures of a Pr(0.60)Ca(0.40)MnO(3) single crystal from neutron diffraction data. The result at low temperature gives a superstructure that cannot be interpreted as Mn(3+)/Mn(4+) charge ordering. The pattern of atom displacements suggests the trapping of electrons within pairs of Mn sites, involving both a local double exchange and a polaronic-like distortion. The two mechanisms act together to form vibronic localized electronic states: Zener polarons. We have confirmed this picture by showing how it elucidates the unconventional paramagnetic behavior of half-doped manganites.

Measuring the magnetization and the specific heat of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $0.11l~xl~0.15$ we have investigated the charge order (CO) transition in lightly doped manganites. Though the properties of the ferromagnetic insulating state at low temperatures hardly depend on x, there is a very strong concentration dependence of the specific heat and the magnetization. While the specific-heat anomaly due to ferromagnetic order strongly increases with x, the jumps of both, entropy and magnetization at the CO transition decreases drastically. The analysis of our data shows that the entropy changes at the CO transition are mainly due to spin degrees of freedom, which strongly suggests that the melting of charge order is driven by magnetic energy.

We present magnetization measurements on lightly doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{MnO}}_{3}$ single crystals with $0.06l~xl~0.15$ in external fields up to 14 T at temperatures between 4.2 and 180 K. In contrast to the ferromagnetic behavior which is found for $xg~0.11,$ the samples with $x=0.06$ and $x=0.09$ exhibit a pronounced hysteresis and a constant high-field susceptibility. By analyzing this particular behavior we can rule out a phase separation into ferromagnetic and antiferromagnetic regions. On the other hand, a model of a canted antiferromagnet can account for the main experimental features.

We have studied the crystal structure of ${\mathrm{LaMnO}}_{3}$ under pressure up to 80 kbar at room temperature by TOF neutron diffraction. The analysis of the crystal structure as a function of pressure has revealed that the Jahn-Teller (JT) distortion is stable up to about 70 kbar. The contraction of the ${\mathrm{MnO}}_{6}$ octahedra is nearly isotropic, but the average tilt angle diminishes with increasing pressure. This behavior shows that ${\mathrm{LaMnO}}_{3}$ evolves towards a wider electronic band system. Above 70 kbar the compound undergoes a transition to an unknown phase. This transition is probably due to the suppression of the Jahn-Teller effect because of the closing of the JT gap: the ionic ${\mathrm{Mn}}^{3+}$ species disappear and the system evolves towards a metalliclike phase. At low temperature the effect of the applied pressure is first to increase the transition temperature and second to suppress magnetic ordering when the metalliclike phase is reached.

The biaxial and uniaxial magnetic anisotropies of epitaxial { 100} oriented La0.7(Sr,Ca)0.3MnO3 films on different substrates and of a sphere-shaped single crystal were measured by torque magnetometry at temperatures ranging from T = 20K to room temperature. In the films the biaxial anisotropy with easy axes 4J/m3. The uniaxial anisotropy was |Ku| ≤ 103 J/m3 and is explained by anisotropic stress. In the single crystal the main anisotropy is uniaxial with a hard axis parallel to [ 111], and with an anisotropy constant KR up to 104J/m3. KR was measured in different planes and explained by the rhombohedral lattice distortion. The biaxial anisotropy in the crystal (K1 = −103 J/m3 at T = lOOK) was smaller than in the films. Differences between film and single crystalline behaviour are discussed in terms of crystal structure and lattice constriction.

We discuss the possibility of a non-collinear magnetic structure to be the ground state of the half-doped manganites, as a natural consequence of the recently proposed alternative model to Mn 3+ /Mn 4+ charge ordering in such systems (Phys. Rev. Lett. 89 (2002) 097205). We show that the new non-collinear model and the collinear arrangement known as the CE-state, are almost two degenerate solutions for the refinement of the neutron powder diffraction studies that we have performed on half-doped manganites.

X-ray powder diffraction was employed to study the effect of chemical (i.e., internal) and external (hydrostatic) pressure in layered cobaltates doped with manganese. The results show that the effects of Mn substitution on the structural parameters of Na2/3(Co1−xMnx)O2 are equivalent to those of an external pressure with dependence of the order of −5 GPa/Mn.

p-type BiCuSeO, a layered oxyselenide composed of conductive (Cu2Se2)2− layers alternately stacked with insulating (Bi2O2)2+ layers, shows an enhancement of the electrical conductivity after substituting Bi3+ by Sr2+, from 470 S m−1 (BiCuSeO) to 4.8×104 S m−1 (Bi0.85Sr0.15CuSeO) at 293 K. Coupled to high Seebeck coefficients, this leads to promising values of the thermoelectric power factor that exceeds 500 μW m−1 K−2 at 873 K. Moreover, the thermal conductivity of these layered compounds is lower than 1 W m−1 K−1 at 873 K. Maximum ZT values reach 0.76 at 873 K, making this family promising for thermoelectric applications in the medium temperature range.

The magnetic properties of (La1 − x Sr x )MnO3 crystals have been studied. The ac susceptibility χ = χ′ + iχ″ has been measured at frequencies ranging from 10 Hz to 1 000 Hz. The x = 0.25 crystal shows a typical ferromagnetic behaviour with a dimensionless SI susceptibility of 6 below T c = 350 K limited by the demagnetizing field. Both x = 0 and x = 0.06 crystals show very sharp peaks in χ′ and χ″ at the Neel points, 139 K and 135 K respectively. This is quite unlike the usual behaviour of an antiferromagnet. Magnetization measurements on an untwinned single crystal of LaMnO3 show a weak moment of 0.18 μ B at 4.2 K which is directed along the c axis. The origin of this canted antiferromagnetic structure is discussed in terms of anisotropy and of Dzyaloshinskii-Moriya interaction. It is concluded that anistropy is insufficient to explain the canted antiferromagnetism.

The iron-pnictide derivatives, superconductors when appropriately doped, present large thermoelectric power factors, with potential application for thermoelectric cooling at low temperatures. Various compounds of REFeAsO family (with RE = Sm, Nd or La, whether doped with fluorine or not) show large values of the Seebeck coefficient, typically of the order of -100 µV/K. A dependence of the Seebeck coefficient with the doping level was observed. The maximum values of the Seebeck coefficient are obtained at around 100 K with similar broad peak shapes independently of the three RE-ions we used.