Synthesis and crystal growth

Abstract:

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M= Ni, Co, Fe were investigated at low temperature. In LiNiPO 4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Néel temperature is approached from below. LiFePO4 shows more conventional behavior, typical for a three-dimensional antiferromagnet. Measurements on Li xFePO4 with x=0.8,0.9, and 1 show evidence for lithium diffusion below ∼250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ∼100 meV and a diffusion constant of DLi∼10-10to10 -9 cm2 s-1 at room temperature. © 2011 American Physical Society.

Abstract:

The magnetic ground states of the isostructural double perovskites Ba 2NaOsO6 and Ba2LiOsO6 are investigated with muon-spin relaxation. In Ba2NaOsO6 long-range magnetic order is detected via the onset of a spontaneous muon-spin precession signal below Tc=7.2±0.2K, while in Ba 2LiOsO6 a static but spatially disordered internal field is found below 8 K. A probabilistic argument is used to show from the observed precession frequencies that the magnetic ground state in Ba 2NaOsO6 is most likely to be low-moment (≈0.2μB) ferromagnetism and not canted antiferromagnetism. Ba2LiOsO6 is antiferromagnetic and we find a spin-flop transition at 5.5T. A reduced osmium moment is common to both compounds, probably arising from a combination of spin-orbit coupling and frustration. © 2011 American Physical Society.

Abstract:

By combining bulk properties, neutron diffraction, and nonresonant x-ray diffraction measurements, we demonstrate that the new multiferroic Cu 3Nb2O8 becomes polar simultaneously with the appearance of generalized helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins-an observation that cannot be reconciled with the conventional theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically induced structural chirality. © 2011 American Physical Society.

Abstract:

We present a study of the thermal conductivity (κ), specific heat (Cp) and Raman spectra of La1-xSrxCoO 3 (x=0,0.1) single crystals. Both the specimens have low thermal conductivity and board Raman peaks, arising from strong scattering of phonons by lattice disorder, produced by (and doping-enhanced) spin-states admixture of the Co3+ ions. The thermal conductivity anomalously deviates from ∼1/T behaviour at high (room) temperatures, expected of an insulator. High-temperature specific heat reveals large decrease in the metal-insulator (M-I) transition temperature with Sr-doping. © 2011 American Institute of Physics.

Abstract:

The origin of colossal magnetoresistance (CMR) in manganese oxides is among the most challenging problems in condensed-matter physics today. The true nature of the low-temperature electronic phase of these materials is heavily debated. By combining photoemission and tunnelling data, we show that in the archetypal bilayer system La 2-2x Sr 1+2x Mn 2 O 7 , polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state of bilayer manganites supports a vanishing coherent quasi-particle spectral weight at the Fermi level throughout k-space. The incoherence of the charge carriers, resulting from strong electron-lattice interactions and the accompanying orbital physics, offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopies. The stacking number N is the key factor for true metallic behaviour, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in this system.