Evolution of magneto-orbital order upon B-site electron doping in Na1-xCaxMn7O12 quadruple perovskite manganites

We present the discovery and refinement by neutron powder diffraction of a new magnetic phase in the Na1-xCaxMn7O12 quadruple perovskite phase diagram, which is the incommensurate analogue of the well-known pseudo-CE phase of the simple perovskite manganites. We demonstrate that incommensurate magnetic order arises in quadruple perovskites due to the exchange interactions between A and B sites. Furthermore, by constructing a simple mean field Heisenberg exchange model that generically describes both simple and quadruple perovskite systems, we show that this new magnetic phase unifies a picture of the interplay between charge, magnetic and orbital ordering across a wide range of compounds.Comment: Accepted for publication in Physical Review Letter

Synthesis and characterization of multiferroic BiMn7_7O12_{12}

We report on the high pressure synthesis of BiMn$_7$O$_{12}$, a manganite displaying a "quadruple perovskite" structure. Structural characterization of single crystal samples shows a distorted and asymmetrical coordination around the Bi atom, due to presence of the $6s^{2}$ lone pair, resulting in non-centrosymmetric space group Im, leading to a permanent electrical dipole moment and ferroelectric properties. On the other hand, magnetic characterization reveals antiferromagnetic transitions, in agreement with the isostructural compounds, thus evidencing two intrinsic properties that make BiMn$_7$O$_{12}$ a promising multiferroic material.Comment: 4 pages, 3 figure

Optical study of the vibrational and dielectric properties of BiMnO3

BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also ferroelectric (FE) due to a non-centro-symmetric C2 structure, until diffraction data indicated that its space group is the centro-symmetric C2/c. Here we present infrared phonon spectra of BMO, taken on a mosaic of single crystals, which are consistent with C2/c at any T > 10 K, as well as room-temperature Raman data which strongly support this conclusion. We also find that the infrared intensity of several phonons increases steadily for decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300 K to 45 at 10 K. At variance with FE materials of displacive type, no appreciable softening has been found in the infrared phonons. Both their frequencies and intensities, moreover, appear insensitive to the FM transition at Tc

Thermoelectric behavior of Ruddlesden-Popper series iridates

The goal of this work is studying the evolution of thermoelectric transport across the members of the Ruddlesden-Popper series iridates Srn+1IrnO3n+1, where a metal-insulator transition driven by bandwidth change occurs, from the strongly insulating Sr2IrO4 to the metallic non Fermi liquid behavior of SrIrO3. Sr2IrO4 (n=1), Sr3Ir2O7 (n=2) and SrIrO3 (n=inf.) polycrystals are synthesized at high pressure and characterized by structural, magnetic, electric and thermoelectric transport analyses. We find a complex thermoelectric phenomenology in the three compounds. Thermal diffusion of charge carriers accounts for the Seebeck behavior of Sr2IrO4, whereas additional drag mechanisms come into play in determining the Seebeck temperature dependence of Sr3Ir2O7 and SrIrO3. These findings reveal close relationship between magnetic, electronic and thermoelectric properties, strong coupling of charge carriers with phonons and spin fluctuations as well as relevance of multiband description in these compounds.Comment: main paper + supplementary informatio

Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMn3_3Mn4_4O12_{12}

By means of neutron powder diffraction, we investigated the effect of the polar Bi$^{3+}$ ion on the magnetic ordering of the Mn$^{3+}$ ions in BiMn$_3$Mn$_4$O$_{12}$, the counterpart with \textit{quadruple} perovskite structure of the \textit{simple} perovskite BiMnO$_3$. The data are consistent with a \textit{noncentrosymmetric} spacegroup $Im$ which contrasts the \textit{centrosymmetric} one $I2/m$ previously reported for the isovalent and isomorphic compound LaMn$_3$Mn$_4$O$_{12}$, which gives evidence of a Bi$^{3+}$-induced polarization of the lattice. At low temperature, the two Mn$^{3+}$ sublattices of the $A'$ and $B$ sites order antiferromagnetically (AFM) in an independent manner at 25 and 55 K, similarly to the case of LaMn$_3$Mn$_4$O$_{12}$. However, both magnetic structures of BiMn$_3$Mn$_4$O$_{12}$ radically differ from those of LaMn$_3$Mn$_4$O$_{12}$. In BiMn$_3$Mn$_4$O$_{12}$ the moments $\textbf{M}_{A'}$ of the $A'$ sites form an anti-body AFM structure, whilst the moments \textbf{M}$_{B}$ of the $B$ sites result from a large and \textit{uniform} modulation $\pm \textbf{M}_{B,b}$ along the b-axis of the moments \textbf{M}$_{B,ac}$ in the $ac$-plane. The modulation is strikingly correlated with the displacements of the Mn$^{3+}$ ions induced by the Bi$^{3+}$ ions. Our analysis unveils a strong magnetoelastic coupling between the internal strain created by the Bi$^{3+}$ ions and the moment of the Mn$^{3+}$ ions in the $B$ sites. This is ascribed to the high symmetry of the oxygen sites and to the absence of oxygen defects, two characteristics of quadruple perovskites not found in simple ones, which prevent the release of the Bi$^{3+}$-induced strain through distortions or disorder. This demonstrates the possibility of a large magnetoelectric coupling in proper ferroelectrics and suggests a novel concept of internal strain engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table

Electron localization and possible phase separation in the absence of a charge density wave in single-phase 1T-VS2_2

We report on a systematic study of the structural, magnetic and transport properties of high-purity 1T-VS$_2$ powder samples prepared under high pressure. The results differ notably from those previously obtained by de-intercalating Li from LiVS$_2$. First, no Charge Density Wave (CDW) is found by transmission electron microscopy down to 94 K. Though, \textit{ab initio} phonon calculations unveil a latent CDW instability driven by an acoustic phonon softening at the wave vector ${\bf q}_{CDW} \approx$ (0.21,0.21,0) previously reported in de-intercalated samples. A further indication of latent lattice instability is given by an anomalous expansion of the V-S bond distance at low temperature. Second, infrared optical absorption and electrical resistivity measurements give evidence of non metallic properties, consistent with the observation of no CDW phase. On the other hand, magnetic susceptibility and NMR data suggest the coexistence of localized moments with metallic carriers, in agreement with \textit{ab initio} band structure calculations. This discrepancy is reconciled by a picture of electron localization induced by disorder or electronic correlations leading to a phase separation of metallic and non-metallic domains in the nm scale. We conclude that 1T-VS$_2$ is at the verge of a CDW transition and suggest that residual electronic doping in Li de-intercalated samples stabilizes a uniform CDW phase with metallic properties.Comment: 22 pages, 10 Figures. Full resolution pictures available at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.89.23512