Magnetic Structure and Interactions in the Quasi-1D Antiferromagnet CaV2_2O4_4

CaV$_2$O$_4$ is a spin-1 antiferromagnet, where the magnetic vanadium ions are arranged on quasi-one-dimensional (1D) zig-zag chains with potentially frustrated antiferromagnetic exchange interactions. High temperature susceptibility and single-crystal neutron diffraction measurements are used to deduce the non-collinear magnetic structure, dominant exchange interactions and orbital configurations. The results suggest that at high temperatures CaV$_2$O$_4$ behaves as a Haldane chain, but at low temperatures, orbital ordering lifts the frustration and it becomes a spin-1 ladder.Comment: 5 pages, 4 figure

Etudes par diffraction des corrélations entre magnétisme et structure des manganites à ordre de charges

Cette thèse se consacre à des études par diffraction des transitions de phases structurales et magnétiques dans les manganites du système Pr_(1-x)Ca_(x)MnO_3. Pour des dopages allant de x = 0.33 à x = 0.5, le diagramme de phases (x,T) n'y présente que des phases isolantes. Sur la base de la théorie du super-échange (SE), leurs propriétés peuvent s'interpréter comme l'existence d'un ordre de charges Mn3+/Mn4+ concomitant à un ordre des orbitales d_z2 sur les ions Mn3+. Ces échantillons présentent par ailleurs des effets de magnétorésitance géante. On comprend alors qualitativement le phénomène par la coexistence de phases antiferromagnétiques (AF) et ferromagnétiques (F), attribuées à des phases isolantes et conductrices respectivement. La magnétorésistance traduirait alors la croissance de la fraction ferromagnétique sous l'application d'un champ magnétique. La conclusion des études structurales et microstructurales menées dans cette thèse contraste singulièrement avec ces images. Les échantillons de poudres et le monocristal étudiés semblent montrer qu'au contraire, leurs phases cristallines sont homogènes à champ nul. Pour résoudre cette contradiction, nous présentons une étude de diffraction sur un monocristal maclé de Pro_0.60Ca_0.40MnO_3 de la structure de la phase d'ordre de charges. Cette étude nous autorise à infirmer l'une des hypothèses de base sur laquelle s'appuie la théorie du SE, et qui stipule que l'état électronique du Mn est à valence mixte Mn3+/Mn4+ dans ces composés. Les distorsions structurales indiquent qu'en fait, l'ordre de charges consiste dans la localisation des électrons sur des dimères de Mn, couplés ferromagnétiquement par le double échange de cet électron. La nouveauté de cette interprétation conduit à reconsidérer plusieurs interprétations des propriétés des manganites : en particulier, elle montre qu'il est nécessaire de ré-interpréter le magnétisme de ces phases isolantes au delà du super échange, en offrant de nouvelles pistes qui mettent en évidence l'importance du rôle de la frustration de l'échange dans ces systèmes.This thesis dedicates itself to diffraction studies of the structural and magnetic transitions in the manganites of the system Pr_(1-x)Ca_xMnO_3. For dopings going from x = 0.33 to 0.5, the (x, T) phase diagram presents only insulating phases. On the basis of the superexchange theory (SE), their fundamental state can be interpreted as the existence of an Mn3+ /Mn4+ -type of charge ordering, concomitant with an ordering of the d_z2 orbitals on the Mn3+ ions. These samples display also giant magnetoresitance properties in this state. One can understand qualitatively the phenomenon by considering that antiferromagnetic (AF) and ferromagnetic (F) phases which are attributed to an insulating and a conductive state respectively, are coexisting. In this scope, the magnetoresitance would result in the growth and percolation of the ferromagnetic fraction of the sample under the application of a magnetic field. The conclusion of the structural and microstructural studies led in this thesis contrasts with these ideas. The powder and the single crystalline samples studied are displaying homogeneous crystalline phases at zero field. To resolve such a contradiction, we present a neutron diffraction study on a twinned single crystal of Pr_O.60Ca_0.4OMnO3 of the structure of the charge ordered phase. This study allows us to invalidate the mixed valence Mn3+/Mn4+ for the electronic state of Mn in these compounds, on which the SE theory is based. The structural distortions we find indicate another type of charge order, that consists in the localization of the electrons on Mn dimers. These electron are involved a double exchange process, coupling ferromagnetically the Mn spins of each pair. The novelty of this interpretation invite to reconsider several interpretations of the properties of manganite : in particular, it shows that the understanding of the magnetism of their insulating phases, put forward the role of the magnetic exchange frustration in these systems, when we go beyond the super-exchange theory.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Magnetically driven dielectric and structural behavior in Bi_0.5La_0.5FeO₃

A detailed structural analysis of the antiferromagnetic (Gz-type) lanthanum doped bismuth ferrite - Bi0.5La0.5FeO3 (Pn′ma′) – using variable-temperature powder neutron diffraction is reported. The analysis highlights a structural link between changes in the relative dielectric permittivity and changes in the FeO6 octahedral tilt magnitudes, accompanied by a structural distortion of the octahedra with corresponding A-site displacement along the c-axis; this behavior is unusual due to an increasing in-phase tilt mode with increasing temperature. The anomalous orthorhombic distortion is driven by magnetostriction at the onset of antiferromagnetic ordering resulting in an Invar effect along the magnetic c-axis and anisotropic displacement of the A-site Bi3+ and La3+ along the a-axis

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

BaFe₉LiO₁₅: A New Layered Antiferromagnetic Ferrite

The new Fe³⁺ oxide BaFe₉LiO₁₅ is isostructural with the magnetically frustrated material BaV₁₀O₁₅, adopting a structure based on the stacking of close-packed pure oxide and BaO₇ layers. Neutron diffraction and Mössbauer spectroscopy shows that BaFe₉LiO₁₅ is long-range antiferromagnetically ordered with a Néel temperature of 460 K. The magnetic ordering of antiferromagnetically coupled ferromagnetic planes is stabilized by 90° and 180° superexchange interactions between the Fe³⁺ cations that supersede the frustrated in-plane direct exchange observed in t_2g-only systems

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted

New Twists on the Perovskite Theme: Crystal Structures of the Elusive Phases R and S of NaNbO₃

The crystal structure of NaNbO₃ has been studied in detail in the temperature regime 360 < <i>T</i> < 520 °C using a combination of high-resolution neutron and synchrotron X-ray powder diffraction, supported by first-principles calculations. A systematic symmetry-mode analysis is used to determine the presence of the key active distortion modes that, in turn, provides a small and an unambiguous set of trial structural models. A unique model for Phase S (480 < <i>T</i> < 510 °C) is elucidated, having a 2 × 2 × 4 superlattice of the aristotype perovskite structure, space group <i>Pmmn.</i> This unusual and unique structure features a novel example of a <i>compound</i> octahedral tilt system in a perovskite. Two possible structural models for Phase R (370 < <i>T</i> < 470 °C) are determined, each having a 2 × 2 × 6 superlattice and differing only in the nature of the complex tilt system along the ‘long’ axis. It is impossible to identify a definitive model from the present study, although reasons for preferring one over the other are discussed. Some of the possible pitfalls in determining such complex, pseudosymmetric crystal structures from powder diffraction data are also highlighted