Magnetodielectric coupling and multi-blocking effect in the Ising-chain magnet Sr2Ca2CoMn2O9

We have demonstrated magnetodielectric (MD) coupling in an Ising-chain magnet Sr2Ca2CoMn2O9, via detailed investigation of ac susceptibility and dielectric constant as a function of temperature, magnetic field and frequency. Sr2Ca2CoMn2O9 consists of spin-chains, made of the regular stacking of one CoO6 trigonal prism with two MnO6 octahedra. The (Co2+ Mn4+ Mn4+) unit stabilizes a (up-down-up) spin-state along the chains which are distributed on a triangular lattice. This compound undergoes a partially disordered antiferromagnetic transition at TN ~ 28 K. The dielectric constant exhibits a clear peak at TN only in presence of an external magnetic field (above 5 kOe), evidencing the presence of MD coupling, which is further confirmed by field-dependent dielectric measurements. We argue that spatial inversion symmetry can be broken as a result of exchange-striction along each spin chain, inducing uncompensated local dipoles. At low temperatures, a dipolar relaxation phenomenon is observed, bearing strong similarities with the blocking effect typical of the spin dynamics in this compound. Such a spin-dipole relationship is referred to as a multi-blocking effect, in relation with the concept of magnetodielectric multiglass previously introduced for related materials.Comment: Multiferroicity, Magnetoelectric coupling, Complex exchange interaction, spin-chain oxid

Non-linear effects and Joule heating in I-V curves in manganites

We study the influence of the Joule effect on the non-linear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit cell parameters as an internal thermometer in X-ray and neutron diffraction. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath are at the origin of the non-linearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.Comment: accepted in Journal of Applied Physic

Non-stoechiometrie par creation de lacunes anioniques : oxydes mixtes de manganese et de fer, de structure apparentee a la perovskite

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Superexchange interactions between Fe-3d and Ln-4f states: long range antiferromagnetism in perovskite derivatives Ba 3 LnFe 2 O 7.5

International audienc

Superexchange interactions between Fe-3d and Ln-4f states: long range antiferromagnetism in perovskite derivatives Ba 3 LnFe 2 O 7.5

International audiencePerovskite derivatives Ba3LnFe2O7.5, with Ln = Sm, Eu, Gd, Dy, Ho, Er have been synthesized. These oxides isotypic to the already known yttrium phase crystallize in the P21/c space group. Their structure can be described as an assemblage of double “FeLn” chains built up of isolated FeO4 tetrahedrons and YO6 octahedrons, interconnected through dimeric tetrahedral Fe2O7 groups. Magnetization and specific heat measurements show that all the members of this series, including the Y-phase, exhibit long range antiferromagnetic ordering, with TN ranging from 6.2 K for Y to 12.5 K for Dy. The evolution of TN with respect to the rare earth size is complex and similar to that observed for the structurally related Ba2LnFeO5 perovskites, but shows significantly higher values. The antiferromagnetism of the Y-phases is ascribed to SSE Fe–O–O–Fe interactions along the chains in both oxides “Ba3Y” and “Ba2Y”, whereas in addition transversal SSE/SE Fe–O–O–Fe–O–Fe–O–O–Fe interactions between the chains are proposed for the “Ba3Y” phase. For magnetic lanthanides, the antiferromagnetic behavior of both series implies strong 3d(Fe)–4f(Ln) interactions, i.e. SE Ln–O–Fe interactions along the chains in both series but in addition transversal SE Ln–O–Fe–O–Fe–O–Ln interactions between the chains in the “Ba3Y” series. The particular role of the magnetic moment and magnetic anisotropy of the lanthanide is also considered

Contrôle de la stoechiométrie en oxygène dans les oxydes "114" de type LnBaFe4O7+d (structure et magnétisme)

Ce travail de thèse est axé sur la caractérisation des propriétés chimiques, structurales et physiques d une nouvelle famille d oxyde de fer de formule LnBa(Fe,M)4O7+ . Ces oxydes, appartenant la famille grandissante des composés dits 114 , possèdent un type structural basé sur la symétrie cubique des 114 , contrairement aux composés à base de cobalt. La description de leur réactivité vis-à-vis de l oxygène, notamment à température ambiante avec le dioxygène de l air, nous a conduit à porter une attention particulière à la caractérisation de la quantité d oxygène excédentaire . Cette approche nous a permis de stabiliser de nouvelles structures pour les composés stœchiométriques LnBaFe4O7.0 ( = 0) avec Ln = Y, Dy-Lu. L étude de ces nouvelles structures à rendu possible la compréhension des phénomènes gouvernant l évolution des propriétés magnétiques et de transport dans cette famille de composés. Nous avons montré grâce à la synthèse de compositions originales, en substituant des atomes de fer par du manganèse, que les capacités d absorption de l oxygène pouvaient être contrôlées par la nature des substituants. Enfin l étude des composés CaBaFe4O7+ et CaBaCo4O7+ à permis de préparer par électrochimie des phases oxydées métastables présentant des propriétés magnétiques originales.This works is based on the characterization of the chemical, structural and physical properties of a new family of iron oxides with the formula LnBa(Fe,M)4O7+ . These oxides, belonging to the growing family of 114 oxides, are based on a cubic cell, conversely to the cobalt containing 114 oxides which crystallized in a hexagonal cell. The description of their reactivity with oxygen, has led us to consider the oxygen excess, . This approach has enable us to stabilize new structures for the stoichiometric compound LnBaFe4O7.0 ( = 0) for Ln = Y, Dy-Lu. The study of these new structures has given opportunity to understand the mechanisms that are governing the trends of the magnetic and transport properties. The synthesis of original compositions, substituting manganese for iron, has led to a significant increase of the oxygen absorption properties. Finally, investigations on CaBaFe4O7+ and CaBaCo4O7+ have allowed us, using electrochemical oxidation methods, to prepare oxidized metastable phases with interesting magnetic properties.CAEN-BU Sciences et STAPS (141182103) / SudocSudocFranceF

Interplay between 3d–3d and 3d–4f interactions at the origin of the magnetic ordering in the Ba 2 LnFeO 5 oxides

International audienc

Interplay between 3d–3d and 3d–4f interactions at the origin of the magnetic ordering in the Ba 2 LnFeO 5 oxides

International audienceA new family of oxides in which 3d–3d and 3d–4f interactions are of comparable strength has been synthesized and characterized both from structural and physical viewpoints. These compounds of formulation Ba2LnFeO5 (Ln  =  Sm, Eu, Gd, Dy, Ho, Er, Yb) are isotypic to the perovskite derivative Ba2YFeO5. They exhibit an original structure consisting of isolated FeO4 tetrahedra linked via LnO6 (or YO6) octahedra. Magnetic and calorimetric measurements show that all these compounds exhibit a unique, antiferromagnetic transition involving both the 3d and 4f ions. The antiferromagnetic properties of the Ln  =  Y phase (non-magnetic Y3+) and of the Ln  =  Eu (non-magnetic ground state multiplet of Eu3+) are ascribed to super-super exchange Fe–O–O–Fe interactions, leading to the lowest TN (5.5 K for Y and 4.6 K for Eu). The introduction of a magnetic lanthanide, i.e. Ln  =  Sm, Gd, Dy, Ho, Er, Yb, in the octahedral sites, leads to larger TN values (up to 9.8 K for Ln  =  Yb). It is found that several mechanisms must be taken into account to explain the complex evolution of the magnetic properties along the Ba2LnFeO5 series. In particular, the super-exchange Ln–O–Fe, as well as the on-site Ln3+ magnetocrystalline anisotropy, are suggested to play crucial roles. This Ba2LnFeO5 series offers a rare opportunity to investigate experimentally a situation where the 3d–3d and 3d–4f interactions co-operate on an equal footing to trigger a unique long-range magnetic ordering in insulating oxides