V. Caignaert

Up to 50 000 per cent resistance variation in magnetoresistive polycrystalline perovskites Ln23Sr13MnO3 (Ln=Nd; Sm)

Abstract A polycrystalline manganite, Sm 2 3 Sr 1 3 MnO 3 , with exceptional magnetoresistive properties has been synthesized. This perovskite-like compound exhibits a resistance variation up to 50 000% at 92.5 K with H=5 T, i.e. of the same order of magnitude as the best films that have been fabricated to date. This large MR ratio compared to other polycrystalline manganites definitely demonstrates that such ceramics can be used for device applications instead of thin films, using liquid nitrogen as a refrigerator. The exploration of other lanthanide manganites Ln 2 3 Sr 1 3 MnO 3 . brings the proof that Tc decreases dramatically with the lanthanidic contraction. The size of the interpolated cation and the Mn (III) Mn(IV) mixed valence should be used in the future as a guide for the exploration and optimization of new magnetoresistive oxides.

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

Abstract:The oxygen deficient cobaltites LnBaCo2O5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at TN∼340 K and at TCO∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co2+ and Co3+ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo2O5 remains conjectural. Two pictures, with Co3+ ions either in intermediate spin state ( t52ge1g) or in high spin state ( t42ge2g), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo2O5 (Ln = Tb, Dy, Ho).

Giant magnetoresistance ratios superior to 1011 in manganese perovskites

Abstract The recent studies of mixed valent manganese perovskites La1−xAxMnO3−δ (A = Ca, Sr, Ba) have shown remarkable giant magneto resistance (GMR) properties. For these oxides fabricated either in the form of thin films [1–8] or of ceramics [9–13], R o R H ratios where Ro and RH are the resistances in a zero and applied magnetic field respectively, ranging from 10 to 104 have been observed at μ0H = 5 or 8T. The recent discovery of the perovskite Pr0.7Sr0.05Ca0.25MnO3−δ [13] with a R o R H ratio superior to 2.5.105 at 88K with μ0H = 5T, states the issue of the limit of the GMR ratio that can be reached in these compounds, that are of the highest importance for device applications. We report herein on the GMR properties of the parent semi-conductor perovskite Pr0.7Sr0.04Ca0.26MnO3−δ that exhibits a spectacular drop of its resistivity of 11 orders of magnitude by applying a magnetic field of 5T at 30K.

La2SrCu2O6: Neutron diffraction study

The oxide La2−xSr1+xCu2O6 is one of the rare layered copper oxides which do not exhibit superconducting properties. In order to understand this behavior a detailed neutron diffraction study of La2SrCu2O6 was undertaken. This study confirms the preliminary X-ray diffraction investigation. However it allows several original features concerning the oxygen atoms to be observed: existence of vacancies in the O(1) sites corresponding to the basal planes of the CuO5 pyramids and of extra oxygen on the O(3) site located between the copper layers. An analysis of the anisotropic thermal parameters shows also anomalies of the anisotropic factors s11 of O(2) and s33 of O(1). These features are interpreted in terms of defects which tend to create a three-dimensionnal framework. Morever the examination of the interatomic distances shows that the intercalation of oxygen between the copper planes is really favoured in this structure owing to the bigger size of the cation interleaved between the copper layers compared to the calcium or yttrium in other layered cuprates.

Sr2Mn2O5, an oxygen-defect perovskite with Mn(III) in square pyramidal coordination

Abstract An oxygen-defect perovskite Sr 2 Mn 2 O 5 was isolated by reduction of SrMnO 3−x perovskites in the presence of zirconium. Its structure, similar to that of Ca 2 Mn 2 O 5 , has been determined by X-ray powder diffraction and HREM. The orthorhombic cell has the parameters : a = 5.523(1) A , b = 10.761(5) A , c = 3.811(1) A . The possible space groups are Pbam and Pba2. The framework is built up from corner-sharing MnO 4 pyramids forming pseudo-hexagonal tunne l s running along 〈001〉 and perovskite tunnels running along 〈110〉 and 〈110〉. This oxide is antiferromagnetic with T N ⋍ 380 ± 10 K and θ P ⋍ 300 ± 10 K .

The manganite Nd0.5Sr0.5MnO3: A rare distortion of the perovskite

Abstract The charge-ordered properties, characterized by a transition from an antiferromagnetic insulating to a ferromagnetic metallic state as temperature increases, have been evidenced for Nd0.5Sr0.5MnO3 with a transition temperature of 160K. The neutron diffraction and HREM study of this manganite at room temperature shows that the space group of this orthorhombic structure ( a = 5.43 A , b = 7.63 A , c = 5.48 A ) is Imma. This new distortion of the perovskite structure is described and compared to that of the manganites with GdFeO3 type structure. The quasi absence of Jahn Teller distortion of manganese in this phase is ascribed to its metallic properties at room temperature.

Effect of Y-Ca substitution upon superconductivity in the oxide YBa2Cu3-xCoxO7-δ

Abstract The coupled substitution Y-Ca / Cu-Co in YBa 2 Cu 3 O 7 has been studied. The oxides Y 1 - y Ca y Ba 2 Cu 3 - x Co x O 7 - δ have been synthesized for 0≤ x ≤0.45 and 0≤ y ≤0.30. They were found tetragonal for x ≥0.09. The oxygen stoichiometry was determined by TGA showing δ values ranging from 6.78 to 7.08. The study of the superconducting properties of these phases shows that for a fixed cobalt content T c increases dramatically as the calcium content increases, leading for instance for the oxide Y 0.64 Ca 0.36 Ba 2 Cu 2.64 Co 0.36 O 6.87 to a T c of 82 K, whereas the non-substituted phase YBa 2 Cu 2.64 Co 0.36 O 7 - δ does not superconduct. As a result, the “compensated oxide” Y 1 - x Ca x Ba 2 Cu 3 - x Co x O 7 - δ , exhibits an almost x -independent T c ranging from 86 to 82 K for 0.15≤ x ≤0.36. A correlation is observed between the T c value and the c / a ratio. This behaviour is interpreted in terms of mobile holes concentration related to bond length changes induced by calcium, assuming that the cobalt is trivalent and sits in Cu 1 sites in agreement with previous neutron diffraction studies of YBa 2 Cu 3 - x Co x O 7 - δ .

YBaMn2O5: crystal and magnetic structure reinvestigation

Abstract The ordered oxygen-deficient perovskite YBaMn2O5 has been synthesized as a monophasic sample. A variable temperature neutron diffraction study of this phase has been carried out. The crystal structure of this manganite was solved in the space group P4/nmm (a = 5.547 A and c = 7.649 A). This oxide, which is closely related to the YBaCuFeO5 structure, forms an alternate stacking of yttrium and barium layers along the c axis. It differs from the latter, however, by the existence of two distinct sites for manganese, corresponding to an ordering of the Mn3+ and Mn2+ cations, respectively. Elucidation of the magnetic structure of YBaMn2O5, from low temperature PND (powder neutron diffraction) data, leads to a G-type antiferromagnetic model; the superimposed Mn2+/Mn3+ charge order results in ferrimagnetic behavior for this phase and explains its magnetic properties, as obtained from susceptibility measurements.

Mixed oxides of cobalt and copper with a double pyramidal layer structure

Abstract The study of layered structures involving transition metal elements with a pyramidal coordination, and especially copper, is of capital importance for the understanding of relationships between superconductivity and magnetism in high Tc superconductors. For this reason, the system Y-Ba-Co-Cu has been investigated. A new phase, YBaCo2−xCuxO5+δ has been isolated with a wide homogeneity range 0.30 ≤ x ≤ 1, which exhibits an excess oxygen for samples annealed in an oxygen flow according to the formula YBaCo2−xCuxO5+δ (δ > 0). The structure model of this phase has been determined for x = 1, showing its similarity with YBaFeCuO5, i.e. forming double pyramidal layers of Co(Cu)O5 pyramids separated by yttrium layers. The oxidation state of cobalt and copper is discussed. Such a material appears promising for magnetic studies.

Remarkable thermal stability of Eu(4-phosphonobenzoate): structure investigations and luminescence properties

A new 3D rare-earth hybrid material Eu(p-O(3)PC(6)H(4)COO) has been synthesised by a hydrothermal route from Eu(NO(3))(3) x 5 H(2)O and the rigid precursor, 4-phosphonobenzoic acid. The structure of Eu(p-O(3)PC(6)H(4)COO) has been solved by X-ray diffraction on a powder sample and is described as an inorganic network in which both carboxylic and phosphonic acid groups are linked to Eu ions forming a three-dimensional architecture. Thermal analysis performed on this compound has underlined its remarkable stability up to 510 degrees C and an optical study has been conducted to examine its luminescence properties that have been related to the structure of the material. The structural and luminescence properties have also been compared with the related material Eu phenylphosphonate.