M. T. Casais

The magnetic structure of YMnO3 perovskite revisited

The magnetic structure of the orthorhombic perovskite YMnO3 has been investigated. A study on a polycrystalline sample based on neutron diffraction data and magnetization measurements has shown that YMnO3 becomes magnetically ordered below TN = 42?K. In the space group Pnma, the sinusoidal magnetic structure is defined by a (Cx,0,0) mode and characterized by the propagation vector k = (kx,0,0). The kx-component increases from 0.420(4), immediately below the ordering temperature, to 0.435(2) at T = 28.7?K. Below 28 K the kx-component remains unchanged. The sinusoidal spin arrangement remains stable down to 1.7?K; at this temperature the amplitude of the sinusoid is Ak = 3.89(6)??B. YMnO3 is the most distorted perovskite of the RMnO3 series (R?=?rare earths); the observed sinusoidal magnetic structure is in contrast with those exhibited by the less-distorted members (i.e.?LaMnO3), which are commensurate-type antiferromagnetic structures.

A structural study from neutron diffraction data and magnetic properties of (R = La, rare earth)

The title compounds (R = La, Pr, Nd, Sm, Eu, Tb, Ho, Er) have been prepared in polycrystalline form by a citrate technique and, excepting the Sm and Eu phases, structurally studied by high-resolution neutron powder diffraction. All the materials are isostructural (space group Pbam, Z = 4) and contain infinite chains of octahedra sharing edges, linked together by and units. The size of the three kinds of coordination polyhedron regularly decreases as R cations become smaller. A bond-valence study allowed us to detect the presence of important tensile and compressive stresses in the crystal structure of , which are progressively released along the series as the rare-earth size decreases. The magnetic properties strongly depend on the nature of R, going from the spin-glass behaviour observed at low temperature in to the field-induced transitions exhibited by . A cusp in the susceptibility curves suggests an antiferromagnetic ordering at low temperatures, which is masked in the compounds containing strongly paramagnetic rare earths (Tb, Ho, Er). At high temperatures the paramagnetic moments are consistent in all cases with the presence of high-spin and cations.

Excess electrical conductivity in polycrystalline Bi-Ca-Sr-Cu-O compounds and thermodynamic fluctuations of the amplitude of the superconducting order parameter

Abstract Measurements of the rounding effects on the electrical resistivity above the superconducting transition in Bi-Ca-Sr-Cu-O polycrystalline compounds are reported, to our knowledge for the first time in this HTSC system. These effects are analyzed in terms of thermodynamic fluctuations of the amplitude of the superconducting order parameter (SCOPF). In the mean-field-like region, the experimental critical exponent seems to be compatible with an order parameter of two components (2d) fluctuating in two dimensions (2D). This contrasts with previous results for A-Ba-Cu-O ( A = Y , Ln ) and Ln-M-Cu-O ( M = Ba , Sr ) superconductors, where SCOPF seem to be 2d-3D in all the different dynamic critical regions.

Crystal and magnetic structure of the complex oxides Sr2MnMoO6, Sr2MnWO6 and Ca2MnWO6: a neutron diffraction study

A study of the crystallographic and magnetic structure of the double perovskites Sr2MnMoO6, Sr2MnWO6 and Ca2MnWO6 has been carried out on polycrystalline samples using neutron powder diffraction (NPD) data. A room temperature analysis of high-resolution NPD patterns has shown that these compounds crystallize, at room temperature, in the monoclinic space group P 21 /n. The three perovskites contain divalent Mn cations. Ca2MnWO6 presents the strongest distortion with respect to the ideal cubic perovskite structure. The low-temperature antiferromagnetic ordering has been followed from sequential NPD data. The magnetic structures are defined by the propagation vectors k = (1/2, 0, 1/2) for Sr2MnMoO6 and Sr2MnWO6, and k = (0, 1/2, 1/2) for Ca2MnWO6. The possible arrangements for the Mn2+ magnetic moments have been derived from a group theory analysis.

Non-stoichiometry, structural defects and properties of LaMnO3+δ with high δ values (0.11≤δ≤0.29)

Strongly oxygenated LaMnO3+δperovskites, with nominal MnIV contents up to 58%, have been prepared by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (200 bar). A high-resolution neutron powder diffraction study of four representative samples with 0.11≤δ≤0.29 reveals the presence of both La and Mn vacancies. Contrary to previous studies, it is found that there are a substantially higher proportion of Mn vacancies, depending rather sensitively on the oxidation conditions. The oxidation state for Mn calculated for the refined stoichiometry La1–xMn1–yO3 is in good agreement with the δ values previously determined by thermal analysis. Further to this, it is also found that as δ increases the Mn–O bond lengths shorten, the Mn–O–Mn angles progressively increase and the perovskite structure becomes more regular, which is consistent with the incorporation of MnIV cations. The presence of Mn vacancies (as much as 13% in samples prepared under high oxygen pressure) perturbs the conduction paths for the transport of holes across Mn–O–Mn, weakening the double-exchange interaction. This structural disorder explains the observed decrease of the ferromagnetic Curie temperature (TC) as δ increases.

Magnetic structures of LaMnO3 + δ perovskites (δ = 0.11, 0.15, 0.26)

Abstract Oxygen excess LaMnO 3 + δ perovskites with a wide range of δ values (and Mn IV contents) have been obtained by thermal decomposition of metal citrates followed by annealings either in air or under high oxygen pressure (up to 200 bar). Three representative samples, with δ values of 0.11, 0.15 and 0.26 (Samples 1–3) have been studied by neutron diffraction in the 2–250 K temperature range. The magnetic structures, ordered magnetic moments and long-range ordering temperatures have been determined. Sample 1 is orthorhombic and exhibits a purely ferromagnetic structure below 165(10) K; Samples 2 and 3 are rhombohedral and show a canted ferromagnetic structure, below T c = 140(10) K and 65(10) K, respectively. The observed decrease of T c and the magnitude of the ordered magnetic moments as δ increases has been interpreted as a consequence of the presence in the crystal structure of Mn vacancies, which perturb the connecting paths for the transport of holes across MnOMn, progressively preventing the double-exchange interaction. Also, the dramatic reduction of the observed magnetic moments for increasing Mn IV contents can be explained in a system which in fact shows spin-glass features.

Raman phonons and light scattering in RMnO3 (R=La, Pr, Nd, Ho, Er Tb and Y) orthorhombic and hexagonal manganites

Abstract RMnO3 compounds present the Pbnm orthorhombic structure for R=La, Pr, Nd while compounds with smaller ionic radii (R=Ho, Er Tb and Y) can be obtained either in the Pbnm or the P63cm hexagonal structure. Their magnetic and transport properties are strongly dependent on the rare earth and, therefore, on their detailed structure. We have studied these compounds through the Raman phonons of both structures as a function of the rare earth ion and temperature. The phonon frequencies of the main peaks of Pbnm structure have been correlated to structural characteristics, such as bond-lengths or bond angles, obtained from the analysis of neutron diffraction data. According to the structural and Raman behavior, these compounds can be classified into two groups: one includes La, Pr, and Nd, and the other includes the rest. This classification also correlates to the different behavior of their magneto transport properties when doped. A structural transition to a cubic or quasi-cubic perovskite, above 800 K, can be deduced from the phonon spectra for YMnO3 orthorhombic compound.

Effects of high vacancy concentrations on the magnetic properties of La1−xMn1−yO3 (0.02⩽x, y⩽0.13)

It has been found that the synthesis conditions and subsequent annealing treatments can significantly change the magnetic and transport behavior of the colossal magnetoresistive materials of the general formula La1−zAzMnO3, where A is a divalent ion. In order to clarify the role of vacancies within this structure, resistivity and magnetization measurements have been carried out on a series of samples derived from the parent compound LaMnO3, with La and Mn vacancies introduced by systematically varying the oxygen annealing conditions. Previous studies have shown that for a given carrier concentration, the Curie temperature of the paramagnetic to ferromagnetic transition (Tc) increases as the tolerance factor of the perovskite structure increases and then begins to decrease slowly for even higher tolerance factors. Generally, Tc also increases with the Mn4+ content, consistent with a double exchange mechanism responsible for ferromagnetism. In this study Tc was found to decrease as the vacancy concentration...

Short-range charge order in R Ni O 3 perovskites ( R = Pr , Nd, Eu, Y) probed by x-ray-absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic

Microscopic nature of the electron doping effects in the double perovskite Sr2−xLaxFeMoO6(0 ≤x≤ 1) series

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