M. J. Martínez-Lope

Origin of neutron magnetic scattering in antisite-disordered Sr 2 FeMoO 6 double perovskites

Antisite disordering in Sr2FeMoO6 double perovskites ~containing Mo atoms at Fe positions, and vice versa! has recently been shown to have a dramatic influence in their magnetic and magnetotransport properties. In the present paper, two polycrystalline Sr 2FeMoO6 samples showing different degrees of antisite disorder ~a nominally ‘‘ordered’’ sample with ;70% of cationic ordering and a nominally ‘‘disordered’’ sample with ;18% of cationic ordering! have been examined by magnetic measurements and neutron powder diffraction techniques in the 15‐500 K temperature range. Our main finding is that the ‘‘disordered’’ sample exhibits a strong magnetic scattering ~noticeable even at 500 K!, comparable to that displayed by the ‘‘ordered’’ one below TC5415 K. For the ‘‘disordered’’ sample, the magnetic scattering exhibited on low-angle Bragg positions, is not to be ascribed to a ~nonexistent! ferrimagnetic ordering: our results suggest that it originates upon naturally occurring groups of Fe cations in which strong antiferromagnetic ~AFM! Fe-O-Fe superexchange interactions are promoted, similar to those existing in the LaFeO3 perovskite. These Fe groups are not magnetically isolated, but coupled by virtue of Fe-O-Mo AFM interactions, which maintain the long-range coherence of this AFM structure. Susceptibility measurements confirm the presence of AFM interactions below 770 K.

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.

Magnetic structure evolution of NdMnO3 derived from neutron diffraction data

The orthorhombic NdMnO3 perovskite (space group Pnma ) has been studied on the basis of magnetization and neutron powder diffraction (NPD) data. Magnetization measurements suggest the coexistence of ferromagnetic and antiferromagnetic interactions: magnetization versus magnetic field curves present a remnant magnetization in the ordered region, which is around 1 µB at T = 6 K. The thermal evolution of the magnetic structure has been followed down to 1.5 K from the NPD data. These measurements show that the Mn sub-lattice becomes ordered below T N 78 K with a spin arrangement (C x ,F y ,0), in such a way that a ferromagnetic component appears along the y -direction. The Nd sub-lattice becomes ordered below T 13 K according to a ferromagnetic arrangement with the moments parallel to the y -direction. At T = 1.5 K the magnetic moment values are 3.22(9) µB for Mn atoms and 1.2(2) µB for Nd atoms.

Enhanced magnetoresistance in the complex perovskite LaCu3Mn4O12

Moderate-pressure techniques (P=2 GPa) have been used to prepare the complex LaCu3Mn4O12 perovskite. It has been characterized by neutron powder diffraction, magnetic, and magnetotransport measurements. This material is ferrimagnetic below TC=361 K. The magnetoresistance (MR) is enhanced with respect to that of CaCu3Mn4O12 due to the effective electronic injection that dramatically reduces the bulk resistivity, thus promoting the grain-boundary contribution to the electrical resistance. Values of low-field MR close to 3% at room temperature are achieved for magnetic fields of 1 T.

In situ high temperature neutron powder diffraction study of oxygen-rich La2NiO4+δ in air: correlation with the electrical behaviour

The knowledge of the thermal evolution of the crystal structure of a cathode material across the usual working conditions in solid oxide fuel cells is essential to understand not only its transport properties but also its chemical and mechanical stability in the working environment. In this regard, high resolution neutron powder diffraction (NPD) measurements have been performed in air from 25 to 700 °C on O2-treated (350 °C, 200 bar) La2NiO4+δ. A structural transition from the orthorhombic Fmmm to the tetragonal F4/mmm space group takes place at about 150 °C. The reversibility of this transition has been determined to be strongly dependent on the sample oxygen content. The structural data have been correlated with the transport properties of this layered perovskite. The electrical conductivity of O2-treated La2NiO4+δ exhibits a dirty-metal (high T)-to-semiconducting (low T) transition as a function of temperature, displaying a maximum value of 82 S cm−1 at around 400 °C. The largest conductivity corresponds, microscopically, to the shortest axial Ni–O2 distance (2.19(1) A), revealing a major anisotropic component for the electronic transport. The interstitial oxygens occupy the 16j and 16e positions in the low and high temperature phases, respectively. The refined oxygen occupancy from NPD data is in quite good agreement with the thermogravimetric data. Good thermal stability of the oxygen content has been observed in the studied temperature range, as required for practical applications.

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.

Record saturation magnetization, Curie temperature, and magnetoresistance in Sr2FeMoO6 double perovskite synthesized by wet-chemistry techniques

Wet-chemistry techniques, involving the previous elaboration of reactive citrate precursors, have been used to prepare Sr2FeMoO6 double perovskite in polycrystalline form. This material has been characterized by x-ray and neutron powder diffraction, magnetic, and magnetotransport measurements. It exhibits a ferromagnetic Curie temperature of 416K, and an almost complete Fe∕Mo ordering at the B sublattice of the perovskite, accounting for the high saturation magnetization at 5K, of 3.97μB per formula unit. The magnetoresistance (MR) is higher than 45% at 5K, for H=9T, and the low-field MR is as high as 6.5% at room temperature for H=0.3T. These excellent features are a consequence of the good homogeneity of the samples, prepared from precursors where a perfect cationic mixing at atomic level is promoted.