Neutron diffraction study of the (BiFeO3)1-x(PbTiO3)x solid solution: nanostructured multiferroic system.

Neutron diffraction studies performed on the solid solution of (BiFeO(3))(1-x)(PbTiO(3))(x) reveal a mixture of two nanoscale phases with different crystal structures: a rhombohedral BiFeO(3)-based phase and a tetragonal PbTiO3-based phase. The ratio of Fe(3)+ and Ti(4)+ ions in the two phases is practically constant; only the proportion of the phases changes. The magnetic moments in the BiFeO(3)-based phase, in contrast to BiFeO(3), deviate from the basal plane. The temperature evolutions of the spin components along the hexagonal axis and within the perpendicular plane are different, leading to a spin re-orientation transition. The antiferromagnetic order in the PbTiO(3)-based phase corresponds to a simple structure with the propagation vector (1/2, 1/2, 1/2). The temperature dependence of the antiferromagnetic moment in the tetragonal phase at x = 0.5 indicates a canted antiferromagnetic order and a net ferromagnetic moment. A strong magnetic coupling between the two constituting phases due to the nanoscale character of the phases and well-developed interface between nanoparticles has been observed. The system of (BiFeO(3))(1-x)(PbTiO(3))(x) demonstrates an interesting scenario, where the proximity effects in the unstable system play a crucial role in the appearance of the unusual magnetic properties.

Effects of A-site disorder in the properties of A2CoMnO6 (A = La, Tb).

We have studied the structural and physical properties of the La2-xTbxCoMnO6 series. The crystal and magnetic structures of these compounds were determined by x-ray and neutron diffraction techniques. All samples belong to the family of double perovskites with space group P21/n, but the Co/Mn ordering is not perfect, and antisite defects are formed. The concentration of these defects increases for intermediate compositions, indicating that La/Tb disorder influences the Co/Mn arrangement. A ferromagnetic ground state is established due to the strong Mn(4+)-O-Co(2+) superexchange interaction. For the intermediate compositions and at low temperature, the Co/Mn ordering is accompanied by the ordering of Tb(3+) moments in the ab-plane, indicating a mutual polarization between both sublattices. Macroscopic magnetic properties reveal that Curie temperature decreases as Tb content increases in correlation with the increase of the structural distortion. All samples show semiconducting behaviour, and overall the electrical resistivity increases with decreasing La-content. The dielectric constant (ε') has a value of around 12 at low temperatures for all samples, revealing the lack of permanent dipoles. The temperature dependence of ε' on warming exhibits a strong increase that depends heavily on the frequency of the electric field. This effect is ascribed to non-intrinsic effects such as contacts or internal barrier-layers.

Effect of cation disorder on structural, magnetic and dielectric properties of La2MnCoO6 double perovskite.

The origin of dielectric anomalies and magnetodielectric response of La(2)MnCoO(6) has been investigated by means of ultra-high resolution synchrotron x-ray powder diffraction, neutron powder diffraction, resistivity, magnetization and dielectric measurements. The study has been performed on two different bulk samples presenting different degrees of Mn/Co order: 95 and 74%. Beside the well-known influence on magnetic properties, our results show that the main effect of disorder lies on the electrical resistivity. Bond distances clearly show Mn(4+)/Co(2+) valence states in the well-ordered sample, while for the disordered one this picture still holds. AC resistivity data show dielectric anomalies and a small magnetodielectric effect, but impedance complex plane analyses prove that these phenomena appear at the frequency-temperature region where extrinsic effects dominate the dielectric response.

Magnetic and structural features of the NdNi(1 - x)Mn(x)O3 perovskite series investigated by neutron diffraction.

Selected members of the perovskite series NdNi(1 - x)Mn(x)O(3) (0 ≤ x ≤ 1) have been prepared by a soft chemistry technique, followed by thermal treatments either under high oxygen pressure (x ≤ 0.5) or in air (x > 0.5). The crystal and magnetic structures have been studied by means of neutron diffraction, complemented with magnetic susceptibility measurements. For x = 0.25, 0.75, the crystal structure of the perovskites can be defined in the orthorhombic Pbnm space group, with Ni and Mn distributed at random over the octahedral sites of the structure. In contrast, the x = 0.5 compound crystallizes in a monoclinic P 2(1)/n structure containing two different octahedral positions, occupied by Ni and Mn, respectively. This is a result of the charge disproportionation of Ni(3+) + Mn(3+) to give Ni(2+) + Mn(4+) cations. The Ni(2+)O(6) octahedra are considerably larger than the Mn(4+)O(6) octahedra. This compound can be considered as a double perovskite of composition Nd(2)NiMnO(6). Unlike NdNiO(3) and NdMnO(3), which exhibit an antiferromagnetic ordering at low temperatures, the intermediate samples for x = 0.25, 0.50, 0.75 exhibit a ferromagnetic arrangement of (Ni, Mn) spins, with the moments aligned along the z axis, as probed using neutron diffraction. A maximum T(C) of 200 K is observed for x = 0.5, whereas T(C) = 150 K and 130 K are observed for x = 0.25 and 0.75, respectively. While NdNiO(3) is metallic above 200 K, a semiconducting behavior is determined between 120-300 K for the intermediate compositions.

The checkerboard pattern of Bi(0.63)Sr(0.37)MnO(3) determined using resonant x-ray scattering at the Mn K edge.

A study using resonant x-ray scattering at the Mn K edge has been carried out on a Bi(0.63)Sr(0.37)MnO(3) single crystal. This compound undergoes a metal-insulator phase transition to the so-called charge ordered (CO) state at about 530 K. Strong resonance signals were observed at room temperature as the energy was tuned through the Mn K edge for several superstructures of the CO phase. The energy, polarization and azimuth angle dependences agree with a checkerboard ordering in the ab plane of manganese atoms of two types, in terms of their different local geometrical structures. One of the sites is anisotropic-a tetragonal distorted oxygen octahedron-and the other is isotropic-a nearly undistorted one, as observed for Bi(0.5)Sr(0.5)MnO(3) and other half-doped manganites. This result indicates that the checkerboard pattern is strongly stable and extends to doping concentrations x<0.5. No superstructures corresponding to the doubling of the c axis were detected. Intermediate valence states lower than 3.5, according to the fractional charge segregation, were deduced for the two non-equivalent Mn atoms, i.e. Mn(3.30+) and Mn(3.44+).

Two-dimensional mapping of chemical information at atomic resolution.

The simultaneous measurement of structural and chemical information at the atomic scale provides fundamental insights into the connection between form and function in materials science and nanotechnology. We demonstrate structural and chemical mapping in Bi(0.5) Sr(0.5) MnO3 using an aberration-corrected scanning transmission electron microscope. Two-dimensional mapping is made possible by an adapted method for fast acquisition of electron energy-loss spectra. The experimental data are supported by simulations, which help to explain the less intuitive features.

Magnetic and electronic properties of Bi(0.75)Ca(0.25)MnO(3).

The magnetic, structural and electronic properties of Bi(0.75)Ca(0.25)MnO(3) have been investigated in comparison with those of Bi(0.75)Sr(0.25)MnO(3). Magnetometry, diffraction and muon spin relaxation (µSR) data confirm different structural, magnetic and electronic transitions in the two compounds. The anisotropic changes of cell parameters across the structural transition in Bi(0.75)Ca(0.25)MnO(3) (275 K) differ markedly from the lattice anomalies in Bi(0.75)Sr(0.25)MnO(3) (600 K) and also from those in Bi(0.50)Ca(0.50)MnO(3) (325 K). The ground state of Bi(0.75)Ca(0.25)MnO(3) is characterized by a high degree of spin disorder and frustrated interactions. There is no evidence of a ferromagnetic component in the ground state of Bi(0.75)Ca(0.25)MnO(3). However, the application of a magnetic field (even of a few gauss) produces a continuous progressive polarization of the Mn moments (≈2 µ(B)/Mn at 5 T, ZFC, 5 K). Differences between Ca and Sr perovskites with x = 1/4 are greater than for the x = 1/2 counterparts, and point to distinct ground states and charge/orbital configurations.