Sylvie Daviero-Minaud

Unprecedented robust antiferromagnetism in fluorinated hexagonal perovskites.

The diversification of antiferromagnetic (AFM) oxides with high Néel temperature is of fundamental as well as technical interest if one considers the need for robust AFM in the field of spin-tronics (exchange bias, multiferroics, etc.). Within the broad series of so-called hexagonal perovskites (HP), the existence of face-sharing octahedral units drastically lowers the strength of magnetic exchanges as compared to corner-sharing octahedral edifices. Here, we show that the partial introduction of F(-) in several Fe-based HP types leads to a drastic increase of the AFM ordering close to the highest values reported in iron oxides (T(N) ≈ 700 K). Our experimental results are supported by ab initio calculations. The T(N) increase is explained by the structural effect of the aliovalent F(-) for O(2-) substitution occurring in preferred anionic positions: it leads to local changes of the Fe-O-Fe connectivity and to chemical reduction into predominant Fe(3+), both responsible for drastic magnetic changes.

Anion-Vacancy-Induced Magneto−Crystalline Anisotropy in Fluorine-Doped Hexagonal Cobaltites

The two cobalt hexagonal perovskites 6H-Ba(6)Co(6)F(0.93)O(16) and 10H-Ba(5)Co(5)F(0.77)O(12.88) were prepared, and their structures were examined by X-ray and neutron diffraction and by (19)F solid state NMR spectroscopy. The magnetic and transport properties of these compounds were probed by magnetic susceptibility and electrical resistivity measurements, and their electronic structures by density functional and tight-binding calculations. The [BaOF(1-x)] layers of these compounds create corner-sharing tetrahedral Co(2)O(7) dimers at the interface between their face-sharing octahedral oligomers. Our density functional calculations leads to an unambiguous charge distribution model, which assigns high-spin Co(3+) ions for the tetrahedral sites and low-spin Co(3+)/Co(4+) ions for the octahedral sites, and this model should be valid for the parent BaCoO(3-delta) and the related oxychlorides and oxybromides as well. The F(-) vacancies in the [BaOF(1-x)] layers cause a strong distortion in the tetrahedral dimer Co(2)O(7), which in turn affects the spin orientation of the high-spin Co(3+) ions of the CoO(4) tetrahedra, i.e., parallel to the c-direction in Ba(6)Co(6)F(1-x)O(16-delta) but perpendicular to the c-direction in Ba(5)Co(5)F(1-x)O(13-delta). This difference in the spin orientations is related to the d-states of the distorted CoO(4) tetrahedra with high-spin Co(3+) (d(6)) ion on the basis of tight binding calculations and spin-orbit coupling as perturbation.

High Dilution of Anionic Vacancies in Sr0.8Ba0.2Fe(O,F)∼2.5

The (Ba,Sr)FeO(3-δ) system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)cubic compounds (generally δ < 0.27, Fe(3/4+)). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (δ ≈ 0.5, Fe(3+)) within the room-temperature stable tetragonal (pseudocubic) Sr(0.8)Ba(0.2)FeF(~0.1)(O,F)(~2.5.) The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/□ constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr(0.8)Ba(0.2)FeO(~2.7) shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe(3+) of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe(3+) cations (M(Fe) ≈ 2.64(1) μB at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.

Selective Metal Exsolution in BaFe2-yMy(PO4)2 (M = Co2+, Ni2+) Solid Solutions

The 2D-Ising ferromagnetic phase BaFe(2+)2(PO4)2 shows exsolution of up to one-third of its iron content (giving BaFe(3+)1.33(PO4)2) under mild oxidation conditions, leading to nanosized Fe2O3 exsolved clusters. Here we have prepared BaFe(2-y)M(y)(PO4)2 (M = Co(2+), Ni(2+); y = 0, 0.5, 1, 1.5) solid solutions to investigate the feasibility and selectivity of metal exsolution in these mixed metallic systems. For all the compounds, after 600 °C thermal treatment in air, a complete oxidation of Fe(2+) to Fe(3+) leaves stable M(2+) ions, as verified by (57)Fe Mössbauer spectroscopy, TGA, TEM, microprobe, and XANES. The size of the nanometric α-Fe2O3 clusters coating the main phase strongly depends on the yM metal concentration. For M-rich phases the iron diffusion is hampered so that a significant fraction of superparamagnetic α-Fe2O3 particles (100% for BaFe(0.5-x)Co(1.5)(PO4)2) was detected even at 78 K. Although Ni(2+) and Co(2+) ions tend to block Fe diffusion, the crystal structure of BaFe(0.67)Co1(PO4)2 demonstrates a fully ordered rearrangement of Fe(3+) and Co(2+) ions after Fe exsolution. The magnetic behaviors of the Fe-depleted materials are mostly dominated by antiferromagnetic exchange, while Co(2+)-rich compounds show metamagnetic transitions reminiscent of the BaCo2(PO4)2 soft helicoidal magnet.

Hydrothermal Synthesis of PbTiO 3 Ferroelectric Films: Characterizations with Large Frequency and Temperature Ranges and Sensor Application

Hydrothermal synthesis is an original way for the deposition of ferroelectric films. It allows to deposit a film with a few micrometers thick on a titanium substrate with various geometrical shapes. X-ray diffraction studies have shown a good nucleation without preferential orientation. S.E.M photographs show a growth of plates in various directions. Ferroelectric characterization has given a hysteresis loop P(E) with a remanent polarization of 3 w C/cm 2 and a coercive field of 10 V/ w m. The pyroelectric coefficient is n = 53 w C · K m 1 · m m 2 . For the dielectric characterizations, the H.F. value of the real permittivity is approximately the one obtained for bulk materials. At low frequencies, a significant conductivity due to the material defects is observed. All types of defects are frozen at 77 K, which highlights well the interest of low temperature measurements.

EXAFS characterization of the gel leading to a hydrothermal deposition of PZT films

Pb(Zr0.52,Ti0.48)O3 (PZT) polycrystalline films were produced by a hydrothermal synthesis on titanium substrates. It uses a single-step process, user-friendly precursors and a sand bath with a temperature lower than 200°C. In order to have a better understanding of the growth phenomena, a study of the precursor gel using X-ray absorption spectroscopy has been made.

Original positively charged nanoflakes by liquid exfoliation of layered oxybromide cobaltites

For the synthesis of novel positively charged 2D-elementary blocks, we have focused on the exfoliation of recent mixed valent Co3+/4+ oxy-bromides, namely the 14H-Ba7Co6BrO17 and 18R-Ba6Co5BrO14 phases built on hexagonal-perovskite blocks separated by [Ba2O2Br]− spacers. Here, we show that the exfoliation in butanol leads to positive nanoblocks in colloidal suspensions, as confirmed Zetametry measurements. Full structural characterization was performed by TEM, EDS, Raman and IR spectroscopies, AFM, X-ray absorption spectroscopy (XANES and DANES) and synchrotron X-ray diffraction (in-plane and out of plane). Combination of all these techniques results into a detailed description of crystalline flat particles (∼100 nm large and ∼10 nm thick) where the Co3+/4+ mixed oxidation state, as well as the local Co environment, are preserved after the exfoliation process. In-plane and out of plane synchrotron X-ray diffraction confirms a strong preferred deposition (001) planes. The original bulk materials showing intrinsic ferromagnetism in the elementary blocks, the magnetic properties of the delaminated nanoparticles are also presented here. Finally, we have prepared new positive nanometric bricks based on hexagonal perovskite structure, rare objects in condensed matter.