Publications

The orthorhombic perovskite, (La(1-x/2)Bi(x/2))(Fe(0.5)Cr(0.5))O(3) was investigated for 0≤x≤1. Its space group, Pnma, compatible with the disordering of iron and chromium in the B sites, confirms previous observations. More importantly this compound is found to be an uncompensated weak ferromagnet, with a very peculiar zero magnetization behaviour, generally observed for ordered magnetic cations in the B sites. It exhibits a magnetic transition at high temperature (T(C)) above 450 K, while the zero magnetization occurs between 100 and 160 K depending on the x-value. The AC magnetic susceptibility study shows that this compound does not exhibit a spin glass or cluster glass behaviour, in contrast to what was suggested for the x = 0 compound. This zero magnetization phenomenon can be interpreted by the fact that this perovskite is an uncompensated weak ferromagnet, which consists of canted weak ferromagnetic domains and clusters of pure chromium and pure iron composition, antiferromagnetically coupled through Cr-O-Fe interactions.

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, Gd$_{0.5}$Dy$_{0.5}$MnO$_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic ac-plane, which is suppressed below 10 K but re-emerges in an applied magnetic field. Neutron diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e. a rotation as well as a strong enhancement of the polarization depending on the magnetic field direction. We show that this unusual behavior originates from the coupling between the transition metal and rare-earth magnetic subsystems.

Interaction of nickel deposited on multilayer films of C60 has been investigated in comparison with that on amorphized graphite by employing photoemission spectroscopy. It is shown that unlike with graphite, Ni interacts strongly with C60 causing large changes in C(1s) and C(2p) binding energies. The increase in the Ni(2p) binding energy at small Ni coverages is also far greater on c60 than on graphite.

Chemical shifts of K absorption discontinuities, Delta E, of several manganese, iron and cobalt oxides with the metal in the formal oxidation states between +2 and +4, have been measured. These data, together with data in the literature on other compounds of these metals, can be fitted into the expression Delta E=aq+bq2, where q is the effective atomic charge on the metal. Theoretical considerations also support this functional relationship between Delta E and q.

We report on the dielectric properties of bismuth aluminate and gallate with Bi:Al(Ga) ratio of 1:1 and 12:1 prepared at high temperature and ambient pressure. These compounds crystallize in a noncentrosymmetric body-centered cubic structure (space group I23) with a ∼ 10.18 Å rather than in the perovskite structure. This cubic phase is related to the γ - Bi 2 O 3 structure which has the actual chemical formula Bi 24 3 + ( Bi 3 + Bi 5 + ) O 40 − δ . In the aluminates and gallates studied by us, the Al and Ga ions are distributed over the 24 f and 2 a sites. These compounds exhibit ferroelectric hysteresis at room temperature with a weak polarization.