Sophie de Brion

Synthesis, optical and magnetic properties of hybrid α,α′-oligothiophenecarboxylates/transition metal hydroxide multilayered compounds

New Cu(II), Ni(II) and Co(II) multilayered systems consisting of alternating transition metal hydroxide and oligothiophenedicarboxylate layers have been elaborated by hydrothermal reaction. The insertion of dicarboxylate ligands comprising 1 to 4 thiophene rings between metal hydroxides and the structures and physical properties of 10 hybrid organic/inorganic systems M–Tn (MCu, Ni, Co, n = 1–4) have been investigated. Controlled hydrothermal synthesis leads to layered compounds of transition metals in octahedral sites only. A wide palette of magnetic behaviors has been observed. The four copper derivatives with interlayer distances of 1.0–2.1 nm show either antiferromagnetic or ferromagnetic intra-layers interactions with a ferromagnetic or canted antiferromagnetic long range ordering. The nickel derivatives are antiferromagnetic and show a metamagnetic transition for interlayer distances below 1.3 nm and are ferrimagnetic above 1.7 nm, with low TN (< 4 K) and coercivity (μ0HC < 30 mT). The cobalt compounds are all canted antiferromagnets at ordering temperatures as high as 40 K. Strong anisotropies induce a hard magnet behavior with coercive fields of 3–6 T. A special case concerns Co-T2 (4), for which the hard magnet state is induced by applying a field. The optical properties of the free ligands, oligothiophenedicarboxylates, were investigated. All ligands are fluorescent. In the hybrids, the main absorption bands observed in the visible region are attributed to the π–π* transition. No emission was observed, the quenching of photoluminescence can be explained by intermolecular interactions or vicinity of the paramagnetic metal ions directly coordinated via the carboxylate moieties.

Coexistence of short-range ferromagnetic and antiferromagnetic correlations in Ge-rich Gd5(SixGe1−x)4 alloys

Isothermal magnetization curves up to 23 T were measured in Gd5(SixGe1−x)4 alloys for x 0.1. We show that a field-induced transition occurs from the antiferromagnetic (AFM) phase to a state in which competing short-range AFM and ferromagnetic (FM) correlations are present. This is based on the abrupt change in the slope of the high-temperature magnetization isotherms at high fields (∼14–15 T). Results suggest that the transition is due to the breaking of the long-range AFM ordering by the magnetic field. This induces short-range FM correlations favouring the formation of coexisting FM and AFM clusters. A further increase of the magnetic field leads to the percolation of the former, giving rise to long-range FM order.

Magnetic frustration in the spinel compounds GeNi2O4 and GeCo2O4

In the spinel compounds AB2O4, the B sites form a pyrochlore lattice. GeCo2O4 and GeNi2O4 have been investigated using high magnetic field magnetization as well as neutron diffraction measurements. Both compounds become antiferromagnetic at low temperature (around 23 and 12K, respectively) with the same propagation vector. The magnetization then presents two spin flop fields and one huge saturation field (above 50T). The Curie–Weiss temperature is positive for the Co spinel and negative for the Ni one. A model taking into account the first neighbor interaction J1 together with two different third neighbor interactions J2 and J3 is able to account for the common observed magnetic behavior. J1 is ferromagnetic while J2 and J3 are antiferromagnetic, in agreement with Goodenough–Kanamori–Anderson rules. These competing interactions are responsible for the frustration in these compounds.