2010

BiFeO₃ is a multiferroic materials in which ferroelectric and anti-ferromagnetic orders coexist well above room temperature (T_N=643 K, T_C=1093 K), with a high polarization (over 100 μC/cm² [1]). We have shown at the LLB by neutron diffraction that these two order parameters interact and that the magnetization of the material can be modified by the application of an electric field. This opens the way towards the implementation of this material in spintronic devices in which magnetization could be controlled by a small electrical voltage rather than by currents or magnetic fields.

Although magnetoelectric materials have been known for decades, no direct proof of the coupling between the two co-existing physical effects had been reported in the literature. We have demonstrated this magneto-electric coupling by neutron diffraction on BiFeO₃ single crystals. One key to success was the exceptional quality of the high purity single crystals which are magnetically and electrically single domain. More...

Measurements of very weak field (down to the femtotesla range) have been mainly addressed by low-Tc SQUIDS. We have proposed in 2004 the principle of a new sensor, based on spin-electronics, to offer extremely good sensitivity for measurements of magnetic fields over a wide range of frequency (dc to hundreds of MHz).

These sensors, called mixed sensors, are based on the association of a sensitive field sensor (Giant Magnetoresistance or Tunnel Magnetoresistance) and of an efficient flux-to-field transformer. The transformer is a cm size superconducting loop containg a micron size constriction. When an external field is applied perpendicular to the loop, a supercurrent is generated, to prevent the entrance of the field in the loop. This supercurrent reaches a very high density when passing through the constriction. The stray field lines due to this current can be locally hundreds to thousands times larger than the applied field. If one locates a GMR or TMR element on top or below the constriction, it will collect this locally enhanced field lines and a change in the resistance of the GMR (TMR) element can be measured (see Figure 1).

This device is fully made out of thin film technology, and can be either using a low-Tc (typically Nb) loop or a High-Tc (YBaCuO) loop. In the first case, liquid helium cooling is required whereas in the second one only nitrogen cooling is necessary.

Guanine rich DNA strands, as those encountered at the extremities of human chromosomes, have the ability to form four-stranded structures (G-quadruplexes) whose building blocks are guanine tetrads. G-quadruplex structures are intensively studied in respect to their biological role, as targets for anticancer therapy and, more recently, to their potential applications in the field of molecular electronics.

We have shown that self-association of guanines gives rise to exciton states. As a consequence, ultrafast excitation transfer takes place within G-quadruplexes. Self-association of guanines slows down the fluorescence decays and increases the fluorescence quantum yield.  The relaxation of ππ* states toward lower energy charge transfer excited  states, involving guanines on different tetrads, depends on the nature cations in the central cavity; it is facilitated by Na⁺ ions, which are smaller and more mobile compared to K⁺ ions.

G-quadruplex publications.pdf