A decade of multiferroic research: new concepts and materials

In the last 10 years, the surge of interest for multiferroics has allowed to identify several new classes of materials in which electric and magnetic degrees of freedom are highly coupled. In particular, in the so-called type II multiferroics, the onset of long range magnetic order induces ferroelectric polarization and the magnetic domains can be controlled by an electric field or conversely the direction of the polarization can be flopped by a magnetic field. I will review the most recent discovery in the field, and show how neutron and X-ray magnetic scattering provide a very detailed understanding of the magnetoelectric coupling mechanism.

The roles of chirality and polarity in novel multiferroics: MnSb₂O₆ and Cu₃Nb₂O₈

At room temperature Cu3Nb2O8 has a centrosymmetric, triclinic crystal structure. If cooled below 24 K, the copper magnetic moments order with a complex, generalized helicoidal magnetic structure that breaks inversion symmetry, giving rise to ferroelectricity. Unusually, the direction of the induced electric polarization vector with respect to the helicoidal spin rotation cannot be reconciled by conventional theories of magneto-electric coupling. Instead, we show that the observed multiferroic properties of Cu3Nb2O8 may be explained through a phenomenological analysis based upon coupling between the magnetic chirality, electric polarity, and a structural axial rotation. Trigonal MnSb2O6 crystallizes with a chiral crystal structure. Typically, magnetic materials with a chiral crystal lattice order with a chiral magnetic structure, where the magnetic exchange interactions and anisotropies follow the symmetry of the lattice. The magnetism of MnSi is a classic example of this scenario, in which exotic skyrmion phases emerge out of a helical magnetic state. To the contrary, we show that the low temperature magnetic structure of MnSb2O6 is cycloidal, described by a magnetic polarity as opposed to a chirality. We demonstrate through ab-initio calculations that this magnetic structure is in fact the ground state of the symmetric-exchange Heisenberg spin Hamiltonian, which has higher symmetry than the underlying crystal lattice. Furthermore, the phenomenology may be understood by considering the coupling between structural chirality, magnetic polarity, and a magnetic axial rotation. As a result, we predict MnSb2O6 to be multiferroic with a weak ferroelectric polarization.

Fast Neutron Laue Diffraction with CCD Detectors

The large area neutron Laue diffractometer based on CCD detectors (CYCLOPS [1]) has been developed and recently completed at the ILL. High-quality Laue patterns covering an angular range of 360⁰ horizontally and 92⁰ vertically, can be obtained in only few seconds. The diffractometer excels for fast survey of reciprocal space and fast data collections through phase transitions as well as in-situ experiments on single crystals with time resolution similar to that obtained with powder diffraction. The detector is being upgraded with new faster CCD cameras having a larger dynamic range. A protocol for detector corrections from spatial distortions and uniformity response has been established which allows to obtain accurate integrated intensities leading to good structural refinements. Examples of results of phase transitions and structural investigations will be presented.