Project

multiferroic; magnetic structure; neutron scattering; polarised neutrons; multiferroic materials; polarimetry

Lead
Lay summary
The magneto­electric effect has a long history that goes back to the end of the 19. century when Pierre Curie suggested the possibility to induce an electric polarisation by a magnetic field or a magnetisation by application of an electric field. The first material that was found to exhibit magneto­electricity (ME) was Cr2O3 [I. E. Dzyaloshinskii, Soviet Phys. JETP 10, 628 (1960); D.N. Astrov, Soviet Phys. JETP 11, 708 (1960)] with a very small value of induced polarization. Following the discovery of the ME effect in Cr2O3 numerous materials were synthesized that exhibit ME behavior. The ME response remained, however, weak. There is currently a renewed interest in the physical properties of multiferroic compounds since a magneto­electric effect was reported in TbMnO3 by Kimura et al. [T. Kimura et al., Nature 426, 55 (2003)] that was followed by the discovery that the similar effect could be observed in other classes of materials like RMn2O5 (R=rare earth), Ni3V2O8 or MnWO4. In these materials the electric polarization is observed in the magnetically ordered phase, which indicates that ferroelectricity originates from magnetic order [G. Lawes et al., Phys. Rev. Lett. 95, 087205 (2005)]. At moment there is no consensus for a microscopic description of the ME effect in these materials and many theoretical models have been proposed. It is well known that symmetry plays a crucial role in the coupling between magnetic and electric properties [see e.g. H. Schmid in Magnetoelectric Interaction Phenomena in Crystals, Ed. A.J. Freeman and H. Schmid (1975); A.B. Harris et al., J. Phys.: Condens. Matter 20, 434202 (2008)]. Therefore a symmetrically correct description of the magnetic ground­state is essential to obtain an understanding of the magneto­electric coupling. It is the scope of this project to find the magnetic ground­state of multiferroic materials of the families BaMF4 (M=Mn, Co, Fe) and boracites (M3B7O13X, M=Cr, Mn, Fe, Co, Cu, Ni; X=Cl, Br, I), by neutron polarimetry as unpolarized neutron diffraction failed in solving the magnetic structure of these compounds unambiguously.

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Last update: 21.02.2013

Active participation

We propose to finance a post-doctoral position at the Laboratory for Neutron Scattering to investigate the magnetic ground-state of complex magneto-dielectric antiferromagnets using spherical neutron polarimetry. The magneto-electric effect has a long history that goes back to the end of the 19. century when Pierre Curie suggested the possibility to induce an electric polarisation by a magnetic field or a magnetisation by application of an electric field. The first material that was found to exhibit magneto-electricity (ME) was Cr2O3 [I. E. Dzyaloshinskii, Soviet Phys. JETP 10, 628 (1960); D.N. Astrov, Soviet Phys. JETP 11, 708 (1960)] with a very small value of induced polarization. Following the discovery of the ME effect in Cr2O3 numerous materials were synthesized that exhibit ME behavior. The ME response remained, however, weak. There is currently a renewed interest in the physical properties of multiferroic compounds since a magneto-electric effect was reported in TbMnO3 by Kimura et al. [T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima and Y. Tokura, Nature 426, 55 (2003)] that was followed by the discovery that the similar effect could be observed in other classes of materials like RMn2O5 (R=rare earth), Ni3V2O8 or MnWO4. In these materials the electric polarization is observed in the magnetically ordered phase, which indicates that ferroelectricity originates from magnetic order [G. Lawes et al., Phys. Rev. Lett. 95, 087205 (2005)]. At moment there is no consensus for a microscopic description of the ME effect in these materials and many theoretical models have been proposed. It is well known that symmetry plays a crucial role in the coupling between magnetic and electric properties [see e.g. H. Schmid in Magnetoelectric Interaction Phenomena in Crystals, Ed. A.J. Freeman and H. Schmid (1975) and J. Phys.: Condens. Matter 20, 434201 (2008); A.B. Harris, A. Aharony and Ora Entin-Wohlmann, J. Phys.: Condens. Matter 20, 434202 (2008)]. Therefore a symmetrically correct description of the magnetic ground-state is essential to obtain an understanding of the magneto-electric coupling. Neutron diffraction is the traditional method to determine the spin arrangement in magnetically ordered materials. The method relies upon the measurement of a set of magnetic Bragg intensities that are then least-square fitted to a given magnetic model. An alternative method was developed in the 90's at ILL, called spherical neutron polarimetry (SNP), that allows a more precise description of magnetically ordered structures as the spin direction can be measured directly. During the last three years we have developed and installed neutron polarimetry at the cold-neutron three-axis spectrometer TASP at SINQ. As will be shown in more details below, neutron polarimetry has proven extremely useful in solving the magnetic structures of multiferroic materials where the magnetic ground-state contains magnetically ordered ions of different types and the spin arrangement is non-collinear.In this project we propose to investigate the magnetic ground-state of multiferroic materials of the families BaMF4 (M=Mn, Co, Fe) and boracites (M3B7O13X, M=Cr, Mn, Fe, Co, Cu, Ni; X=Cl, Br, I), as unpolarized neutron diffraction failed in solving the magnetic structure of these compounds unambiguously.