The incommensurate magnetic structure of YMn2O5: a stringent test of the multiferroic mechanism
ArXiv 0808.2237 (2008)
Abstract:
We have determined the magnetic structure of the low-temperature incommensurate phase of multiferroic YMn2O5 using single-crystal neutron diffraction. By employing corepresentation analysis, we have ensured full compliance with both symmetry and physical constraints, so that the electrical polarization must lie along the b axis, as observed. The evolution of the spin components and propagation through the commensurate-incommensurate phase boundary points unambiguously at the exchange-striction mechanism as the primary driving force for ferroelectricity.Electric field switching of antiferromagnetic domains in YMn2O5: a probe of the multiferroic mechanism.
Phys Rev Lett 101:6 (2008) 067205
Abstract:
We employ neutron spherical polarimetry to determine the nature and population of the coexisting antiferromagnetic domains in multiferroic YMn2O5. By applying an electric field, we prove that reversing the electrical polarization results in the population inversion of two types of in-plane domains, related to each other by inversion. Our results are completely consistent with the exchange-striction mechanism of ferroelectricity, and support a unified model where cycloidal ordering is induced by coupling to the main magnetic order parameter.Orbital ordering promotes weakly-interacting S=1/2 dimers in the triangular lattice compound Sr3Cr2O8
ArXiv 0807.0877 (2008)
Abstract:
The weakly interacting S=1/2 dimers system Sr3Cr2O8 has been investigated by powder neutron diffraction and inelastic neutron scattering. Our data reveal a structural phase transition below room temperature corresponding to an antiferro-orbital ordering with nearly 90 degrees arrangement of the occupied 3z^2-r^2 d-orbital. This configuration leads to a drastic reduction of the inter-dimer exchange energies with respect to the high temperature orbital-disorder state, as shown by a spin-dimer analysis of the super-superexchange interactions performed using the Extended Huckel Tight Binding method. Inelastic neutron scattering reveals the presence of a quasi non-dispersive magnetic excitation at 5.4 meV, in agreement with the picture of weakly-interacting dimers.A Neutron diffraction study of multiferroics RMn2O5
ArXiv 0806.4128 (2008)
Abstract:
The magnetic properties of RMn2O5 multiferrroics as obtained by unpolarized and polarized neutron diffraction experiments are reviewed. We discuss the qualitative features of the magnetic phase diagram both in zero magnetic field and in field and analyze the commensurate magnetic structure and its coupling to an applied electric field. The origin of ferrolectricity is discussed based on calculations of the ferroelectric polarization predicted by different microscopic coupling mechanisms (exchange striction and cycloidal spin-orbit models). A minimal model containing a small set of parameters is also presented in order to understand the propagation of the magnetic structure along the c-direction.From single-molecule magnetism to long-range ferromagnetism in Hpyr [Fe17 O16 (OH) 12 (py) 12 Br4] Br4
Physical Review B - Condensed Matter and Materials Physics 77:22 (2008)