The magnetic properties of the high temperature alpha form of the CaCr2O4 compound have been investigated for the first time by magnetic susceptibility, specific heat and powder neutron diffraction. The system undergoes a unique magnetic phase transi
tion at 43K to a long range order incommensurate helical phase with magnetic propagation vector k=(0,0.3317(2),0). The magnetic model proposed from neutron diffraction data shows that the plane of rotation of the spins is perpendicular to the wave-vector, and that the magnetic modulation is consistent with two modes belonging to distinct irreducible representations of the group. The magnetic point group 2221 is not compatible with ferroelectricity unlike the CuCrO2 delafossite [Kimura et al., Phys. Rev. B, 78 140401 (2008)] but predicts the existence of quadratic magnetoelectric effects, discussed based on a Landau analysis.
We have studied the frustrated system YBaCo4O7 generally described as an alternating stacking of Kagome and triangular layers of magnetic ions on a trigonal lattice, by single crystal neutron diffraction experiments above the Neel ordering transition
. Experimental data reveals pronounced magnetic diffuse scattering, which is successfully modeled by direct Monte-Carlo simulations. Long-range magnetic correlations are found along the c-axis, due to the presence of corner-sharing bipyramids, creating quasi one-dimensional order at finite temperature. In contrast, in the Kagome layers ab-plane, the spin-spin correlation function -displaying a short-range 120 degrees configuration- decays rapidly as typically found in spin-liquids. YBaCo4O7 experimentally realizes a new class of two-dimensional frustrated systems where the strong out-of-plane coupling does not lift the in-plane degeneracy, but instead act as an external field.
The origin of the long-wavelength incommensurate magnetic structure of Ca3Co2O6 is discussed considering possible inter-chains super-superexchange paths. The experimental value of the propagation vector k=(0,0,$Delta$) with $Delta$ $>$ 1 can be repro
duced only if one considers the next nearest super-superexchange interaction. A spin-dimer analysis using the Extended-Huckel Tight-Binding method confirms that, despite longer interatomic Co-Co distances, the latter interaction is indeed much stronger. The stability of the observed structure with respect to certain commensurate states is discussed.
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 phys
ical 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.
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 order
ing 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.
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 ana
lyze 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.
The commensurate phase of multiferroic HoMn2O5 was studied by X-ray magnetic scattering, both off resonance and in resonant conditions at the Ho-L3 edge. Below 40 K, magnetic ordering at the Ho sites is induced by the main Mn magnetic order parameter
, and its temperature dependence is well accounted for by a simple Curie-Weiss susceptibility model. A lattice distortion of periodicity twice that of the magnetic order is also evidenced. Azimuthal scans confirm the model of the magnetic structure recently refined from neutron diffraction data for both Mn and Ho sites, indicating that the two sublattices interact via magnetic superexchange.
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 popul
ation 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.
Precise magnetic structures of RMn2O5, with R= Y, Ho, Bi in the commensurate/ferroelectric regime, have been determined by single-crystal neutron diffraction. For each system, the integrated intensities of a large number of independent magnetic Bragg
reflections have been measured, allowing unconstrained least-squares refinement of the structures. The analysis confirms the previously reported magnetic configuration in the ab-plane, in particular the existence of zig-zag antiferromagnetic chains. For the Y and Ho compounds additional weak magnetic components parallel to the c-axis were detected which are modulated in phase quadrature with the a-b components. This component is extremely small in the BiMn2O5 sample, therefore supporting symmetric exchange as the principal mechanism inducing ferroelectricity. For HoMn2O5, a magnetic ordering of the Ho moments was observed, which is consistent with a super-exchange interaction through the oxygens. For all three compounds, the point symmetry in the magnetically ordered state is m2m, allowing the polar b-axis found experimentally.
