L. C. Chapon

The nature of the magnetic order in Ca3Co2O6

We present a detailed powder and single-crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order coexists with a shorter-range order with a correlation length scale of approximately 180 angstroms in the ab plane. Remarkably, on cooling, the volume of material exhibiting short-range order increases at the expense of the long-range order.

X-Ray Imaging and Multiferroic Coupling of Cycloidal Magnetic Domains in Ferroelectric Monodomain BiFeO 3

Magnetic domains at the surface of a ferroelectric monodomain BiFeO(3) single crystal have been imaged by hard x-ray magnetic scattering. Magnetic domains up to several hundred microns in size have been observed, corresponding to cycloidal modulations of the magnetization along the wave vector k=(δ,δ,0) and symmetry equivalent directions. The rotation direction of the magnetization in all magnetic domains, determined by diffraction of circularly polarized light, was found to be unique and in agreement with predictions of a combined approach based on a spin-model complemented by relativistic density-functional simulations. Imaging of the surface shows that the largest adjacent domains display a 120° vortex structure.

Slow magnetic order-order transition in the spin chain antiferromagnet Ca3Co2O6.

Using powder neutron diffraction, we have discovered an unusual magnetic order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering the temperature, an antiferromagnetic phase with a propagation vector k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure with k=(0,0,1.01). This transition occurs over an unprecedented time scale of several hours and is never complete.

One-Dimensional Magnetic Fluctuations in the Spin-2 Triangular Lattice α-NaMnO2

The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T< or =45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta approximately equal to 7.5 meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k(B) approximately 73 K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k(B) approximately 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.

Electric Field Switching of Antiferromagnetic Domains in YMn2O5 : A Probe of the Multiferroic Mechanism

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.

Neutron scattering and muon spin relaxation measurements of the noncentrosymmetric antiferromagnet CeCoGe3

The magnetic states of the noncentrosymmetric pressure-induced superconductor CeCoGe3 have been studied with magnetic susceptibility, muon spin relaxation (?SR), single-crystal neutron diffraction, and inelastic neutron scattering (INS). CeCoGe3 exhibits three magnetic phase transitions at TN1=21,TN2=12, and TN3=8K. The presence of long-range magnetic order below TN1 is revealed by the observation of oscillations of the asymmetry in the ?SR spectra between 13 and 20 K and a sharp increase in the muon depolarization rate. Single-crystal neutron-diffraction measurements reveal magnetic Bragg peaks consistent with propagation vectors of k=(0,0,23) between TN1 and TN2,k=(0,0,58) between TN2 and TN3 and k=(0,0,12) below TN3. An increase in intensity of the (110) reflection between TN1 and TN3 also indicates a ferromagnetic component in these phases. These measurements are consistent with an equal moment two-up two-down magnetic structure below TN3 with a magnetic moment of 0.405(5)?B/Ce. Above TN2, the results are consistent with an equal moment two-up one-down structure with a moment of 0.360(6)?B/Ce. INS studies reveal two crystal-electric-field (CEF) excitations at ?19 and ?27meV. From an analysis with a CEF model, the wave functions of the J=52 multiplet are evaluated along with a prediction for the magnitude and direction of the ground-state magnetic moment. Our model correctly predicts that the moments order along the c axis, but the observed magnetic moment of 0.405(5)?B is reduced compared to the predicted moment of 1.0?B. This is ascribed to hybridization between the localized Ce3+ f electrons and the conduction band. This suggests that CeCoGe3 has a degree of hybridization between that of CeRhGe3 and the noncentrosymmetric superconductor CeRhSi3

Helical spin waves, magnetic order, and fluctuations in the langasite compound Ba3NbFe3Si2O14

We have investigated spin fluctuations in the langasite compound Ba3NbFe3Si2O14 both in the ordered state and as a function of temperature. The low-temperature magnetic structure is defined by a spiral phase characterized by magnetic Bragg peaks at (q) over right arrow = (0,0,tau similar to 1/7) onset at T-N = 27 K as previously reported by Marty et al. [Phys. Rev. Lett. 101, 247201 (2008)]. The nature of the fluctuations and temperature dependence of the order parameter is consistent with a classical second-order phase transition for a two-dimensional triangular antiferromagnet. We show that the physical properties and energy scales including the ordering wave vector, Curie-Weiss temperature, and spin waves can be explained through the use of only symmetric exchange constants without the need for a dominant Dzyaloshinskii-Moriya interaction. This is accomplished through a set of helical exchange pathways along the c direction imposed by the chiral crystal structure and naturally explains the magnetic diffuse scattering, which displays a strong vector chirality up to high temperatures, well above the ordering temperature. This illustrates a strong coupling between magnetic and crystalline chirality in this compound.

Multiferroicity and spiral magnetism inFeVO4with quenched Fe orbital moments

FeVO{sub 4} has been studied by heat capacity, magnetic susceptibility, electric polarization and single-crystal neutron-diffraction experiments. The triclinic crystal structure is made of S-shaped clusters of six Fe{sup 3+} ions, linked by VO{sub 4}{sup 3-} groups. Two long-range magnetic ordering transitions occur at T{sub N1}=22 K and T{sub N2}=15 K. Both magnetic structures are incommensurate and below T{sub N2}, FeVO{sub 4} becomes weakly ferroelectric coincidentally with the loss of the collinearity of the magnetic structure in a very similar fashion than in the classical TbMnO{sub 3} multiferroic material. However we argue that the symmetry considerations and the mechanisms invoked to explain these properties in TbMnO{sub 3} do not straightforwardly apply to FeVO{sub 4}. First, the magnetic structures, even the collinear structure, are all acentric so that ferroelectricity in FeVO{sub 4} is not correlated with the fact magnetic ordering is breaking inversion symmetry. Regarding the mechanism, FeVO{sub 4} has quenched orbital moments that questions the exact role of the spin-orbit interactions.

Spin correlations in the geometrically frustrated RBaCo{sub 4}O{sub 7} antiferromagnets : mean-field approach and Monte-Carlo simulations.

Spin correlations in the geometrically frustrated RBaCo{sub 4}O{sub 7} compounds, usually described as an alternating stacking of Kagome and triangular layers on a hexagonal lattice, have been studied by mean-field approach and by Monte Carlo simulations. The behavior of the system was modeled with an isotropic Heisenberg Hamiltonian as a function of the relevant parameter J{sub out}/J{sub in}, representing the ratio between exchange integrals inside the Kagome layers, J{sub in}, and between Kagome and triangular layers, J{sub out}. This ratio can be varied in real systems by appropriate chemical substitutions. At the mean-field level, long-range magnetic order with the wave vector at the K point of symmetry (k=a*/3+b*/3) has been found for J{sub out}/J{sub in} > 0.7. Below this value, the dominant Fourier modes are completely degenerate in the entire Brillouin zone. The Monte Carlo simulations revealed that the long-range ordered configuration found in the mean-field calculations becomes the ground state of the system for J{sub out}/J{sub in} > 1.5. Below this critical ratio, quasi-one-dimensional magnetic ordering along the c axis, involving spins of the triangular sublattice was observed. The correlations in the (ab) plane were found to have a short-range 120{sup o} character with correlation length dependent on J{submorexa0» out}/J{sup in}.«xa0less

MnSb 2 O 6 : A Polar Magnet with a Chiral Crystal Structure

Structural and magnetic chiralities are found to coexist in a small group of materials in which they produce intriguing phenomenologies such as the recently discovered Skyrmion phases. Here, we describe a previously unknown manifestation of this interplay in