Guiding antiferromagnetic transitions in Ca[Formula: see text]RuO[Formula: see text].

Understanding and controlling the transition between antiferromagnetic states having different symmetry content with respect to time-inversion and space-group operations are fundamental challenges for the design of magnetic phases with topologically nontrivial character. Here, we consider a paradigmatic antiferromagnetic oxide insulator, Ca[Formula: see text]RuO[Formula: see text], with symmetrically distinct magnetic ground states and unveil a novel path to guide the transition between them. The magnetic changeover results from structural and orbital reconstruction at the transition metal site that in turn arise as a consequence of substitutional doping. By means of resonant X-ray diffraction we track the evolution of the structural, magnetic, and orbital degrees of freedom for Mn doped Ca[Formula: see text]RuO[Formula: see text] to demonstrate the mechanisms which drive the antiferromagnetic transition. While our analysis focuses on a specific case of substitution, we show that any perturbation that can impact in a similar way on the crystal structure, by reconstructing the induced spin-orbital exchange, is able to drive the antiferromagnetic reorganization.

High-pressure developments for resonant X-ray scattering experiments at I16.

An experimental setup to perform high-pressure resonant X-ray scattering (RXS) experiments at low temperature on I16 at Diamond Light Source is presented. The setup consists of a membrane-driven diamond anvil cell, a panoramic dome and an optical system that allows pressure to be measured in situ using the ruby fluorescence method. The membrane cell, inspired by the Merrill-Bassett design, presents an asymmetric layout in order to operate in a back-scattering geometry, with a panoramic aperture of 100° in the top and a bottom half dedicated to the regulation and measurement of pressure. It is specially designed to be mounted on the cold finger of a 4 K closed-cycle cryostat and actuated at low-temperature by pumping helium into the gas membrane. The main parts of the body are machined from a CuBe alloy (BERYLCO 25) and, when assembled, it presents an approximate height of 20-21 mm and fits into a 57 mm diameter. This system allows different materials to be probed using RXS in a range of temperatures between 30 and 300 K and has been tested up to 20 GPa using anvils with a culet diameter of 500 µm under quasi-cryogenic conditions. Detailed descriptions of different parts of the setup, operation and the developed methodology are provided here, along with some preliminary experimental results.

Coherent Magnetoelastic Domains in Multiferroic BiFeO_{3} Films.

The physical properties of epitaxial films can fundamentally differ from those of bulk single crystals even above the critical thickness. By a combination of nonresonant x-ray magnetic scattering, neutron diffraction and vector-mapped x-ray magnetic linear dichroism photoemission electron microscopy, we show that epitaxial (111)-BiFeO_{3} films support submicron antiferromagnetic domains, which are magnetoelastically coupled to a coherent crystallographic monoclinic twin structure. This unique texture, which is absent in bulk single crystals, should enable control of magnetism in BiFeO_{3} film devices via epitaxial strain.

Negative thermal expansion in hybrid improper ferroelectric Ruddlesden-Popper perovskites by symmetry trapping.

We present new results on the microscopic nature of the ferroelectricity mechanisms in Ca3 Mn2O7 and Ca3Ti2O7. To the first approximation, we confirm the hybrid improper ferroelectric mechanism recently proposed by Benedek and Fennie for these Ruddlesden-Popper compounds. However, in Ca3Mn2O7 we find that there is a complex competition between lattice modes of different symmetry which leads to a phase coexistence over a large temperature range and the "symmetry trapping" of a soft mode. This trapping of the soft mode leads to a large uniaxial negative thermal expansion (NTE) reaching a maximum between 250 and 350 K (3.6×10^(-6) K^{-1}) representing the only sizable NTE reported for these and related perovskite materials to date. Our results suggest a systematic strategy for designing and searching for ceramics with large NTE coefficients.

Noncoplanar and counterrotating incommensurate magnetic order stabilized by Kitaev interactions in γ-Li(2)IrO(3).

Materials that realize Kitaev spin models with bond-dependent anisotropic interactions have long been searched for, as the resulting frustration effects are predicted to stabilize novel forms of magnetic order or quantum spin liquids. Here, we explore the magnetism of γ-Li(2)IrO(3), which has the topology of a three-dimensional Kitaev lattice of interconnected Ir honeycombs. Using magnetic resonant x-ray diffraction, we find a complex, yet highly symmetric incommensurate magnetic structure with noncoplanar and counterrotating Ir moments. We propose a minimal Kitaev-Heisenberg Hamiltonian that naturally accounts for all key features of the observed magnetic structure. Our results provide strong evidence that γ-Li(2)IrO(3) realizes a spin Hamiltonian with dominant Kitaev interactions.

X-ray imaging and multiferroic coupling of cycloidal magnetic domains in ferroelectric monodomain BiFeO3.

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.

Giant tunability of ferroelectric polarization in GdMn2O5.

Giant tunability of ferroelectric polarization (ΔP=5000 µC/m2) in the multiferroic GdMn2O5 with external magnetic fields is discovered. The detailed magnetic model from x-ray magnetic scattering results indicates that the Gd-Mn symmetric exchange striction plays a major role in the tunable ferroelectricity of GdMn2O5, which is in distinction from other compounds of the same family. Thus, the highly isotropic nature of Gd spins plays a key role in the giant magnetoelectric coupling in GdMn2O5. This finding provides a new handle in achieving enhanced magnetoelectric functionality.

Cu3Nb2O8: a multiferroic with chiral coupling to the crystal structure.

By combining bulk properties, neutron diffraction, and nonresonant x-ray diffraction measurements, we demonstrate that the new multiferroic Cu(3)Nb(2)O(8) becomes polar simultaneously with the appearance of generalized helicoidal magnetic ordering. The electrical polarization is oriented perpendicularly to the common plane of rotation of the spins-an observation that cannot be reconciled with the conventional theory developed for cycloidal multiferroics. Our results are consistent with coupling between a macroscopic structural rotation, which is allowed in the paramagnetic group, and magnetically induced structural chirality.

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.

Direct observation of charge order in triangular metallic AgNiO2 by single-crystal resonant X-ray scattering.

We report resonant x-ray scattering measurements on a single crystal of the orbitally degenerate triangular metallic antiferromagnet 2H-AgNiO2 to probe the spontaneous transition to a triple-cell superstructure at temperatures below T(S)=365 K. We observe a strong resonant enhancement of the supercell reflections through the Ni K edge. The empirically extracted K-edge shift between the crystallographically distinct Ni sites of 2.5(3) eV is much larger than the value expected from the shift in final states, and implies a core-level shift of ∼1 eV, thus providing direct evidence for the onset of spontaneous honeycomb charge order in the triangular Ni layers. We also provide band-structure calculations that explain quantitatively the observed edge shifts in terms of changes in the Ni electronic energy levels due to charge order and hybridization with the surrounding oxygens.

Resonant x-ray scattering investigation of magnetic ordering in NpAs(1 - x)Se(x) (x = 0.05, 0.10).

A resonant x-ray scattering investigation of the NpAs(1 - x)Se(x) system with single crystals of 5 and 10% Se content is reported. The main features of the magnetic phase diagram previously studied by neutron scattering were confirmed. The coexistence within a single domain of ferro- and antiferro-components in the low-T ferrimagnetic phase was established, as well as the single-k character of the incommensurate phase and of the antiferromagnetic component of the ferrimagnetic phase. A tetragonal lattice distortion was found in the ferro- and ferrimagnetic phases which is not compatible with the proposed model for the ferromagnetic phase. The study of ferromagnetism was carried out using polarization analysis of the diffracted beam to separate the scattering intensities originating from magnetism and charge, which are superimposed in reciprocal space. The magnetic character of the ferromagnetic signal calculated from the measured intensities in the polarization analysis σπ and σσ channels was confirmed by analysis of the corresponding temperature dependence.

Interplay of inequivalent atomic positions in resonant x-ray diffraction of Fe(3)BO(6).

'Forbidden' Bragg reflections of iron orthoborate Fe(3)BO(6) were studied theoretically and experimentally in the vicinity of the iron K edge. Their energy spectra are explained as resulting from the interference of x-rays scattered from two inequivalent crystallographic sites occupied by iron ions. This particular structure property gives rise to complex azimuthal dependences of the reflection intensities in the pre-edge region as they result from the interplay of site specific dipole-quadrupole and quadrupole-quadrupole resonant scattering. Also evidenced is an anisotropic character of the absorption spectrum. Self-absorption correction to the diffraction data, as well as possible contributions of thermal vibrations and magnetic order, are discussed. Particular care is given to extracting clean spectra from the data, and it is demonstrated that excellent results can be obtained even from measurements that appear corrupted by several effects such as poor crystal quality and multiple scattering.

ID20: a beamline for magnetic and resonant X-ray scattering investigations under extreme conditions.

A new experimental station at ESRF beamline ID20 is presented which allows magnetic and resonant X-ray scattering experiments in the energy range 3-25 keV to be performed under extreme conditions. High magnetic field up to 10 T, high pressure up to 30 kbar combined with low temperatures down to 1.5 K are available and experiments can be performed at the M-edges of actinide elements, L-edges of lanthanides and K-edges of transition metals.

Direct determination of the magnetic ground state in the square lattice S = 1/2 antiferromagnet Li2VOSiO4.

Powder neutron diffraction and resonant x-ray scattering measurements from a single crystal have been performed to study the low-temperature state of the 2D frustrated, quantum-Heisenberg system Li2VOSiO4. Both techniques indicate a collinear antiferromagnetic ground state, with propagation vector k=(1 / 2 1 / 2 0), and magnetic moments in the a-b plane. Contrary to previous reports, the ordered moment at 1.44 K, m=0.63(3)micro(B), is very close to the value expected for the square lattice Heisenberg model ( approximately 0.6micro(B)). The magnetic order is three dimensional, with antiferromagnetic a-b layers stacked ferromagnetically along the c axis. Neither x-ray nor neutron diffraction shows evidence for a structural distortion between 1.6 and 10 K.

Local chiral-symmetry breaking in globally centrosymmetric crystals.

A thorough tensor analysis of the Bragg-forbidden reflection (00.3)(h) in corundum systems having a global center of inversion, such as V2O3 and alpha-Fe2O3, shows that anomalous x-ray resonant diffraction can access chiral properties related to the dipole-quadrupole (E1-E2) channel via an interference with the pure quadrupole-quadrupole (E2-E2) process. This is also confirmed by independent ab initio numerical simulations. In such a way, it becomes possible to detect chiral quantities in systems where dichroic absorption techniques are ineffective.