We report direct experimental evidence for a room-temperature, ∼130  μC/cm(2) ferroelectric polarization from the tetragonal-like BiFeO(3) phase. The physical origin of this remarkable enhancement of ferroelectric polarization has been investigated by a combination of x-ray absorption spectroscopy, scanning transmission electron microscopy, and first principles calculations. A large strain-induced Fe-ion displacement relative to the oxygen octahedra, combined with the contribution of Bi 6s lone pair electrons, is the mechanism driving the large ferroelectric polarization in this tetragonal-like phase.

A new orthorhombic phase of the multiferroic BiFeO3 has been created via strain engineering by growing it on a NdScO(3)(110)(o) substrate. The tensile-strained orthorhombic BiFeO3 phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane {110}(pc) directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism.

The material class of rare earth nickelates with high Ni3+ oxidation state is generating continued interest due to the occurrence of a metal-insulator transition with charge order and the appearance of non-collinear magnetic phases within this insulating regime. The recent theoretical prediction for superconductivity in LaNiO3 thin films has also triggered intensive research efforts. LaNiO3 seems to be the only rare earth nickelate that stays metallic and paramagnetic down to lowest temperatures. So far, centimeter-sized impurity-free single crystal growth has not been reported for the rare earth nickelates material class since elevated oxygen pressures are required for their synthesis. Here, we report on the successful growth of centimeter-sized LaNiO3 single crystals by the floating zone technique at oxygen pressures of up to 150 bar. Our crystals are essentially free from Ni2+ impurities and exhibit metallic properties together with an unexpected but clear antiferromagnetic transition.

The strong coupling between antiferromagnetism and ferroelectricity at room temperature found in BiFeO3 generates high expectations for the design and development of technological devices with novel functionalities. However, the multi-domain nature of the material tends to nullify the properties of interest and complicates the thorough understanding of the mechanisms that are responsible for those properties. Here we report the realization of a BiFeO3 material in thin film form with single-domain behaviour in both its magnetism and ferroelectricity: the entire film shows its antiferromagnetic axis aligned along the crystallographic b axis and its ferroelectric polarization along the c axis. With this we are able to reveal that the canted ferromagnetic moment due to the Dzyaloshinskii–Moriya interaction is parallel to the a axis. Furthermore, by fabricating a Co/BiFeO3 heterostructure, we demonstrate that the ferromagnetic moment of the Co film does couple directly to the canted moment of BiFeO3.

Barium thio-oxocobaltate(II), Ba[CoS2/2 O2/2 ], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4) Å) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below TN =222 K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.

We studied the local Ru 4d electronic structure of α-RuCl3 by means of polarization dependent xray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is −12 ± 10 meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a J eff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, α-RuCl3 is an ideal candidate for the realization of Kitaev physics.Geometrically frustrated quantum spin systems are important owing to the fact that frustration often results in a suppression of conventional mean field ground states in favor of more exotic phases of matter. Current research focuses on the effect of spin-orbit coupling (SOC) and the role it plays in the realization of different exotic phases such as unconventional superconductivity or quantum spin liquids [1-3]. Especially, quantum spin liquids can result in topological states with fractional excitations. An important, theoretically solvable model is the Kitaev model with spin-1/2 on a honeycomb lattice, where the coupling between neighboring spins is highly anisotropic with bond-dependent spin interactions. In contrast to spin liquids arising from usual geometrical frustrated spin arrangements, the bond-dependent spin interactions within the Kitaev model frustrate the spin configuration on a single site [4].The search for fractionalized excitations and the identification of a Kitaev spin liquid state has been experimentally quite difficult. Increased attention has been focussed on the honeycomb iridates [5,6], starting from the assumption that large spin-orbit coupling is the leading energy scale in determining the ground state such that the Ir 5d t 2g orbitals are described in terms of J eff = 1/2 and 3/2 orbitals. However, the real iridate systems exhibit trigonal distortion (D trig = 0.1 eV [6]) and a significant itinerant character of the Ir 5d orbitals [7-9], which complicates the electronic ground state. Despite a flurry of both theoretical and experimental studies, the nature of the ground state in honeycomb iridates are being fiercely debated and the occurrence of Kitaev physics is still far from clear.Recently, α-RuCl 3 has been suggested as a promising candidate material for the realization of the Kitaev model [10] and excitations observed via Raman [11,12] and in-elastic neutron scattering [13] have been presented as evidence that α-RuCl 3 may be close to a quantum spin liquid ground state. In the last two years a number of publications discussing the realization of the Kitaev physics in α-RuCl 3 has appeared in literature [3,[14][15][16][17][18][20][21][22][23][24][25][26][27][28][29]...

Controlling ferroic orders (ferroelectricity, ferromagnetism, and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between order parameter coupling strengths and incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in epitaxial mixed-phase BiFeO 3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light driven flexoelectric

The coupled valence and spin state transition (VSST) taking place in (Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by soft x-ray absorption spectroscopy (XAS) experiments carried out at the Pr-M4,5, Co-L2,3, and O-1s edges. This VSST is found to be composed of a sharp Pr/Co valence and Co spin state transition centered at T*=89.3 K, followed by a smoother Co spin-state evolution at higher temperatures. At T < T*, we found that the praseodymium displays a mixed valence Pr3+/Pr4+ with about 0.13 Pr4+/f.u., while all the Co3+ is in the low-spin (LS) state. At T around T*, the sharp valence transition converts all the Pr4+ to Pr3+ with a corresponding Co3+ to Co4+ compensation. This is accompanied by an equally sharp spin state transition of the Co3+ from the low to an incoherent mixture of low and high spin (HS) states. An involvement of the intermediate spin (IS) state can be discarded for the Co3+. While above T* and at high temperatures the system shares rather similar properties as Sr-doped LaCoO3, at low temperatures it behaves much more like EuCoO3 with its highly stable LS configuration for the Co3+. Apparently, the mechanism responsible for the formation of Pr4+ at low temperatures also helps to stabilize the Co3+ in the LS configuration despite the presence of Co4+ ions. We also found out that that the Co4+ is in an IS state over the entire temperature range investigated in this study (10-290 K). The presence of Co3+ HS and Co4+ IS at elevated temperatures facilitates the conductivity of the material.Comment: 19 pages, 7 figures, Accepted in PR