Magnetic control of ferroelectric polarization

Influence of antiferromagnetic ordering on ferroelectric polarization switching of YMnO3 epitaxial thin films

In this study, the switching behavior of ferroelectric polarization of (0001) YMnO(3) epitaxial films at around Néel temperature was investigated. From the experimental results of the frequency and temperature dependences of coercive electric filed (E(c)) obtained from polarization-electric field (P-E) hysteresis loop, the crosscorrelation phenomena between magnetics and ferroelectrics are discussed in detail. The P-E hysteresis loops of the films were measured in the frequency range from 1 Hz to 10 kHz, and the temperature was varied from 10 to 150 K. Frequency dependence of Ec accorded with Ishibashi-Orihara's theory at the measured temperature range. However, temperature dependence of E(c) disagreed with Devonshire's theory below 120 K, which is close to the Néeel temperature of the YMnO(3) epitaxial film. disagreed with Devonshire's theory below 120 K, which is close to the Neel temperature of the YMnO(3) epitaxial film.

Dual nature of improper ferroelectricity in a magnetoelectric multiferroic

Using first-principles calculations, we study the microscopic origin of ferroelectricity (FE) induced by magnetic order in the orthorhombic HoMnO3. We obtain the largest ferroelectric polarization observed in the whole class of improper magnetic ferroelectrics to date. We find that the two proposed mechanisms for FE in multiferroics, lattice and electronic based, are simultaneously active in this compound: a large portion of the ferroelectric polarization arises due to quantum-mechanical effects of electron orbital polarization, in addition to the conventional polar atomic displacements. An interesting mechanism for switching the magnetoelectric domains by an electric field via a 180 degrees coherent rotation of Mn spins is also proposed.

Ferrodistortive instability at the (001) surface of half-metallic manganites

We present the structure of the fully relaxed (001) surface of the half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional theory. Two relevant ferroelastic order parameters are identified and characterized. The known tilting of the oxygen octahedra, which is present in the bulk phase, decreases towards the surface. A ferrodistortive Mn off-centering, triggered by the surface and not reported before, decays monotonically into the bulk. This distortion affects neither the half-metallicity nor the zero-temperature magnetization, but does change the effective spin-spin interactions, and thus the temperature dependence of the magnetic properties.

Polarization reversal in multiferroic TbMnO3 with a rotating magnetic field direction

For the memory application of magnetoelectric multiferroics, not only bistability (i.e., ferroelectricity) but also the switching of the polarization direction with some noneverlasting stimulus is necessary. Here, we report a novel method for the electric polarization reversal in TbMnO3 without the application of an electric field or heat. The direction of the magnetic-field-induced polarization along the a axis (Pa) is memorized even in the zero field where Pa is absent. The polarization direction can be reversed by rotating the magnetic-field direction in the ab plane.

Observation of ferrotoroidic domains

Domains are of unparalleled technological importance as they are used for information storage and for electronic, magnetic and optical switches. They are an essential property of any ferroic material. Three forms of ferroic order are widely known: ferromagnetism, a spontaneous magnetization; ferroelectricity, a spontaneous polarization; and ferroelasticity, a spontaneous strain. It is currently debated whether to include an ordered arrangement of magnetic vortices as a fourth form of ferroic order, termed ferrotoroidicity. Although there are reasons to expect this form of order from the point of view of thermodynamics, a crucial hallmark of the ferroic state--that is, ferrotoroidic domains--has not hitherto been observed. Here ferrotoroidic domains are spatially resolved by optical second harmonic generation in LiCoPO4, where they coexist with independent antiferromagnetic domains. Their space- and time-asymmetric nature relates ferrotoroidics to multiferroics with magnetoelectric phase control and to other systems in which space and time asymmetry leads to possibilities for future applications.

Magnetic field switching between the two orbital-ordered states in DyVO3

The critical phase competition between different spin-orbital-ordered states has been investigated for the DyVO3 single crystal. As temperature is lowered, the compound exhibits a reentrant spin and orbital ordering (SO and OO) transition: C-->G-->C type for SO and G-->C-->G type for OO. It was found that a magnetic field also drives the phase transition from C to G for OO and concomitantly from G to C for SO, the latter of which is coupled with the metamagnetic transition of the Dy 4f moments. The mechanism of this novel magnetic-field-induced orbital switching is discussed.

Electric polarization, magnetoelectric effect, and orbital state of a layered iron oxide with frustrated geometry

A layered iron oxide RFe2O4 (R denotes rare-earth-metal elements) is an exotic dielectric material with charge-order (CO) driven electric polarization and magnetoelectric effect caused by spin-charge coupling. In this paper, a theory of electronic structure and dielectric property in RFe2O4 is presented. Charge frustration in paired-triangular lattices allows a charge imbalance without inversion symmetry. Spin frustration induces reinforcement of this polar CO by a magnetic ordering. We also analyze an orbital model for the Fe ion which does not show a conventional long-range order.

Non-resonant and resonant x-ray scattering studies on multiferroic TbMn2O5

Comprehensive x-ray scattering studies, including resonant scattering at Mn L, Tb L, and M edges, were performed on single crystals of TbMn2O5 for crystallographic data to elucidate the nature of its commensurate and incommensurate phases. The scattering results provide direct evidence of symmetry lowering to the ferroelectric phase driven by magnetically induced lattice modulations and show the presence of multiple magnetic orders. The competing orders under spin-frustrated geometry are believed to cause discommensuration and result in the commensurate-to-incommensurate phase transition around 24 K. It is proposed that the low temperature incommensurate phase consists of commensurate domains separated by antiphase domain walls which change both signs of spontaneous polarizations and x-ray scattering amplitudes for forbidden reflections.

Coupling of frustrated ising spins to the magnetic cycloid in multiferroic TbMnO(3)

We report on diffraction measurements on multiferroic TbMnO(3) which demonstrate that the Tb- and Mn-magnetic orders are coupled below the ferroelectric transition T(FE) = 28 K. For T<T(FE) the magnetic propagation vectors (tau) for Tb and Mn are locked so that tau(Tb) = tau(Mn), while below T(N)(Tb) = 7 K we find that tau(Tb) and tau(Mn) lock-in to rational values of 3/7b* and 2/7b*, respectively, and obey the relation 3tau(Tb)-tau(Mn)=1. We explain this novel matching of wave vectors within the frustrated anisotropic next-nearest-neighbor Ising model coupled to a periodic external field produced by the Mn-spin order. The tau(Tb)=tau(Mn) behavior is recovered when Tb magnetization is small, while the tau(Tb) = 3/7 regime is stabilized at low temperatures by a peculiar arrangement of domain walls in the ordered state of Ising-like Tb spins.

Ferroelectricity driven by the noncentrosymmetric magnetic ordering in multiferroic TbMn(2)O(5): a first-principles study

The ground state structural, electronic, and magnetic properties of multiferroic TbMn(2)O(5) are investigated via first-principles calculations. We show that the ferroelectricity in TbMn(2)O(5) is driven by the noncentrosymmetric magnetic ordering, without invoking the spin-orbit coupling and noncollinear spins. The intrinsic electric polarization in this compound is calculated to be 1187 nC cm(-2), an order of magnitude larger than previously thought.

Magnetic interactions in the geometrically frustrated triangular lattice antiferromagnet CuFeO2

The spin-wave excitations of the geometrically frustrated triangular lattice antiferromagnet CuFeO2 have been measured using high resolution inelastic neutron scattering. Antiferromagnetic interactions up to third nearest neighbors in the ab plane (J1, J2, J3, with J{2}/J{1} approximately 0.44 and J{3}/J{1} approximately 0.57), as well as out-of-plane coupling (J{z}, with J{z}/J{1} approximately 0.29) are required to describe the spin-wave dispersion relations, indicating a three-dimensional character of the magnetic interactions. Two energy dips in the spin-wave dispersion occur at the incommensurate wave vectors associated with multiferroic phase and can be interpreted as dynamic precursors to the magnetoelectric behavior in this system.

Prediction for new magnetoelectric fluorides

We use symmetry considerations in order to predict new magnetoelectric fluorides. In addition to these magnetoelectric properties, we discuss which among these fluorides are the ones susceptible to present multiferroic properties. We emphasize that several materials exhibit ferromagnetic properties. This ferromagnetism should enhance the interplay between the magnetic and dielectric properties in these materials.

Electronic structure and magnetism of EuTiO(3): a first-principles study

Density-functional theory calculations were carried out for the multiferroic EuTiO(3) using the LDA+U approach. Total-energy calculations for ferromagnetic (F), and antiferromagnetic A-, C-, and G-type arrangements in the cubic phase shows that the ground-state magnetic configuration is G-type antiferromagnetic for U≤6 eV and ferromagnetic for U≥7 eV. Values of first- and second-neighbour exchange integrals have been calculated by mapping the energy difference between the different magnetic configurations to a Heisenberg Hamiltonian. The system seems to be critically balanced between ferromagnetic and antiferromagnetic states for realistic values of U, and switches from antiferromagnetic to a ferromagnetic ground state on hydrostatic expansion of volume.

Magnetic Properties of Bulk BiCrO3 Studied with dc and ac Magnetization and Specific Heat

Single-phased powder BiCrO(3) sample was prepared at 6 GPa and 1653 K. Its magnetic properties were investigated by dc/ac magnetization, magnetic relaxation, and specific heat measurements. Four anomalies of magnetic origin were found near 40, 75, 109, and 111 K. The long-range antiferromagnetic order with weak ferromagnetism occurs at T(N) = 109 K. The ac susceptibilities showed that the transition near T(N) is a two-step transition. Additional frequency-independent broad anomalies were observed on the real part of the ac susceptibilities near 75 K, likely, caused by the change in the magnetic easy axis. The dc magnetic susceptibilities also had anomalies at 75 K, and the isothermal magnetization curves and relaxation curves changed their behavior below 75 K. Below 40 K, frequency-dependent anomalies with very large temperature shifts were observed on both the real and imaginary parts of the ac susceptibilities. The monoclinic-to-orthorhombic structural phase transition near 420 K was investigated by magnetization and differential scanning calorimetry measurements.

Polaron melting and ordering as key mechanisms for colossal resistance effects in manganites

Polarons, the combined motion of electrons in a cloth of their lattice distortions, are a key transport feature in doped manganites. To develop a profound understanding of the colossal resistance effects induced by external fields, the study of polaron correlations and the resulting collective polaron behavior, i.e., polaron ordering and transition from polaronic transport to metallic transport is essential. We show that static long-range ordering of Jahn-Teller polarons forms a polaron solid which represents a new type of charge and orbital ordered state. The related noncentrosymmetric lattice distortions establish a connection between colossal resistance effects and multiferroic properties, i.e., the coexistence of ferroelectric and antiferromagnetic ordering. Colossal resistance effects due to an electrically induced polaron solid-liquid transition are directly observed in a transmission electron microscope with local electric stimulus applied in situ using a piezo-controlled tip. Our results shed light onto the colossal resistance effects in magnetic field and have a strong impact on the development of correlated electron-device applications such as resistive random access memory (RRAM).

Chiral spin pairing in helical magnets

A concept of chiral spin pairing is introduced to describe a vector-chiral liquid-crystal order in frustrated spin systems. It is found that the chiral spin pairing is induced by the coupling to phonons through the Dzyaloshinskii-Moriya interaction and the four-spin exchange interaction of the Coulomb origin under the edge-sharing network of magnetic and ligand ions. This produces two successive second-order phase transitions upon cooling: an O(2) chiral spin nematic, i.e., spin cholesteric, order appears with an either parity, and then the O(2) symmetry is broken to yield a helical magnetic order. Possible candidate materials are also discussed as new multiferroic systems.

Direct transition from a disordered to a multiferroic phase on a triangular lattice

We report the first direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)(2). Ferroelectricity is observed only when the magnetic structure has chirality and breaks inversion symmetry. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that chiral magnetic order can give rise to a pseudoelectric field, whose temperature dependence agrees with experiment.

Multiferroicity induced by dislocated spin-density waves

We uncover a new pathway towards multiferroicity, showing how magnetism can drive ferroelectricity without relying on inversion symmetry breaking of the magnetic ordering. Our free-energy analysis demonstrates that any commensurate spin-density-wave ordering with a phase dislocation, even if it is collinear, gives rise to an electric polarization. Because of the dislocation, the electronic and magnetic inversion centers do not coincide, which turns out to be a sufficient condition for multiferroic coupling. The novel mechanism explains the formation of multiferroic phases at the magnetic commensurability transitions, such as the ones observed in YMn(2)O(5) and related compounds. We predict that in these multiferroics an oscillating electrical polarization is concomitant with the uniform polarization. On the basis of our theory, we put forward new types of magnetic materials that are potentially ferroelectric.

Charge order and the origin of giant magnetocapacitance in LuFe2O4

The nature of the charge order in the charge frustrated compound LuFe(2)O(4) and its effect on magnetocapacitance were examined on the basis of first-principles electronic structure calculations and Monte Carlo simulations of electrostatic energy. Our work shows that two different types of charge order of almost equal stability (i.e., square root of 3 x square root of 3 and chain types) occur in the Fe(2)O(4) layers of LuFe(2)O(4), and that the ground state of LuFe(2)O(4) has a ferrielectric arrangement of the Fe(2)O(4) layers with square root of 3 x square root of 3 charge order. The giant magnetocapacitance effect of LuFe(2)O(4) at room temperature is accounted for in terms of charge fluctuations arising from the interconversion between the two types of charge order, that becomes hindered by an applied magnetic field.

Influence of oxygen defects on the crystal structure and magnetic properties of the (Tb1-xNax)MnO3-y (0<or=x<or=0.3) system

The crystallographic and magnetic behaviors of (Tb1-xNax)MnO3-y (0<or=x<or=0.3) have been studied by neutron powder diffraction (NPD), synchrotron X-ray powder diffraction, and Raman spectroscopy techniques. Although Na+ ions have larger ionic radii than Tb3+ ions, analysis of NPD data reveals a decrease in cell volume upon Na-doping, which can be explained solely by the occurrence of oxygen deficiencies and not by the size effect. The Raman spectrum represents the variation in bond length and bond angle, which originates from the balance of ions, asymmetric structure, and defects in the system. Na-doping causes an oxygen deficiency, and consequently, a peak shift is seen in the Raman spectrum because of the structural adjustment resulting from the doping. The observed effective moments decrease with increasing x because of the replacement of Tb3+ ions by Na+ ions. The well-defined peak at approximately 45 K (labeled TMn) of the x=0.3 sample is associated with Mn spin ordering, while the magnetic responses associated with TMn are not clearly present in the x=0.15 and x=0 samples.

Ferroelectricity driven by Y d0-ness with rehybridization in YMnO3

We investigated electronic structure of hexagonal multiferroic YMnO3 using the polarization dependent x-ray absorption spectroscopy (XAS) at O K and Mn L(2,3) edges. The spectra exhibit strong polarization dependence at both edges, reflecting anisotropic Mn 3d orbital occupation. Moreover, the O K edge spectra show that Y 4d states are strongly hybridized with O 2p ones, resulting in large anomalies in Born effective charges on off-centering Y and O ions. These results manifest that the Y d(0)-ness with rehybridization is the driving force for the ferroelectricity, and suggest a new approach to understand the multiferroicity in the hexagonal manganites.

Giant sharp and persistent converse magnetoelectric effects in multiferroic epitaxial heterostructures

Magnetoelectric coupling between magnetic and electrical properties presents valuable degrees of freedom for applications. The two most promising scenarios are magnetic-field sensors that could replace low-temperature superconducting quantum interference devices, and electric-write magnetic-read memory devices that combine the best of ferroelectric and magnetic random-access memory. The former scenario requires magnetically induced continuous and reversible changes in electrical polarization. These are commonly observed, but the coupling constants thus obtained are invalid for data-storage applications, where the more difficult to achieve and rarely studied magnetic response to an electric field is required. Here, we demonstrate electrically induced giant, sharp and persistent magnetic changes (up to 2.3 x 10(-7) s m(-1)) at a single epitaxial interface in ferromagnetic 40 nm La(0.67)Sr(0.33)MnO(3) films on 0.5 mm ferroelectric BaTiO(3) substrates. X-ray diffraction confirms strain coupling via ferroelastic non-180( composite function) BaTiO(3) domains. Our findings are valid over a wide range of temperatures including room temperature, and should inspire further study with single epitaxial interfaces.

First-principles study of structural, electronic, and multiferroic properties in BiCoO3

Electronic and magnetic properties of BiCoO(3) have been investigated using the ab initio density-functional calculations with local spin density approximation (LSDA) and LSDA+U methods. The structural stability and the origin of the multiferroism for ferroelectronic and ferromagnetic existence were addressed. It was shown that the stability of the C-type antiferromagnetic (C-AFM) structure is better than that of other possible configurations. The hybridization between Bi-O and Co-O with interplay and a local magnetic moment on the Co(3+) play important roles for the nature of the ferroelectricity and ferromagnetism. Theoretical calculations predict the insulating ground state with a band gap of 2.11 eV in the C-AFM ordering for BiCoO(3) originated from the antiferromagnetic interaction in the ab plane, which is in well agreement with experiments.

Symmetry of multiferroicity in a frustrated magnet TbMn2O5

Based on measurements of soft-x-ray magnetic scattering and symmetry considerations, we demonstrate that the magnetoelectric effect in TbMn2O5 arises from an internal field determined by S-->q--> x S-->-q--> with S-->q--> being the magnetization at modulation vector q-->, whereas the magnetoelastic effect in the exchange energy governs the response to external electric fields. Our results set fundamental symmetry constraints on the microscopic mechanism of multiferroicity in frustrated magnets.

Electric control of spin helicity in a magnetic ferroelectric

Magnetic ferroelectrics or multiferroics, which are currently extensively explored, may provide a good arena to realize a novel magnetoelectric function. Here we demonstrate the genuine electric control of the spiral magnetic structure in one such magnetic ferroelectric, TbMnO3. A spin-polarized neutron scattering experiment clearly shows that the spin helicity, clockwise or counterclockwise, is controlled by the direction of spontaneous polarization and hence by the polarity of the small electric field applied on cooling.

Tunnel junctions with multiferroic barriers

Multiferroics are singular materials that can exhibit simultaneously electric and magnetic orders. Some are ferroelectric and ferromagnetic and provide the opportunity to encode information in electric polarization and magnetization to obtain four logic states. However, such materials are rare and schemes allowing a simple electrical readout of these states have not been demonstrated in the same device. Here, we show that films of La(0.1)Bi(0.9)MnO(3) (LBMO) are ferromagnetic and ferroelectric, and retain both ferroic properties down to a thickness of 2 nm. We have integrated such ultrathin multiferroic films as barriers in spin-filter-type tunnel junctions that exploit the magnetic and ferroelectric degrees of freedom of LBMO. Whereas ferromagnetism permits read operations reminiscent of magnetic random access memories (MRAM), the electrical switching evokes a ferroelectric RAM write operation. Significantly, our device does not require the destructive ferroelectric readout, and therefore represents an advance over the original four-state memory concept based on multiferroics.

Magnetic excitations in multiferroic TbMnO3: evidence for a hybridized soft mode

The magnetic excitations in multiferroic TbMnO3 have been studied by inelastic neutron scattering in the spiral and sinusoidally ordered phases. At the incommensurate magnetic zone center of the spiral phase, we find three low-lying magnons whose character has been fully determined using neutron-polarization analysis. The excitation at the lowest energy is the sliding mode of the spiral, and two modes at 1.1 and 2.5 meV correspond to rotations of the spiral rotation plane. These latter modes are expected to couple to the electric polarization. The 2.5 meV mode is in perfect agreement with recent infrared-spectroscopy data giving strong support to its interpretation as a hybridized phonon-magnon excitation.

Enhanced ferroelectric polarization by induced Dy spin order in multiferroic DyMnO3

Neutron powder diffraction and single crystal x-ray resonant magnetic scattering measurements suggest that Dy plays an active role in enhancing the ferroelectric polarization in multiferroic DyMnO3 above T(Dy)(N)=6.5 K. We observe the evolution of an incommensurate ordering of Dy moments with the same periodicity as the Mn spiral ordering. It closely tracks the evolution of the ferroelectric polarization. Below T(Dy)(N), where Dy spins order commensurately, the polarization decreases to values similar for those of TbMnO3. The higher P(s) found just above T(Dy)(N) arises from the contribution of Dy spins so as to effectively increase the amplitude of the Mn spin spiral.

Ferroelectricity in an s=1/2 chain cuprate

We report our discovery of ferroelectricity in the spiral-magnetic state in the quantum quasi-one-dimensional (1D) S=1/2 magnet of LiCu2O2. Electric polarization (P) emerges along the c direction below the spiral-magnetic order temperature, but changes from the c to a axis when magnetic fields (H) are applied along the b direction. We also found that P(c) increases with H(c), and P(a) appears with H(a). LiCu2O2 in zero field appears to be the first ferroelectric cuprate and also a prototypical example of the "1D spiral-magnetic ferroelectrics." However, the unexpected behavior in H may demonstrate the complexity of the ordered spin configuration, inherent in the 1D S=1/2 magnet of LiCu2O2.

Dynamical magnetoelectric coupling in helical magnets

We develop a theory of collective mode dynamics in the helical magnets coupled to electric polarization via spin-orbit interaction. The low-lying modes associated with the ferroelectricity are not the transverse optical phonons, but are the spin waves hybridized with the electric polarization. This hybridization leads to the Drude-like dielectric function epsilon(omega) in the limit of zero magnetic anisotropy. There are two additional low-lying modes: phason of the spiral and rotation of helical plane along the polarization axis. Role of these low-lying modes in the neutron scattering and antiferromagnetic resonance is revealed, and a novel experiment to detect the dynamical magnetoelectric coupling is discussed.

Electromagnons in multiferroic YMn2O5 and TbMn2O5

Based on temperature dependent far infrared transmission spectra of YMn2O5 and TbMn2O5 single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the steplike anomaly of the static dielectric constant at the commensurate--incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.

Multiferroics: progress and prospects in thin films

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties - and in turn promise new device applications - as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Multiferroics: a magnetic twist for ferroelectricity

Magnetism and ferroelectricity are essential to many forms of current technology, and the quest for multiferroic materials, where these two phenomena are intimately coupled, is of great technological and fundamental importance. Ferroelectricity and magnetism tend to be mutually exclusive and interact weakly with each other when they coexist. The exciting new development is the discovery that even a weak magnetoelectric interaction can lead to spectacular cross-coupling effects when it induces electric polarization in a magnetically ordered state. Such magnetic ferroelectricity, showing an unprecedented sensitivity to ap plied magnetic fields, occurs in 'frustrated magnets' with competing interactions between spins and complex magnetic orders. We summarize key experimental findings and the current theoretical understanding of these phenomena, which have great potential for tuneable multifunctional devices.

Magnetic and electric phase control in epitaxial EuTiO(3) from first principles

We propose a design strategy--based on the coupling of spins, optical phonons, and strain--for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO(3).

Ferroelectricity in the magnetic E-phase of orthorhombic perovskites

We show that the symmetry of the spin zigzag chain E phase of the orthorhombic perovskite manganites and nickelates allows for the existence of a finite ferroelectric polarization. The proposed microscopic mechanism is independent of spin-orbit coupling. We predict that the polarization induced by the E-type magnetic order can potentially be enhanced by up to 2 orders of magnitude with respect to that in the spiral magnetic phases of TbMnO3 and similar multiferroic compounds.

Selective Synthesis of TbMn2O5 Nanorods and TbMnO3 Micron Crystals

Multiferroic rare-earth manganates TbMn2O5 and TbMnO3 were synthesized selectively via a one-pot hydrothermal route. The different morphologies can be obtained by changing the ratio of reactants MnCl2.4H2O and KMnO4. SEM and TEM images showed a high quality for the products that was also confirmed by XPS patterns and Raman spectra.

Electron localization or delocalization in incommensurate helical magnets

The electronic states in incommensurate helical magnets are studied theoretically from the viewpoint of the localization or delocalization. It is found that in the multiband system with a relativistic spin-orbit interaction, the electronic wave functions show both an extended and localized nature along the helical axis depending on the orbital, helical wave number, and the direction of the plane on which spins rotate. The possible realization of this localization is discussed.

Ferroelectric polarization flop in a frustrated magnet MnWO4 induced by a magnetic field

The relationship between magnetic order and ferroelectric properties has been investigated for MnWO4 with a long-wavelength magnetic structure. Spontaneous electric polarization is observed in an elliptical spiral spin phase. The magnetic-field dependence of electric polarization indicates that the noncollinear spin configuration plays a key role for the appearance of the ferroelectric phase. An electric polarization flop from the b direction to the a direction has been observed when a magnetic field above 10 T is applied along the b axis. This result demonstrates that an electric polarization flop can be induced by a magnetic field in a simple system without rare-earth 4f moments.

Multiferroic and magnetoelectric materials

A ferroelectric crystal exhibits a stable and switchable electrical polarization that is manifested in the form of cooperative atomic displacements. A ferromagnetic crystal exhibits a stable and switchable magnetization that arises through the quantum mechanical phenomenon of exchange. There are very few 'multiferroic' materials that exhibit both of these properties, but the 'magnetoelectric' coupling of magnetic and electrical properties is a more general and widespread phenomenon. Although work in this area can be traced back to pioneering research in the 1950s and 1960s, there has been a recent resurgence of interest driven by long-term technological aspirations.

Tracking the motion of charges in a terahertz light field by femtosecond X-ray diffraction

In condensed matter, light propagation near resonances is described in terms of polaritons, electro-mechanical excitations in which the time-dependent electric field is coupled to the oscillation of charged masses. This description underpins our understanding of the macroscopic optical properties of solids, liquids and plasmas, as well as of their dispersion with frequency. In ferroelectric materials, terahertz radiation propagates by driving infrared-active lattice vibrations, resulting in phonon-polariton waves. Electro-optic sampling with femtosecond optical pulses can measure the time-dependent electrical polarization, providing a phase-sensitive analogue to optical Raman scattering. Here we use femtosecond time-resolved X-ray diffraction, a phase-sensitive analogue to inelastic X-ray scattering, to measure the corresponding displacements of ions in ferroelectric lithium tantalate, LiTaO(3). Amplitude and phase of all degrees of freedom in a light field are thus directly measured in the time domain. Notably, extension of other X-ray techniques to the femtosecond timescale (for example, magnetic or anomalous scattering) would allow for studies in complex systems, where electric fields couple to multiple degrees of freedom.