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.

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Multiferroic and magnetoelectric materials

The magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

Magnetic control of ferroelectric polarization

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.

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Multiferroics: a magnetic twist for ferroelectricity

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: progress and prospects in thin films.

This article reviews the physics of high-temperature superconductors from the point of view of the doping of a Mott insulator. The basic electronic structure of cuprates is reviewed, emphasizing the physics of strong correlation and establishing the model of a doped Mott insulator as a starting point. A variety of experiments are discussed, focusing on the region of the phase diagram close to the Mott insulator (the underdoped region) where the behavior is most anomalous. The normal state in this region exhibits pseudogap phenomenon. In contrast, the quasiparticles in the superconducting state are well defined and behave according to theory. This review introduces Anderson's idea of the resonating valence bond and argues that it gives a qualitative account of the data. The importance of phase fluctuations is discussed, leading to a theory of the transition temperature, which is driven by phase fluctuations and the thermal excitation of quasiparticles. However, an argument is made that phase fluctuations can only explain pseudogap phenomenology over a limited temperature range, and some additional physics is needed to explain the onset of singlet formation at very high temperatures. A description of the numerical method of the projected wave function is presented, which turns out to be a very useful technique for implementing the strong correlation constraint and leads to a number of predictions which are in agreement with experiments. The remainder of the paper deals with an analytic treatment of the $t\text{\ensuremath{-}}J$ model, with the goal of putting the resonating valence bond idea on a more formal footing. The slave boson is introduced to enforce the constraint againt double occupation and it is shown that the implementation of this local constraint leads naturally to gauge theories. This review follows the historical order by first examining the U(1) formulation of the gauge theory. Some inadequacies of this formulation for underdoping are discussed, leading to the SU(2) formulation. Here follows a rather thorough discussion of the role of gauge theory in describing the spin-liquid phase of the undoped Mott insulator. The difference between the high-energy gauge group in the formulation of the problem versus the low-energy gauge group, which is an emergent phenomenon, is emphasized. Several possible routes to deconfinement based on different emergent gauge groups are discussed, which leads to the physics of fractionalization and spin-charge separation. Next the extension of the SU(2) formulation to nonzero doping is described with a focus on a part of the mean-field phase diagram called the staggered flux liquid phase. It will be shown that inclusion of the gauge fluctuation provides a reasonable description of the pseudogap phase. It is emphasized that $d$-wave superconductivity can be considered as evolving from a stable U(1) spin liquid. These ideas are applied to the high-${T}_{c}$ cuprates, and their implications for the vortex structure and the phase diagram are discussed. A possible test of the topological structure of the pseudogap phase is described.

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Doping a Mott insulator: Physics of high-temperature superconductivity

Negative thermal expansion (NTE) materials, which shrink upon heating, are required in modern technologies. In this study, new lead-free NTE compounds were obtained by substitution of La in PbTiO3 ...

Negative Thermal Expansion in Lead-Free La-Substituted Bi0.5Na0.5VO3

Transport properties of PbVO3, a material whose structural and electronic properties bear similarities with high-temperature copper superconductors, were investigated between 2 and 300 K and in the pressure range of 0.1 MPa and 11.3 GPa. There is a structural phase transition from about 2 GPa at room temperature from a super-tetragonal phase to a cubic phase with a significant drop of resistivity by about 5 orders of magnitude. Nevertheless, the cubic phase exhibited a semiconducting behavior of resistivity between 2 and 300 K up to 11.3 GPa. The pressure dependence of lattice parameters of the tetragonal and cubic phases was in very good agreement with results of first-principle calculations from J. Phys.: Condens. Matter 24, 435403 (2012). Fitting with the Birch–Murnaghan equation of states gave a bulk modulus K0 = 38.0(1.1) GPa and a unit cell volume V0 = 67.63(6) A3 for the tetragonal phase and K0 = 179.5(1.4) GPa and V0 = 58.272(14) A3 for the cubic phase.

Absence of Metallic Conductivity in Tetragonal and Cubic PbVO3 at High Pressure

Provided are a piezoelectric material without using lead or an alkali metal, the piezoelectric material having a stable crystal structure in a wide temperature range, high insulation property, and high piezoelectric property, and a piezoelectric element using the piezoelectric material, in which the piezoelectric material is made of a metal oxide having a tetragonal crystal structure and expressed by Ba(Si x Ge y Ti z )O 3 (here, 0≦x≦1, 0≦y≦1, and 0≦z≦0.5), the piezoelectric element includes the piezoelectric material and a pair of electrodes sandwiching the piezoelectric material, and at least one of the pair of electrodes is made of SrRuO 3 or Ni.

Metal oxide, piezoelectric material and piezoelectric element

The crystal structure of isostructural SrCuV2O7 and BaCuV2O7 consists of one-dimensional (1D) zigzag chains of Cu atoms with next-nearest-neighbor interaction. The main intrachain interaction was found to be ferromagnetic and estimated at 4.6 K (Hamiltonian H ∼ −2J). SrCuV2O7 and BaCuV2O7 are new examples in the scanty family of 1D ferromagnets. Isothermal magnetization measurements at 0.08 K and specific heat data showed that MCuV2O7 exhibits antiferromagnetic long-range ordering at TN = 1.36 K for SrCuV2O7 and TN = 1.47 K for BaCuV2O7. Spin-flop transitions were observed in the antiferromagnetic state at 0.08 K near 0.5 kOe in SrCuV2O7 and 2 kOe in BaCuV2O7. In air, SrCuV2O7 and BaCuV2O7 melted incongruently above 983 and 1018 K, respectively.

Masaki Azuma

Papers

Negative Thermal Expansion in Lead-Free La-Substituted Bi0.5Na0.5VO3

Absence of Metallic Conductivity in Tetragonal and Cubic PbVO3 at High Pressure

Metal oxide, piezoelectric material and piezoelectric element

Low-dimensional ferromagnetic properties of SrCuV2O7 and BaCuV2O7.

Direct observation of negative thermal expansion in SrCu3Fe4O12