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

Enhancement of polarization and related properties in heteroepitaxially constrained thin films of the ferroelectromagnet, BiFeO3, is reported. Structure analysis indicates that the crystal structure of film is monoclinic in contrast to bulk, which is rhombohedral. The films display a room-temperature spontaneous polarization (50 to 60 microcoulombs per square centimeter) almost an order of magnitude higher than that of the bulk (6.1 microcoulombs per square centimeter). The observed enhancement is corroborated by first-principles calculations and found to originate from a high sensitivity of the polarization to small changes in lattice parameters. The films also exhibit enhanced thickness-dependent magnetism compared with the bulk. These enhanced and combined functional responses in thin film form present an opportunity to create and implement thin film devices that actively couple the magnetic and ferroelectric order parameters.

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Epitaxial BiFeO3 multiferroic thin film heterostructures.

Recent research activities on the linear magnetoelectric (ME) effect?induction of magnetization by an electric field or of polarization by a magnetic field?are reviewed. Beginning with a brief summary of the history of the ME effect since its prediction in 1894, the paper focuses on the present revival of the effect. Two major sources for 'large' ME effects are identified. (i) In composite materials the ME effect is generated as a product property of a magnetostrictive and a piezoelectric compound. A linear ME polarization is induced by a weak ac magnetic field oscillating in the presence of a strong dc bias field. The ME effect is large if the ME coefficient coupling the magnetic and electric fields is large. Experiments on sintered granular composites and on laminated layers of the constituents as well as theories on the interaction between the constituents are described. In the vicinity of electromechanical resonances a ME voltage coefficient of up to 90?V?cm?1?Oe?1 is achieved, which exceeds the ME response of single-phase compounds by 3?5 orders of magnitude. Microwave devices, sensors, transducers and heterogeneous read/write devices are among the suggested technical implementations of the composite ME effect. (ii) In multiferroics the internal magnetic and/or electric fields are enhanced by the presence of multiple long-range ordering. The ME effect is strong enough to trigger magnetic or electrical phase transitions. ME effects in multiferroics are thus 'large' if the corresponding contribution to the free energy is large. Clamped ME switching of electrical and magnetic domains, ferroelectric reorientation induced by applied magnetic fields and induction of ferromagnetic ordering in applied electric fields were observed. Mechanisms favouring multiferroicity are summarized, and multiferroics in reduced dimensions are discussed. In addition to composites and multiferroics, novel and exotic manifestations of ME behaviour are investigated. This includes (i) optical second harmonic generation as a tool to study magnetic, electrical and ME properties in one setup and with access to domain structures; (ii) ME effects in colossal magnetoresistive manganites, superconductors and phosphates of the LiMPO4 type; (iii) the concept of the toroidal moment as manifestation of a ME dipole moment; (iv) pronounced ME effects in photonic crystals with a possibility of electromagnetic unidirectionality. The review concludes with a summary and an outlook to the future development of magnetoelectrics research.

https://iopscience.iop.org/article/10.1088/0022-3727/38/8/R01/pdf

Revival of the Magnetoelectric Effect

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3–PbTiO3 and Pb(Mg1/3Nb2/3)O3–PbTiO3 were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3, morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33’s)>2500 pC/N and subsequent strain levels up to >0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for 〈001〉 oriented rhombohedral crystals, although 〈111〉 is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics, could be achieved being related to an E-field induced phase transformation. High electromechanical coupling (k33)>90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals

BiFeO3 is perhaps the only material that is both magnetic and a strong ferroelectric at room temperature. As a result, it has had an impact on the field of multiferroics that is comparable to that of yttrium barium copper oxide (YBCO) on superconductors, with hundreds of publications devoted to it in the past few years. In this Review, we try to summarize both the basic physics and unresolved aspects of BiFeO3 (which are still being discovered with several new phase transitions reported in the past few months) and device applications, which center on spintronics and memory devices that can be addressed both electrically and magnetically.

Physics and Applications of Bismuth Ferrite

In this work, a modified lumped parameter method has been discussed which, we believe, is especially suitable for microwave measurements of dielectric dispersion in high dielectric materials in a particular frequency region. The method combines some of the features of both the lumped parameter method and the distributed parameter method. Dielectric dispersions for Pb(Zr1-xTix)O3, BaTiO3 and BaxSr1-xTiO3 ceramics in the frequency domain from 1 MHz to 1GHz are presented. This paper also discusses the principle of the measurement and the accuracy of the experimental data which is retrieved.

Quasi lumped parameter method for microwave measurements of dielectric dispersion in ferroelectric ceramics

Dielectric and mechanical loss in the system Pb3MgNb2O9 - Pb3NiNb2O9

A simple Devonshire form has been derived for the phenomenological elastic Gibbs function to describe the elasto-dielectric parameters of simple proper ferroelectrics in the tungsten bronze structure family which has 4/mmm prototypic point symmetry. For the assumption that all temperature dependence is carried by the Curie-Weiss behaviour implicit in the quadratic term and that the expansion may be terminated at the first sixth-order term, reasonable agreement between calculated and derived Ps against T curves in the ferroelectric phase can be obtained for a wide range of bronze compositions. From the fitting it is clear that second and sixth rank terms are remarkably constant over a very wide range of bronze compositions. Variation in the negative fourth rank term is larger, but this is to be expected since it contains large contributions from electrostrictive and elastic terms which will depend on boundary conditions. These initial studies suggest that the phenomenological method may be used to derive expectation values for tensor parameters across the whole family of ferroelectric bronzes. The study also points up the need for more careful detailed studies of lattice strain, birefringence and permittivity as a function of temperature in model bronze compounds to provide more detailed checks of the method.

Phenomenological analysis of tetragonal tungsten bronze ferroelectrics

Note: Penn State Univ,Mat Res Lab,University Pk,Pa 16802E6501Times Cited:0Cited References Count:0 Reference LC-ARTICLE-1986-001 Record created on 2006-08-21, modified on 2017-05-10

Electromechanical Anisotropy in Modified Lead Titanate Ceramics

The electrostrictive property and dielectric behavior of the stretched poly (vinylidene fluoride–trifluoroethylene) copolymers, denoted as P(VDF–TrFE), have been studied at cryogenic temperatures. Polarization hysteresis loops and high electrostrictive performance (strain ∼1%) are obtained at room temperature. An obvious dielectric relaxation process is observed and the relaxation rate follows the Vogel–Fulcher relation with TVF=160 K, U=0.18 eV, ν0=5.7×1012 Hz. The dielectric relaxation behavior and high strain level decrease quickly with decreasing temperature, and below 160 K only a small strain level (∼0.1%) and an “intrinsic” dielectric constant sustains. The correlation of dielectric behavior and strain activity and the physical mechanism of the quick suppression of the strain with decreasing temperature are briefly discussed. The results indicate that a high dielectric constant is critical for obtaining high electrostrictive strains in the polymers at a reasonable electric field.

L. E. Cross

Papers

Quasi lumped parameter method for microwave measurements of dielectric dispersion in ferroelectric ceramics

Dielectric and mechanical loss in the system Pb3MgNb2O9 - Pb3NiNb2O9

Phenomenological analysis of tetragonal tungsten bronze ferroelectrics

Electromechanical Anisotropy in Modified Lead Titanate Ceramics

Electrostrictive and dielectric properties of stretched poly(vinylidene fluoride–trifluoroethylene) copolymers at cryogenic temperatures