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

This letter reports the dielectric properties of the cubic pyrochlore (Bi3/4Zn1/4)2(Zn1/4Ta3/4)2O7, monoclinic zirconolite Bi2(Zn1/3Ta2/3)2O7, and their composites in the temperature range of 10–400 K from 102 to ∼1010 Hz. Composites with a near zero temperature coefficient of capacitance (TCC) consisting of the cubic pyrochlore and monoclinic zirconolite phases have been obtained. The observed microwave dielectric properties (e=∼60, Q=∼325, and TCC=∼30 ppm/K at ∼5.1 GHz) as well as their low sintering temperatures (950–1100 °C) show that the compounds are promising materials for recently developed microwave devices, which demand that dielectric ceramics can be cofired with silver electrodes.

Dielectric properties of Bi2O3–ZnO–Ta2O5 pyrochlore and zirconolite structure ceramics

Domains in 0.9Pb(Zn 1/3 Nb2/3)O3 − 0.1 PbTiO3 or “PZN-PT single crystals under the application of electric field were investigated with an optical microscope. The dynamic response of the domains was studied under an electric field on PZNPT crystals is reported. In this study, electric field applied along [001]. The domain formation with the changing temperature showed the successive phase transitions from cubic to tetragonal and then to rhombohedral with decreasing temperature. The room temperature configuration showed the co-existence of both rhombohedral and tetragonal domains. Under the given experimental conditions we found that a single domain state could be induced only in the tetragonal state. Even though the temperature was lowered to −100°C, complete transformation to the rhombohedral state was not observed.

In situ observation of domains in 0.9Pb(Zn1/3 Nb2/3)O3 − 0.1PbTiO3 single crystals

Ordering in Pb(Mg13Nb23)O3Pb(Mg12W12)O3 solid solutions

A phase region of variable order-disorder [VOD] has been identified in this study for compositions in the range [x- 0.0–0.075] (x: mole fraction PbTiO3). Materials from this region retain some degree of long-range ordering of 5-site cations as evidenced by x-ray and electron diffraction which is enhanced with post-sintering annealing. Annealed ceramics exhibit sharp dielectric responses [K(T] and near-normal first-order ferroelectric behavior while as-fired specimens with lesser degrees of long-range ordering show the more diffuse and dispersive characteristics of a relaxor ferroelectric.

Variable structural ordering in lead scandium tantalate-lead titanate materials

The dielectric behavior of 95.5% Pb(Zn1/3Nb2/3)O3–4.5% PbTiO3 single crystals oriented along 〈001〉 direction with and without dc electric field has been studied at cryogenic temperatures. A pronounced low-temperature dielectric relaxation process was observed below 200 K; the relaxation rate follows the Arrhenius law (τ0=∼1.0×10−15 s and U=0.24 eV). An additional dielectric anomaly showed up around 250 K at 10 kHz under a dc electric field. These results indicate rather complicated polarization mechanisms at cryogenic temperatures which clearly need more detailed study. The strain levels at cryogenic temperatures suggest that this material is very promising for space applications, in which the performance at cryogenic temperatures is critical.

L. E. Cross

Papers

Dielectric properties of Bi2O3–ZnO–Ta2O5 pyrochlore and zirconolite structure ceramics

In situ observation of domains in 0.9Pb(Zn1/3 Nb2/3)O3 − 0.1PbTiO3 single crystals

Ordering in Pb(Mg13Nb23)O3Pb(Mg12W12)O3 solid solutions

Variable structural ordering in lead scandium tantalate-lead titanate materials

Dielectric relaxation and strain behavior of 95.5% Pb(Zn1/3Nb2/3)O3–4.5%PbTiO3 single crystals at cryogenic temperatures