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

Earlier studies1-3 have shown that in Pb(Sc1/2Ta1/2)O3 PST and Pb(Sc1/2Nb1/2)O3 which are simple cubic perovskites, the degree of ordering of the different B site cations in the ABO3 structure can be controlled thermally. Quenched disordered crystals and ceramics show diffuse dispersive dielectric properties, while well annealed ordered materials exhibit “normal” sharp first order transitions. In this work the solid solutions between PST and PSN are shown to permit control of both the centroid and the span of the diffuse Curie range. Since the depolarization curve can be modified by both composition and cation ordering, interesting pyroelectric effects are possible.

Diffuse Ferroelectric Phase-Transition and Cation Order in the Solid-Solution System Pb(Sc1/2nb1/2)O3-Pb(Sc1/2ta1/2)O3

Thin films of lead zirconate titanate have been fabricated for application to a new family of flexure-wave piezoelectric micromotors that are characterized by low speed and high torque. The high relative dielectric constant and breakdown strength of the films lead to high stored energy densities. Evaluation of the film as a bimorph yielded a value of -88 pC/N for the transverse piezoelectric strain coefficient, d31; the relevant electromechanical coupling factor, k31, calculated thereupon was 0.22. The development of the piezoelectric ultrasonic micromotors from the PZT thin films, and the architecture of the stator structure are described. Nonoptimized prototype micromotors show rotational velocities of 100–300 rpm at drives of 3–5 V.

Piezoelectric Thin Film Ultrasonic Micromotors

Recently-developed electrostrictive materials have been classified on the basis of the origin of their diffuse phase transitions for the purpose of seeking how to improve the temperature characteristics of transducers. A simple interrelation between the temperature coefficient of electrostriction and the phase transition diffuseness is demonstrated in the solid solution systems Pb(Mg1/3Nb2/3)O3–Pb(Mg1/2W1/2)O3 and Pb(Mg1/3Nb2/3)O3–PbTiO3, in conjunction with the short-range order in B-site cations. A cluster model, extended from the random compositional fluctuation model by Smolensky and Rolov, is proposed for a diffuse phase transition in complex perovskites with short-range ordering of the cation. Based on this model, the improvement of the thermostability has been made in the Pb(Mg1/3Nb2/3)O3–PbTiO3 ceramics by adding Pb(Mg1/2W1/2)O3 or Ba(Zn1/3Nb2/3)O3, resulting in a temperature coefficient less than 1% deg-1.

Interrelation of Electrostriction with Phase Transition Diffuseness –Improvement of the Temperature Characteristics of Electrostriction–

Electric-field-induced strains of polyurethane elastomers have been measured. The mechanical constrains and flexure motion of the samples which can cause large experimental error were excluded from the measured strain signals. The longitudinal and transverse strain coefficients are evaluated as functions of frequency. The dielectric and elastic properties and their temperature and frequency dependencies have also been studied. The experimental results show that strain larger than 2% can be obtained and that the sample processing conditions can affect the strain level. The contributions from Maxwell stress effect and electrostriction to the strain responses are examined. The results suggest that Maxwell stress effect plays an important role in electromechanical response of the materials.

The origins of electromechanical response in polyurethane elastomers

Based on the direct piezoelectric effect and signal comparison method, a quasi-static, high sensitivity, phase sensitive d33 meter has been developed. The system is capable of measuring the complex piezoelectric d33 constant down to about 1 pC/N and has a phase resolution of ±0.05°. The advantage of this apparatus is that its operation is almost as easy as that of the commercial Berlincourt d33 meter, hence, it is suitable for everyday routine usage.

L. E. Cross

Papers

Diffuse Ferroelectric Phase-Transition and Cation Order in the Solid-Solution System Pb(Sc1/2nb1/2)O3-Pb(Sc1/2ta1/2)O3

Piezoelectric Thin Film Ultrasonic Micromotors

Interrelation of Electrostriction with Phase Transition Diffuseness –Improvement of the Temperature Characteristics of Electrostriction–

The origins of electromechanical response in polyurethane elastomers

A High Sensitivity, Phase Sensitive d33 Meter for Complex Piezoelectric Constant Measurement