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

Monolithic Multilayer Electromechanical Transducers for Optical Applications

A nonspontaneous, high permittivity, “dielectrically soft” symmetry axis possessing large piezoelectric activity was found to exist for predominantly ferroelectric tetragonal FT compositions in the relaxor lead magnesium niobate–ferroelectric lead titanate, (1−x)Pb(Mg1∕3Nb2∕3)O3–(x)PbTiO3, single crystal system. The symmetry axis is not unique. It is a function of the lead titanate concentration x. It becomes the [011] for tetragonal compositions with x>0.35 and the [001] for rhombohedral compositions with 0.30⩽x⩽0.34. The two axial systems are degenerate for x∼0.35 and likely to converge at x∼0.29. Domain engineering for maximum piezoelectric response is easily accomplished for electric fields applied along this axis. The observations were confirmed by (1) isothermal resonance-antiresonance experiments on length extensional bars fabricated from multidomain (1−x)Pb(Mg1∕3Nb2∕3)O3–(x)PTiO3 single crystals and (2) numerical computation using Devonshire’s theory. The results are discussed in terms of recent f...

Evolution of a nonspontaneous, high piezoelectric coupling symmetry axis in relaxor-ferroelectric single crystals

contribute to aging in normal macro-domain ferroelectrics No obvious dielectric aging effects have been observed in pure .9Pb(Mgl13Nb2,3)03:. 1PbTi03 relaxor ferroelectric ceramics in this study. The addition of small amounts of MnO however, induces aging effects. Aging effects preferentially reduce the dispersive component of the weak field permittivities above the aging temperature. Isothermal aging rates are observed to increase with increasing doping level. Dielectric constants vs the logarithm of time at constant temperature level out with time. The origin of aging for this type of relaxor material can be explained in terms of the volume stabilization of the polar vectors in polar micro-regions by the defect dipoles introduced by the doping.

Dielectric Aging Effects in Doped Lead Magnesium Niobate: Lead Titanate Relaxor Ferroelectric Ceramics

An ion polarizability additivity rule (R. D. Shannon)1 was applied to calculate the dielectric constants of a large family of substrate materials (for HTSC films) and to compare with the measured dielectric constant values. The applicability and the limitations of the model were examined in light of experimental results of new substrate materials. The suitability and the limitations of the ion polarizability additivity rule in predicting dielectric constants of ionic materials were discussed. Possible causes of discrepancies found between the calculated and experimental values of the dielectric polarizabilities and dielectric constants were examined and modifications on the current additivity approach were proposed.

L. E. Cross

Papers

Monolithic Multilayer Electromechanical Transducers for Optical Applications

Evolution of a nonspontaneous, high piezoelectric coupling symmetry axis in relaxor-ferroelectric single crystals

Dielectric Aging Effects in Doped Lead Magnesium Niobate: Lead Titanate Relaxor Ferroelectric Ceramics

Ion polarizability additivity rule and its application on HTSC substrate materials

Electrostriction tensor components of Alkaline-earth fluoride single crystals