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

The effect of aging on the temperature dependence of permittivity (K) and tan δ as a function of frequency was examined for both ferroelectric and antiferroelectric PLZT compositions. All compositions aged near their transition temperatures exhibited a marked decrease in dispersion between the aging temperature and the temperature of the high frequency (100 kHz) permittivity peak. These results are discussed in terms of the Smolenskii model for dielectric relaxation.

Aging of dielectric dispersion in plzt relaxor ceramics

The nonlinear dielectric response in [001] and [111] (1−x)Pb(Zn1∕3Nb2∕3)O3-xPbTiO3 (x=0045 and 008) single crystals was investigated as a function of frequency between 30 and 250 °C For comparison, a parallel study was carried out on a BaTiO3 single crystal Both second and third harmonic polarization components were detected in Pb(Zn1∕3Nb2∕3)O3-PbTiO3 and BaTiO3 single crystals, and were observed to go through maxima at the phase transition temperatures The third harmonic signal was nearly frequency independent for BaTiO3, but strongly dispersive in Pb(Zn1∕3Nb2∕3)O3-PbTiO3 The strong frequency dependence appears to be characteristic of some relaxor substructure of the domains

Nonlinear dielectric response in (1−x)Pb(Zn1∕3Nb2∕3)O3-xPbTiO3 (x=0.045 and 0.08) single crystals

Dielectric behavior of Cd 2 Nb 2 O 7 has been studied from room temperature down to 10 K with and without dc electric fields. Three dielectric relaxation modes I, II, and III were reexamined. By using the Cole - Cole equation fitting of the frequency dependence of the dielectric constant and loss of Cd 2 Nb 2 O 7 , we have obtained more precise relaxation time data, compared to the data reported in the literature. The results indicate that the relaxation time follows both the Arrhenius law and Vogel - Fulcher relation in the frequency range of 10 2 - 10 5 Hz. With increasing dc bias, the relaxation modes were suppressed and eventually eliminated. The results show a significant suppression of the dielectric constant and loss by the application of dc fields. At above 10 kV/cm, only a dielectric peak remains. The high tunability (37 - 64%) under 15 kV/cm at 5 kHz in the wide temperature range of 50 - 180 K and very low dielectric loss (0.4 - 6 2 10 m 3 )) are observed.

Dielectric Relaxation Behavior and High Tunability in Cd 2 Nb 2 O 7

(Sr1-xPbx)TiO3 (SPT)-MgO composites are promising for room temperature electric field tunable applications. The composites show a significant processing dependence of dielectric properties. This paper reviews dielectric characteristics of SPT-MgO composites, such as permittivity, loss tangents, dielectric maximum temperature, tunability, and hysteresis under bias. In addition, factors which influence tunability and dielectric properties are described.

(Sr1-xPbx)TiO3-MgO composites for field tunable applications

An alternative form of a flexural amplificaiton type sensor can be proposed using and amplified d 15 response. The mechanically amplified flexi-distortional system may improve the d h g h figure of merit by transferring the compressional stress on the device to a lateral shear on the electroceramic element. This type of amplified system is mechanically classified as distortional and, there fore, without volume change. By transforming the hydrostatic stress to trancsverse strain, as in the flexi-extensional devices, transverse forces can be used to develop shear strain and activate the d 15 coefficient. Electrodes are placed along the sides of the sensing element rather than along the top and bottom surfaces. Flexi-distortional devices are modeled in order to check for the limits of sensitivity in their performance.

L. E. Cross

Papers

Aging of dielectric dispersion in plzt relaxor ceramics

Nonlinear dielectric response in (1−x)Pb(Zn1∕3Nb2∕3)O3-xPbTiO3 (x=0.045 and 0.08) single crystals

Dielectric Relaxation Behavior and High Tunability in Cd 2 Nb 2 O 7

(Sr1-xPbx)TiO3-MgO composites for field tunable applications

Flexi-distoritional piezoelectric composites