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.

/pdf/multiferroic-and-magnetoelectric-materials-2fpof7uzog.pdf

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.

/pdf/epitaxial-bifeo3-multiferroic-thin-film-heterostructures-2ns67gte2l.pdf

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

Reversible pyroelectric effect in Pt(Sc1/2Ta1/2)O3 ceramics under dc bias

A significant temperature dependence of the electrostrictive coefficients of K(Ta 0.55 Nb 0.45 )O 3 has been observed in a narrow temperature range above the ferroelectric Cuire point ( T - T C <30°C). The reciprocal permittivity versus temperature relation deviates from the Curie-Weiss law in the same temperature range. A simple model is proposed based on the polarization critical-fluctuation.

Anomalous Temperature Dependence of Electrostrictive Coefficients in K(Ta0.55Nb0.45)O3

Piezoelectric actuators and sensors made of tubular structure can provide a great agility of the effective response in the radial direction. For a radially poled piezoelectric tube, the effective piezoelectric constant in that direction can be tuned to be positive, zero, and negative by varying the ratio of the outer radius (R0) to the inner radius (r0) of the tube. For a suitable ratio of R0/r0, this effective constant can also be changed in sign or set to zero by adjusting the dc bias field level for tubes made of electrostrictive materials. Therefore, one can make a piezoelectric transducer with all the effective piezoelectric tensile constants having the same sign. The end capped thin wall tubes also exhibit exceptionally high hydrostatic response and the small size of the tubular structure makes it very suitable for integration into 1 - 3 composite which possesses low acoustic impedance and high hydrostatic response.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Piezoelectric tubes and 1-3-type tubular composites as tunable actuators and sensors

The electric field induced antiferroelectric-to-ferroelectric phase transition of lead zirconate titanate stannate ceramics was investigated by means of dielectric, polarization, and strain hysteresis measurements. Compositions of varying titanium and tin within the general formula (Pb0.98La0.02) (Zr0.66Ti0.11-xSn0.23+x)O3, located in the tetragonal antiferroelectric phase field and near the ferroelectric rhombohedral boundary were prepared. As the applied electric field increased, a sudden increase in both longitudinal and transverse strain was observed with a corresponding change in dielectric constant, loss, and polarization, indicating the transition from antiferroelectric to ferroelectric phase. The longitudinal strain increased continuously into the ferroelectric phase, whereas the transverse strain became negative after the phase change. By defining the phase change field from polarization and high field dielectric constant and loss measurements, the longitudinal strains associated with the phase change for all of the compositions were less than 0.2%. For some compositions, however, the longitudinal strain increased to levels greater than 0.5% with increasing applied field. Owing to the relatively small decrease in transverse strain in the ferroelectric region, the volume strain continued to increase even after antiferroelectric to ferroelectric phase change.

Field-induced lead zirconate titanate stannate antiferroelectric-to-ferroelectric phase-switching ceramics

Lead magnesium niobate (PMN) thin films were prepared by a solution sol-gel (SSG) technique. X-ray diffraction analysis indicated that the perovskite phase of PMN formed at a temperature as low as 775°C. A highly oriented film along (111) direction was obtained on silicon substrate. The orientation and crystallization rate were strongly influenced by the type of substrate and heating conditions.

L. E. Cross

Papers

Reversible pyroelectric effect in Pt(Sc1/2Ta1/2)O3 ceramics under dc bias

Anomalous Temperature Dependence of Electrostrictive Coefficients in K(Ta0.55Nb0.45)O3

Piezoelectric tubes and 1-3-type tubular composites as tunable actuators and sensors

Field-induced lead zirconate titanate stannate antiferroelectric-to-ferroelectric phase-switching ceramics

Solution-sol-gel processing of lead magnesium niobate thin films