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

A laser interferometer was used to study the phase delay of converse piezoelectric response. The real and imaginary parts of the piezoelectric coefficients were calculated from the measured strain amplitude and the phase delay of the strain with respect to the applied electric field for PZT-5 ceramics. The same coefficients were also calculated from measured resonance properties by using Smits's iterative method. The results were in good agreement. The phase delay of the strain with respect to the applied electric field was also studied near resonance frequencies; the result was in excellent agreement with the classic vibration theory.

Laser Interferometer for Studying Phase Delay of Piezoelectric Response

We present a method for calculating the dielectric constant and the dissipation factor for thin-film ferroelectric capacitors from scattering parameter measurements at multi-gigahertz frequencies. Physical measurement is discussed along with description of the model upon which the calculation is based. Experimental results for 0.5 micron films of PZT are reported up to 15 GHz. Direct measurement is compared with indirectly calculated values at the 100 MHz to 1.5 GHz range, showing excellent agreement.

High frequency dielectric properties of thin-film PZT capacitors

Electrostrictive terms have been introduced into the Kittel free energy expression by means of a fourth-order cross term of the two-sublattice polarizations, giving rise to multiple phase transitions. Schematic p-T diagrams of the nonpolar, polar and antipolar phases are classified by the electrostrictive Ω coefficient. Spontaneous strain curves are derived for each of the four types of phase diagrams.

Electrostructive Effects in Paraelectric-Ferroelectric-Antiferroelectric Phase Transitions

The dielectric and piezoelectric properties of lead zirconate titanate (PZT) thin films were investigated to examine its viability for application to a new family of flexure-wave piezoelectric micromotors, characterized by low speed and high torque. Stored energy densities higher than the air-gap electrostatic actuators accrue from high relative dielectric permittivity (1300) and breakdown strength (1 MV/cm) of the films. The piezoelectric longitudinal and transverse strain coefficients were measured by a nonresonance technique through a modified Mach-Zehnder laser interferometer; at a dc bias of 75 k V/cm, d33 was measured to be 220 pC/N; evaluation of the film as a bimorph yielded a value of —88 pC/N for d31 at 200 kV/cm ac field. The electromechanical coupling factors calculated thereupon were k33 = 0.49, k31 = 0.22, and kp = 0.37. The development of the piezoelectric ultrasonic micromotors from the PZT thin films, and the architecture of the stator structures are described. Nonoptimized prototype micr...

Ferroelectric thin films for piezoelectric micromotors

In this study, a simple and direct method has been proposed, which can to be used for quantitatively distinguishing the mechanisms of domain reorientation processes in polycrystalline materials. By using this method, the 90° -domain reorientation in the Pb(ZrxTi1-x)O3 ceramic under the electric field was examined through the X-ray diffraction analysis. It was found that the polarization switching in the PZT ceramic with the composition near the morphotropic phase boundary is predominantly controlled by the two successive 90° domain processes rather than only the 180° domain reversal process. Experimental results also indicate that the coercive field of ferroelectric ceramics is related to the cooperative deformation associated with each grain. This cooperative deformation arises from 90°-domain reversal process.

L. E. Cross

Papers

Laser Interferometer for Studying Phase Delay of Piezoelectric Response

High frequency dielectric properties of thin-film PZT capacitors

Electrostructive Effects in Paraelectric-Ferroelectric-Antiferroelectric Phase Transitions

Ferroelectric thin films for piezoelectric micromotors

90°-Domain reversal in Pb(Zrx, Ti1-x)O3 ceramics