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 temperature dependence of the pyroelectric coefficients of lead barium niobate Pb1−xBaxNb2O6 (PBN) single crystals were investigated using the Byer-Roundy technique. Pyroelectric coefficients were found to be enhanced in single crystals of the near-morphotropic phase boundary (MPB) compositions. High pyroelectric coefficients (336 μC/m2−K, 1 − × = 0.684) and switchable polarization vectors between the two perpendicular crystallographic directions ([001] and [110]) in crystal of near-morphotropic phase boundary composition (1 − x = 0.615) were found to be of interest for pyroelectric device applications.

Pyroelectric properties of lead barium niobate single crystals

Composites containing micron-sized lead zirconate titanate and nanometer-sized lead sulfide in a glass matrix were prepared using a sol-gel technique. The PbS size ranged from 3.5 to 15.3 nm. It was possible to pole some of these nanocomposites. The measured values of saturation polarization, remanent polarization, and coercive field showed an increase as the PbS particle size was enhanced. The composites exhibited improvement in the piezoelectric and pyroelectric behavior as compared to that of the base glass-ceramic.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400049463

Piezoelectric and pyroelectric behavior of lead zirconate titanate–lead sulfide–glass nanocomposites

A new type of barrier layer capacitor is described utilizing thin glass layers on highly conducting ceramics of barium plumbate and barium bismuth plumbate. The frequency dispersion of the apparent dielectric constant has been explained using a modified version of the Maxwell–Wagner model. These capacitors have a low temperature coefficient of capacitance and a high dispersion frequency in the megahertz range. Simple processing conditions together with low firing temperature make it possible to produce the barrier layer capacitors inexpensively.

Barrier Layer Capacitor Using Barium Bismuth Plumbate and Barium Plumbate.

An improved quantitative pyroelectric characterization system has been developed in an effort to more thoroughly investigate the dynamic pyroelectric response of selected materials at particular temperatures of interest, such as at transition, or to study electrical aging effects. Specifically, the system is capable of measuring pyroelectric current responsivity of a wide variety of materials ranging from ferroelectric ceramics to polymers at constant temperature utilizing a modulated heating source technique.' Effectiveness of this system was determined through measurements of lithium tantalate and lithium niobate samples of various geometries. The result of the study was a comprehensive evaluation of dynamic pyroelectric current responsivity of these samples with a particularly good signal-to-noise enhancement for evaluating high noise materials such as polymer gels which are extremely difficult to evaluate using other techniques. These measurements revealed a significant dependence of the pyro...

An improved quantitative method for determining dynamic current response of pyroelectric materials

Collocated smart structures provide the most effective technique of influencing the stimuli. Such systems can also be used for absorbing underwater acoustic waves. A collocated system is designed by mounting piezoelectric sensor on a multilayer PZT discs stack. Using PZT sensor for narrowband response and modified lead titanate sensor for broadband response, the effectiveness of these smart structures in absorbing acoustic waves is experimentally determined by using an acoustic tube.

L. E. Cross

Papers

Pyroelectric properties of lead barium niobate single crystals

Piezoelectric and pyroelectric behavior of lead zirconate titanate–lead sulfide–glass nanocomposites

Barrier Layer Capacitor Using Barium Bismuth Plumbate and Barium Plumbate.

An improved quantitative method for determining dynamic current response of pyroelectric materials

Underwater acoustic absorption by collocated smart materials