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

In this paper we report on synthesis of SBT thin-films by sol-gel processing. Sr metal, Bi, 2-ethyl hexanoate and Ta-ethoxide were used as precursors. Thin-films with nominal composition SrBi/sub 2/Ta/sub 2/O/sub 9/ and SBT +10% excess Bi content were made. Films were annealed at various temperatures to study the microstructure, crystallization temperature and the polarization values. Good crystallization of SBT was obtained by annealing at 700/spl deg/C-2 hrs, independent of the Bi content in the films. Films annealed in oxygen atmosphere at 800/spl deg/C-2 hrs did not show any significant change in the polarization value. Crack free films were made with film thicknesses of 0.4 /spl mu/m. Films annealed at 800/spl deg/C-2 hrs showed a grain size of /spl sim/0.2 /spl mu/m, and reasonably good polarization values of 5 /spl mu/C/cm/sup 2/. In contrast, films prepared with 10% excess Bi showed a very fine grain size <0.1 /spl mu/m with a lower polarization values of 1.5 /spl mu/C/cm/sup 2/.

The effect of annealing temperature on the formation of SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) thin films

The piezoelectric performance of 1-3-type composite depends critically on the stress transfer between the two constituents phases. The results of an investigation of the elastic and piezoelectric behavior of composites with 2-2 and 1-3 connectivities are presented. By taking into account the nonuniform strain profiles in the constituent phases, the theoretical model presented can quantitatively predict the performance of these composites. Theoretical predictions agree quantitatively with the experimental results. >

Strain profile and piezoelectric performance of piezocomposites with 2-2 and 1-3 connectivities

The aim of the present paper is to give a contribution to the characterization of piezoelectric composite materials, which are to be operated at high mechanical or electric power level by means of the determination of higher order elastic and dielectric coefficients.

Elastic and dielectric nonlinearities of piezoelectric composite materials

Experimental evidence show that the piezoelectric effect in ferroelectric ceramics cannot be accounted for by the average of the intrinsic piezoelectric effect of the oriented ferroelectric domains. The measured values of piezoelectric moduli dij are usually 60-70% larger than the calculated average values at room temperature.1,2,3 The difference as well as the absolute values of dij become smaller when the system is cooled,4 which suggests that reversible ferroelectric/ferroelastic domain wall movement may be the cause for the discrepancy, because the walls are more mobile at higher temperatures.

The twin structures and the piezoelectric effect in ferroelectric ceramics

Ferroelectric relaxor thin films Pb[(Mg 1/3 Nb 2/3 ) 0.9 Ti 0.1 ]O 3 (90PMNIOPT) have been fabricated by the sol-gel technique. The dielectric, piezoelectric and electrostrictive characteristics have been investigated. Experimental results show that the dielectric permittivity and effective piezoelectric coefficients of the films can be tuned by varying dc bias fields which offers the useful features in designing smart micro transducer and actuator systems. Dielectric constants of the films are in the range of 5000 to 6000. The electric field induced strain is on the order of 10 −3 , and the maximum effective d 33 coefficient of 90PMINI0PT films is as large as 265 pC/N at 31 kV/cm. The electrostrictive coefficient Q 11 is on the order of 1.2 c×10 −2 m 4 /C 2 and M 11 is 3.5 c×10 −16 m 2 /V 2 .

L. E. Cross

Papers

The effect of annealing temperature on the formation of SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) thin films

Strain profile and piezoelectric performance of piezocomposites with 2-2 and 1-3 connectivities

Elastic and dielectric nonlinearities of piezoelectric composite materials

The twin structures and the piezoelectric effect in ferroelectric ceramics

Tunable Dielectric and Piezoelectric Characteristics of Lead Magnesium Niobate Titanate Relaxor Thin Films