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.

/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

The abnormal aging behavior, i.e., severe aging in the electric field induced piezoelectric coefficient while very weak dielectric aging, observed in the relaxor ferroelectric lead magnesium niobate-lead titanate (PMN-PT) ceramics under a dc electric bias field, can be significantly reduced by hot isostatic pressing (HIP) treatment on presintered samples. The aging can also be reduced by doping suitable amounts of either La (donor) or Mn (acceptor). We suggest that the reduction in the aging is due to the introduction of additional random fields into the material, which reduces the probability of the growth of micropolar regions into metastable and/or stable macropolar domains. The abnormal aging behavior and the effectiveness of HIP in reducing it indicate the importance of the elastic energy in controlling the aging and relaxor behavior in PMN-PT relaxor ferroelectrics.

The effect of ferroelastic coupling in controlling the abnormal aging behavior in lead magnesium niobate-lead titanate relaxor ferroelectrics

The dielectric properties of the strontium titanate aluminosilicate glass-ceramics described in the previous paper have been investigated over the frequency range of 10 to 1000 kHz and temperature range of −170 to 200° C. The dielectric properties were strongly dependent on the crystallization conditions, which determined the amounts of SrTiO3 and secondary crystalline phases, and the microstructure of the glass-ceramics. Room temperature values of the dielectric constant and temperature coefficient varied from 13.5 and +125 p.p.m. ° C−1 in uncrystallized glass to 47 and −600 p.p.m. ° C−1, respectively, in glass-ceramics crystallized for 16 h at 1100° C.

SrTiO3 glass ceramics

The transmission electron microscope (TEM) was used to directly observe ordered microdomains in Pb(Sc0.5Ta0.5)O3 (PST). The microstructure of both single crystal and polycrystalline PST was studied as a function of S, the degree of order (which was controlled by thermal treatment). The observed domain sizes were in good agreement with the results of prior X-ray studies. It was found that the domain coarsening kinetics are much slower for the single crystal than the polycrystalline material. Also evidence was obtained for the presence of short range order within PST.

http://iopscience.iop.org/1347-4065/24/S2/550/pdf/1347-4065_24_S2_550.pdf

TEM of the Relaxor Material Pb(Sc0.5Ta0.5)O3

In this paper, we present a new type of 2-2 piezoelectric ceramic-polymer composite operated at the transverse piezoelectric mode (TP). The new TP mode piezocomposite has exceptionally high hydrostatic piezoelectric response, high reliability, and can be made at low cost. Based on the isostrain approximation, an analytical model is developed to analyze and optimize the effective piezoelectric and the effective elastic properties of the composite. Several composites with the new structure were fabricated and tested. The calculated and experimental results show good agreement. It is shown that a TP 2-2 composite made of PZT plates in a soft polymer matrix with elastically unidirectional face plates yields an effective hydrostatic piezoelectric coefficient d/sub h/ of 6000 (pC/N) and a hydrostatic figure of merit d/sub h/g/sub h/ higher than 30000 (10/sup -15/ m/sup 2//N). >

A new transverse piezoelectric mode 2-2 piezocomposite for underwater transducer applications

The elastic response of a [001]-oriented and -poled ferroelectric rhombohedral lead zinc niobate-lead titanate single crystal close to the morphotropic phase region is examined under thermal, electrical, and mechanical boundaries similar to those used in sound projectors. Resonance measurements yielded a monotonically decreasing Young’s modulus as a function of temperature in the ferroelectric rhombohedral state with a sudden stiffening near the ferroelectric rhombohedral (FR)-ferroelectric tetragonal (FT) transition. The quasistatic, zero-field stress-strain response revealed a FR instability under uniaxial compression. A dc bias field stabilized the FR state under compressive stress. Young’s modulus derived from the linear elastic response below instability agrees well with resonance data for FR. A larger modulus than expected for FT was observed above instabilities. The elastic response is analyzed in terms of ferroelectric-ferroelectric transitions as predicted by a high-order Devonshire theory. Impli...

L. E. Cross

Papers

The effect of ferroelastic coupling in controlling the abnormal aging behavior in lead magnesium niobate-lead titanate relaxor ferroelectrics

SrTiO3 glass ceramics

TEM of the Relaxor Material Pb(Sc0.5Ta0.5)O3

A new transverse piezoelectric mode 2-2 piezocomposite for underwater transducer applications

Elasticity of high coupling relaxor-ferroelectric lead zinc niobate-lead titanate crystals