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 ferroelectric tungsten bronze single crystal of Pb0.33Ba0.70Nb2O6 was grown from a melt using the Czochralski technique. The crystal belongs to the tetragonal point group 4mm with the spontaneous polarization parallel to the ‘c’ axis. The room temperature lattice parameters were a = 12.50 A and c = 3.995 A. The spontaneous polarization was found to be 0.40 C/m2°C. The Curie transition was 350°C as determined from the temperature dependence of the dielectric constants. The pyroelectric properties were found to be typical of other tetragonal ferroelectric bronzes. Dielectric constant ϵ11 and piezoelectric constant d15 show a strong enhancement from the approach of the tetragonal:orthorhombic morphotropic phase boundary and it is clear that crystals with composition in the tetragonal phase field closer to this boundary will be of major interest for piezoelectric and electro-optic applications.

Ferroelectric properties of tungsten bronze lead barium niobate (PBN) single crystals

A review and some case studies on the interatomic distances of candidate substrates in comparison to the high Tc superconductor (HTSC) phase YBa2Cu3O7−δ (YBCO) is presented, in an attempt to enhance the basis for substrate selection for YBCO film epitaxy. This preliminary study was carried out by examining a variety of interatomic distances in the structure rather than merely matching the lattice parameters. Interatomic structure matching planes of selected YBCO orientations in contact with substrates were identified. The surface termination of the substrate was found to be a crucial parameter in determining the oriented or epitaxial growth. Possible composition dependence of the orientation of the films at the nucleation stage was also anticipated depending on the comparison. Several currently most commonly used substrates are discussed in some detail.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400074409

Surface crystallographic structure compatibility between substrates and high Tc (YBCO) thin films

The shape-memory effect in PLZT (Lead Lanthanum Zirconate Titanate) ceramics with composition x/65/35 (4.0  x  8.0) has been investigated using bending experiments and temperature cycling. Relationships between load and the degree of bending, together with their temperature dependence, have been determined, and characteristic temperatures associated with the onset and disappearance of the pseudo-plastic shape change are compared with the observed dielectric anomalies. Effects of mechanical stress and electric field on the shape-memory temperatures and the magnitude of strain have also been investigated. It is concluded that domain alignment and the temperature-dependence of spontaneous strain are important factors governing the shape-memory effect in ferroelectrics. Preferred-orientation effects have been confirmed by X-ray diffraction analysis.

Shape-memory effect in PLZT ferroelectric ceramics

Glass ceramics in the system Li2O‐SiO2‐B2O3 with oriented crystallites were prepared by crystallizing the glasses in a temperature gradient. Piezoelectric and electromechanical properties of the glass ceramics are reported. Some of the compositions exhibit low temperature coefficient of resonance, comparable to LiTaO3 single crystals. Advantages of fabricating glass‐ceramic piezoelectric resonators are described and the possibility of using them in surface acoustic wave (SAW) devices is suggested.

Piezoelectric properties of lithium borosilicate glass ceramics

The c-axis oriented thin films of SbSI are prepared by the recrystallization of the amorphous SbSI films. The recrystallized films show the pyroelectric properties and have a dielectric capacitance between 0.2-0.5 μF/cm2 and the tan δ <1%.

L. E. Cross

Papers

Ferroelectric properties of tungsten bronze lead barium niobate (PBN) single crystals

Surface crystallographic structure compatibility between substrates and high Tc (YBCO) thin films

Shape-memory effect in PLZT ferroelectric ceramics

Piezoelectric properties of lithium borosilicate glass ceramics

Preparation and electrical properties of thin films of antimony sulphur iodide (SbSI)