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

(Pb(Mg1/3Nb2/3)O3)0.9(PbTiO3)0.1(or 0.9PMN:0.1PT) ceramics with different impurity content were prepared to study the ageing effects in this type of relaxor ferroelectrics. Dielectric, pyroelectric and electrostrictive properties of the different ceramics were measured and compared. The pure, stoichiometrc and lattice sites fully “stuffed” ceramic showed no obvious ageing effect. However, the MnO doped samples and the samples prepared from the oxides with certain impurity content showed ageing effects. It can be concluded that the ageing effects in this type of relaxor ferroelectrics are exclusively resulted from the defects introduced by the impurity content and deliberate doping. The ageing effects may be avoided by preparing pure, stoichiometric and lattice site fully “stuffed” ceramic compositions.

Ageing effects in lead magnesium niobate type of relaxor ferroelectric ceramics

Polarization reversal in PLZT 8/65/35 under hydrostatic pressures

10 μm thick LiNbO3 crystal films have been fabricated from a poled bulk single crystal by the technique of Crystal Ion Slicing (CIS). The CIS technique is a promising approach to fabricate the free standing ferroelectric films for various integration applications and thin film structures. In this paper, the dielectric and pyroelectric properties of CIS LiNbO3 films as well as their temperature dependence were investigated in order to evaluate the ferroelectric properties of the films compared to the parent bulk single crystals. The investigation of the dielectric and pyroelectric responses were carried out in the temperature range of 30-100°C. The results suggest that the film retains the ferroelectric properties of the bulk crystal with magnitudes close to those of single crystal values at room temperature. However, the virgin CIS film showed an additional distinct phase transition in the temperature range around 75°C, as determined from the dielectric and pyroelectric responses. This phase tran...

Dielectric and pyroelectric properties of crystal ion sliced (CIS) LiNbO3 thin film

Interconnect systems which are required in high-speed GaAs digital ICs use stripline techniques for signal traces which must be deposited over very low dielectric constant substrates. Materials with relative dielectric constant k less than 3 and having low-loss tangent up to microwave frequencies are required for ceramic materials. Such values are impossible to achieve in single-phase ceramic monoliths; composite approaches are necessary. A technique for preparing porous silica films 1-10- mu m thick is presented. The dielectric constant of these films is in the range of 2.4 to 2.8 with a loss tangent less than 0.005 at 1 kHz. >

Ultra-low dielectric constant porous silica thick films for high-speed IC packaging

In many applications such as hydrophone and ultrasonic transducers, materials with large piezoelectric anisotropy are preferred in order to suppress the interfering signals from lateral modes. It has been shown that piezoelectric anisotropy can be significantly improved by structure design. For instance, for a radially poled cylindrical tube, the effective transverse piezoelectric response can be tuned to zero. In this work, the effective piezoelectric responses of lead zirconate titanate (PZT) and lead magnesium niobate-lead titanate (PMN-PT) ceramic cylindrical tubes were studied. Large piezoelectric anisotropy with a high effective uniaxiai coefficient has been obtained for both materials. It has been shown that near zero effective d 31 can be achieved for a PZT tube with a proper dimension ratio of ro/Ro , where ro , andRo are inner and outer radii of the tube, respectively. While for a PMN-PT tube, the effective piezoelectric responses can be tuned by the ratio of ro/RO as well as the bias f...

L. E. Cross

Papers

Ageing effects in lead magnesium niobate type of relaxor ferroelectric ceramics

Polarization reversal in PLZT 8/65/35 under hydrostatic pressures

Dielectric and pyroelectric properties of crystal ion sliced (CIS) LiNbO3 thin film

Ultra-low dielectric constant porous silica thick films for high-speed IC packaging

Tailoring material properties by structure design–radially poled piezoelectric cylindrical tube