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 piezoelectric strain coefficients have been measured as a function of composition for films in the PbZrO3–PbTiO3 (PZT) solid solution system, using a double‐beam laser interferometry technique. This compositional dependence of piezoelectric, and the associated dielectric and ferroelectric properties for films 1 μm in thickness with varying Zr/Ti ratio, deposited on platinized silicon substrates using a modified sol‐gel route, corresponds to data reported for undoped PZT ceramics with respect to the effective morphotropic phase boundary composition. Films with composition near the morphotropic phase boundary, Pb(Zr0.52Ti0.48)O3, show enhanced values of the longitudinal piezoelectric coefficient, 194 pC/N; dielectric permittivity, 1310; and remanent polarization, 36 μC/cm2.

Electrical properties’ maxima in thin films of the lead zirconate–lead titanate solid solution system

A magnetoelectric laminate composite consisting of magnetostrictive Terfenol-D (Tb1–xDyxFe2–y) and piezoelectric Pb(Zr,Ti)O3 layers has an extremely high voltage gain effect of ≈300 at its resonant state, offering potential for high-voltage miniature transformer applications.

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A strong magnetoelectric voltage gain effect in magnetostrictive-piezoelectric composite

A modified Michelson interferometer is used to study the strain properties of piezoelectric and electrostrictive materials. For small displacement, a feedback loop is introduced to stabilize the system against the low‐frequency optical path‐length drifting and the system is capable of resolving displacements of the order of 10−3 A. For the strain induced by domain switching, a dual‐channel signal detection scheme is used which automatically reads out the displacement of the sample. The effect on the measurement of the sample bonding to a substrate and other related problems are discussed.

Laser interferometer for the study of piezoelectric and electrostrictive strains

The electrostrictive strains induced by high electric fields have been measured for compositions in the solid solution system Pb(Mg1/3Nb2/3)O3: PbTiO3 (PMN:PT) containing up to 20 mole% PT Large anhysteretic quadratic strains (up to 1,000 × 10−6) can be induced at room temperature with no evidence of remanence In pure Pb (Mg1/3Nb2/3) O3 the electrostriction constants Q 11 and Q 12 are significantly smaller than in normal perovskites and the large induced strains may be traced to the high polarizability of the relaxor With the addition of PT, the Q constants increase and the relaxation range shortens, with compositions close to 10 mole% PT providing maximum quadratic sensitivity Over the relaxation range of temperatures, the thermal expansion is extremely small and comparable to the best thermally stable glasses and ceramics

Electrostrictive behavior of lead magnesium niobate based ceramic dielectrics

In this study, a piezoelectric composite is fabricated from a Ba0.67Sr0.33TiO3 (BST) composition and air, and measured at room temperature (24°C) where the BST is in its paraelectric centric phase. The charge separation mechanism is flexoelectricity, driven by the strain gradient introduced by the pyramidal boundary shapes of each BST building unit of the composite. In spite of the macro (millimeter) dimensions of the building units that inhibit a steep strain gradient, the composite is sufficiently active to be measured by using a conventional Berlincourt d33 meter. The measured values are in good agreement with expectation from the earlier measured flexoelectric coefficient μ11 of the BST and the finite element simulation results of the strain gradient in the pyramid BST units.

L. E. Cross

Papers

Electrical properties’ maxima in thin films of the lead zirconate–lead titanate solid solution system

A strong magnetoelectric voltage gain effect in magnetostrictive-piezoelectric composite

Laser interferometer for the study of piezoelectric and electrostrictive strains

Electrostrictive behavior of lead magnesium niobate based ceramic dielectrics

Piezoelectric composite based on the enhanced flexoelectric effects