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

Values of coefficients related to the tilting of the oxygen octahedra in the low-temperature rhombohedral phase of the lead zirconate-titanate (PZT) solid solution system are needed in the development of a thermodynamic theory of the PZT system. In this paper these coefficients will be determined from experimental spontaneous strain and oxygen octahedral tilt angle data. Values of the tilt angle related coefficients β1,β11, and φ are assumed to be independent of temperature, and are then related to P L, θL, and T R, where P L and θL are the spontaneous polarization and tilt angle of the low-temperature rhombohedral phase at the transition temperature T R between tilted and untilted structures. P L and θL are derived from experimental data for the PZT 90/10 composition, and used together with spontaneous strain values at this composition to determine electrostrictive (Q 44) and rotostrictive (R 44) constants. These constants are then in turn used to derive P L and θL values for other PZT compositi...

Thermodynamic theory of the lead zirconate-titanate solid solution system, part IV: Tilting of the oxygen octahedra

Grain-oriented glass-ceramics of Li2Si2o5, fresnoite (Ba2TiSi2O8), and its isomorphs Sr2TiSi2O8, and Ba2TiGe2O8, were prepared by recrystallizing glasses in a temperature gradient. Electromechanical and hydrostatic piezoelectric properties of these glass-ceramics were measured. Piezoelectric voltage coefficients g33 and hydrostatic voltage coefficient gh of these glass-ceramics are comparable to those of polyvinylidene fluoride and an order of magnitude higher than the corresponding values of lead zirconate-titanate. These glass-ceramics seem to be attractive candidate materials for hydrophones and several piezoelectric devices. Hydrostatic piezoelectric properties of Ba2TiSi2O8 and Ba2TiGe2O8 single crystals were also measured. The unusually high values of gh in fresnoite single crystals and glass-ceramics are supposed to be due to positive d31 in these materials. A composite model is proposed to explain the positive sign of d31 in fresnoite based on its crystal structure and internal Poisson's ratio stress.

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

The decrease in remnant polarization, P r , and increase in coercive field, E c , of ferroelectric ceramics subjected to a large a.c. electric field is termed electric fatigue. In transparent hot-pressed lanthanum doped lead zirconate titanate (PLZT) ceramics, electric fatigue was not detected after 10 9 cycles when grain size is small (∼ 5 μ m) but became increasingly severe as the grain size increases to 10 μm and beyond. The electric field causes large strains (⩾ 10 −3 ) due to the anisotropic piezoelectric and electro-strictive coefficients of the material, which results in intergranular microcracking due to incompatible deformations. The effective field on a grain in a cracked sample is less than the applied field, limiting domain switching and polarization. The microcracking occurs only in the electroded, fatigued region of the sample and is initiated on the high voltage terminal side. The microscopic and acoustic emission data support the proposed mechanism of electric fatigue. An estimate of the critical grain size for microcracking due to electric stress, based on energy criterion, is in good agreement with experiment. The process of crack growth during electric fatigue is also elucidated.

Grain size dependence of electric fatigue behavior of hot pressed PLZT ferroelectric ceramics

The preparation of Ba(Mg1/3Ta2/3)O3 (BMT) ceramics and the first successful growth of BMT single crystals by the laser heated pedestal growth technique (LHPG) is reported. The single crystal has a simple cubic perovskite structure in comparison to the ordered hexagonal structure normally found in ceramics of the same phase. The dielectric properties of single crystals are examined, the dielectric Q value increases with the increase in B‐site ordering, while the dielectric constant κ is relatively independent of ordering. Our results also show that BMT is a candidate substrate for high Tc superconductor thin films as it has one of the highest Q values known for the oxide perovskite family, along with its matching thermal expansion coefficient α=9.0×10−6/ °C and twin‐free cubic perovskite structure with a=4.0877 A. BMT single crystals grown by the LHPG technique are probably the highest melting oxide compounds grown to date.

Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique

The dc electric-field dependence of dielectric constant in the paraelectrics, ${\mathrm{KTaO}}_{3}$ single crystals, ${\mathrm{CaTiO}}_{3}$ and ${(Ln}_{1/2}{\mathrm{Na}}_{1/2}){\mathrm{TiO}}_{3}$ $(Ln=\mathrm{La}$ and Nd) ceramics is studied at cryogenic temperatures in the dc electric-field range 0--60 kV/cm. The results indicate that ${\mathrm{KTaO}}_{3}$ possesses a high tunability $K{=[\ensuremath{\epsilon}{(E}_{0})\ensuremath{-}\ensuremath{\epsilon}(E)]/\ensuremath{\epsilon}{(E}_{0})=\ensuremath{\sim}50%}$ at 20 kV/cm, and low dielectric loss $\mathrm{tan}\ensuremath{\delta}$ $(\ensuremath{\sim}0.003);$ while ${\mathrm{CaTiO}}_{3}$ and ${(Ln}_{1/2}{\mathrm{Na}}_{1/2}){\mathrm{TiO}}_{3}$ $(Ln=\mathrm{La}$ and Nd) show low tunability of $\ensuremath{\sim}1%$ and $\ensuremath{\sim}0.1%,$ respectively, at 20 kV/cm; however, the dielectric loss $\mathrm{tan}\ensuremath{\delta}$ is very small $(\ensuremath{\sim}0.0004--0.003).$ The possible physical mechanism of the behavior observed is discussed.

L. E. Cross

Papers

Thermodynamic theory of the lead zirconate-titanate solid solution system, part IV: Tilting of the oxygen octahedra

Grain-Oriented Glass-Ceramics for Piezoelectric Devices

Grain size dependence of electric fatigue behavior of hot pressed PLZT ferroelectric ceramics

Ba(Mg1/3Ta2/3)O3 single crystal fiber grown by the laser heated pedestal growth technique

Dielectric behavior of paraelectric KTaO 3 , CaTiO 3 , and ( L n 1 / 2 Na 1 / 2 ) TiO 3 under a dc electric field