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

Sol-gel synthesis of Ba(Mg1/3Ta2/3)O3: Phase pure powder and thin films

The pyroelectric response of selected compositions from this solid solution system have been investigated. The nature of the temperature dependence of the pyroelectric coefficient for the compositions examined indicate the presence of a relatively broad, curved morphotropic phase boundary, [x = 0.3–0.45]. The high values of pyroelectric coefficient achievable, in particular for low x compositions [x = 0–0.1], suggest potential application of these materials as highly efficient and versatile field-stabilized devices.

Pyroelectric response and depolarization behavior of (1−x)Pb(Sc1/2Ta1/2)O3-(x)PbTiO3 materials

The dielectric and conduction behavior of SrTiO3 thin films deposited on a SrTiO3 single-crystal substrate is studied. Without dc bias, an obvious dielectric “defect mode” in the dielectric loss is observed in the temperature range of ∼100–200 K; however, no noticeable corresponding dielectric constant peak is observed. By applying a high dc bias (⩾40 kV/cm), a dielectric constant peak with frequency dispersion appears in the same temperature range, the dielectric loss is increased, and simultaneously high dc conduction is observed. The induced dielectric constant peak is related to dc conduction and attributed to the coupling effect of the mobile carriers with the dielectric defect mode.

Dielectric relaxation and conduction in SrTiO3 thin films under dc bias

The hydrostatic-pressure dependence of the refractive index and the low-frequency dielectric constant of a perovskite-type single crystal, KMg${\mathrm{F}}_{3}$, have been determined at room temperature. The refractive index $n$ for $\ensuremath{\lambda}=589.3$ nm increases monotonously in proportion to pressure $p$ with a slope of $\frac{\ensuremath{\partial}n}{\ensuremath{\partial}p}=2.46\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{kbar}}^{\ensuremath{-}1}$. On the other hand, the dielectric constant at 10 kHz decreases with increasing pressure, from which the electric-displacement-related electrostrictive coefficient ${Q}_{h}$ ($={Q}_{11}+{2Q}_{12}$) is calculated as 0.24 ${\mathrm{m}}^{4}$ ${\mathrm{C}}^{\ensuremath{-}2}$. These data are compared with the $\frac{\ensuremath{\partial}n}{\ensuremath{\partial}p}$ values and the ${Q}_{h}$ coefficients of various alkali fluorides and perovskite oxides.

Pressure dependence of the refractive index and dielectric constant in a fluoroperovskite, KMg F 3

The composition-dependent electrical properties in (Pb,La)(Zr,Ti,Sn)O3 antiferroelectric-ferroelectric phase switching thin and thick films have been systematically studied and compared with bulk ceramics. The films were deposited on Pt-buffered silicon substrates by a sol-gel method. The results show that the dependence of low-field dielectric properties on compositions in the films is similar to that in bulk ceramics but the variation of high field properties (polarization or hysteresis loops) is quite different, which may be attributed to the special mechanical boundary condition of the films. While all the films with compositions in the antiferroelectric tetragonal region in the phase diagram demonstrate the existence of remanent polarization in the hysteresis loops, the films with zero remanent polarization can be obtained in the antiferroelectric orthorhombic region. This is because the films are under high tensile stress due to the thermal mismatch between the film and substrate, which tends to stabilize the ferroelectric phase and causes the retention of ferroelectric phase for the films in the antiferroelectric tetragonal region because of their relatively small free energy difference between the antiferroelectric phase and ferroelectric phase.

L. E. Cross

Papers

Sol-gel synthesis of Ba(Mg1/3Ta2/3)O3: Phase pure powder and thin films

Pyroelectric response and depolarization behavior of (1−x)Pb(Sc1/2Ta1/2)O3-(x)PbTiO3 materials

Dielectric relaxation and conduction in SrTiO3 thin films under dc bias

Pressure dependence of the refractive index and dielectric constant in a fluoroperovskite, KMg F 3

Effect of compositional variations on electrical properties in phase switching (Pb, La)(Zr, Ti, Sn)O3 thin and thick films