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

Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties — and in turn promise new device applications — as a result of the coupling between their dual order parameters. We review recent progress in the growth, characterization and understanding of thin-film multiferroics. The availability of high-quality thin-film multiferroics makes it easier to tailor their properties through epitaxial strain, atomic-level engineering of chemistry and interfacial coupling, and is a prerequisite for their incorporation into practical devices. We discuss novel device paradigms based on magnetoelectric coupling, and outline the key scientific challenges in the field.

Multiferroics: progress and prospects in thin films.

A large body of work has been reported in the last 5 years on the development of lead-free piezoceramics in the quest to replace lead–zirconate–titanate (PZT) as the main material for electromechanical devices such as actuators, sensors, and transducers. In specific but narrow application ranges the new materials appear adequate, but are not yet suited to replace PZT on a broader basis. In this paper, general guidelines for the development of lead-free piezoelectric ceramics are presented. Suitable chemical elements are selected first on the basis of cost and toxicity as well as ionic polarizability. Different crystal structures with these elements are then considered based on simple concepts, and a variety of phase diagrams are described with attractive morphotropic phase boundaries, yielding good piezoelectric properties. Finally, lessons from density functional theory are reviewed and used to adjust our understanding based on the simpler concepts. Equipped with these guidelines ranging from atom to phase diagram, the current development stage in lead-free piezoceramics is then critically assessed.

Perspective on the Development of Lead-free Piezoceramics

An applied electric field can reversibly change the temperature of an electrocaloric material under adiabatic conditions, and the effect is strongest near phase transitions. We demonstrate a giant electrocaloric effect (0.48 kelvin per volt) in 350-nanometer PbZr 0.95 Ti 0.05 O 3 films near the ferroelectric Curie temperature of 222°C. A large electrocaloric effect may find application in electrical refrigeration.

http://science.sciencemag.org/content/sci/311/5765/1270.full.pdf

Giant Electrocaloric Effect in Thin-Film PbZr0.95Ti0.05O3

Synthesis methods for nanosized hydroxyapatite with diverse structures.

Despite intensive studies on Pb(Mg(1/3)Nb(2/3))O(3) (PMN) relaxor, understanding the exact nature of its giant dielectric response and of its physical ground state is a fundamental issue that has remained unresolved for decades. Here, we report a comprehensive study of PMN relaxor crystal, and show that (i) its anomalous dielectric behavior in a broad temperature range results from the reorientation of polarization in the crystal, and (ii) the PMN relaxor is essentially a nanosized ferroelectric material with multiscale inhomogeneities of domain structure in addition to the well-known inhomogeneities of chemical composition and local symmetry. Such inhomogeneities are believed to play a crucial role in producing the huge and enigmatic physical effects in relaxor system, and may be used to design other new systems with giant effects such as a relaxor system.

Relaxor Pb(Mg(1/3)Nb(2/3))O3: a ferroelectric with multiple inhomogeneities.

Polarization measurements reveal that AgNbO3 has an extremely large polarization, which can reach a value of 52μC∕cm2 in polycrystals. Experiments also show that the large internal atom distortion in AgNbO3 is also strongly coupled to the electric field, indicating that high piezoelectric performance can be realized in AgNbO3 system. This finding opens the way to designing a new class of lead-free, high-performance piezoelectric materials based on AgNbO3.

AgNbO3: A lead-free material with large polarization and electromechanical response

Crystal structure of ferroelectric silver niobate AgNbO3 was determined (Pmc21) by convergent beam electron, electron, neutron, and synchrotron diffraction techniques and first-principles calculations. The atomic displacements along the c axis in Pmc21 AgNbO3 are responsible for the spontaneous polarization, ferroelectricity, and the paraelectric−ferroelectric phase transition.

Structure of ferroelectric silver niobate AgNbO3

We report the anomalous phase evolution in ferroelectric single crystals ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{TiO}}_{3}$ ($0.02lxl0.34$), and demonstrate the significant effects of quantum fluctuation on the ferroelectric phase transition. In addition, large electromechanical responses in this class of crystals are also demonstrated. Our results indicate that an effective approach to control the ferroelectricity of perovskite oxide can be realized not only by the covalency between $A$ site atom and oxygen but also by the substitution of $A$ site with small ions with off-centering nature. Theoretical calculations support the idea that the off-center displacements of the smaller Ca ions in the Ba-site play an important role in the exotic natures of ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{TiO}}_{3}$.

Anomalous phase diagram of ferroelectric (Ba,Ca)TiO3 single crystals with giant electromechanical response.

LiNbO3-type MnMO3 (M = Ti, Sn) were synthesized under high pressure and temperature; their structures and magnetic, dielectric, and thermal properties were investigated; and their relationships were discussed. Optical second harmonic generation and synchrotron powder X-ray diffraction measurements revealed that both of the compounds possess a polar LiNbO3-type structure at room temperature. Weak ferromagnetism due to canted antiferromagnetic interaction was observed at 25 and 50 K for MnTiO3 and MnSnO3, respectively. Anomalies in the dielectric permittivity were observed at the weak ferromagnetic transition temperature for both the compounds, indicating the correlation between magnetic and dielectric properties. These results indicate that LiNbO3-type compounds with magnetic cations are new candidates for multiferroic materials.

Desheng Fu

Papers

Relaxor Pb(Mg(1/3)Nb(2/3))O3: a ferroelectric with multiple inhomogeneities.

AgNbO3: A lead-free material with large polarization and electromechanical response

Structure of ferroelectric silver niobate AgNbO3

Anomalous phase diagram of ferroelectric (Ba,Ca)TiO3 single crystals with giant electromechanical response.

High-Pressure Synthesis and Correlation between Structure, Magnetic, and Dielectric Properties in LiNbO3-Type MnMO3 (M = Ti, Sn)