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

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 magnetoelectric materials, which simultaneously exhibit ferroelectricity and ferromagnetism, have recently stimulated a sharply increasing number of research activities for their scientific interest and significant technological promise in the novel multifunctional devices. Natural multiferroic single-phase compounds are rare, and their magnetoelectric responses are either relatively weak or occurs at temperatures too low for practical applications. In contrast, multiferroic composites, which incorporate both ferroelectric and ferri-/ferromagnetic phases, typically yield giant magnetoelectric coupling response above room temperature, which makes them ready for technological applications. This review of mostly recent activities begins with a brief summary of the historical perspective of the multiferroic magnetoelectric composites since its appearance in 1972. In such composites the magnetoelectric effect is generated as a product property of a magnetostrictive and a piezoelectric substance. A...

Multiferroic magnetoelectric composites: Historical perspective, status, and future directions

We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO3-CoFe2O4 ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO3 (001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe2O4 nanopillars embedded in a BaTiO3 matrix. The CoFe2O4 nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.

http://science.sciencemag.org/content/sci/303/5658/661.full.pdf

Multiferroic BaTiO3-CoFe2O4 Nanostructures.

Connectivity and piezoelectric-pyroelectric composites

A eutectic composite material with the mixed spinel cobalt ferrite-cobalt titanate and the perovskite barium titanate as co-existing phases has been prepared, which shows a magnetoelectric effect due to the mechanical coupling of the piezomagnetic spinel and the piezoelectric perovskite. The maximum value of the magnetoelectric effect ΔE/ΔH obtained up till now is 5.0 × 10−2 V cm−1 Oe−1 at room temperature.

An in situ grown eutectic magnetoelectric composite material

In a composite material consisting of a piezoelectric and a piezomagnetic phase which are mechanically coherent, electric and magnetic fields are converted into each other by a two-step process involving mechanical deformation as the intermediate step. We have developed such a material on the basis of a Co ferrite-Ba titanate eutectic which is unidirectionally solidified so as to obtain an aligned two-phase structure. The maximum value of the magneto-electric effect obtained at present is 0.13 V/cm Oe at room temperature. The dimensionless quantity 4π(dP/dH) is 0.2 or a factor of 200 higher than the best single-crystal material known at room temperature (Cr2O3).

Magnetoelectricity in piezoelectric-magnetostrictive composites

In a previous paper1 we described the magnetoelectiic effect in Co ferrite-Ba titanate unidirectionally solidified eutectic, which can reach a conversion factor (α factor = 4π(dP/dH) 200 times higher than that of the best single-phase material at room temperature, Cr2O3. We subsequently developed sintered composites with an inherently less ideal structure and therefore lower conversion factor, but easier fabrication. We also developed a new poling procedure, which minimizes ageing effects. This paper is focused on the last two items.

Piezoelectric-piezomagnetic composites with magnetoelectric effect

The existence region and the magnetic and electrical properties of MnSb

The consequences of a simple type of exchange interaction between charge carriers in a broad energy band and localized magnetic moments will be discussed for a ferromagnetic semiconductor. The interaction causes (1) a splitting of the energy band into two bands for the different spin directions, (2) spin disorder scattering of the charge carriers. The calculated temperature and field dependence of the magnetoresistance are compared with experimental data of CdCr2Se4. The magnetoresistance of compounds MeCr2S4, Me=Fe, Co, Cd was measured. For n‐type CdCr2S4 and for p‐type FeCr2S4 and p‐type CoCr2S4, the magnetoresistance −Δρ/ρ0 was found to be 0.78, 0.05, and 0.05, respectively, at 12 kOe near Tc, whereas n‐type FeCr2S4 and CoCr2S4 show no effect. The magnetoresistance of the system Fe1−xCdxCr2S4 was also measured. The temperature dependence of the polar magneto‐optical Kerr effect of CdCr2Se4 is discussed in connection with the described model for the energy bands.

A.M.J.G. Van Run

Papers

An in situ grown eutectic magnetoelectric composite material

Magnetoelectricity in piezoelectric-magnetostrictive composites

Piezoelectric-piezomagnetic composites with magnetoelectric effect

The existence region and the magnetic and electrical properties of MnSb

Magnetoresistance in Chalcogenide Spinels