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 magnetoelectric effect--the induction of magnetization by means of an electric field and induction of polarization by means of a magnetic field--was first presumed to exist by Pierre Curie, and subsequently attracted a great deal of interest in the 1960s and 1970s (refs 2-4). More recently, related studies on magnetic ferroelectrics have signalled a revival of interest in this phenomenon. From a technological point of view, the mutual control of electric and magnetic properties is an attractive possibility, but the number of candidate materials is limited and the effects are typically too small to be useful in applications. Here we report the discovery of ferroelectricity in a perovskite manganite, TbMnO3, where the effect of spin frustration causes sinusoidal antiferromagnetic ordering. The modulated magnetic structure is accompanied by a magnetoelastically induced lattice modulation, and with the emergence of a spontaneous polarization. In the magnetic ferroelectric TbMnO3, we found gigantic magnetoelectric and magnetocapacitance effects, which can be attributed to switching of the electric polarization induced by magnetic fields. Frustrated spin systems therefore provide a new area to search for magnetoelectric media.

Magnetic control of ferroelectric polarization

Multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. As a result, they have a spontaneous magnetization that can be switched by an applied magnetic field, a spontaneous polarization that can be switched by an applied electric field, and often some coupling between the two. Very few exist in nature or have been synthesized in the laboratory. In this paper, we explore the fundamental physics behind the scarcity of ferromagnetic ferroelectric coexistence. In addition, we examine the properties of some known magnetically ordered ferroelectric materials. We find that, in general, the transition metal d electrons, which are essential for magnetism, reduce the tendency for off-center ferroelectric distortion. Consequently, an additional electronic or structural driving force must be present for ferromagnetism and ferroelectricity to occur simultaneously.

Why Are There so Few Magnetic Ferroelectrics

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

Premises of creation of Internet portal designed to provide access to participants of educational and scientific process for the joint creation, consolidation, concentration and rapid spreading of educational and scientific information resources in its own depository are considered. CMS-based portal content management systems’ potentiality is investigated. Architecture for Internet portal of MES of Ukraine’s information resources is offered.

Створення порталу інформаційно-довідкових ресурсів міністерства освіти і науки україни

RECENTLY, superconductivity at >130 K was reported1,2 in multiphase samples of the compound system HgBa2Can−1CunO2n+2+δ (Hg-12(n−l)n) with n = 1, 2, 3,.... Single-phase Hg-1223 was subsequently synthesized and found to be superconducting at a record temperature of 135 K (ref. 3). It remains to be seen if a much higher transition temperature (Tc) than 135K can be achieved in this compound system. The application of pressure generally increases the Tc of underdoped layered cuprates, with a pressure derivative of Tc that decreases with increasing doping4,5. Preliminary studies on Hg-12236 showed a pressure-induced increase in Tc without any sign of saturation up to 17 kbar. Here we report the observation of superconductivity at up to 153 K in Hg-1223 at 150 kbar, with a main transition at 147 K. This observation provides an indication that superconductivity at >150 K may be possible in this system at ambient pressure, if suitable forms of chemical substitution can be found.

Superconductivity above 150 K in HgBa2Ca2Cu3O8+δ at high pressures

Anomalies in the dielectric constant and loss tangent have been observed in the ferroelectromagnet YMnO{sub 3} near its N{acute e}el temperature of {approximately}80 K and below its ferroelectric Curie temperature of {approximately}914 K. These anomalies are indicative of coupling between the ferroelectric and antiferromagnetic orders in this compound. A small but distinct magnetoelectric effect and a magnetoresistive effect up to {approximately}15{percent} were also detected in a magnetic field at 5 T. The results will be contrasted with previous theoretical predictions. {copyright} {ital 1997} {ital The American Physical Society}

Coupling between the ferroelectric and antiferromagnetic orders in YMnO 3

The high temperature superconducting Hg-Ba-Ca-Cu-O (HBCCO) compound system reported by Schilling et al. has been reproduced with an onset transition-temperature ( T c0 ) up to ≁140 K and a main transition at ≁135 K, after proper heat-treatment. We found a positive pressure-effect on T c with a rate of 0.17 to 0.18 K/kbar and an irreversibility line at temperatures higher than that in the double Bi and Tl layered high temperature superconductors (HTS's). The observation suggests that the compounds synthesized by us are underdoped and further T c enhancement (although not drastic) is possible by proper doping, and the HBCCO system appears to be a better candidate than the double Bi and Tl layered HTS's for applications based on the higher irreversibility line.

Study of superconductivity in the Hg-Ba-Ca-Cu-O system

Reversible and irreversible magnetizations (M) have been measured on a polycrystalline sample of ${\mathrm{HgBa}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{6+\mathrm{\ensuremath{\delta}}}$ (Hg 1:2:1:2) before and after oxygenation. For the preoxygenated sample, the superconducting transition temperature (${\mathit{T}}_{\mathit{c}}$) is \ensuremath{\sim}114 K with a transition width of \ensuremath{\sim}2 K. The irreversibility line (${\mathit{H}}_{\mathrm{irr}}$) is approximated by a power law ${\mathit{H}}_{\mathrm{irr}}$\ensuremath{\sim}(1-${\mathit{T}}_{\mathrm{irr}}$/${\mathit{T}}_{\mathit{c}}$${)}^{\mathit{n}}$ with n\ensuremath{\sim}5/2 between 0.1 and 5.5 T. For the oxygenated sample, ${\mathit{T}}_{\mathit{c}}$ is \ensuremath{\sim}120 K with a width \ensuremath{\sim}4 K and the n for ${\mathit{H}}_{\mathrm{irr}}$ is almost the same. The values of n, which are \ensuremath{\sim}3/2 for Y 1:2:3:, \ensuremath{\sim}5/2 for Hg 1:2:1:2, and \ensuremath{\sim}11/2 for the Bi and Tl families, seem to correlate well with the distance between adjacent ${\mathrm{CuO}}_{2}$ blocks. A scaling law M\ensuremath{\propto}[1-(T/${\mathit{T}}^{\mathrm{*}}$${)}^{2}$]F(H), is observed for reversible M up to a few degrees below ${\mathit{T}}_{\mathit{c}}$ with a ${\mathit{T}}^{\mathrm{*}}$\ensuremath{\sim}112 K before oxygenation. This suggests that Hg 1:2:1:2 is a quasi-two-dimensional system, where the effects of thermal fluctuations have been previously demonstrated.

Irreversibility line of the HgBa2CaCu2O6+ delta high-temperature superconductors.

The superconducting phase HgBa2CaCu2O6+δ with a sharp transition has been prepared and studied. Through studies of X-ray diffraction patterns, transmission electron microscopy, magnetization, and resistivity, we found that it has a tetragonal symmetry with lattice parameters a=3.862(1) A and c=12.707(5) A , with a main superconducting transition at ≈ 112 K prior to oxygenation and ≈ 117 K after prolonged oxygenation, and has a rather low irreversibility line. No superstructures of the type previously reported by Schilling et al. and Gao et al. in the >130 K phase were detected in these samples.

Zhao Huang

Papers

Superconductivity above 150 K in HgBa2Ca2Cu3O8+δ at high pressures

Coupling between the ferroelectric and antiferromagnetic orders in YMnO 3

Study of superconductivity in the Hg-Ba-Ca-Cu-O system

Irreversibility line of the HgBa2CaCu2O6+ delta high-temperature superconductors.

Superconductivity at 112–117 K in HgBa2CaCu2O6+δ