Multiferroic magnetoelectric composite systems such as ferromagnetic-ferroelectric heterostructures have recently attracted an ever-increasing interest and provoked a great number of research activities, driven by profound physics from coupling between ferroelectric and magnetic orders, as well as potential applications in novel multifunctional devices, such as sensors, transducers, memories, and spintronics. In this Review, we try to summarize what remarkable progress in multiferroic magnetoelectric composite systems has been achieved in most recent few years, with emphasis on thin films; and to describe unsolved issues and new device applications which can be controlled both electrically and magnetically.

Recent Progress in Multiferroic Magnetoelectric Composites: from Bulk to Thin Films

Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. This class of micro- and nano-structured artificial media have attracted great interest during the past 15 years and yielded ground-breaking electromagnetic and photonic phenomena. However, the high losses and strong dispersion associated with the resonant responses and the use of metallic structures, as well as the difficulty in fabricating the micro- and nanoscale 3D structures, have hindered practical applications of metamaterials. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response (e.g. scattering amplitude, phase, and polarization), mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A review of metasurfaces: physics and applications.

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

Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response, mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A review of metasurfaces: physics and applications

Materials with a coexistence of magnetic and ferroelectric order — multiferroics — provide an efficient route for the control of magnetism by electric fields. The study of multiferroics dates back to the 1950s, but in recent years, key discoveries in theory, synthesis and characterization techniques have led to a new surge of interest in these materials. Different mechanisms, such as lone-pair, geometric, charge-ordering and spin-driven effects, can support multiferroicity. The general focus of the field is now shifting into neighbouring research areas, as we discuss in this Review. Multiferroic thin-film heterostructures, device architectures, and domain and interface effects are explored. The violation of spatial and inversion symmetry in multiferroic materials is a key feature because it determines their properties. Other aspects, such as the non-equilibrium dynamics of multiferroics, are underrated and should be included in the topics that will define the future of the field. Multiferroic materials exhibit magnetic and ferroelectric order at the same time and provide a way to control magnetism with electric fields. We discuss the mechanisms supporting multiferroicity, multiferroic thin films and heterostructures, the non-equilibrium dynamics of multiferroics, fundamental symmetry issues and the impact of multiferroics on other research areas.

The evolution of multiferroics

Two-phase multiferroic nano-composite thin films have been a topic of research interests in the last few years This is because of their expected magnetoelectric coupling, as well as potential applications This review focuses on recent findings in self-assembled nano-structure composite thin films, and various efforts to realize and improve their magnetoelectricity Topics include: (i) nano-pillar and maze structures, and their formation mechanisms, and a nano-belt structure oriented in-plane found by our research group; (ii) the ferroelectric properties of composite thin films, and how they can be enhanced by epitaxial engineering; (iii) a magnetic anisotropy that is induced by constraint stress, and by the nano-structures of the ferromagnetic phase; and (iv) a magnetoelectric coupling that was first observed via a change in magnetization near the Curie temperature of the ferroelectric phase, a magnetization switching assisted by electric field, and recently direct measurements using a magnetic cantilever method yielding values of 18 mV/cm Oe in BiFeO3–CoFe2O4

Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films

We report the direct measurement of a magnetoelectric (ME) exchange between magnetostrictive CoFe2O4 nanopillars in a piezoelectric BiFeO3 matrix for single-layer nanocomposite epitaxial thin films grown on (001) SrTiO3 substrates with SrRuO3 bottom electrodes The ME coefficient was measured by a magnetic cantilever method and had a maximum value of ∼20 mV/cm Oe The films possessed saturation polarization (60 μC/cm2) and magnetization (410 emu/cc) properties equivalent to bulk values, with typical hysteresis loops

/pdf/direct-measurement-of-magnetoelectric-exchange-in-self-42v1drva4r.pdf

Direct measurement of magnetoelectric exchange in self-assembled epitaxial BiFeO3–CoFe2O4 nanocomposite thin films

The domain structures of Na${}_{1/2}$Bi${}_{1/2}$TiO${}_{3}$-Ba${}_{x}$TiO${}_{3}$ (NBT-$x$%BT) crystals for $x$$=$0, 4.5, and 5.5 have been investigated by polarized light and piezoresponse force microscopies. The results show that BaTiO${}_{3}$ (i) refines the size of polar nanoregions and enhances their self-organization, and (ii) suppresses the formation of proper ferroelastic domains at high temperatures in the paraelectric state, and it rather favors the formation of improper ones that form below the ferroelectric Curie temperature and that elastically accommodate the ferroelectric ones.

/pdf/evolution-of-domain-structures-in-na-1-2-bi-1-2-tio-3-single-4tgmqnreo4.pdf

Evolution of domain structures in Na 1 / 2 Bi 1 / 2 TiO 3 single crystals with BaTiO 3

Resonant bending-mode Tb1−xDyxFe2−y∕elastic-steel∕Pb(Zr,Ti)O3 magnetoelectric (ME) laminate composites have been investigated. An elastic-steel layer with a relatively high Qm significantly increases the resonant enhancement of the ME coefficient due to an increased effective Qm of the laminate. The three-phase ME laminates have a low first-order bending frequency of ∼5kHz, with a resonance-enhanced ME coefficient of ∼40V∕cmOe.

/pdf/resonant-bending-mode-of-terfenol-d-steel-pb-zr-ti-o3-1syigj1777.pdf

Resonant bending mode of Terfenol-D∕steel∕Pb(Zr,Ti)O3 magnetoelectric laminate composites

We report the ferroelectric, ferromagnetic, and magnetoelectric (ME) properties of self-assembled epitaxial BiFeO3–CoFe2O4 (BFO–CFO) nanostructure composite thin films deposited on (001), (110), and (111) SrTiO3 (STO) single crystal substrates. These various properties are shown to depend on orientation. The maximum values of the relative dielectric constant, saturation polarization, longitudinal piezoelectric coefficient, saturation magnetization, and ME coefficient at room temperature were 143, 86 μm/cm2, 50 pm/V, 400 emu/cc, and 20 mV/cm Oe, respectively.

/pdf/magnetoelectric-and-multiferroic-properties-of-variously-9pfi63o6yy.pdf

Li Yan

Papers

Review of magnetoelectric perovskite–spinel self-assembled nano-composite thin films

Direct measurement of magnetoelectric exchange in self-assembled epitaxial BiFeO3–CoFe2O4 nanocomposite thin films

Evolution of domain structures in Na 1 / 2 Bi 1 / 2 TiO 3 single crystals with BaTiO 3

Resonant bending mode of Terfenol-D∕steel∕Pb(Zr,Ti)O3 magnetoelectric laminate composites

Magnetoelectric and multiferroic properties of variously oriented epitaxial BiFeO3-CoFe2O4 nanostructured thin films