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New Iron-Based Multiferroics with Improper Ferroelectricity E-Mail: Sdong@Seu.Edu.Cn IOP Journal of Physics D: Applied Physics Journal of Physics D: Applied Physics J. Phys. D: Appl. Phys. J. Phys. D: Appl. Phys. 51 (2018) 243002 (15pp) https://doi.org/10.1088/1361-6463/aac345 51 New iron-based multiferroics with improper 2018 ferroelectricity © 2018 IOP Publishing Ltd Jin Peng1 , Yang Zhang1, Ling-Fang Lin1, Lin Lin2, Meifeng Liu2,3, Jun-Ming Liu2,4 and Shuai Dong1 JPAPBE 1 School of Physics, Southeast University, Nanjing 211189, People’s Republic of China 2 Laboratory of Solid State Microstructures and Innovative Center of Advanced Microstructures, Nanjing 243002 University, Nanjing 210093, People’s Republic of China 3 Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, People’s J Peng et al Republic of China 4 Institute for Advanced Materials, South China Normal University, Guangzhou 510006, People’s Republic of China New iron-based multiferroics with improper ferroelectricity E­mail: [email protected] Printed in the UK Received 26 January 2018, revised 3 April 2018 Accepted for publication 9 May 2018 Published 23 May 2018 JPD Abstract In this contribution to the special issue on magnetoelectrics and their applications, we 10.1088/1361-6463/aac345 focus on some single phase multiferroics, which have been theoretically predicted and/ or experimentally discovered by the authors in recent years. In these materials, iron is the Paper common core element. However, these materials are conceptually different from the mostly­ studied BiFeO3, since their ferroelectricity is improper. Our reviewed materials are not simply repeating one magnetoelectric mechanism, but cover multiple branches of improper 1361-6463 ferroelectricity, including the magnetism­driven ferroelectrics, geometric ferroelectric, as well as electronic ferroelectric driven by charge ordering. In this sense, these iron­based improper ferroelectrics can be an encyclopaedic playground to explore the comprehensive physics of multiferroics and magnetoelectricity. Furthermore, the unique characteristics of iron’s 3d 24 orbitals make some of their magnetoelectric properties quite prominent, comparing with the extensively­studied Mn­based improper multiferroics. In addition, these materials establish the crossover between multiferroics and other fields of functional materials, which enlarges the application scope of multiferroics. Keywords: multiferroics, improper ferroelectricity, iron oxide, iron selenide, iron fluoride (Some figures may appear in colour only in the online journal) 1. Introduction between ferroelectricity and magnetism not only generates emergent physics, but also provides more functionalities for 1.1. What are improper ferroelectrics applications, e.g. electric control of magnetism. Therefore, the field of multiferroic materials and magnetoelectricity is quite Ferroelectrics are important functional materials, playing attractive and great progress has been made since the begin­ an irreplaceable role in sensors, information storage, trans­ ning of the new century [2–7]. ducers, and other applications. Magnetic materials are even Based on the origin of ferroelectricity, ferroelectrics can be more extensively used, especially in the information storage classified into two families: proper ferroelectrics and improper area. Generally speaking, multiferroics denote a class of mat­ ferroelectrics. The ferroelectrics that have been studied and erials that combine these two different characteristics. A more applied to industries in the past century are basically proper rigorous definition of a multiferroic material is the simulta­ ferroelectrics. Ferroelectricity originates from ‘ferroelectric 0 2 neous presentation of more than one primary ferroic order active’ ions (e.g. those with the d configuration or with the 6s parameter in a single phase [1]. The coexisting and crossover lone pair), as found in BiFeO3, PbTiO3, and Pb(Fe1/2Nb1/2)O3. 1361-6463/18/243002+15$33.00 1 © 2018 IOP Publishing Ltd Printed in the UK J. Phys. D: Appl. Phys. 51 (2018) 243002 J Peng et al Figure 1. Illustration of three types of improper multiferroics. (a) Magnetic ferroelectrics; (b) geometric ferroelectrics; (c) electronic ferroelectrics. The top and bottom panes denote the positive and negative ferroelectric state. Although most of the proper ferroelectrics are not magn etic, 1.2. Magnetoelectricity in improper multiferroics there are exceptions, such as BiFeO [8]. 3 Magnetoelectricity is the correlation between magnetic Improper ferroelectrics form an emerging area with the moment and electric dipole moment. In particular, positive research upsurge of multiferroic materials. Most improper magnetoelectric effect means that the electric dipole moment ferroelectrics are multiferroics, although not all. Improper fer­ can be controlled by magnetic field, while the inverse magne­ roelectricity is not caused by ferroelectric active ions, but ‘ ’ toelectric effect denotes that the control of magnetic moment the phase transitions of other order parameters, e.g. structural is by an electric field. The magnetoelectric coupling effect transition (the so­called geometric ferroelectricity), charge­ has valuable applications, especially the inverse magneto­ ordering (the so­called electronic ferroelectricity), or magn­ electric effect. The control of magnetic moment by electric etic ordering (the so­called magnetic ferroelectricity) [3]. field can be energy conservative and efficient. It can overcome Magnetic ferroelectrics are achieved by some special the technical bottleneck of current magnetic storage and fer­ magnetic orders, such as spiral magnetic ordering as shown roelectric storage. Magnetoelectric couplings widely exist in in figure 1(a). The displacement of electronic cloud and/or single phase bulks, surfaces/interfaces, even in nonmagnetic ions can be achieved via the spin orbit coupling and/or spin­ – and nonferroelectric topological insulators [6]. However, the lattice coupling. These materials are also called the type­II magnetoelectric coupling effects are very weak in most of multiferroics [9]. The representative material is orthorhombic these systems. TbMnO [10]. 3 Multiferroics are an ideal platform for the pursuit of strong Geometric ferroelectricity exists in some special lattices magnetoelectric coupling because of its intrinsic magnetism with geometric frustration, as shown in figure 1(b). In these and electric dipoles. However, the general mutual exclusion materials, ferroelectricity is generated by collaborative mul­ between magnetic moment and electric dipole at the quantum tiple nonpolar modes of lattice distortion. Hybrid improper level makes the pursuit of desirable multiferroic materials a ferroelectrics, which have attracted great attentions recently, challenging problem in condensed matter physics. also belong to the category of geometric ferroelectrics [11]. For those proper ferroelectrics with magnetism, such Some of these materials are nonmagnetic. However, since as BiFeO , the magnetic and ferroelectric properties origi­ transition metal elements are mostly magnetic, most geo­ 3 nate from independent order parameters. Thus the Landau metric ferroelectrics are multiferroics. The representative free energy of the phase transitions in these systems can be material is hexagonal YMnO [12, 13]. 3 abstractly expressed as: Electric dipoles and eventual ferroelectricity can be 2 4 2 4 achieved via charge ordering, leading to so called electronic F(P, L)=αFeP + βFeP + ...... + αAFML + βAFML + ..., ferroelectrics [14]. Many transition metal ions have multiple (1) valence states. In some special lattice environments, some where P is the ferroelectric order parameter; L is the antifer­ elements can have ordered two valence states, forming the romagnetic (AFM) order parameter; α/β are corre sponding charge ordering, as illustrated in figure 1(c). Some charge coefficients. Therefore, these systems can easily achieve good ordering can be switched between two ordered states, magnetic/ferroelectric properties. For example, BiFeO3 has giving rise to ferroelectricity. The representative material is a large ferroelectric polarization (∼90 μC cm−2), high fer­ Pr0.5Ca0.5MnO3 [14]. roelectric transition temperature (∼1100 K), and high magn­ These three types of improper ferroelectrics are mostly etic transition temperature (∼660 K) [15]. However, due to magnetic (although there are a few exceptions in geometric the independency of magnetic and ferroelectric order param­ ferroelectrics, such as Ca3Ti2O7), so they are mostly multifer­ eters, these properties must be coupled by high­order effects, roics, which generally have magnetoelectric coupling. e.g. indirect and weak magnetoelectric coupling via lattice 2 J. Phys. D: Appl. Phys. 51 (2018) 243002 J Peng et al distortions. Recent theoretical work found that a small protion Second, Mn has good chemical activity. Mn­based oxides of BiFeO3 polarization is improper [16]. This kind of multi­ display abundant crystal structures, including quasi­one ferroics can be considered as magnetoelectric composites in dimensional, quasi­two dimensional, three­dimensional, the atomic scale. square lattice and triangular lattice. It offers a fertile ground In contrast, it is hopeful to realize strong magnetoelectric for improper ferroelectrics. coupling in improper ferroelectrics. The essence is to ‘down­ Third, the multiple stable valence states is the prerequi­ 3 grade’ the ferroelectric order parameter from the primary one site for the formation of charge ordering, for example, Mn + / to an dependent
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