Transition Metal Oxides for Magnetic and Energy Applications

Rada Savkina and Aleksej Smirnov

INTRODUCTION

As it is known, a broad structural variety as well as the unusual properties of transition metal oxides is due to the unique nature of the transition metal cation outer d- and /-electrons. These are the elements of the three rf-group transition series and the two /-group series in the periodic table consisted from a total of 60 chemical elements (see Figure 6.1. rZ-block and /-block). Although strictly speaking, the most common definition of a transition metal is accepted by the ШРАС (International Union of Pure and Applied Chemistry). These are the elements with a partially filled d subshell or have the capacity to produce cations with an incomplete d subshell. The /-block lanthanide and actinide series are called the “inner” transition metals.

Because valence electrons are present in more than one shell, transition metals often exhibit multiple stable oxidation states that can give rise to a large number of oxide types such as monoxide (AO), dioxide (AC)2), perovskite (AB03), and spinel (AB204). Most important structure types for transition metal oxides as well as the relationship between structural features and physical and chemical properties of these materials are described, for example, by Greedan (2017).

Transition metal oxides are considered to be very fascinating functional materials in which the nature of metal-oxygen bonding can vary between nearly ionic to highly covalent or metallic one reflected in phenomenal range of electronic and magnetic properties exhibited by these materials. Their unique features involve colossal dielectric constant (Lunkenheimer et al., 2009), efficient charge separation (Kaspar et al., 2016), enhanced surface reactivity, as well as magnetization and polarization properties. There are oxides with metallic properties (e.g., Ru02, Re03, LaNi03) and oxides with highly insulating behavior (e.g. BaTi03, Cr203), oxides which are characterized with charge ordering (e.g. La2_vSrvNi04, La2_vSrvCu04, Fe304) (Mizokawa, & Fujimori, 2002) and defect ordering (e.g. Ca2Mn205, Ca2Fe2Os) (Rao, 1993). It is also necessary to mention oxides with different magnetic properties - from ferromagnetics (e.g. СЮ2, y- Fe203, Y3Fe5012) to anti-ferromagnetics (e.g. a- Fe203, NiO, LaCr03), as well as superconductive cuprates (Waldram, 2017) and materials with a close coupling of magnetization and polarization via magnetoelectric and magnetodielectric effects - multiferroics (Hill, 2000; Lawes, & Srinivasan, 2011).

Unique properties of transition metal oxides develop when their spatial dimensions are reduced to the nanoscale (Rajesh Kumar, Raj, & Venimadhav, 2019). Besides, applications of composite structures including two or more transition metal oxides allow even wider diversity in their electronic properties and chemical behavior. For example, in single-phase multiferroic metal-oxide-based materials, the magnetoelectric coupling is very weak and the ordering temperature is too low. In contrast, multiferroic composites incorporated

The transition metals (d-block) and the “inner” transition metals (/block) on the periodic table, (wvvw.technologyuk.net)

FIGURE 6.1 The transition metals (d-block) and the “inner” transition metals (/:block) on the periodic table, (wvvw.technologyuk.net).

ferroelectric and ferromagnetic phases are characterized with giant magnetoelectric coupling response above room temperature.

Thus, studies of materials based on transition metal oxides and, especially, nanocomposite structures remain relevant in terms of practical application and are described in numerous scientific publications including monographs and reviews. The role of different ceramic oxide systems and their surface nano-architecture in governing the efficacy of a supercapacitor are presented in Balakrishnan and Subramanian (2017). Huge potential of metal oxide nanoparticles in technological field of current gas sensing tools is described by Eranna (2019). It focuses on the materials, devices, and techniques used for gas sensing applications, such as resistance and capacitance variations. Properties and applications of perovskite-type oxides are overviewed by Tejuca and Fierro (2019). This chapter is intended to provide recent results in the field of magnetic properties of transition metal oxides. In particular, we will consider such an interesting class of materials known as magnetoelectric multiferroics. In addition, we will look at recent advances in the use of transition metal oxides for solar fuel production.

MAGNETIC PHENOMENA IN OXIDE-BASED MATERIALS

Magnetic phenomena and materials are important and relevant in terms of practical application. since many decades they are the basis for mass storage devices as well as the subject of study in the field of spintronics. We are talking about sensors and memories based on giant magnetoresistive magnetic multilayers, in which magnetic fields cause order of magnitude changes in conductivity (Hartmann. 2000; Reig, Cubells-Beltran, & Ramirez Munoz, 2009). Other devices, for example, field-effect spin transistors using the ferromagnetic source and drain, are still under development (Sugahara, & Nitta, 2010; Sugahara, Takamura, Shuto, & Yamamoto, 2014).

It should be noted that the two very important factors that determine the progress in the field of research of magnetic materials and phenomena are advances in the characterization and growth techniques. In particular, in the surface probing methods such as the scanning tunneling microscopy and scanning tunneling spectroscopy (Wiesendanger, 2009), which not only provide spatial resolution on the atomic scale but also allow to investigate the atomic structure and the local density of states at the surface. Moreover, closely related methods such as the spin-polarized scanning tunneling microscopy (Bode, 2003) and the magnetic exchange force microscopy (Schwarz, Kaiser, Schmidt, & Wiesendanger, 2009) allow to obtain information about magnetic properties of materials. A flurry of activity in atomic and nanoscale growth techniques in the past decades has also led to the production of modern composite and hybrid magnetic materials that reveal a range of fascinating phenomena.

Among them, multiferroic magnetoelectrics are materials that are both ferromagnetic and ferroelectric in the same phase. Because of the combination of magnetic and electric properties, they are attractive materials for various electrically and magnetically cross- coupled devices in next generation of electronics and energy harvesting technologies, and at the same time they also represent a grand scientific challenge on understanding complex solid-state systems with strong correlations between multiple degrees of freedom (Lu, Hu, Tian, & Wu, 2015). Based on the type of ordering and coupling, multiferroic oxides have drawn increasing interest for a variety of device applications, such as magnetic field and electric current sensors (Palneedi, Annapureddy, Priya, & Ryu, 2016), ferroelectric photovol- taics, nanoelectronics (Ortega, Kumar, Scott, & Katiyar, 2015), and biomedicine (Kargol, Malkinski, & Caruntu, 2012). Such materials have the potential applications that include memory elements (Roy, Gupta, & Garg, 2012; Scott, 2007), in which data are stored both in the electric and the magnetic polarizations, or novel memory media, which might allow the writing of a ferroelectric data bit and the reading of the magnetic field generated by association. Since single-phase materials with strong cross-coupling properties exist rarely in nature, intensive research activity is directed toward the development of new multiferroic materials with strong magneto-electric coupling. In turn, the appearance of innovative materials with desired properties leads to the elaboration of new applications. Thus, the application-driven research in the field of multiferroics has focused on alternative materials such as composites from magnetic and ferroelectric materials coupled with mechanical strain, electric field effects, or exchange bias at the interfaces. It was recently published in the new book on multiferroics where the theory, materials, devices, design, and application of the ones are presented (Stojanovic, 2018).

WHAT IS A MULTIFERROIC?

“Crystals can be defined as multiferroic when two or more of the primary ferroic properties are united in the same phase.” Hans Schmid (University of Geneva, Switzerland) in Fiebig, Eremenko, and Chupis (2004). We know three basic ferroic orders - ferromagnetism (FM) (spontaneous magnetization), ferroelasticity (spontaneous strain), and ferroelectricity (FE) (spontaneous polarization) (see Figure 6.2). Along with ferromagnets, ferroelectrics, and ferroelastics there is a fourth class of primary ferroics based on spontaneous magnetic vortex - ferrotoroidicity (Toledano et al., 2015).

Multiferroicity is determined by a number of material parameters, including crystal symmetry, electronic, and chemical behavior. These materials can be divided into single-phase type with widely separated ferroelectric and magnetic ordering temperatures (type I multiferroics) and single-phase multiferroics having a magnetic transition with concurrent ferroelectric ordering (type II multiferroics) (Lawes, & Srinivasan, 2011; Lu et ah, 2015).

If strong coupling between ferroic orders exists, such materials are named magnetoelectrics (ME), which is the property that in certain materials a magnetic field induces an electric polarization and, conversely, an electric field induces a magnetization. Magnetoelectric coupling may arise directly between the two-order parameters, or indirectly via strain. It is important to point out also that not every magnetic ferroelectric exhibits a linear magneto-electric effect and that not every material that exhibits a linear magneto-electric effect is

Schematic illustration of the magnetic-elastic-electric coupling in multiferroic materials

FIGURE 6.2 Schematic illustration of the magnetic-elastic-electric coupling in multiferroic materials: M is magnetization, S is mechanical strain, and P is dielectric polarization.

Reproduced from Palncedi ct al. (2016) under the terms of the Creative Commons Attribution License.

also simultaneously multiferroic. Relationship between magnetic and electric materials shows that very small overlapping between these fields exists (Eerenstein, Mathur, & Scott, 2006). There is a fundamental reason behind the scarcity of ferroelectricity/ferromagnetism phenomena coexistence in the single-phase multiferroics. This is mutual exclusivity of the origins of magnetism and electric polarization - ferromagnetism needs unpaired 3d electrons and unfilled 3d orbitals, while ferroelectric polarization needs filled 3d orbitals of transition metals. Thus, only limited number of monolithic ME materials (in particular, transition metal oxides based) exhibit nonzero coupling at room temperature and an additional electronic or structural driving force must be present for FM and FE to occur simultaneously.

SINGLE-PHASE MULTIFERROIC COMPOUNDS

Single-phase multiferroic materials can be classified according to the multiferroicity origin. In type 1 multiferroics, the magnetic and electric orderings originate from different units (A-site driven, geometrically, and charge ordering-driven ferroelectricity). Ferroelectricity typically appears at higher temperatures than magnetism, and the spontaneous polarization P is often rather large (of order 100 gC/cnr) while the ME coupling is weak. Examples are BiFe03 (rN= 643 K. 90 цС/cm2) and YMn03 {TN= 76 К,P~ 6 gC/cm2, TN is Neel temperature) The multiferroic properties of bulk BiFe03 are fairly weak, but in thin-film form they are greatly enhanced.

In type 11 multiferroics, magnetic field can cause changes in symmetry through spin interactions, inducing ferroelectricity, ME coupling is much greater than that in type I, but the ferroelectric polarization in type II multiferroics is usually much smaller (-10 pC/cnr). The important difference between multiferroics of types I and II is the nature of domain walls. FE and antiferromagnetic (AFM) domain walls may coincide in type I multiferroics. In multiferroics of type II, magnetoelectric coupling is originated from the interaction of magnetic and ferroelectric domain walls. One of the best-known examples of this behavior is TbMnOj, an insulating perovskite that orders antiferromagnetically at Tn= 41 К and then undergoes a second magnetic transition at the Curie temperature 7c= 28 К (Lawes, & Sri- nivasan, 2011). Most of these materials are characterized with AFM spin configuration.

One of the most common ways of magnetic and ferroelectric phase coexistence in AB03 perovskites is hybridization between 2p orbitals of oxygen and 6p orbitals of A ions and localization of 6s2 electrons on А-site ions. These are ferrites, manganites, and chromites of Bi. Pb and rare earth elements: BiFe03, BiMn03, BiCr03, and (RE)(Fe, Mn, Cr)03 as well as mixed perovskites with transitional metals such as (Bi, Pb, RE)2(B B')06. In the following sections, we will review the research progresses of some single-phase multiferroics and composite structures predominantly based on transition metal oxides.

 
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