Nanoscale characterization of emergent phenomena in multiferroics
Highlights
► Multiferroics exhibit intriguing physical properties and promise new device applications. ► Photoemission electron microscopy (PEEM) as a powerful tool to study multiferroics. ► PEEM being extensively used in study different orders in (anti-)ferroic materials. ► Outlook of PEEM study in contribution to future multiferroics research.
Introduction
Ferroics are materials with a spontaneous, reversible ordering. The best known are ferromagnets, where the ordering of spins can be reversed by a magnetic field. Similarly, in ferroelectrics, aligned electric-dipoles can be reversed by an electric field and in ferroelastics, strain alignment can be altered by a stress field. Ferroics have been of great interest both for their fundamental physics and for their potential technological applications. In the last decade there has been a significant amount of research focused on magnetoelectric multiferroics, i.e. the phenomenon of inducing magnetic or electric polarization by applying an external electric or magnetic field [1], [2]. These systems exhibit unusual physical properties — which in turn promise new device applications — as a result of the coupling between their order parameters [3], [4], [5]. The correlation of order parameters required of ferroic materials to be classified as multiferroic is shown schematically in Fig. 1 [6]. Only a small subgroup of magnetically and electrically polarizable materials are either ferromagnetic or ferroelectric and fewer still simultaneously exhibit both order parameters. In these materials, however, there is the possibility that electric fields cannot only reorient the polarization but also control magnetization; similarly, a magnetic field can change the electric polarization. These functionalities offer intriguing degrees of freedom and we refer to such materials as magnetoelectrics. Current interests focus more on materials that combine ferroelectricity with ferromagnetism or, more loosely, with any kind of magnetism, i.e. ferri- and antiferromagnetism are also considered. The terminology is often extended to include composite heterostructures, such as ferroelectrics interlayered with magnetic materials and ferromagnets embedded in ferroelectric matrix [5]. The promise of coupling magnetic and electronic order parameters and the potential to manipulate one through the other has captured the attention of researchers worldwide. In terms of applications, the prospect of electric-field control of magnetism is particularly exciting, as it could lead to smaller, more energy-efficient devices.
The key to realize the application of multiferroics into real devices depends on the basic understanding of the material systems. Advanced characterization techniques, such as X-ray diffraction (XRD), Raman spectroscopy, second-harmonic generation (SHG), neutron scattering, and transmission electron microscopy (TEM), have been applied and served in the study of multiferroics to extract critical information of various multiferroics. XRD has been widely employed to characterize the crystal structure of the materials, as well as probing the structural phase transitions. Recently, giant magneto-elastic coupling in multiferroic manganites has been carefully observed with XRD and other diffraction methods. Temperature-dependent XRD reciprocal space mapping also has been used to find the concurrent transition of ferroelectric and magnetic ordering near room temperature [7], [8], [9]. Neutron scattering, an excellent probe of both structure and magnetic order, has been applied to study the magnetic structure and the coupling between the ferroelectric and antiferromagnetic directions for single crystals of multiferroic BiFeO3 [10]. Sensitive to symmetry breaking, SHG is a powerful tool to study ferroelectric and magnetic orders in multiferroics, especially at surfaces and buried interfaces. Researchers have observed a giant coupling of SHG to the spontaneous polarization in compounds with magnetically-driven ferroelectricity, such as TbMn2O5 [11]. Utilizing spin waves in multiferroics is one pathway to low-power high frequency application, especially in the terahertz range. Recently, electric-field controllable magnonics is realized in multiferroic BiFeO3 at room temperature by probing with Raman spectroscopy [12]. However, ferroics typically show complex domain structures, most of these techniques do not offer the spatial resolution to probe multiferrocity in nanoscale region. This is crucial, especially when we want to take full advantages of the coupling between order parameters in magnetoelectric multiferroics for device applications [13], [14].
Domain formation occurs to minimize the total free energy of a system. In a ferromagnet, the single domain state will minimize the exchange interaction energy. For example, the energetically most favorable magnetic state of a thin film is to align all the spins parallel in the film plane. However, the magnetic stray field associated with this configuration around any finite sample leads to a huge the magnetostatic energy. As a consequence, closure domains form and reduce the stray field but increase the exchange energy at the domain walls and anisotropy energy for the spins oriented away from their magnetic easy axis. This domain configuration is the most common in magnetic materials [15]. Similarly, in the case of ferroelectric materials, the competition of several energy terms, i.e. dipole–dipole interaction energy and anisotropy energy, lead to a complex domain configuration in the system [16]. Domain structures in ferromagnets and ferroelectrics can be studied by magnetic force microscopy and piezo-response force microscopy (PFM), respectively. However, in order to study multiferroicity, a technique, which can simultaneously study the domain structures of different order parameters is highly desirable. Second harmonic generation has been demonstrated to be a powerful tool to study multiferroic single crystals, which typically have large domains [17]. The characterization down to nano-size region remains elusive. In this review article, we will show that photoemission electron microscopy (PEEM) [18] using synchrotron radiation for excitation provides a powerful tool to study multiferroicity with nano-scale spatial resolution.
Section snippets
Physics of photoemission electron microscopy
PEEM can employ soft X-rays generated at a synchrotron for excitation of electrons from the sample of interest using the photoelectric effect. The spatial distribution of the electrons is then observed. In the X-ray absorption process, electrons are excited from core levels to unoccupied valence states, leaving empty core states. The decay of these core holes generates Auger electrons and fluorescence photons. The primary Auger electrons lose their energy through inelastic scattering processes
Ferroelectricity
Ferroelectric oxides exhibit a spontaneous, stable, and switchable electric polarization that is due to atomic displacements of positive transition metal ions and negative oxygen ions in opposite directions, which reduces the symmetry of the crystal lattice [26]. The ferroelectric PbZr0.2Ti0.8O3 (PZT) for example has a tetragonal perovskite structure where the Ti4+ and Zr4+ ions occupy the centers of a cube, the Pb2+ ions are located at the corners and the O2− ions are centered on each face of
Antiferromagnetism
PEEM is also a unique tool for the study of antiferromagnetic domains in thin films and single crystals on the nanometer scale. Due to the lack of observable net magnetization, the techniques that can be used to study the antiferromagnetic domain structures are limited. Although the antiferromagnetic domain structure in bulk crystal has been determined with neutron diffraction topography (∼70 μm) and X-ray diffraction topography (1–2 μm) [30], little was known about the antiferromagnetic domain
Multiferroism
Multiferroism in materials can be achieved through different pathways [36]. For example, a multiferroic can be a material in which ferroelectricity and magnetism have different sources and appear largely independent of one another. This can be created by site-engineering the functionality in model systems like perovskites (ABO3) where one can make use of the stereo-chemical activity of an A-site cation with a lone pair (i.e., 6 s electrons in Bi or Pb) to induce a structural distortion and
Outlook
Several challenges remain in multiferroic research, and PEEM will keep serving as an important characterization tool providing critical information to understand the fundamental science. For instance, in order to push the ferromagnet/multiferroic heterostructures into the real device applications, several key issues need to be addressed, such as power consumption, switching speed, the scalability. Especially for scale-down issue, nano-size device needs to be constructed, the competition between
Acknowledgments
The authors acknowledge the support of the Director, Office of Basic Energy Sciences, Materials Science Division of the US Department of Energy under Contract No. DE-AC02-05CH11231 and previous contracts, ONR-MURI under Grant No. E21-6RU-G4 and previous contracts, and the Western Institute of Nanoelectronics program as well as significant intellectual and financial support from scientists and engineers at Intel. Over the past 8–10 years, R.R. has also benefitted significantly through funding
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