Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B09: Ordering in Ferroic Oxides IFocus
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Sponsoring Units: DMP Chair: Sylvia Matzen, Univ of Paris - Sud 11 CNRS Room: LACC 301A |
Monday, March 5, 2018 11:15AM - 11:51AM |
B09.00001: Backscattered Scanning Electron Microscopy Domain Imaging of Ferroelectric Films: in operando Ferroelectric Domain Structure Characterization Invited Speaker: Jon Ihlefeld There are a variety of methods that can be used to image domain structure in bulk ceramics and single crystalline ferroelectrics. However, as crystal dimensions decrease, the number of available methods also decreases to just a few and are even more limited if non-destructive evaluation is desired. In this presentation, we will show how backscatter scanning electron microscopy utilizing differential channeling and backscatter electron yield can be used to non-destructively image nano-scale domains in ferroelectric thin films. Using lead zirconate titanate (PZT) bilayer and epitaxial thin films as examples, we will outline the necessary imaging conditions and ultimate resolution limits of the technique. It will be shown that by combining backscattered electron imaging with electron backscatter diffraction one can identify individual grain and domain orientation information that is complementary information to that collected by more conventional methods to image domain structure in ferroelectric films, such as piezoresponse force microscopy (PFM). An additional advantage of this technique is the utility to observe ferroelastic domain structure changes through top electrodes in actual capacitor device structures. This is demonstrated in a bilayer PZT film by applying electric fields using 3 nm thick platinum electrodes. Interestingly, it is observed that the for most grains, the number of domains increases in the actual capacitor while the electric field is applied and that the number of domains decreases when the field is removed. Imaging of ferroelastic domain structure through an electrically conductive electrode would be difficult by other means and presents a new opportunity for understanding domain structure in functioning devices. Finally, we will show that 90° domains with domain wall spacings as small as 10 nm can be resolved in epitaxial films, making this technique a high-resolution alternative to PFM |
Monday, March 5, 2018 11:51AM - 12:03PM |
B09.00002: Microscopic Theory of Spin Toroidization in Periodic Crystals Yang Gao, David Vanderbilt, Di Xiao Spin toroidization has been proposed to be a primary order parameter in systems that break time reversal and inversion symmetry but preserve the combined symmetries. It has been related to various magnetoelectric phenomenon. However, previous theories of spin toroidization have been based on the multipole expansion in the electromagnetic theory, which is a classical theory by its nature. Here we provide a microscopic quantum-mechanical theory of spin toroidization in periodic crystals using Bloch functions. Our result is gauge invariant, and more importantly, it directly connects to the antisymmetric part of the spin magnetoelectric polarizability. To illustrate our result, we calculate the spin toroidization in a toy model and demonstrate that it is a genuine bulk property. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B09.00003: Magnetoelectric response in square cupola antiferromagnets Kenta Kimura, Peter Babkevich, Masayuki Toyoda, Kunihiko Yamauchi, Takehito Nakano, Yasuo Nozue, Henrik Ronnow, Tsuyoshi Kimura A newly discovered magnetic insulator A(TiO)Cu4(PO4)4 (A = Ba, Sr, etc.) crystalizes into a tetragonal chiral crystal structure with P4212 space group [1]. It consists of a unique magnetic M4O12 cluster with square cupola geometry, in which four corner-sharing MO4 plaquettes (M = transition metal) form a noncoplanar buckled structure. Here, we report on the experimental results indicating that this asymmetric square cupola cluster provides a source for unique magnetoelectric (ME) effects. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B09.00004: Electronic Excitations of Polar magnet (Fe1-xZnx)2Mo3O8 Probed by Resonant Inelastic X-ray Scattering Hsiao-Yu Huang, Amol Singh, Kunchala Ramesh Babu, Jun Okamoto, Ting-Chun Huang, Ashish Chainani, Yu-Miin Sheu, Takashi Kurumaji, Yoshinori Tokura, Guang-Yu Guo, C. Chen, Di-Jing Huang Multiferroic materials which possess more than one ferroic order have attracted much interest because of the spontaneous coexistence of electricity and (anti)magnetism. The magnetoelectric (ME) effect is a key in designing novel electronic devices. The polar magnet Fe2Mo3O8 shows not only strong ME coupling but also chemical-doping controllability of the distinct ME phases. To study the change of electronic structure with doping, we measured the high-resolution resonant inelastic X-ray scattering (RIXS) at Fe L-edge of (Fe1-xZnx)2Mo3O8 with x = 0 and 0.125. In combination with charge-transfer multiplet calculations, our RIXS results disentangle the electronic properties of octahedral and tetrahedral Fe2+ and provide spectroscopic evidence that the doped Zn favors to replace tetrahedral Fe in the doped compound, in agreement with LDA+U calculations. |
Monday, March 5, 2018 12:27PM - 12:39PM |
B09.00005: Hartree-Fock Pseudopotentials for Improved Band Gap Description of Density Functional Theory Hengxin Tan, Yuanchang Li, Shengbai Zhang, Duan Wenhui Density functional theory (DFT) can run into serious difficulties with localized states in elements such as transition metals with occupied-d states and oxygen. In contrast, Hartree-Fock (HF) method can be a better approach for such localized states. Here, we develop HF pseudopotentials (PPs) to be used alongside with DFT for solids. Calculations for a range of semiconductors with diverse physical properties show noticeably improved band gaps, especially for those late-transation-metal-based ones. We also find the HF PP-based HSE calculations yield significantly improved band gap for such systems with no need to tune any parameter, compared to the PBE PP-based HSE calculations. Moreover, by taking ZnO as an example, we also showed how the levels of the Zn 3p, Zn 3d affect the electronic properties such as the fundamental band gap and the d-band width. |
Monday, March 5, 2018 12:39PM - 12:51PM |
B09.00006: Soft x-ray dichroism study of the magnetic phase diagram of hexaferrite Ba0.5Sr1.5Mg2Fe12O22. Francis Chmiel, Roger Johnson, Noah Waterfield Price, Dharmalingam Prabhakaran, Paul Steadman, Paolo Radaelli The family of hexaferrites possess the largest known magnetoelectric coefficients of any material, and understanding the origins of the strong coupling between magnetism and electrical polarization relies upon knowledge of its intricate magnetic structure. By characterising the magnetic phase diagram of the novel hexaferrite Ba0.5Sr1.5Mg2Fe12O22 by SQUID magnetometry and magnetocurrent measurements, we revealed a number of metamagnetic states, one of which exhibits striking magnetoelectric effects. This phase has a commensurate, k=(0,0,1.5), fan-type magnetic structure, which we have studied in detail by spatially resolved soft x-ray circular dichroism. We find that the observed circular dichroism is of pure magnetic origin, as opposed to arising from charge- magnetic interference suggested in previous studies, and hence it is directly sensitive to magnetic polarity. This has enabled us to spatially resolve the evolution of the microscopic magnetoelectric polarity of the system as a function of magnetic field and temperature. |
Monday, March 5, 2018 12:51PM - 1:03PM |
B09.00007: Exploring the Effects of Pressure on the Magnetic Phases in the Co doped MnWO4 system Melissa Gooch, Narayan Poudel, Bernd Lorenz, Jinchen Wang, Feng Ye, Jaime Fernandez-Baca, Paul C. W. Chu Cobalt doped MnWO4 is a type 2 multiferroic compound and is also known to have one of the most complex and rich phase diagrams to date. Multiferroic materials are well understood to be sensitive to small perturbations which may be introduced through chemical substitution, magnetic and electric fields, or external pressure which can result in new states. Now these new states can have different magnetic orders and/or ferroelectric properties, which can lead to a wonderfully complex phase diagram to explore. Previously completed neutron measurements conducted at ambient and high pressure have resolved the complex magnetic phase diagram. |
Monday, March 5, 2018 1:03PM - 1:15PM |
B09.00008: Ferroelectric Ferrimagnetic LiFe2F6: Charge Ordering Mediated Magnetoelectricit Ling-Fang Lin, Qiao-Ru Xu, Jun-Jie Zhang, Yan-Ping Liang, Shuai Dong The realization of magnetoelectric coupling between large polarization (P) and large magnetization (M) remains a big challenge despite tremendous efforts in the past decades. Here, we proposed a new bulk charge ordering (CO) multiferroic material, trirutile-type LiFe2F6, which can realize this desired magnetoelectric coupling [1]. The magnetic ground state was obtained to be A+-type antiferromagnetism, which is in agreement with previous neutron study [2]. Our DFT calculations indicate that LiFe2F6 is a charge-ordered material with Fe2+/Fe3+ configuration, which can lead to room-temperature ferroelectricity. More interestingly, the ferrimagnetic A− state with a net magnetization of 0.5 μB/Fe can be stabilized by moderate compressive strain. Thus, LiFe2F6 can be a rare single phase multiferroic system with both P and M. Most importantly, since the charge ordering is the common ingredient for both P and M, the net M can be fully switched by flipping P, rendering an intrinsic strong magnetoelectric effect. |
Monday, March 5, 2018 1:15PM - 1:27PM |
B09.00009: Anomalous Behavior of Spin Dynamics in the Metal-Organic Framework (MOF) Dimethylammonium Manganese Formate (DMMnF) Rhea-Donna Reyes, Sanath Ramakrishna, John Haddock, Arneil Reyes, Nar Dalal Dimethylammonium Manganese Formate(DMMnF) is of interest as a multiferroic metal-organic framework. For the first time, 1H and 55Mn NMR are used to study the known dielectric transition at TFE ~ 184K and antiferromagnetic (AF) transition at TN ~ 8.5K. A broad and strong zero-field 55Mn NMR signal was found below TN, with a calculated 8T internal field due to ordered Mn moments. This signal is suppressed above 0.3T, consistent with the known spin-flop transition. Dramatic changes in 1H spectra correlate to the magnetic ordering with a critical exponent of β = 1/4, in contrast to the mean field prediction. The spin-lattice relaxation T1 recovery exhibits a double exponential behavior, with the long and short components differing by 3 orders of magnitude. The long T1 component dips near 150K and 8.5K, while the short component exhibits a huge enhancement approaching the critical regime followed by an exponential decay at low temperatures. This behavior is suggestive of an opening of a spin gap, Δ ~ 4.45K, about half of TN. Implications of these results will be discussed. |
Monday, March 5, 2018 1:27PM - 1:39PM |
B09.00010: Weak ferromagnetism and short range polar order in NaMnF3 thin films Amit KC, Pavel Borisov, David Lederman The orthorhombically distorted perovskite fluoride NaMnF3 (Pnma space group) has been predicted to have a polar instability at low temperatures if a = c distortion is imposed. Epitaxial NaMnF3 thin films were grown on cubic SrTiO3 (100) substrates via molecular beam epitaxy (MBE) to test such prediction. Thin film structural quality as a function of the substrate temperature and film thickness was investigated using X-ray diffraction (XRD), in-situ reflection high-energy electron diffraction (RHEED), and atomic force microscopy (AFM). The best films were smooth and single phase grown with four different twin domains. In-plane magnetization measurements revealed antiferromagnetic ordering with weak ferromagnetism below the Néel temperature TN = 66 K. For the dielectric studies, NaMnF3 films were grown on 30 nm SrRuO3 (100) layer used as bottom electrode grown via pulsed laser deposition. The complex permittivity as a function of frequency indicated a strong Debye-like relaxation times. A power-law divergence of characteristic relaxation time revealed an order-disorder phase transition at 8 K. The slow relaxation dynamic indicated the formation of super-dipoles (superparaelectric moments) that extend over several unit cells, similar to polar nanoregions of relaxor ferroelectrics. |
Monday, March 5, 2018 1:39PM - 1:51PM |
B09.00011: Complex phase transitions in hexagonal manganites: the Landau theoretical approach Kunlun Yang, Yang Zhang, Lin Lin, Junming Liu Improper ferroelectric hexagonal manganites have been extensively investigated for the coexistence of ferroelectricity and magnetism and the potential for ferroelectric memory. However, the exact understanding about the experienced phases and their sequence in the phase transition in hexagonal manganites has been the subject of much debate. In the line with the well-established Landau theory YMnO3, we find that the competition between the anisotropy of the trimerization and the coupling of the trimerization to the polarization plays a key role in the determination of the low-temperature structure of hexagonal manganites. By adding a higher order term on Q to the existing expression of Landau free energy, the P3c1 structure which is allowed to emerge at low temperature based on group theoretical analysis is accessed. A kind of topological twelvefold vortex domain pattern corresponding to the P3c1 phase is achieved using phase-field simulation. Our result provides a new prospective for revealing the mystery of the complex phase transition in hexagonal manganites. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B09.00012: High-temperature optical diode effect without magnetic order in polar FeZnMo3O8 Shukai Yu, Bin Gao, Jae Wook Kim, Sang-Wook Cheong, Michael Man, Julien Madéo, Keshav Dani, Diyar Talbayev We find a giant high-temperature optical diode effect on polar ferrimagnet FeZnMo3O8 using terahertz spectroscopy, where the transmitted light intensity in one direction is higher than the intensity transmitted in the opposite direction by over 100 times. Differently from all existing reports of the optical diode effect in other magnetoelectric materials where the long-range magnetic ordering is a necessary prerequisite, the effect in FeZnMo3O8 takes place in the paramagnetic phase with no long-range magnetic order. In FeZnMo3O8, the effect is observed as a resonance with a strong magnetic dipole active transition centered at 1.27 THz which is assigned as electron spin resonance between the eigenstates of the single-ion anisotropy Hamiltonian. We propose that the optical diode effect in paramagnetic FeZnMo3O8 is driven by single-ion terms in magnetoelectric free energy. |
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