Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session L32: Focus Session: Multiferroics I |
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Sponsoring Units: GMAG DMP Chair: Dennis Meier, ETH Zuerich Room: 207B |
Wednesday, March 4, 2015 8:00AM - 8:12AM |
L32.00001: Huge spin-driven polarizations at room temperature in bulk BiFeO$_{3}$ Jun Hee Lee, Randy Fishman Although BiFeO$_{3}$ is one of the most investigated multiferroics, its magnetoelectricity and spin-driven polarizations are barely understood on an atomistic level. By combining a first-principles approach with a spin-cycloid model, we report hidden but huge spin-driven polarizations at room temperature in bulk BiFeO$_{3}$. One of the polarizations reaches $\sim$ 0.03 C/m$^{2}$, which is larger than any other spin-driven polarization in a bulk material by one order of magnitude. By comparing our results with intrinsic measurements such as neutron scattering, Raman spectroscopy, IR directional dichroism, and high magnetic-field measurements, we disentangle all the hidden spin-driven polarizations due to exchange-striction, spin-current, and single-ion-anisotropy. We find that the broken inversion symmetries of the $R$3c structure of BiFeO$_{3}$ induce the strong response of the magnetic couplings to an electric field and are responsible for the associated huge spin-driven polarizations. [Preview Abstract] |
Wednesday, March 4, 2015 8:12AM - 8:24AM |
L32.00002: Theory for the Spectroscopic Mode Frequencies and THz Absorption of Multiferroic BiFeO$_{3}$ Randy Fishman, Jun Hee Lee A microscopic model for BiFeO$_{3}$ that includes two Dzyaloshinskii-Moriya interactions and easy-axis anisotropy along the ferroelectric polarization predicts both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field [1]. Due to simultaneously broken time-reversal and spatial-inversion symmetries, the absorption of light changes slightly as the magnetic field or the direction of light propagation is reversed. We discuss three sets of physical mechanisms that contribute to the THz abosorption and directional dichroism (DD) of BiFeO$_{\mathrm{3}}$: spin current, magnetostriction, and anisotropy. First-principles calculations are used to obtain relationships among some of the polarization matrix elements induced by broken inversion symmetries in R3c structure. While our model nicely describes the DD along the magnetic field direction [1,-1,0], it fails to predict the weak DD observed for field along [1,1,0].\\[4pt] [1] U. Nagel, R.S. Fishman, T. Katuwal, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong, and T. Room, \textit{Phys. Rev. Lett.} \textbf{110}, 257201 (2013). [Preview Abstract] |
Wednesday, March 4, 2015 8:24AM - 8:36AM |
L32.00003: Atomic-Scale Tunneling Spectra across BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Heterointerfaces Ya-Ping Chiu, Bo-Chao Huang, Pu Yu, Chia-Seng Chang, Ying-Hao Chu Atomic-level evolution of electronic structures across BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ complex oxide heterointerfaces has been demonstrated by cross-sectional scanning tunneling microscopy and spectroscopy in this work. Analysis of scanning tunneling spectroscopy results exploits how the change in the terminated interface brings the influence to the electrostatic configurations across the BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ heterointerfaces. Spatially unit-cell-by-unit-cell resolved electronic states at the atomic level reveal that the control of material interfaces at the atomic level determines the ferroelectric polarization in BiFeO$_{3}$. The precise electronic information therefore provides a clear realization about the electronic state at these complex-oxide heterointerfaces, which is crucial to understand and design a host of novel functionalities at complex oxide heterointerfaces. [Preview Abstract] |
Wednesday, March 4, 2015 8:36AM - 8:48AM |
L32.00004: Structural evolution from Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts to BiFeO$_{3}$ nanochains and their multiferroic properties Sining Dong, Xiaoguang Li, Xinyu Liu, Malgorzata Dobrowolska, Jacek Furdyna In this study, we reported the structural evolution of Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts to BiFeO$_{3}$ nanochains and the related variations of multiferroic properties. By using in-situ transmission electron microscopy with comprehensive characterization, it was found that the layered perovskite multiferroic Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts were very unstable in a vacuum environment with Bi being easily removed. Based on this finding, a simple vacuum annealing method was designed which successfully transformed the Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts into one-dimensional BiFeO$_{3}$ nanochains. Both the Bi$_{4.2}$K$_{0.8}$Fe$_{2}$O$_{9+\delta}$ nanobelts and the BiFeO$_{3}$ nanochains showed multiferroic behaviors, with their ferroelectric and ferromagnetic properties clearly established by piezoresponse and magnetic measurements, respectively. Interestingly, the BiFeO$_{3}$ nanochains exhibited a surprisingly large exchange bias with small training effects. This one-dimensional BiFeO$_{3}$ multiferroic nanostructure characterized by a relatively stable exchange bias offers important functionalities that may be attractive for device applications. [Preview Abstract] |
Wednesday, March 4, 2015 8:48AM - 9:00AM |
L32.00005: Correlated Switching Dynamics in the Nanoscale Proximity of 90$^{\circ}$ Ferroelectric Domain Walls Shiming Lei, Xueyun Wang, S.W. Cheong, L.Q. Chen, Sergei Kalinin, Venkatraman Gopalan Ferroelectrics are materials which have a built in polarization in their crystal structure even in the absence of an electric field. Domain walls themselves can possess dramatically different properties than the bulk ferroelectrics themselves. Previously we discovered that the universally present 180$^{\circ}$ walls have an order of magnitude lower domain switching threshold field than the bulk. This effect extends up to many microns around a wall, though the wall itself is unit cell thick. Here we present new results on 90$^{\circ}$ walls in PbTiO$_{3}$ single crystals that show similar proximity effect and correlated switching. Our SSPFM imaging across the $a/c/a$ domain walls suggests a strong correlated switching behavior in the proximity of the inclined 90$^{\circ}$ domain walls, even at a small AC driving voltage of 1V without DC bias on the tip. Consistent with phase-field modeling results, the inclined extended domain walls is found to act as nucleation sites in ferroelectric materials, and give rise to the domain wall asymmetrical broadening across the domain wall. [Preview Abstract] |
Wednesday, March 4, 2015 9:00AM - 9:12AM |
L32.00006: Magnetic-induced polarization in charge-ordered CaMn$_{7}$O$_{12}$ system Diomedes Saldana-Greco, Jin Soo Lim, Andrew M. Rappe The electronic properties of CaMn$_{7}$O$_{12}$ yield interesting physical phenomena including charge-ordering, non-collinear magnetism, and improper ferroelectricity. The charge-ordered CaMn$_{7}$O$_{12}$ is a complex quadruple perovskite whose chemical formula is (CaMn$^{3+}_{3}$)(Mn$^{3+}_{3}$Mn$^{4+}$)O$_{12}$ where three Mn$^{3+}$ are on the $A$-site (Mn1), three Mn$^{3+}$ on the $B$-site (Mn2), and one Mn$^{4+}$ on the $B$-site (Mn3). Three parallel $c$-chains with alternating Mn1 and Mn2 form a Kagome lattice with either Mn3 or Ca at the center of the hexagonal rings. The non-collinear magnetic state consists of spin moments lying on the $ab$-plane, forming a helical pattern along the $c$-axis. Our DFT+$U$+$J$ study shows that the Mn3 spins adopt a (90$^{\circ}$,30$^{\circ}$) spin configuration with respect to the surrounding (Mn1,Mn2) spins, breaking the inversion symmetry and generating a Berry-phase computed ferroelectric polarization of 2975 $\mu$C/m$^2$ along the $c$-axis. We demonstrated that when the magnetic helicity of the system is reversed, the ferroelectric polarization flips. This study aims to explore how the electronic and magnetic properties are intertwined to give rise to a multiferroic, charge-ordered state. [Preview Abstract] |
Wednesday, March 4, 2015 9:12AM - 9:48AM |
L32.00007: Giant spin-driven ferroelectric polarization and magnetoelectric effect in perovskite rare-earth maganites under high pressure Invited Speaker: Tsuyoshi Kimura The discovery of ferroelectricity in TbMnO$_{3}$ triggered extensive studies on a type of multiferroics, ``spin-driven ferroelectrics.'' Unlike conventional ferroelectrics such as BaTiO$_{3}$, spin-driven ferroelectrics exhibit remarkable magnetoelectric (ME) effects. However, the ferroelectric polarization $P$ in spin-driven ferroelectrics ever reported (\textless 10$^{-1} \mu $C/cm$^{2}$) is much smaller than that in conventional ferroelectrics (typically 10$^{0}$ $\sim$ 10$^{1} \mu $C/cm$^{2}$). Thus, the quest for robust magnetically-controllable $P$ comparable to that in conventional ferroelectrics is still a major challenge in the research on multiferroics. In this study, we utilized the ``high-pressure'' to attain a magnetically-controllable spin-driven $P$ with its magnitude being comparable to that in conventional ferroelectrics [T. Aoyama \textit{et al.}, Nature Commun. 5, 4927 (2014)]. With a home-made high-pressure measurement system with a diamond anvil cell, we investigated high-pressure effects on ME properties of perovskite $R$MnO$_{3}$ ($R = $ Gd, Tb, and Dy). Our study revealed that these manganites exhibit a pressure-induced ME phase transition and that the high-pressure phase shows the largest $P$ (e.g., 1 $\mu $C/cm$^{2}$ in TbMnO$_{3}$) among spin-driven ferroelectrics ever reported. Moreover, $P$ is further enhanced by applying a magnetic field. Our study demonstrates that it is possible to attain giant spin-driven ferroelectric polarization which comes close to that in conventional ferroelectrics, and to control it magnetically.\\[4pt] This work has been done in collaboration with T. Aoyama, K. Yamauchi, A. Iyama, S. Picozzi, A. Miyake, and K. Shimizu. [Preview Abstract] |
Wednesday, March 4, 2015 9:48AM - 10:00AM |
L32.00008: Terahertz electromagnons in spin-diluted cupric oxide: dynamics of twisted spin states James Lloyd-Hughes, Samuel Jones, Nicola Wurz, Michele Failla, Chris McConville, Dharmalingham Prabhakaran Understanding the physics of magnetoelectric materials may lead to their application in actuators, sensors and solid state memories. Improper multiferroics also have novel quasiparticle excitations: electromagnons form when spin-waves become electric-dipole active. We investigated magnons, electromagnons and spin-lattice coupling in Cu(1-x)Zn(x)O (0\textless x\textless 0.05), an improper ferroelectric. Terahertz time-domain spectroscopy demonstrated electromagnons only in the multiferroic phase, and established the selection rule and that the oscillator strength tracks the static polarisation [1]. The impact of non-magnetic Zn-substitution on lattice dynamics was elucidated by Raman and Fourier-transform spectroscopy, showing strong spin-lattice coupling in Cu(1-x)Zn(x)O. While the phonon and magnon modes broaden and shift as a result of alloy-induced disorder, the electromagnon was found to be insensitive to Zn substitution and the induced disorder in the local spin structure. The results demonstrate that electromagnon excitations and dynamic magnetoelectric coupling can be maintained even in disordered spin systems, and at elevated temperatures [2]. [1] Nature Communications 5, 3787 (2014). [2] Physical Review B 90, 064405 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 10:00AM - 10:12AM |
L32.00009: Terahertz study of potential multiferroic materials Sr$_2$FeSi$_2$O$_7$ and BaFe$_{12}$O$_{19}$ Thuc T. Mai, M.T. Warren, J. Brangham, T-H. Hwan, S-W. Cheong, J. Yan, R. Vald\'es Aguilar We present data on the complex dielectric function of two iron-based quantum magnets in the terahertz frequency range. We study the phase transitions in Sr$_2$FeSi$_2$O$_7$ at low temperature when it becomes a collinear antiferromagnet, in contrast with the canted antiferromagnetic state of iso-structural multiferroic Ba$_2$CoGe$_2$O$_7$. We compare the terahertz response of these two materials in light of the recent observation of electromagnon excitations in Ba$_2$CoGe$_2$O$_7$. We have also studied BaFe$_{12}$O$_{19}$, a hexagonal ferromagnet that is predicted to have an antiferroelectric state. We will report experiments at terahertz frequencies probing this predicted state. [Preview Abstract] |
Wednesday, March 4, 2015 10:12AM - 10:24AM |
L32.00010: Enhancement of Magnetoelectric Coupling in CoGa$_{x}$Fe$_{2-x}$O$_{4}$/BaTiO$_{3}$ Composite Yan Ni, Zhen Zhang, David Jiles, Cajetan Nlebedim Multiferroic materials exhibit magnetoelectric coupling and promise new device applications including magnetic sensors, generators and filters. An effective method for developing magnetoelectric (ME) materials with enhanced ME effect is achieved by the coupling through the interfacial strain between piezoelectric and magnetostrictive materials. In this study, enhancement of magnetoelectric coupling was found by systematically studying the electrical and magnetic properties of CoGa$_{x}$Fe$_{2-x}$O$_{4}$/BaTiO$_{3}$ composite. It is found that Ga doping not only stabilizes the magnetic phase of composites but also increases the sensitivity of magnetoelastic response by 30{\%}. Moreover, Ga doping reduces the electrical conductivity and the dielectric loss of composite. An enhancement of the electrostrain with doping Ga is also observed in CoGa$_{x}$Fe$_{2-x}$O$_{4}$/BaTiO$_{3}$ (x$=$0.3). As both the sensitivity of magnetostriction and the change in the electric field with strain increase, the ME voltage coefficient also increase. Thus, our work is beneficial for the application of CoFe$_{2}$O$_{4}$/BaTiO$_{3}$-based multiferroic materials. [Preview Abstract] |
Wednesday, March 4, 2015 10:24AM - 10:36AM |
L32.00011: Ultrafast Optical Spectroscopy of BiFeO3-BaTiO3 Based Structures B.A. Magill, M.A. Meeker, G.A. Khodaparast, S. Priya, Y. Zhou, Hyun-Cheol Song, M. Bishop, S. McGill Ultrafast optical spectroscopy can provide insight into fundamental microscopic interactions, dynamics and the coupling of several degrees of freedom. Pump/ probe studies can reveal the answer to questions like ``What are the achievable switching speeds in multiferroics?'' In this talk, we report on two color (400/800nm) pump-probe differential reflectance spectroscopy of BiFeO3-BaTiO3 based structures to probe the coupling between optical and acoustic phonons to spin waves, in these material systems. We present the results of several different time resolved transient reflectivity measurements to probe both the carrier and spin dynamics. [Preview Abstract] |
Wednesday, March 4, 2015 10:36AM - 10:48AM |
L32.00012: Linear to quadratic magnetoelectric effect in Fe langasite Sergey Artyukhin, David Vanderbilt, Sang-Wook Cheong Materials with coexisting and interacting switchable ferroic orders -- multiferroics -- are the subject of intense investigations due to their existing and potential applications in spintronics and information storage technology. Here we investigate the much debated magnetically induced polarization and magneto-electric effect in the hexagonal Fe-langasite Ba$_3$NbFe$_3$Si$_2$O$_14$ with distorted triangular layers of magnetic Fe ions. We propose a simple model for these phenomena, and discuss how application of the magnetic field induces a toroidal moment, responsible for the peculiar magnetoelectric effect in this material. [Preview Abstract] |
Wednesday, March 4, 2015 10:48AM - 11:00AM |
L32.00013: Enhanced Multiferroicity in LuFeO$_{3}$ Through Sc Doping Steven Disseler, Yoon Seok Oh, Rongwei Hu, Xuan Luo, Jeff Lynn, Sang-Wook Cheong, William Ratcliff Hexagonal manganites of the type \textit{R}MnO$_{3}$ are well known examples of single-phase multiferroic materials, but suffer from low magnetic ordering temperatures and weak magnetoelectric coupling making them unsuitable for implementation in devices. Recently, the isostructural ferrites \textit{R}FeO$_{3}$ have been proposed as promising materials to exhibit greatly enhanced magnetic properties, including a much stronger coupling mechanism between ferromagnetic moment and ferroelectic polarization. Here we present a magnetometry and neutron scattering investigation of LuFeO$_{3}$ forced into the ferroelectric structure through Sc-doping. We find the magnetic ordering temperature dramatically increases relative to pure hexagonal LuFeO$_{3}$ and LuMnO$_{3}$, as well as an unusual spin-reorientation at low temperatures. We will discuss possible mechanisms for this reorientation and how it provides insight into the enhanced magnetic properties Limit of the \textit{R}FeO$_{3}$ series. [Preview Abstract] |
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