Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A41: Focus Session: Multiferroics and Magnetoelectrics |
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Sponsoring Units: DMP DCOMP Chair: Manuel Bibes, Unite Mixte de Physique CNRS Room: Mile High Ballroom 3C |
Monday, March 3, 2014 8:00AM - 8:12AM |
A41.00001: Systematic investigation of the growth, structure, and ferroic properties of strained epitaxial Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ thin films with multiferroic potential T. Varga, T.C. Droubay, M.E. Bowden, S.A. Stephens, S. Manandhar, V. Shutthanandan, R.J. Colby, B.C. Kabius, E. Apra, S.A. Chambers Ferroelectrically induced weak ferromagnetism has been predicted a few years back in perovskite MTiO$_{3}$ (M$=$Fe,Mn,Ni). We set out to stabilize this metastable perovskite structure by growing NiTiO$_{3}$ epitaxially on different substrates in an attempt to achieve the multiferroic properties in these compounds. Epitaxial Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ films of different thicknesses were deposited on Al$_{2}$O$_{3}$, Fe$_{2}$O$_{3}$/Al$_{2}$O$_{3}$, and LiNbO$_{3}$ substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of substrate choice, film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy, suggest that the predicted perovskite structure was made. Our physical property characterization showing lattice polarization, ferromagnetism, and a likely coupling between the ferroic order parameters indicate that \textit{R3c} NiTiO$_{3}$ with potential multiferroic properties has been synthesized. Lattice strain from mismatch has a marked effect on the structure of the films. Film stoichiometry and the choice of substrate were found to affect the observed ferroic properties. These results suggest that the properties of the films can be controlled by the choice of substrate and film stoichiometry. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A41.00002: Engineered phase competition in $A$-site-ordered manganites $R$BaMn$_2$O$_6$ ($R$=Y and rare earth elements) from first principles Jiangang He, Craig J. Fennie ($A/A^\prime$)MnO$_3$ manganites for which the $A$-site cations order in layers, e.g., $R$BaMn$_2$O$_6$ ($R$=Y and rare earth elements) show higher charge and orbital ordering temperatures as compared with $A$-site disordered manganites. The degree of MnO$_6$ octahedra rotation, and therefore the Mn-O-Mn angle and Mn-O bond length, in $R$BaMn$_2$O$_6$ varies strongly with the ionic size of the rare earth ion. In fact $R$BaMn$_2$O$_6$ spans from a ferromagnetic metal ($R$=La) to an $A$-type antiferromagnetic metal ($R$=Pr and Nd), to a CE-type charge/orbital-ordered insulator ($R$=Sm-Y). The tuning of the electronic and magnetic ground states coincides with changes in the rotation patters and structural transitions from tetragonal ($R$=La-Nd), to orthorhombic ($R$=Sm-Gd), to monoclinic ($R$=Tb-Y) as the radius of $R$ decreasing, reflecting the competition among charge, orbital, magnetic, Jahn-Teller, and lattice degrees of freedom. In this talk we present the epitaxial phase diagram calculated from first-principles for these $A/A^\prime$ layered manganites and discuss the possibility of using an electric-field to control the competition among these phases via octahedral rotation induced ferroelectricity. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 9:00AM |
A41.00003: Spectroscopic signatures of domain walls in multiferroic ErMnO$_3$ Invited Speaker: Janice Musfeldt We investigated the spectroscopic response of stripe- and vortex-containing ErMnO$_3$ in order to uncover the dynamic signatures of the domain walls. We quantify Born effective charge and polarization differences using the lattice behavior, analyze the local rare earth environment from the f-manifold excitations, and reveal how shifts in the charge transfer excitations impact the band gap. These findings are unified with a discussion of hybridization and domain wall density effects. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A41.00004: Magnetic-field-induced spin flop transition and magnetoelectric effect in Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$ Nobuyuki Abe, Taka-hisa Arima, Nguyen Khanh, Takahiko Sasaki Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$ compounds with x \textgreater\ 0.5 have the same crystal structure as brownmillerite, where (Fe,Al)O$_{6}$ octahedron layers and (Fe,Al)O$_{4}$ tetrahedron layers alternately stacks. The space group is orthorhombic Ibm2, which allows the presence of spontaneous polarization along the c-axis. These materials also exhibit the antiferromagnetic transition at the 350K $\sim$ 570K. We have investigated the magnetoelectric effect of single crystals. In a magnetic field applied along the spin easy axis, a metamagnetic transition is observed to accompany an anomaly of the electric polarization and the dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A41.00005: Multiferroic Pr$_2$Ti$_2$O$_7$: A candidate material to search for the electric dipole moment of the electron Maribel Nunez Valdez, Nicola Spaldin We use density functional theory (DFT) to explore the suitability of the A$_n$B$_n$O$_{3n+2}$ perovskite oxides [1] as materials for searching for the electric dipole moment (eEDM). The experimental search for the eEDM is of interest as its observation would confirm the violation of charge-parity (CP) symmetry in the Universe. Experiments involving electric-field-correlated measurements in solids are promising. In particular, multiferroic Eu$_{0.5}$Ba$_{0.5}$TiO$_3$, which was designed specifically to search for the eEDM, set an improved limit compared with previous solid-state searches [2], but suffered from hysteretic heating [3]. Here we show that the A$_n$B$_n$O$_{3n+2}$ layered perovskites ($n=4$, A=Pr,Gd and B=Ti) have an alternative mechanism for ferroelectricity plus appropriate magnetic interactions, suggesting that they are suitable candidates for an eEDM search. [1] F. Lichtenberg, A. Herrnberger, K. Wiedenmann, Prog. Solid State Chem. \textbf{36} (2008). [2] K.Z. Rushchanskii, S. Kamba, V. Goian, \textit{et al.}, Nature Mater. \textbf{9}, 649 (2010). [3] S. Eckel, A.O. Sushkov, and S.K. Lamoreaux, Phys. Rev. Lett. \textbf{109}, 193003, (2012). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A41.00006: High temperature ferrielectricity and ferrimagnetism in LnACrOsO$_{6}$ by design Hena Das, Saurabh Ghosh, Martha Greenblatt, Tanusri Saha-Dasgupta, Craig Fennie Despite intense efforts over the last decade, there are surprisingly few multiferroics in which a net magnetization coexists with a switchable polarization at room temperature. Since magnetism tends to be the harder problem, one approach to solve this challenge is to start with a material that is magnetically ordered at room temperature and drive it ferroelectric. In this regard, the double perovskite Sr$_{2}$CrOsO$_{6}$ is a promising candidate; it is ferromagnetic and insulating with a $T_{\mathrm{c}} =$ 725 K, the highest known $T_{\mathrm{c}}$ of any magnetic insulating oxide with appreciable uncompensated magnetic moment. Here we discuss our first-principles study of the ferroic properties of as not yet synthesized 3$d$-5$d$ double perovskites, LnACrOsO$_{6}$ (Ln $=$ La, Y, Ce-Lu; A $=$ Na, K). We identify polar compounds that have moderate polarization switching barriers and display ferrimagnetism that is expected to persist above room temperature. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A41.00007: Probing spin fluctuations in the paramagnetic phase of EuTi$O_{3}$ by muon spin rotation techniques Zurab Guguchia, Hugo Keller, Alexander Shengelaya, Jurgen Kohler, Annette Bussmann-Holder The muon spin rotation (${\mu}$SR) technique was used to search for theoretically predicted spin fluctuations in EuTi$O_{3}$ (ETO) deep in the paramagnetic phase. ETO is a perovskite with cubic structure above $T_{S}$=282 K, followed by a tetragonal phase below $T_{S}$ and shows antiferromagnetic (AFM) ordering at $T_{N}$=5.7 K. A strong spin-lattice coupling exists at low temperatures. Even though it is not apparent that this spin-lattice coupling continues to high temperatures, model calculations predict a strong paramagnon-phonon coupling at elevated temperatures. In order to test these predictions, ${\mu}$SR studies on ETO have been performed at temperatures above and below $T_{S}$. While the AFM phase is clearly observed in the ${\mu}$SR signal, a finite signal remains also in the paramagnetic phase, following closely the temperature dependence of the zone boundary soft mode. This unusual finding demonstrates that spin fluctuations are present deep in the paramagnetic phase and are tied to the soft zone boundary mode. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A41.00008: Large dynamical magnetic charges driven by exchange striction Meng Ye, David Vanderbilt Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential applications. First-principles methods have only recently been developed to calculate the full ME response tensor $\alpha$ including both electronic and ionic contributions.\footnote{A. Malashevich et al., Phys. Rev. B, {\bf 86}, 094430 (2012).} In several materials, the dominant contribution to the ME response has been shown to be the ionic term $\alpha_{\rm ion}$, which is proportional to both the Born charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101}, 117201 (2008).} Here we present a theoretical study of mechanisms that could enhance the magnetic charge $Z^{\rm m}$. The KITP\-ite structure is reported with large ME response arising from exchange striction and spin frustration.\footnote{K. Delaney et al., Phys. Rev. Lett., {\bf 102}, 157203 (2009).} Using first-principles density-functional methods, we calculate the atomic $Z^{\rm m}$ tensors in KITP\-ite and conclude that even when SOC is completely absent, the exchange striction acting on the non-collinear spin structure induces much larger magnetic charges than in the case when $Z^{\rm m}$ is driven by SOC as in $\rm{Cr_2O_3}$. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A41.00009: Correlation between bulk magnetoelectricity and boundary magnetization in Cr$_{2}$O$_{3}$ Junlei Wang, Christian Binek Boundary magnetization is a roughness insensitive net magnetization. It emerges at the surface or interface of a magnetoelectric antiferromagnet in a single-domain state and has been utilized in voltage controlled spintronic system for potential ultra-low power application based on exchange bias system with Cr$_{2}$O$_{3}$. Previous work has lacked to demonstrate the direct relation between the bulk spin structure and the boundary magnetization. In this work, we use magneto-optical Faraday effect to observe boundary magnetization and correlate it with the bulk magnetoelectric response of a Cr$_{2}$O$_{3}$ single crystal on an applied electric field, $E$. Our method discriminates the $E$- dependent bulk Faraday rotation, $\theta $, from the stationary boundary magnetization. To this end we investigate $\theta $ vs. $E$ in two distinct antiferromagnetic single-domain states which are prepared via magnetoelectric annealing. Temperature dependence of the boundary magnetization, $m_{BM} \propto \Theta (E =$ 0), as well as the corresponding bulk magnetoelectric susceptibility, $\alpha \propto $ d$\Theta $/d$E$, is obtained from separate investigations of $\theta $ vs. $E $for the two single domain states. Our magneto-optical setup uses a near-infrared laser so that transmission loss is admissible for our sample of 500 $\mu $m thickness. We utilize lock-in and compensation techniques to maximize measurement precision and to enable absolute Faraday rotation measurement which is gauged with respect to magnetization. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A41.00010: Electromagnons: Electrically active spin excitations in multiferroics Stanislav Kamba, Veronica Goian, Filip Kadlec, Christelle Kadlec, Premysl Vanek, Martin Kempa, Marti Gich In some multiferroics spin wave can be excited by electric component of elmg. radiation and such excitations activated by dynamic magnetoelectric coupling are called electromagnons. We will discuss mechanism of electromagnon activation in the THz spectra of three different compounds: In the multiferroic TbMnO$_{3}$, the ferroelectricity is induced by inverse Dzyaloshinskii-Moriya interaction, but two electromagnons are activated by the magnetostriction. Second example is CaMn$_{7}$O$_{12}$, whose polarization is the highest among all spin-induced ferroelectrics. In this material we observed three electromagnons, whose frequencies correspond to maxima of magnon density of states, so they should correspond to magnons from Brillouin zone boundary. Finally we will demonstrate that electromagnons are not limited to spin-induced ferroelectrics. We have observed an electromagnon in nanograin ceramics of epsilon-Fe$_{2}$O$_{3}$. This material is below 490 K a pyroelectric ferrimagnet and the electromagnon activates in the THz spectra only below 110 K, when the magnetic structure becomes incommensurately modulated. We will show how by combining infrared, THz and inelastic neutron scattering experiments, the electromagnons can be discerned from magnons or phonons. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A41.00011: Multiferroicity in Cu$_{2}$OSeO$_{3}$? Eugen Ruff, Stephan Krohns, Helmuth Berger, Peter Lunkenheimer, Alois Loidl Topological spin textures in solids are in the focus for applications in future spin-electronic technology, like high-density magnetic storage devices. Prominent materials are metallic alloys with B20 structure, such as MnSi [1], where skyrmions, vortex-like objects of nanometer scale, have been experimentally detected. In these materials, it is well known that low currents can drive skyrmion switching. In contrast, the discovery of magnetoelectric skyrmions in an insulating chiral-lattice magnet Cu$_{2}$OSeO$_{3}$ leads to another promising route to electric control [2]. This system is suggested to carry a local electric dipole, which implies that the skyrmions should be controllable by the external electric field without losses due to joule heating. Here we provide a thorough analysis of the magnetic and polar phases, using SQUID and pyrocurrent measurements. In order to investigate the possible ferroelectric properties of Cu$_{2}$OSeO$_{3}$, we have performed dielectric spectroscopy in various magnetic fields in a broad frequency range below 70 K. Combining all these different techniques, we address the question whether Cu$_{2}$OSeO$_{3}$ is magnetoelectric or multiferroic.\\[4pt] [1] S.M\"{u}hlbauer \textit{et al}., Science \textbf{323}, 915 (2009).\\[0pt] [2] S.Seki \textit{et al}., Science \textbf{336}, 198 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A41.00012: Probing the Origin of Large Magnetic Field coupled Electric Polarization in the RAl$_{3}$(BO$_{3}$)$_{4}$ system Han Zhang, Tian Yu, Trevor Tyson, Christine Nelson, Leonard Bezmaternykh The multiferroic system RAl$_{3}$(BO$_{3})_{4}$ (R$=$rare earth) is known to exhibit a strong coupling of the magnetic field to the electrical polarization. To understand the origin of this behavior, detailed structural studies on single crystals and powders derived from crystals were conducted. The structure as a function of temperature, magnetic field and pressure was explored. The results are compared with magnetic field dependent electric polarization and heat capacity measurements. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A41.00013: Growth and Characterization of the Multiferroic Barium Transition Metal Fluorides Ba$M$F$_{4}$ Trent Johnson, Pavel Borisov, David Lederman We have investigated the temperature dependent growth, as well as the magnetic and ferroelectric properties of thin films of the isostructural compounds Ba$M$F$_{4}$, where $M=$Fe, Co, Ni. The films were grown by molecular beam epitaxy to thicknesses of 50 or 100 nm on single crystal Al$_{2}$O$_{3}$ (0001) substrates. X-ray diffraction shows that this family of films grow epitaxially in the (010) orientation, but are twinned in the plane, with three domain orientations rotated by 120$^{\circ}$ relative to one another. Measurements of the remanent hysteresis via interdigitated electrodes show that the compounds $M=$Co, and Ni are ferroelectric, but no switching behavior was observed in the Fe system at electric fields up to 400 kV/cm. Measurements of the field-cooled and zero-field-cooled magnetic moment confirm the existence of low temperature magnetic behavior. [Preview Abstract] |
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