Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H33: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides: Multiferroics & Magnetoelectrics I |
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Sponsoring Units: DMP DCOMP Chair: Dipanjan Mazumdar, University of Alabama Room: C143/149 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H33.00001: Changing Dielectrics into Multiferroics---Alchemy Enabled by Strain Invited Speaker: Ferroelectric ferromagnets are exceedingly rare, fundamentally interesting multiferroic materials. The properties of what few compounds simultaneously exhibit these phenomena pale in comparison to useful ferroelectrics or ferromagnets: their spontaneous polarizations ($P_{s})$ or magnetizations ($M_{s})$ are smaller by a factor of 1000 or more. The same holds for (magnetic or electric) field-induced multiferroics. Recently, however, Fennie and Rabe proposed a new route to ferroelectric ferromagnets\footnote{C.J. Fennie and K.M. Rabe, \textit{Phys. Rev. Lett.} \textbf{97} (2006) 267602.}---transforming magnetically ordered insulators that are neither ferroelectric nor ferromagnetic, of which there are many, into ferroelectric ferromagnets using a single control parameter: strain. The system targeted, EuTiO$_{3}$, was predicted to simultaneously exhibit strong ferromagnetism ($M_{s}$~$\sim $~7~$\mu _{B}$/Eu) and strong ferroelectricity ($P_{s}$~$\sim $~10~$\mu $C/cm$^{2})$ under large biaxial compressive strain. These values are orders of magnitude higher than any known ferroelectric ferromagnet and rival the best materials that are solely ferroelectric or ferromagnetic. Hindered by the absence of an appropriate substrate to provide the desired compression, we show$^{3}$ both experimentally and theoretically the emergence of a multiferroic state under biaxial \textit{tension} with the unexpected benefit that even lower misfits are required, thereby enabling higher quality crystalline films. The resulting genesis of a strong ferromagnetic ferroelectric points the way to high temperature manifestations of this spin-phonon coupling mechanism.\footnote{J.H. Lee and K.M. Rabe, \textit{Phys. Rev. Lett.} \textbf{104} (2010) 207204.} Our work\footnote{J.H. Lee, L. Fang, E. Vlahos, X. Ke, Y.W. Jung, L. Fitting Kourkoutis, J-W. Kim, P.J. Ryan, T. Heeg, M. Roeckerath, V. Goian, M. Bernhagen, R. Uecker, P.C. Hammel, K.M. Rabe, S. Kamba, J. Schubert, J.W. Freeland, D.A. Muller, C.J. Fennie, P. Schiffer, V. Gopalan, E. Johnston-Halperin, and D.G. Schlom, \textit{Nature} \textbf{466} (2010) 954-958.} demonstrates that a single experimental parameter, strain, simultaneously controls multiple order parameters and is a viable alternative tuning parameter to composition for creating multiferroics. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 8:48AM |
H33.00002: Magnetoelectric coupling in the strain-induced multiferroic BiMnO$_{3}$ Patrick Mickel, Hyoungjeen Jeen, Amlan Biswas, Arthur Hebard BiMnO$_{3}$ is a rare single phase, multiferroic compound which displays both ferromagnetic and ferroelectric properties. We have grown thin films of BiMnO$_{3 }$on SrTiO$_{3}$ (100) substrates using pulsed laser deposition that display the presence of both order parameters. The ferroelectricity is found to be highly tunable, modulated by both magnetic fields (decreasing by more than 10{\%}) and external strain (increasing by more than 50{\%}). Time dependent ferroelectric measurements in addition to dielectric characterizations reveal BiMnO$_{3}$ is a relaxor ferroelectric. The polar-nano-regions (PNRs) responsible for relaxor ferroelectricity are shown to reside at the island edges where strain is inherently high. Understanding the PNR properties is shown to be essential for understanding the magnetoelectric and strain couplings. [Preview Abstract] |
Tuesday, March 22, 2011 8:48AM - 9:00AM |
H33.00003: Magnetoelectric coupling in layered perovskites from first principles Turan Birol, Craig J. Fennie The rational design of a multiferroic with a large polarization and a strong coupling between the polarization and the magnetization remains a challenge. Recognizing the limitations of bulk materials, we attempt to design a strongly coupled multiferroic by focusing on artificial layered materials. In particular, strained Sr-Ti-O layered perovskites have recently been shown to have ferroelectric lattice instabilities that can be controlled by altering the effective dimensionality of the layered system. We use a combination of density-functional theory and group theoretical methods to investigate the interplay of magnetization with ferroelectricity when a layer of magnetic transition metal ions are introduced into this highly tunable dielectric superlattice. [Preview Abstract] |
Tuesday, March 22, 2011 9:00AM - 9:12AM |
H33.00004: Electrical field control of interface magnetic anisotropy Lei Xu, Shufeng Zhang The interface magnetic anisotropy of ferromagnetic metals comes from the spin-orbit interaction. By explicitly taking into account the interaction between the symmetry-broken interface potential and the spin-dependent electric dipoles of the Bloch states, we find that the interface spin-orbit coupling can be modeled by the Rashba spin-orbit Hamiltonian (RSOH). Due to the presence of the RSOH, the spin up and down states of the ferromagnet are spin mixed at the interface. Among other consequences, the RSOH induces a perpendicular surface magnetic anisotropy whose magnitude is comparable to the observed values in transition metals. When an external electric field is applied across the interface, the induced screening potential modifies the RSOH and thus the perpendicular anisotropy can be manipulated. Our calculated results are in agreement with the experiments [1]. \\[4pt] [1] Endo et al., Appl. Phys. Lett. 96, 212503 (2010); T. Nozaki et al, Appl. Phys. Lett. 96, 022506 (2010). [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:24AM |
H33.00005: First Principles Studies of Electronic structure and Lattice Dynamics of Multiferroic GaFeO$_{3}$ Amritendu Roy, Rajendra Prasad, Sushil Auluck, Ashish Garg GaFeO$_{3}$ (GFO) is a room temperature piezoelectric material with antiferromagnetic ordering in the ground state. However, experimental observation reports ferrimagetic behavior below the magnetic transition temperature, attributed to the site disorder of Fe and Ga sites. This transition occurs at temperatures close to room temperature, depending upon the Fe content of the material. Previous structural characterization studies indicate that the room temperature crystal structure (\textit{Pc2}$_{1}n)$ is retained at least until 4 K. While there are a few experimental studies on this compound, there is no well established understanding of its electronic structure and lattice dynamics which can give insight into the piezoelectric and magnetic properties of the material. From this perspective, we started our calculations with the experimental lattice parameters of stoichiometric GFO assuming no partial occupancies of the constituent ions. The calculations are carried out using local spin density approximation (LSDA+U). Electronic structure and Born effective charges were calculated based on the ground state structure. First principles density functional theory based calculations using small displacement method was adopted to calculate the phonon dispersion relations for the material. On the basis of the dispersion relations modes were assigned. [Preview Abstract] |
Tuesday, March 22, 2011 9:24AM - 9:36AM |
H33.00006: Ferroelectricity-ferromagnetism coexistence and electromagnons in multi-band electron systems Takahiro Mikami, Takashi Oka, Hideo Aoki While it has been established that noncoliner spin textures can realize multiferroics through magnetoelectric effect, here we look into another senario. Namely, in multi-band systems that comprise odd- and even-parity orbits, homogeneous ferromagnetism and ferroelectricity can coexist as proposed by Batista. In multi-orbital systems Hund's exchange coupling is obviously expected to play an important role, but this has yet to be studied in the above scenario. We have here determined the finite-temperature phase diagram for the quarter-filled two-band Hubbard model in the strong coupling limit. A ferroelectric-ferromagnetic phase appears in multi-band insulator phases, where Hund's coupling, neglected in previous researches, is indeed found to be important for the multiferroic phase. We have further explored low-lying excitation spectrum in the multiferroic phase, since collective excitations should be an interesting experimental probe in the multiferroics. Similar to previous studies for the SU(4) Kugel-Khomskii model, magnon-pseudomagnon bound states appear as electromagnetic excitations due to a cross correlation in the homogeneous multiferroic phase. [Preview Abstract] |
Tuesday, March 22, 2011 9:36AM - 9:48AM |
H33.00007: Electromagnon in TbMnO$_3$ under magnetic field by Raman scattering Pauline Rovillain, Maximilien Cazayous, Yann Gallais, Alain Sacuto, Marie-Aude Measson, Hideaki Sakata Magnetoelectric excitations in the multiferroic TbMnO$_3$ have been investigated by Raman spectroscopy. Our observations reveal electromagnons excitations at 30 cm$^{-1}$ and at 60 cm$^{-1}$ with electric polarization of light parallel to the a axis [1]. When a magnetic field is applied along the c axis, no flop of the spiral plane or polarization is observed but TbMnO$_3$ becomes paraelectric and a simple antiferromagnetic phase is developed. We show that the dipole character of the electromagnons disappears whereas their magnon compound appears immediately when the spins spiral is destabilized with a magnetic field along the c axis. The magnon dispersion curve associated with the electromagnons is preserved before the construction of the magnon dispersion of the simple antiferromagnetic phase at higher magnetic field. The effect of the phase transition on the phonon modes shows that the Mn-O distance is the key that controls the polar character of the electromagnons.\\[4pt] [1] P. Rovillain et al., PRB 81, 054428 (2010) [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:00AM |
H33.00008: Dielectric and Resistive Response in Multiferroic Superlattices Sandra Dussan, Ashok Kumar, Ram S. Katiyar Building superlattices (SLs) with alternate layers of ultra thin films of ferroelectric and ferromagnetic materials is one of the ways to engineer magnetoelectric multiferroic materials. Alternate thin layers of ferroelectric PbZr$_{0.52}$Ti$_{0.48}$O$_{3}$ (PZT) and ferromagnetic La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) materials were grown on LaAlO$_{3}$ (001) substrates using Pulsed laser deposition technique. X-ray diffraction patterns displayed the typical satellite peaks confirming SLs formation. The surface topography indicates homogeneous films with average surface roughness of $\sim $ 1.5 nm. Well saturated polarization and high dielectric tunability were observed at room temperature. Piezo-force microscopy (PFM) measurements revealed switching of polarization under the external DC bias field. The zero field cooled (ZFC) and field cooled (FC) magnetic measurements revealed the ferromagnetic behavior of SLs, and it undergoes phase transition at lower temperature compared to the bulk and thin films of pure LSMO. To gain further understanding of the electrical properties of the SLs, impedance spectroscopy, dielectric permittivity and ac conductivity were investigated. We observed dynamic magneto-resistive and magneto-dielectric effects around the LSMO metal-insulator and ferromagnetic phase transition temperature [Preview Abstract] |
Tuesday, March 22, 2011 10:00AM - 10:12AM |
H33.00009: Enhanced resonant magnetoelectric coupling in frequency-tunable composite multiferroic bimorph structures Peter Finkel, Sam Lofland, Ed Garrity, Dwight Viehland We report on a giant tunable enhanced resonant magnetoelectric (ME) coupling in multiferroic magnetostrictive/piezoelectric composite based on Fe-Ni/PVDF and Metglas/PZT-fiber bimorph structures. The approach was shown to provide more than a tenfold gain in the ME coefficient, and a magnetic/electric field assisted stress-reconfigurable resonance frequency tuning, up to 100{\%}. The studies were performed by laser Doppler spectroscopy. We also show that this principle of a continuously tuned resonance that might be used to enhance sensitivity and to reject noise for ME magnetic sensors. [Preview Abstract] |
Tuesday, March 22, 2011 10:12AM - 10:24AM |
H33.00010: Interface magnetoelectric effect in ferroelectric/antiperovskite heterostructures Pavel Lukashev, Kirill Belashchenko, Evgeny Tsymbal, Renat F. Sabirianov We present results of the first principles calculations of the magnetoelectric effect in thin film layered heterostructures of typical ferroelectric (FE), such as PbTiO3, with Mn-based antiperovskite (AP), such as Mn3GaN. Mn-based antiperovskite materials are interesting due to a non-trivial magnetic order and a linear magnetic response to applied strain that makes them piezomagnetic. The symmetry breaking produces a net magnetization at the FE/AP heterostructure interfaces. This magnetization can be controlled by reversing the polarization of the FE layer. Our calculations show that for the positive FE polarization the induced net magnetization is 3.8 $\mu_{B}$ at the PbO/GaMn and 0.6 $\mu_{B}$ at the TiO2/Mn2N interface, while the corresponding values are 1.6 $\mu_{B}$ and 1.2 $\mu_ {B}$ for the negative FE polarization and 2.2 $\mu_{B}$ and 0.4 $\mu_{B}$ for the zero FE polarization. Thus, the FE/AP interface magnetization exhibits a strong dependence on the direction of the FE polarization, with difference as large as by a factor of 2. The presented novel approach to electrically control the magnetic properties of thin-film layered ferroelectric/piezomagnetic heterostructures may be interesting for practical applications. Therefore, we hope that our results will stimulate experimental work on the FM/AP thin-film layered heterostructures. [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 10:36AM |
H33.00011: Electric field tuning of magnetic domains in permalloy thin films using elastic coupling with ferroelectric PZT bilayers Anbusathaiah Varatharajan, Arun Luykx, Luz Sanchez, R. Polcawich, Ichiro Takeuchi We are investigating electric field controlled magnetic domain motion in permalloy films deposited on Pb(Zr$_{x}$Ti$_{(1-x)}$) O$_{3}$ (PZT) bilayers. Previously, we have shown that bilayered heterostructures consisting of a tetragonal PbZr$_ {0.3}$Ti$_{0.7}$O$_{3}$ film (70 nm) deposited on a rhombohedral PbZr$_{0.7}$Ti$_{0.3}$O$_{3}$ (70 nm) display large ferroelastic domains in the top tetragonal PZT layer (Adv. Mat. 21, 3497, 2009). The reversible non-volatile twin boundary motion in this layer can serve as a basis for inducing controlled strain on magnetic thin films deposited on top. We find that permalloy films (50 nm) sputtered on top of the ferroelastic layer exhibit out-of-plane magnetization whose domains can be imaged by magnetic force microscopy (MFM). Voltage pulses are applied between patterned pads of the permalloy film and the bottom electrode underneath the PZT bilayer. This results in different twin configurations in the tetragonal PZT layer, which in turn leads to changes in magnetic domains in the permalloy film as monitored by MFM. [Preview Abstract] |
Tuesday, March 22, 2011 10:36AM - 10:48AM |
H33.00012: Magnetism of Fe nanostructure on ultrathin BaTiO$_{3}$ film X. Chen, J. Kim, S. Yang, J.S Kim, G. Rojas, R. Skomski, M. Bode, A. Bhattacharya, T. Santos, N. Guisinger, H. Lu, A. Gruverman, C. Binek, V. Sessi, J. Honolka, A. Enders A study of Fe nanostructure on BaTiO$_{3}$ (BTO) thin films with variable temperature scanning tunneling microscopy (STM) and X-Ray Magnetic Circular Dichroism (XMCD) under ultrahigh vacuum is presented. Fe/BTO is a prototype system for the study of magneto-electric effects but it is experimentally challenging to achieve high quality metal-oxide interfaces. Our approach is to use atomically flat, unreconstructed and stoichiometric BTO films of 13 unit cell thickness on SrTO$_{3}$, and to deposit Fe impurity atoms and small clusters with molecular beam epitaxy at T = 8 K and compact nanometer clusters by buffer layer assisted growth for comparison. The magnetism of both systems was studied by XMCD at the Fe L3/2 absorption edges. The key observation is that even isolated Fe atoms on the BTO have a sizeable magnetic moment, which quickly increases with increasing coverage. This, together with a detailed analysis of the L 3/2 line shape, is evidence that intermixing and oxidation at the Fe/BTO interface is strongly suppressed. The interface quality achieved can thus potentially be exploited to experimentally observe a magneto-electric interface effect predicted by Tsymbal et al. [Phys. Rev. Lett. 97, 047201 (2006)]. [Preview Abstract] |
Tuesday, March 22, 2011 10:48AM - 11:00AM |
H33.00013: Voltage Manipulation of Magnetic Anisotropy in MgO/Ferromagnet/Ag system Jared Wong, Adrian Swartz, wei han, Roland Kawakami Recently, the development of new types of memory storage and processing devices has led to great interest in voltage-induced manipulation of magnetic properties in ferromagnetic metals (FM). We investigate the voltage-induced changes in the magnetic properties of a FM in an Indium Tin Oxide (ITO)/Poly(methyl methacrylate) (PMMA)/MgO/FM/Ag system. Samples are fabricated through molecular beam epitaxy (MBE) synthesis and PMMA resist is used as a dielectric layer. ITO is used for the top transparent conductive electrode and magnetic properties are examined through magneto-optic Kerr effect (MOKE) measurements. We report our results and observations of voltage-induced manipulation of the magnetic anisotropy in ITO/PMMA/MgO/FM/Ag system. [Preview Abstract] |
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