Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H11: Focus Session: Multiferroic Heterostructures |
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Sponsoring Units: DMP Chair: Nicola Spaldin, University of California, Santa Barbara Room: Colorado Convention Center Korbel 1F |
Tuesday, March 6, 2007 8:00AM - 8:12AM |
H11.00001: Multiferroic BiFeO$_{3}$/BiCrO$_{3}$ superlattices Mark Huijben, Colleen Kantner, Qian Zhan, Joseph Orenstein, Ramamoorthy Ramesh There is currently an increasing interest into multiferroic materials. Although a large number of potential applications can be envisaged, there are currently no known single-phase materials that show large, robust magnetization and polarization at room temperature. Theoretical calculations of artificially constructed (111) layered double perovskite Bi$_{2}$FeCrO$_{6}$ predict them to be ferrimagnetic (with a magnetic moment of 2 $\mu _{B}$ per formula unit) and ferroelectric (with a polarization of $\sim $80 $\mu $C/cm$^{2})$. A high degree of control over the layer composition is required to accomplish this. In this work we fabricated such epitaxial BiFeO$_{3}$/BiCrO$_{3}$ superlattices by laser-MBE during which the growth was controlled on the atomic scale by reflection high energy electron diffraction. We will report results of structural, chemical, electrical and magnetic measurements of such superlattices. [Preview Abstract] |
Tuesday, March 6, 2007 8:12AM - 8:24AM |
H11.00002: Electric Field Controlled Magnetism in BiFeO$_{3}$/Ferromagnet Films M. Barry, K. Lee, Y.H. Chu, P.L. Yang, L.W. Martin, C.A. Jenkins, R. Ramesh, A. Scholl, A. Doran BiFeO$_{3}$ is the only single phase room temperature multiferroic that is currently known. Not only does it have applications as a lead-free replacement for ferroelectric memory cells and piezoelectric sensors, but its interactions with other materials are now attracting a great deal of attention. Its multiferroic nature has potential in the field of exchange bias, where it could allow electric-field control of the ferromagnetic (FM) magnetization. In order to understand this coupling, an understanding of the magnetization in BiFeO$_{3}$ is necessary. X-ray linear and circular dichroism images were obtained using a high spatial resolution photoelectron emission microscope (PEEM), allowing elemental specificity and surface sensitivity. A piezoelectric force microscope (PFM) was used to map the ferroelectric state in micron-sized regions of the films, which were then probed using crystallographic measurements and temperature dependent PEEM measurements. Temperature dependent structural measurements allow decoupling of the two order parameters, ferroelectric and magnetic, contributing to the photoemission signal. Careful analysis of linear and circular dichroism images allows determination of magnetic directions in BiFeO$_{3}$ and FM layers. [Preview Abstract] |
Tuesday, March 6, 2007 8:24AM - 9:00AM |
H11.00003: Ferroelectricity in (BaTiO$_{3}$)$_{n}$/(SrTiO$_{3}$)$_m$ Superlattices Containing as Few as one BaTiO$_{3}$ Layer (n=1) Invited Speaker: The question of how thin a ferroelectric can be and still be ferroelectric has been the source of an intensive research effort over the past decade. Several studies, both theoretical and experimental, have concluded that with appropriate boundary conditions ferroelectricity can exist in superlattices containing BaTiO$_{3}$ or PbTiO$_{3}$ layers as thin as one unit cell. In this talk I will show the results of experiment and theory for BaTiO$_{3}$/SrTiO$_{3}$ superlattices grown by reactive molecular-beam epitaxy (MBE) on three different substrates: TiO$_{2}$-terminated (001) SrTiO$_{3}$, (110) DyScO$_{3}$, and (110) GdScO$_{3}$. With the aid of reflection high-energy electron diffraction (RHEED), precise single-monolayer doses of BaO, SrO, and TiO$_{2}$ were deposited sequentially to create commensurate BaTiO$_{3}$/SrTiO$_{3}$ superlattices with a variety of periodicities. The superlattices consist of an $n$ unit-cell-thick slab of BaTiO$_{3}$ followed by an $m$ unit-cell-thick slab of SrTiO$_{3}$, which are designated [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$, where $q$ is the number of times the bilayer is repeated. X-ray diffraction (XRD) measurements exhibit clear superlattice peaks and the narrowest rocking curves ever reported for oxide superlattices. High-resolution transmission electron microscopy reveals nearly atomically abrupt interfaces. UV Raman results show that the BaTiO$_{3}$ in these [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ superlattices is tetragonal and the SrTiO$_{3}$ is polar due to strain. Temperature-dependent UV Raman and XRD reveal the paraelectric-to-ferroelectric transition temperature ($T_{C})$. Our results* demonstrate (1) that [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ superlattices containing as few as one strained BaTiO$_{3}$ layer ($n$=1) are ferroelectric and (2) the sensitivity of $T_{C}$ to the boundary conditions. Comparisons to \textit{ab initio} and phase-field modeling of the properties of these [(BaTiO$_{3})_{n}$/(SrTiO$_{3})_{m}$]$_{q}$ ferroelectric superlattices will be made and the importance of strain demonstrated. In addition to probing finite size effects and the importance of mechanical boundary conditions, these heterostructures may be relevant for novel phonon devices, including mirrors, filters, and cavities for coherent phonon generation and control. * D.A. Tenne, A. Bruchhausen, N.D. Lanzillotti-Kimura, A. Fainstein, R.S. Katiyar, A. Cantarero, A. Soukiassian, V. Vaithyanathan, J.H. Haeni, W. Tian, D.G. Schlom, K.J. Choi, D.M. Kim, C.B. Eom, H.P. Sun, X.Q. Pan, Y.L. Li, L.Q. Chen, Q.X. Jia, S.M. Nakhmanson, K.M. Rabe, and X.X. Xi, ``Probing Nanoscale Ferroelectricity by Ultraviolet Raman Spectroscopy,'' \textit{Science} \textbf{313} (2006) 1614-1616. [Preview Abstract] |
Tuesday, March 6, 2007 9:00AM - 9:12AM |
H11.00004: Magnetoelectric Phase Control in Epitaxial Oxides from First Principles Craig Fennie, Karin Rabe We propose a design strategy - based on the coupling of spins, optical phonons, and strain - for systems in which magnetic (electric) phase control can be achieved by an applied electric (magnetic) field. Using first-principles density-functional theory calculations, we present a realization of this strategy for the magnetic perovskite EuTiO$_3$. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H11.00005: Interfacial multiferroism and giant magnetoelectricity in nano-capacitors James Rondinelli, Massimiliano Stengel, Nicola Spaldin We present results of density functional calculations of the dielectric and magnetic responses of SrRuO$_3$/SrTiO$_3$/SrRuO$_3$ oxide heterostructures. Our calculations indicate the spatial coexistence of magnetic and polar behavior at the metal-insulator interface, suggesting a route to a new type of {\it interfacial multiferroic}. We also find a giant magnetoelectric response, and comment on the control of this magnetoelectric effect as a means to realizing new devices and sensors. Such devices should be feasible as the ability to fabricate and functionalize new complex materials continues to grow. By controlling the interacting charge, spin and lattice degrees of freedom at interfaces, it is then possible to tune the interactions between layers to create new electric or magnetic phases that are accessible with applied fields. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H11.00006: Non-Linear Optical probing of MultiFerroicity and Phase Transitions in BiFeO$_{3}$ Thin Films Amit Kumar, Lane Martin, R. Ramesh, Venkatraman Gopalan Bismuth Iron Oxide BiFeO$_{3}$ is being studied extensively by researchers to utilize its multiferroic properties for designing multi-state memory devices. In this work , we present the first results of the non linear optical probing of this material to study the simultaneous ferroelectric and antiferromagnetic ordering below the Neel's temperature. Optical second harmonic generation (SHG) has been employed to determine crystal and magnetic symmetries of thin BiFeO$_{3}$ films grown in different orientations at temperatures ranging from room temperatures to above the Neel's Temperature. We show that SHG can separate the antiferromagnetic and ferroelectric order parameters cleanly and probe each of these phenomena. The coupling between the ferroelectric and antiferromagnetic ordering under electric and magnetic fields will be presented. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H11.00007: Optical spectroscopic study on new magnetoelectric hexagonal \textit{RE}MnO$_{3}$ (\textit{RE}=Gd, Tb, Dy, and Ho) thin films Woo Seok Choi, Sung Seok A. Seo, Jung Hyuk Lee, Daesu Lee, Tae Won Noh, Yunsang Lee Recently, magnetoelectric effects in various oxides have been attracting lots of attentions and are being extensively investigated due to their intriguing couplings between the magnetic and electric order parameters. Here we report optical spectroscopic investigations on new hexagonal \textit{RE}MnO$_{3}$ (\textit{RE} = Gd, Tb, Dy, and Ho) thin films, which are fabricated by epi-stabilization technique [1]. From the in-plane optical conductivity spectra of the hexagonal \textit{RE}MnO$_{3}$, we observe a dramatic increase of the optical transition related to Mn 3d a$_{1g}$ energy level, as the ionic radius of the $R$ ion increases. The optical transition at 1.64 eV for DyMnO$_{3}$ shifts to 1.67 and 1.81 for TbMnO$_{3}$ and GdMnO$_{3}$ respectively. For natural hexagonal \textit{RE}MnO$_{3}$ (\textit{RE} = Y, Er, Lu, and Sc) with smaller ionic sizes, the same optical transitions occur at $\sim $1.6 eV. The large peak shift in new hexagonal \textit{RE}MnO$_{3}$ is understood by local flattening of Mn-O bipyramid, which will enhance the crystal field energy of a$_{1g}$, as the \textit{RE} ionic size increases. \newline [1] J. H. Lee \textit{et al}., Adv. Mat., to be published (2006). [Preview Abstract] |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H11.00008: Electronic reconstruction at SrMnO$_{3}$-LaMnO$_{3}$ superlattice interfaces Serban Smadici, Peter Abbamonte, Anand Bhattacharya, Xiaofang Zhai, James Eckstein, Andrivo Rusydi Progress in molecular beam epitaxy made possible the growth of manganese oxide superlattices with the dopant ions arranged in separate regular layers. Little is known about how this ``structural'' doping is reflected into the MnO$_{2}$ planes; for instance what is the plane effective hole concentration. We studied superlattices made of SrMnO$_{3}$ and LaMnO$_{3}$ layers with a doping of x=0.33 using resonant soft x-ray scattering. For scattering momenta at which the non-resonant contribution is suppressed by symmetry, i. e. L=3 in units of the superlattice period, resonant soft x-ray scattering probes the distribution of doped holes, and, in particular, the nature of the interface between the doped and undoped layers. Our measurements at the O K edge show a temperature-dependent hole distribution. This electronic reconstruction which occurs with cooling below T$_{c}$ =220~K is related to the transition of the superlattice to a ferromagnetic state. Resonant scattering spectra at the Mn L edges from spin and orbital distributions have complex shapes providing additional insights into these new materials. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H11.00009: Transport and magnetic properties of digital superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n}$. A. Bhattacharya, J. Kavich, S.G.E. te Velthuis, X. Zhai, M. Warusawithana, J. Freeland, Sam Bader, J.N. Eckstein Superlattices of (LaMnO$_{3})_{2n}$/(SrMnO$_{3})_{n }$, $n$=1-5, were synthesized using O$_{3}$-assisted MBE. Both constituents are antiferromagnetic insulators at low temperatures. The overall stoichiometry is the same as for La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ random alloys, but the $A$-site disorder is eliminated. For $n$=1, 2 a metallic ferromagnetic state is obtained at low T. For $n$=1, all measured properties are nearly identical to those of the random alloy. The emergence of a metallic state is interpreted in terms of a Mott transition driven by the proximity between LaMnO$_{3}$/SrMnO$_{3}$ interfaces. For $n\ge $3, a transition to an insulating state occurs, with a suppression of $T_{C }$ and $M_{s}$ and an increase in $H_{c}$. Using neutron and resonant x-ray scattering, we observe a modulation of the ferromagnetism commensurate with the superlattice period for $n$=5. We propose that magnetic frustration at the AF/F interfaces drives the insulating state. Ack: DOE BES contract {\#}DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H11.00010: Interface Magnetization in Digitally Layered (LaMnO$_{3})_{2n }$/(SrMnO$_{3})_{n}$ Superlattices J.J. Kavich, A. Bhattacharya, J.W. Freeland, J.N. Eckstein Interfaces in solids have been an enduring theme in materials physics due to the exciting new physics that can emerge in interface regions arising from a variety of competing interactions. Because of this, multilayer materials can provide a novel means to separate disorder physics from other competing interactions. The samples used in this experiment were grown using ozone-assisted atomic layer by layer molecular beam epitaxy (ALL-MBE) on (100) oriented SrTiO$_{3}$. Using X-ray Resonant Magnetic Scattering, we present a comparison of the magnetic properties at interfaces of a series of (LaMnO$_{3})_{2n }$/(SrMnO$_{3})_{n}$ superlattices as a function of n ranging from 1 to 5 unit cells. Combined with transport measurements, the magnetic properties appear to be correlated with a change in the electronic properties of the superlattice. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H11.00011: Optically-induced coherent spin precession in manganite superlattices Haibin Zhao, Kevin Smith, Gunter Luepke The artificial perovskite superlattices composed of LaMnO$_{3}$ (LMO) and SrMnO$_{3}$ (SMO) have been investigated to elucidate the interface ferromagnetic order created by adjoining the two antiferromagnetic Mott insulators. Optically-induced coherent spin precessions are studied by time-resolved magneto-optical Kerr effect in a superlattice composed of 4 u. c. (unit cells) LMO and 2 u. c. SMO for a total of 13 double-layers grown on a single-crystalline (001) substrate of SrTiO$_{3}$ (STO). We found that the demagnetization field and the anisotropy field are very similar in the LMO/SMO superlattice (H$_{d}$ = 0.75 T, H$_{a}$ = -0.14 T) and in a La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ thin film (H$_{d}$ = 0.72 T, H$_{a}$ = -0.2 T) both grown on STO substrates indicating that the strain and magnetic character are very similar in the two manganite structures. However, the low field precession behavior is distinctly different which shows that pinning by antiferromagnetic spins in the LMO layers and/or surface anisotropy of the superlattice may contribute significantly to the effective field at low applied fields, thus modifying the mode profile and precession frequency. We will discuss this exchange interaction in LMO/SMO superlattices with different periods in zero-field cooling and field cooling. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H11.00012: Magnetism and electronic structure at the interface of a metal CaRuO$_{3 }$ and Mott insulator CaMnO$_{3}$. Alexander Boris, John Freeland, Jerald Kavich, Ho Nyung Lee, Petar Yordanov, Giniyat Khaliullin, Bernhard Keimer, Jak Chakhalian Recent advances in fabrication of ultra-thin complex oxide heterostructures have opened new opportunities to investigate possible novel quantum states at the correlated interfaces. With this aim we fabricated ultra-thin superlattices of CaMnO$_{3}$(CMO)/CaRuO$_{3}$(CRO) with the thickness of CRO layers from 1 to 12 unit cells by laser MBE. Electronic properties of CRO/CMO were investigated by soft x-ray spectroscopies at the L-edges of Mn and Ru. SQUID and optical reflectivity revealed a ferromagnetic thickness-independent transition at T$_{c }\approx $100K and CRO thickness-dependent negative magnetoresistance. This behavior is in marked contrast to the individual layers. At the interface we found a clear sign of net magnetic moment on Mn, which saturates only at magnetic field of 5T. Unlike CMO, similar measurements at the Ru L$_{3}$-edge showed no detectable magnetism in the field up to 5T. Comparison with Ru references confirmed Ru(IV) oxidation state. These findings are in the sharp contrast with previously suggested models involving Ru(IV-V) valency exchange and thus reveal intricate nature of the interface between a metal and Mott insulator. [Preview Abstract] |
Tuesday, March 6, 2007 10:48AM - 11:00AM |
H11.00013: Probing multiferroicity and spin-spin interactions via angular dependent dielectric measurements on $Y$-doped \textit{HoMnO}$_{3}$ in high magnetic fields Relja Vasic, Haidong Zhou, Chris Wiebe, James Brooks Dielectric measurements are used to characterize magnetic phase transitions in the doped ferrielectric oxides \textit{Ho}$_{1-x}Y_{x}$\textit{MnO}$_{3}$ ($x$ = 0, 0.4, 0.5, 0.6, 0.7, 0.8, 1). The \textit{T-B-$\theta $} phase diagram of the ferrielectric material \textit{Ho}$_{1-x}Y_{x}$\textit{MnO}$_{3}$ has been determined from the dielectric constant angular dependence between the crystallographic $c$-axis and applied magnetic field. The re-entrant temperature-magnetic field phase transitions which involve in- plane \textit{Mn} spin rotations in the antiferromagnetic state below the N\'{e}el temperature are driven by the interaction with the \textit{Ho} subsystem. We describe this behavior in terms of the interaction of the \textit{Ho} sublattice spin system with the underlying, robust \textit{YMnO}$_{3}$ antiferromagnetic triangular lattice, where the \textit{Ho}-spin interactions are highly sensitive to $Y$ concentration and field direction. The magnetic field anisotropy study is an important step towards understanding of magnetic and electric phase competition in the diluted $4f$ system by non-magnetic Yttrium($Y)$. [Preview Abstract] |
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