Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X17: Focus Session: Magnetic Oxide Thin Films - Multiferroic Heterostructures and Europium Oxide |
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Sponsoring Units: GMAG DMP Chair: Tiffany Santos, Argonne National Laboratory Room: D174 |
Thursday, March 24, 2011 2:30PM - 3:06PM |
X17.00001: Electric field control of magnetism in multiferroic heterostructures Invited Speaker: Much interest is being devoted to designing systems where magnetic and ferroelectric orders coexist (multiferroics), and where the presence of magnetoelectric coupling could enable the electrostatic control of magnetism in the solid state. In particular, proximity effects can be tailored to design novel electronic structures with enhanced magnetoelectric couplings in composite heterostructures [1]. A striking example of this approach is our recent demonstration of a large, charge-mediated, magnetoelectric coupling in epitaxial PZT/LSMO heterostructures [2], which explores the sensitivity of the magnetic properties of the doped manganites to charge. Through magnetic, electric, structural and spectroscopic characterization, we demonstrate that the magnetoelectric coupling in PZT/LSMO heterostructures is electronic in origin, and results from the modulation in the valency of the Mn upon switching the PZT ferroelectric polarization [3]. In particular, we conclude that the interfacial spin ordering is modified upon charge doping, which explains the large magnetoelectric response found in this system [4]. This ability to control spin via electric fields opens a new pathway for the development of novel spin-based technologies. \\[4pt] [1] Vaz et al. Adv. Mater. 22:2900, 2010.\\[0pt] [2] Molegraaf et al. Adv. Mater. 21:3470, 2009.\\[0pt] [3] Vaz et al. Phys. Rev. Lett., 104:127202, 2010.\\[0pt] [4] Vaz et al. Appl. Phys. Lett., 97:042506, 2010. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X17.00002: Ferroelectric field effect modulation of magnetism in composite multiferroics Jason Hoffman, Carlos Vaz, Yaron Segal, Matthew Marshall, Fred Walker, Charles Ahn This work harnesses the strong charge-driven magnetoelectric coupling in Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_{3}$/La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ (PZT/LSMO) heterostructures to explore the sensitivity to charge of the electron transport and magnetic behavior of complex oxides. Epitaxial LSMO films that exhibit a highly ordered crystalline structure, as determined by \emph{in situ} reflection high energy electron diffraction (RHEED) measurements and \emph{ex situ} x-ray diffraction and transmission electron microscopy are grown by oxide molecular beam epitaxy. Off-axis RF magnetron sputtering is used to grow the PZT gate dielectric, which is characterized by square polarization-electric field hysteresis loops, with a large polarization and low leakage current. We use a combination of low-field magneto-transport and magneto-optic Kerr effect (MOKE) magnetometry to study the ferroelectric field effect induced changes in the magnetic coercive field in PZT/LSMO bilayers. A reversible shift in the coercive field is observed for the two polarization states of the ferroelectric, with a larger coercive field in polarization state that accumulates hole carriers at the PZT/LSMO interface. The reversible electrical control of magnetism in engineered heterostructures is opens new directions in the field of spintronics. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X17.00003: Pulsed Laser Deposition of Cr$_{2-x}$Fe$_{x}$TeO$_{6}$ Thin Film Junlei Wang, Kirill D. Belashchenko, Peter A. Dowben, Christian Binek Promising spintronic concepts such as Cr$_{2}$O$_{3}$ based voltage-controlled exchange bias system [1] employ electric controlled boundary magnetization. Symmetry arguments reveal that equilibrium boundary magnetization is a generic property of magnetoelectric antiferromagnets [2]. However, experimental evidence of the boundary magnetization is scarce. Here we explore non-traditional growth of magnetoelectric oxides with tri-rutile structure using pulsed laser deposition (PLD) methodology. We grow and characterize structurally and magnetically various magnetoelectric thin films of the Cr$_{2-x}$Fe$_{x}$TeO$_{6}$ family starting from x=2 in order to take advantage of the reduced chemical complexity of Fe$_{2}$TeO$_{6}$ and the beneficial high temperature onset of antiferromagnetic order at 230K in comparison to 90K of Cr$_{2}$TeO$_{6}$. Our investigation aims on an experimental test of the predicted generality of the equilibrium boundary magnetization in magnetoelectric antiferromagnets. \\[4pt] [1]. He, Xi et al., Nature Materials 9, 579 - 585 (2010) \\[0pt] [2]. Belashchenko, K.D., Phys. Rev. Lett. 105, 147204 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X17.00004: Properties of the Predicted Multiferroic Ca$_{3}$Mn$_{2}$O$_{7}$ - Experiment R. Misra, C. Adamo, N.A. Benedek, S.A. Denev, A. SenGupta, J.A. Mundy, J.H. Lee, D.A. Muller, V. Gopalan, C.J. Fennie, D.G. Schlom, P. Schiffer We have studied the properties of epitaxial films of Ca$_{3}$Mn$_{2}$O$_{7}$, an $n=2$ Ruddlesden-Popper phase. This material has been predicted to have novel multiferroic properties, including electric field switching of the magnetization [1]. 50 nm thick unstrained Ca$_{3}$Mn$_{2}$O$_{7}$ films were grown by reactive MBE on (110) YAlO$_{3}$ single crystal substrates. XRD shows that the Ca$_{3}$Mn$_{2}$O$_{7}$ films are single phase and epitaxial with (001) Ca$_{3}$Mn$_{2}$O$_{7}$ // (110) YAlO$_{3}$. Our films show a transition to a weakly ferromagnetic or canted antiferromagnetic state below 120K. The magnetic properties have strong anisotropy with a clear transition visible with an in-plane applied field, but none along the out of plane direction. Second harmonic generation results show that a weak polar order exists at room temperature and it persists until $\sim $700$^{\circ}$C. We also report on the low temperature dielectric properties of the material. \\[4pt] [1] N. A. Benedek and C. J. Fennie, arXiv:1007.1003v1. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X17.00005: Robust isothermal electric control of exchange bias at room temperature Invited Speaker: Voltage-controlled spintronics is of particular importance to continue progress in information technology through reduced power consumption, enhanced processing speed, integration density, and functionality in comparison with present day CMOS electronics. Almost all existing and prototypical solid-state spintronic devices rely on tailored interface magnetism, enabling spin-selective transmission or scattering of electrons. Controlling magnetism at thin-film interfaces, preferably by purely electrical means, is a key challenge to better spintronics. Currently, most attempts to electrically control magnetism focus on potentially large magnetoelectric effects of multiferroics. We report on our interest in magnetoelectric Cr$_{2}$O$_{3}$ (chromia). Robust isothermal electric control of exchange bias is achieved at room temperature in perpendicular anisotropic Cr$_{2}$O$_{3}$(0001)/CoPd exchange bias heterostructures. This discovery promises significant implications for potential spintronics. From the perspective of basic science, our finding serves as macroscopic evidence for roughness-insensitive and electrically controllable equilibrium boundary magnetization in magnetoelectric antiferromagnets. The latter evolves at chromia (0001) surfaces and interfaces when chromia is in one of its two degenerate antiferromagnetic single domain states selected via magnetoelectric annealing. Theoretical insight into the boundary magnetization and its role in electrically controlled exchange bias is gained from first-principles calculations and general symmetry arguments. Measurements of spin-resolved ultraviolet photoemission, magnetometry at Cr$_{2}$O$_{3}$(0001) surfaces, and detailed investigations of the unique exchange bias properties of Cr$_{2}$O$_{3}$(0001)/CoPd including its electric controllability provide macroscopically averaged information about the boundary magnetization of chromia. Laterally resolved X-ray PEEM and temperature dependent MFM reveal detailed microscopic information of the chromia (0001) surface magnetization and provide a coherent interpretation of our results on robust isothermal electric control of exchange bias. The latter promise a new route towards purely voltage-controlled spintronics and an exciting way to electrically control magnetism. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X17.00006: Europium Chalcogenide Magnetic Semiconducting Nanocrystals Invited Speaker: This abstract not available. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X17.00007: First-principles analysis of magnetic interaction in electron-doped EuO Joonhee An, Kirill Belashchenko Using linear response calculations within the linear muffin-tin orbital method, we analyze the exchange interaction in electron-doped EuO. The 4f shell is treated within the LDA+U method. Calculations in the virtual crystal approximation show that the RKKY interaction mediated by the conduction band qualitatively explains the observed doping dependence of the Curie temperature in EuO. Further, to understand the role of a particular rare earth dopant, we consider EuO supercells with a substitutional Gd atom, as well as with an oxygen vacancy. Important differences with the virtual crystal approximation are found. The behavior of the exchange interaction in real space is analyzed, and its mechanisms are sorted out. The applicability of the magnetic polaron picture to Gd-doped EuO is evaluated. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X17.00008: Curie temperature of electron-doped EuO - is there an intrinsic limit? A. Schmehl, T. Mairoser, A. Melville, T. Heeg, L. Canella, P. B\"oni, W. Zander, J. Schubert, D.E. Shai, E.J. Monkman, K. M. Shen, D.G. Schlom, J. Mannhart Increasing the Curie temperature ($T_{C})$ of the ferromagnetic semiconductor europium monoxide is the key problem to make this versatile material attractive for wide use. Its half-metallic behavior and its structural and electronic compatibility with Si, GaN and GaAs make EuO a promising material for semiconductor-based spintronics. By doping EuO with donor impurities, $T_{C}$ can substantially be increased. This increase is attributed to an additional exchange interaction that is mediated via the conduction electrons. Here we report on Hall measurements on Gd doped EuO films grown over a wide range of doping concentrations and growth conditions. We demonstrate that only a small fraction of the introduced impurities actually act as donors even for optimized growth parameters. Too high growth temperatures even render the dopants completely inactive. These results open the exciting question, if further raising the charge carrier density will elevate the Curie temperature way above today's maximum value of 170 K. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X17.00009: The Effect of Lu Doping on Ferromagnetic EuO Alexander Melville, Thomas Mairoser, Andreas Schmehl, Jochen Mannhart, Darrell Schlom Europium Oxide (EuO) is a poorly understood ferromagnetic semiconductor whose spin-ordering temperature (T$_{C})$ can be greatly influenced by the inclusion of dopants such as oxygen vacancies or one of several trivalent ions. The ability to grow high-quality crystalline and stoichiometric EuO by adsorption-controlled growth using molecular-beam epitaxy is imperative in separating the effect of oxygen vacancies from that of trivalent dopants. In this study, we have prepared 5{\%} Lu-doped EuO and characterized the effects of this doping on the magnetic and electronic properties. We show for the first time that Lu is a viable dopant material for EuO, increasing the T$_{C}$ up to 120K as a result of an increase in the carrier concentration to 1.8x10$^{26 }$m$^{-3}$ from 1.0x10$^{23 }$m$^{-3}$. This is on par with other EuO films grown in an adsorption-controlled environment and doped with La or Gd. Furthermore, we find that EuO maintains a high spin-polarization ($>$80{\%}) at this doping level. As a result of the simultaneously high T$_{C}$ and high spin-polarization, EuO can be considered for spintronic applications at much higher temperatures than possible for undoped EuO. [Preview Abstract] |
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