Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q1: Magnetic Tunnel Junctions
Session Chairs
Tuesday PM, April 18, 2006
Room 3020 (Moscone West)
9:30 AM - **Q1.1
Giant Tunneling Magnetoresistance and Tunneling Spin Polarization in Magnetic Tunnel Junctions Using MgO Tunnel Barriers.
Stuart Parkin 1 , See-Hun Yang 1 , Hyunsoo Yang 1 , Christian Kaiser 1 , Brian Hughes 1 , Phil Rice 1
1 , IBM Almaden Research Center, San Jose, California, United States
Show AbstractRecent advances in generating, manipulating and detecting spin-polarized electrons and electrical current make possible new classes of spin based sensor, memory and logic devices [1]. One key component of many such devices is the magnetic tunneling junction (MTJ) - a sandwich of thin layers of metallic ferromagnetic electrodes separated by a tunneling barrier, typically an oxide material only a few atoms thick. The magnitude of the tunneling current passing through the barrier can be adjusted by varying the relative magnetic orientation of the adjacent ferromagnetic layers. As a result, MTJs can be used to sense the magnitude of magnetic fields or to store information. The electronic structure of the ferromagnet together with that of the insulator determines the spin polarization of the current through an MTJ -- the ratio of 'up' to 'down' spin electrons. Using conventional amorphous alumina tunnel barriers tunneling spin polarization (TSP) values of up to ~55% are found for conventional 3d ferromagnets, such as CoFe, but using highly textured crystalline MgO tunnel barriers TSP values of more than 92% can be achieved for otherwise the same ferromagnet [2]. Such TSP values rival those previously observed only with half-metallic ferromagnets. Corresponding giant values of tunneling magnetoresistance are found, exceeding 350% at room temperature and nearly 600% at 3K. Perhaps surprisingly the MgO tunnel barrier can be quite rough: its thickness depends on the local crystalline texture of the barrier, which itself is influenced by structural defects in the underlayer. The tunneling probability in an MTJ may depend on the symmetry of the electronic wave-functions which can lead to spin filtering, as theoretically predicted for Fe/MgO/Fe [3], when the symmetry of the majority and minority bands are distinct. The tunneling probability is also strongly influenced by chemical bonding at the ferromagnet/ barrier interface so accounting, for example, for little change of TSP and TMR when Co is diluted with significant amounts of Pt [4] and for high tunneling spin polarization in rare-earth transition-metal alloys with nearly zero magnetization [5]. 1S. Parkin, X. Jiang, C. Kaiser, A. Panchula, K. Roche, and M. Samant, Proc. IEEE 91, 661 (2003).2S. S. P. Parkin, C. Kaiser, A. Panchula, P. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Mater. 3, 862-867 (2004).3W. H. Butler, X.-G. Zhang, T. C. Schulthess, and J. M. MacLaren, Phys. Rev. B 63, 054416 (2001).4C. Kaiser, S. van Dijken, S.-H. Yang, H. Yang, and S. S. P. Parkin, Phys. Rev. Lett. 94, 247203 (2005).5C. Kaiser, A. F. Panchula, and S. S. P. Parkin, Phys. Rev. Lett. 95, 047202 (2005).
10:00 AM - Q1.2
How Many Crystalline Interface Layers are Necessary to Create High TMR?
Christian Heiliger 1 , Peter Zahn 1 , Ingrid Mertig 1
1 Department of Physics, Martin-Luther-University Halle-Wittenberg, Halle Germany
Show AbstractRecent experiments [1,2,3] based on epitaxially grown Fe/MgO/Fe samples shed light on the subject of tunneling magnetoresistance (TMR). First of all, the obtained TMR ratios exceed the predictions by Julliere's model [4]. Second, experimentally obtained bias voltage characteristics show features which could be related to the electronic structure of the system in the ballistic limit of tunneling. The high crystallinity of the samples [1,2,3] seemed to be the reason. New experiments [5], however, demonstrate that even amorphous electrodes attached to a crystalline MgO barrier show a TMR of more than 130%. The question that is addressed in this talk is: How many crystalline metal layers close to the interface are necessary to obtain high TMR.A screened Korringa-Kohn-Rostoker (KKR) method based on density functional theory was applied to calculate the electronic and magnetic structure of the different junctions self-consistently. The Landauer conductance of planar junctions was calculated using the Baranger-Stone scheme by means of Green's functions in the limit of coherent tunneling.[1] J. Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, Appl. Phys. Lett. 82, 4507 (2003).[2] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Materials 3, 868 (2004).[3] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials 3, 862 (2004).[4] M. Julliere, Phys. Lett. 54A, 225 (1975)[5] K. Tsunekawa, D. D. Djayaprawira, M. Nagai, H. Maehara, S. Yamagata, N. Watanabe, S. Yuasa, Y. Suzuki, and K. Ando, Appl. Phys. Lett. 87, 072503 (2005)
10:15 AM - Q1.3
Finite Bias Calculations on Electronic Transport Through Fe/MgO/FeTunneling Junctions
Ivan Rungger 1 , Oleg Mryasov 2 , Alexandre Rocha 1 , Olle Heinonen 2 , Stefano Sanvito 1
1 School of Physics, Trinity College Dublin, Dublin Ireland, 2 , Seagate Research, Pittsburgh, Pennsylvania, United States
Show AbstractIn this work we calculate the I-V characteristics of Fe/MgO/Fe(100) tunneling junctions using our recently developed code Smeagol [1]. Smeagol interfaces the non-equilibrium Green's functions (NEGF) method with density functional theory (DFT) using the numerical implementation contained in the SIESTA code [2], which is based on a linear combination of atomic orbitals basis set. Smeagol calculates self-consistently the current for a finite applied bias, and it has been tailored for simulations of magnetic materials.First we investigate the zero-bias transmission coefficients T(E) as a function of the in-plane k-vector and the energy for either collinear parallel or antiparallel configurations of the magnetic leads, as well as for non-collinear orientations. Then the zero bias calculations are compared to the self-consistent calculations at finite bias, and the charging behavior of the interface is analyzed. It has been found that for small biases the minority current in the parallel configuration is mainly carried by surface states, but that it saturates for voltages above about 20 meV. In contrast, for the other spin channels of the different leads configurations the zero-bias predictions are found to be a good approximation of the self-consistent finite bias calculations.Furthermore we analyze the effect of geometrical asymmetry due to either partial asymmetric interface oxidation or due to asymmetric lattice distortions. It has been shown that these asymmetries are important for the understanding of the observed I-V curves. Finally we investigate how above results are sensitive to the inclusion of a pseudo self-interaction correction for the standard LDA exchange and correlation potential. This correction increases the bandgap of MgO compared to the LDA result, bringing it very close to the experimental value.References[1] A. R. Rocha, V. M. Garcia-Suarez, S. W. Bailey, C. J. Lambert, J. Ferrer and S. Sanvito, cond-mat/0510083 (2005)[2] J. M. Soler, E. Artacho, J. D. Gale, A. Garcia, J. Junquera, P. Ordejon and D. Sanchez-Portal, J. Phys.: Condens. Matter 14, 2745 (2002)
10:30 AM - **Q1.4
Tunnel Conductance Oscillation Induced by the Quantum Well in fully Epitaxial Double Barrier Magnetic Tunnel Junctions.
Koichiro Inomata 1 2 , Takayuki Nozaki 1 , Nobuki Tezuka 1 2
1 Materials Science, Tohoku University, Sendai, Miyagi, Japan, 2 CREST, JST, Kawaguchi, Saitama, Japan
Show AbstractQuantum oscillation of the tunneling conductance was observed in fully epitaxial double barrier magnetic tunnel junctions (DMTJs) consisting of a MgO seed layer (10)/Fe (50)/MgO(2)/Fe(t)/MgO(2)/Fe(15)(the numbers are film thickness in nm), deposited on a single crystal MgO(001) substrate using the molecular beam epitaxy (MBE)[1]. The designed thickness of a middle Fe layer was varied from 1.0 nm to 1.5 nm. All layers were evaporated by electron-beam bombardment at room temperature (RT) and annealed at 300 C for the bottom Fe electrode and 200 C for the middle and top Fe electrodes for 20 min. in order to improve the morphology of the layer surface. The epitaxial growth of each layer was confirmed by observing the reflection high-energy electron diffraction (RHEED) patterns during deposition and after the annealing treatment. The cross-sectional transmission electron microscopy (TEM) exhibits the layered isolated islands of 10 ~ 40 nm in diameter and 5 nm in height for the middle Fe layer, which is about three times thicker than the designed thickness. We confirmed that the Fe islands grow epitaxially on the MgO barrier with very flat interface, demonstrating parallel-connected tunneling for the DMTJs. We have observed the tunnel magnetoresistance (TMR) of 110% and 135% at 5K and RT, respectively, which are comparable to that for the single magnetic tunnel junctions (SMTJs). The large TMR in the DMTJs over the value estimated by the Julliere model using the Fe spin polarization is consistent with the coherent tunneling as in the SMTJs using a MgO barrier. The conductance of the DMTJs oscillates as a function of the bias voltage in the positive bias direction for various middle Fe layer thicknesses. The bias voltage where the conductance shows maximum changes into a higher value with decreasing the middle layer thickness. We can observe the oscillatory feature even at RT for the DMTJ with t = 1.2 nm (actual thickness is estimated to be about 2 nm from the cross-sectional TEM analysis) in the positive bias direction, although the oscillation amplitude attenuates with increasing temperature. For the case of t = 1.5 nm, on the other hand, the oscillatory feature disappears at RT due to the small energy gap between the quantum levels. These characteristics demonstrate the creation of the QW states in the middle Fe layer. The conductance oscillation was observed only in the parallel magnetization configuration. This result can be explained by the QW states formed in only one of the two spin bands, because in our structure only up spin electrons of Δ1 band in the Fe(001) electrode is expected to form the QW states due to the absence of the down spin Δ1 states at the Fermi energy. Further improvement of the quality of the quantum well layer will provides us the realization of the spin-dependent resonant tunneling effect.Reference[1]T. Nozaki, S. Nakamura, N. Tezuka and K. Inomata, Phys. Rev. Lett. submitted
11:30 AM - **Q1.5
Magnetic Materials for High-Performance Toggle MRAM
Jon Slaughter 1 , R. Dave 1 , S. Pietambaram 1 , J. Sun 1 , G. Grynkewich 1 , M. DeHerrera 1 , K. Smith 1 , N. Rizzo 1 , S. Tehrani 1
1 Technology Solutions Organization, Freescale Semiconductor, Inc., Chandler, Arizona, United States
Show AbstractMagnetoresistive random access memory (MRAM) employs a magnetoresistive device integrated with standard silicon-based microelectronics, resulting in a combination of qualities not found in other memory technologies. For example, MRAM is non-volatile, has unlimited read and write endurance, and has demonstrated high-speed read and write operations. Here we present an overview of the characteristics of our 4Mb Toggle-MRAM circuit based on magnetic tunnel junction (MTJ) devices, and outline paths for improving performance and scaling to higher densities. Specific technology demonstrations for improving performance and scalability of both the read and write operations will be presented. For example, high-MR, MgO-based tunnel junction material has been integrated with 180nm and 90nm CMOS circuitry to improve the read performance. The high MR values obtainable with MgO-based MTJ devices can enable significant read access time improvements, but are useful only if all the other requirements that enable fully functional MRAM circuits are met. These requirements include: narrow bit-to-bit resistance distributions, free-layer magnetic properties that produce narrow switching distributions, low interlayer coupling, good bias dependence, reliability, and thermal endurance. Results for MgO tunnel barriers grown with different processes show how the barrier structure affects the growth of the subsequently deposited magnetic material. Tests comparing properties of the 4Mb arrays with MgO-based and AlOx-based MTJ material show improved useable signal with MgO, although this polycrystalline barrier results in wider distributions for several array parameters. Paths to reduced power consumption that employ novel magnetic materials also will be presented.
12:00 PM - **Q1.6
STM, XPS and Spin-Torque Studies of Magnetic Tunnel Junctions
Robert Buhrman 1
1 School of Applied and Engineering Physics, Cornell University, Ithaca, New York, United States
Show AbstractThe recent advances in magnetic tunnel junction (MTJ) technology together with the demonstration of the ability of spin-polarized currents to excite and switch nanomagnets have opened up a wide range of new opportunities regarding the study and technological applications of nanomagnetism and metallic spintronics. To further advance these prospects we need new and more detailed understanding and enhanced control of these MTJ materials systems, and also of the details of spin transport through these tunnel barriers. In this presentation I will discuss results from recent STM and XPS studies in our laboratory of the chemical and electronic structure of half-formed and fully-formed magnetic tunnel junctions, which identify the nature of the electronic defects in these ultra-high oxide barrier layers and point to possible pathways for further improvement. I will also report on spin-torque experiments with MTJs which have provided a new approach to the measurement and understanding of the bias dependence of spin transport and TMR. The spin torque results demonstrate that the polarization of the tunnel current does not, at least in the systems we have studied, decrease strongly with bias, contrary to the bias behavior of the TMR. This result is quite positive for spin-torque applications of MTJs and is also consistent with the electronic structure of ultra-thin tunnel barriers as revealed by the STS and XPS studies.
12:30 PM - **Q1.7
Novel Transport Phenomena in Ballistic Conductors and Tunnel Junctions.
Evgeny Tsymbal 1
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractIn this talk we overview our recent results on modeling transport phenomena in ballistic conductors and tunnel junctions. For ferromagnetic ballistic conductors we predict that electronic transport exhibits ballistic anisotropic magnetoresistance (BAMR) – a change in the ballistic conductance with the direction of magnetization [1]. This phenomenon originates from the effect of the spin-orbit interaction on the electronic band structure which leads to a change in the number of bands crossing the Fermi energy when the magnetization direction changes. For epitaxial Co/SrTiO3/Co magnetic tunnel junctions with bcc Co(001) electrodes we predict a very large tunneling magnetoresistance(TMR), originating from a mismatch of majority- and minority-spin states contributing to the conductance [2]. In agreement with the experimental data, we find that the spin polarization of the tunneling current across the Co/SrTiO3 is negative. We attribute this property to the complex band structure of SrTiO3 which is formed from localized 3d states of Ti and, unlike MgO- and Al2O3-based MTJs, allows efficient tunneling of the minority d electrons of Co. Stimulated by experimental observations of ferroelectricity in thin films of a nanometer thickness, we consider a new type of tunnel barriers based on ferroelectric materials. We predict that in such ferroelectric tunnel junctions (FTJs) the conductance depends strongly on the direction of the electric polarization [3]. This giant electroresistance effect is the consequence of a different potential profile seen by transport electrons for the two opposite polarization orientations. Using a ferroelectric barrier in a magnetic tunnel junction makes it multiferroic, where ferromagnetic electrodes are separated by a ferroelectric barrier. Multiferroic tunnel junctions (MFTJs) have the potential to provide an additional degree of freedom in controlling the conductance. We discuss possible implications following from the interplay between ferroelectric and ferromagnetic properties of the two ferroic constituents in these junctions. This work is supported by NSF and Nebraska Research Initiative. [1] J. Velev, R. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 94, 127203 (2005).[2] J. Velev, K. D. Belashchenko, D. Stewart, M. van Schilfgaarde, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 95 (2005), in press.[3] M. Ye. Zhuravlev, R. F. Sabirianov, S. S. Jaswal, and E. Y. Tsymbal, Phys. Rev. Lett. 94, 246802 (2005).
Q2: Ultrafast Magnetization Dynamics and Damping
Session Chairs
Tuesday PM, April 18, 2006
Room 3020 (Moscone West)
2:30 PM - **Q2.1
Spin Torques and Pumping in Itinerant Ferromagnets: From Heterostructures to Bulk Dynamics.
Yaroslav Tserkovnyak 1 , Arne Brataas 2 , Gerrit Bauer 3
1 Physics Department, Harvard University, Cambridge, Massachusetts, United States, 2 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway, 3 Kavli Institute of NanoScience, Delft University of Technology, Delft Netherlands
Show Abstract3:00 PM - Q2.2
Modeling The Common Origin Of Fs Demagnetization And Gilbert Damping.
Francesco Dalla Longa 1 , Bert Koopmans 1 , Wim de Jonge 1
1 Dept. of Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractAll-optical techniques exploiting femtosecond laser pulses have opened the way towards the exploration of the ultimate limits of magnetization dynamics. It has been found that magnetic order in ferromagnetic transition metals can be quenched within a few hundred femtoseconds after laser heating. However, the microscopic interpretation of the phenomena at the sub-ps level have been a mystery until recently. In this presentation we present a microscopic model that successfully explains the ultrafast equilibration of magnetic order in ferromagnetic metals at a time scale τM of only a few hundred femtoseconds after pulsed laser excitation. We found that τM can be directly related to the so-called Gilbert damping factor α that describes damping of GHz precessional motion of the magnetization vector, thereby unifying two apparently unrelated issues in applied magnetism. The crucial ingredient in our approach is the inclusion of spin-flip processes accompanying momentum scattering with impurities or phonons. A simple model Hamiltonian is used to derive analytical expressions for both the Gilbert damping and the demagnetization. Independent of the spin-scattering mechanism, an appealingly simple equation relating the two key parameters via the Curie temperature TC is derived, τM ~ c0h / 2πkTC α, with h and k the Planck and Boltzmann constant, resp., and the prefactor c0 ~ 1/4. This readily predicts a demagnetization time of ~ 100 fs for reasonable values of Gilbert damping in ferromagnetic nickel. A comparison with experiments, in which both parameters are measured on the same sample, will be discussed.Results from numerical extensions of the model, incorporating an applied magnetic field and heat diffusion out of the irradiated volume, will be presented as well. In particular, this approach allows us to simulate ultrafast laser-induced reversal of magnetization in a reverse magnetic field. Our results demonstrate that phonon-mediated spin-flip scattering is a good candidate to explain the sub-ps magnetic response. However, although the lattice is an essential ingredient, it is found that the spin temperature is following the electron temperature (rather than the lattice temperature), and that reversal can be achieved even though the lattice temperature stays below the Curie temperature. Having established this crucial insight, a wide range of future experimental investigations can be envisioned – as presently being implemented.
3:15 PM - Q2.3
Reduced Spin-lattice Coupling and Gilbert Damping in Epitaxial Fe1-xVx (50nm) Thin Films.
Lili Cheng 1 , Christian Scheck 1 , Yong Feng Guan 1 , William Bailey 1
1 APAM, Columbia University, New York, New York, United States
Show AbstractThe reduction of Gilbert damping in magnetic thin films is an important materials goal for several applications in >1 GHz spin electronics, improving the signal to noise ratio (SNR) in nanoscale GMR sensors and the efficiency in spin momentum transfer (SMT) devices. Fe-low Z alloys, with average composition near Z = 25, are of potential interest for reduced damping due to an expected reduction in spin-lattice coupling, according to previous Einstein-de Haas experiments. We have deposited epitaxial MgO(100)/Fe1-xVx thin films by ultra high vacuum (UHV) cosputtering at a base pressure of 2×10-9 Torr, with Vanadium concentrations x up to 52%, spanning the Z = 25 composition at x = 33%. The GHz relaxation rate (damping) and anisotropy constants were measured using broadband (0–40 GHz) and angle-dependent X–band (10GHz) ferromagnetic resonance (FMR). Relaxation rates were measured directly by the frequency-swept FMR linewidth Df; these drop from ~ 270 MHz for pure Fe to 200 MHz for Fe67V33 over the range of 4 to 14 GHz. 10 GHz field-swept linewidths for the pure Fe films are measured as low as DHpp = 29 Oe, close to the lowest literature measured values, indicating a minor role for inhomogenous (extrinsic) broadening/relaxation. In angle-dependent measurements, we find a dramatic reduction in the cubic magnetocrystalline anisotropy constant K1 up to 42% V; K1 drops by nearly two orders of magnitude from 4.8×105 erg/cm3 for pure Fe to 1.5×104 erg/cm3 for Fe58V42. The concurrent reduction of relaxation rate and magnetocrystalline anisotropy is suggestive of a common origin in reduced spin-lattice coupling for the alloy.
3:30 PM - Q2.4
Spin-torque Effects in Single-crystalline Fe Nanomagnets and Nanopillars.
Henning Dassow 1 2 , Ronald Lehndorff 1 2 , Daniel Buergler 1 2 , Matthias Buchmeier 1 2 , Peter Gruenberg 1 2 , Claus Schneider 1 2
1 Institut fuer Festkoerperforschung, Forschungszentrum Juelich GmbH , Juelich Germany, 2 cni - Center of Nanoelectronic Systems for Information Technology, Forschungszentrum Juelich GmbH , Juelich Germany
Show Abstract4:15 PM - **Q2.5
Ultrafast Magnetization Dynamics, a New Frontier in X-ray Science
Andreas Scholl 1
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractUltrafast magnetic writing using field and optical pulses has caught the interest of scientists because of the fascinating physics underlying the exchange of energy and angular momentum in a magnetic material and because of possible applications in data storage. Experiments in ultrafast magnetism put stringent requirements onto the experimental technique, both in terms of temporal and of spatial resolution. The availability of pulsed x-ray sources and fast detectors have now opened the door to such studies using x rays as a probe.First, I will discuss time-resolved imaging of magnetic patterns with 100 nm spatial resolution using the Photoemission Electron Microscope PEEM-2 at beamline 7.3.1.1 of the Advanced Light Source [1]. The bunch length of the storage ring sets the time resolution of 80 ps. Magnetic vortices appear in soft-magnetic micron-size structures and are characterized by a curling magnetization. We observed that the chirality or handedness of the vortex, which is determined by the out-of-plane magnetization of the nanometer-size vortex core, governed the dynamics of the structure in response to fast field pulses. The field pulses were generated by a laser-triggered Auston switch.On a faster time scale, magnetization dynamics experiments often rely on direct laser excitation of the material. Experiments using the time-resolved magneto-optical Kerr effect (TR-MOKE) [2] demonstrated the possibility of manipulating magnetism using a femtosecond laser pulse. The origin and the implications of the fsec-laser-driven demagnetization of Ni have been widely debated. X-ray magnetic dichroism is an ideal tool to probe such dynamics in complex materials because x-ray sum rules quantify spin moment, orbital moment and magnetic anisotropy of each element in a multi-element system. Furthermore, x-rays are sensitive to both ferromagnetic and antiferromagnetic order. I will present measurements of the transient spin and orbital moment of Fe/Gd following a f-sec laser excitation. The experiments were conducted using an x-ray streak camera with a temporal resolution of 2 ps at BL 4.0.2 of the Advanced Light Source.This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 at Lawrence Berkeley National Laboratory.[1] S.B. Choe et al., Science 304, 420 (2004)[2] E. Beaurepaire et al., Phys. Rev. Lett. 76, 4250 (1996)
4:45 PM - Q2.6
Measurement of Ferromagnetic Resonance (FMR) by Time-resolved XMCD: Element- and Layer-resolved Magnetization Dynamics at 2 ps and 0.1 deg. Resolution.
Yongfeng Guan 1 , William Bailey 2 1 , Chi-Chang Kao 3 , Elio Vescovo 3 , Dario Arena 3
1 Applied Physics Program, Department of Applied Physics, Columbia University, New York, New York, United States, 2 Materials Science Program, Department of Applied Physics, Columbia University, New York, New York, United States, 3 National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, United States
Show AbstractWe demonstrate an x-ray magnetooptical measurement of ferromagnetic resonance (FMR), enabling highly sensitive studies of the coupled precession of elements in an alloy and layers in a heterostructure. Small-angle (< 1.0 deg.) precession could be measured magnetooptically for the first time due to the high magnetic contrast of time-resolved x-ray magnetic circular dichroism (tr-XMCD) in the transmission geometry. Basic expectations of driven FMR precession could be verified in Ni81Fe19, driven to ~ 0.7 deg. cone angles at 2.3 GHz. Weak ferromagnetic coupling (~ 5 Oe), on the verge of detectability in variable-frequency FMR measurements, could be revealed clearly in the phase and amplitude of the layer-resolved in a Ni81Fe19(25 nm)/Cu(20 nm)/Co93Zr7(25 nm) trilayer precession. The technique enables studies of the microscopic mechanisms in relaxation / damping, either in complex materials or heterostructures.
5:00 PM - Q2.7
Interface Magnetization Precession and Switching in Fe/AlGaAs (001).
Gunter Luepke 1 , Haibin Zhao 1 , Diyar Talbayev 1 , Aubrey Hanbicki 2 , Conny Li 2 , George Kioseoglou 2 , Berry Jonker 2
1 Applied Science, College of William and Mary, Williamsburg, Virginia, United States, 2 , Naval Research Laboratory, Washington, District of Columbia, United States
Show Abstract5:15 PM - Q2.8
Field-driven Crossover in Precesssional Dynamics of Lateral Ferromagnetic Heterostructures.
Nikolay Polushkin 1 , Steven Michalski 1 , Lanping Yue 1 , Roger Kirby 1
1 Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show Abstract5:30 PM - **Q2.9
Spin Dynamics in Magnetic Vortices
Jooho Park 1 , Robert Compton 1 , Mun Chan 1 , Paul Crowell 1
1 Physics, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractA vortex is the simplest topological defect that can form in a magnetic system. We have used time-resolved Kerr microscopy to study vortex dynamics in ferromagnetic disks with diameters from 500 nm to 2 microns and thicknesses from 20 nm to 50 nm. In addition to ordinary spin waves, these systems show a low-frequency excitation corresponding to the translational degree of freedom of the vortex core.[1] We show how the motion of the vortex core couples to the spin wave spectrum, breaking the degeneracies that exist in the case of cylindrical symmetry.[2] I will also discuss recent work on excitations in elliptical particles, focusing on the relation between spin dynamics and inhomogeneous magnetic microstructure. This work was supported by NSF DMR 04-06029 and the NSF MRSEC program under DMR 02-12032.1. J. P. Park, P. Eames, D. M. Engebretson, J. Berezovsky, and P. A. Crowell, Phys. Rev. B 67, 020403R (2003).2. J. P. Park and P. A. Crowell, Phys. Rev. Lett. 95, 167201 (2005).
Q3: Poster Session: Magnetic Materials, Mostly Metallic
Session Chairs
Wednesday AM, April 19, 2006
Salons 8-15 (Marriott)
9:00 PM - Q3.10
Magnetic Properties of Ni Nanoinclusion in Alumina.
Annika Pohl 1 , Claudio Sangregorio 1 , Claudia Innocenti 1
1 Dipartimento di Chimica, Universita' di Firenze, Sesto Fiorentino (Fi) Italy
Show Abstract9:00 PM - Q3.11
Self-Assembled Growth of Ferrimagnetic Spinel Nanometric Pyramids
Florencio Sanchez 1 , Nico Dix 1 , Ulrike Luders 2 , Jean Francois Bobo 2 , Josep Fontcuberta 1
1 , ICMAB-CSIC, Bellaterra Spain, 2 , Onera, Toulouse France
Show AbstractWhereas self-assembled growth of semiconductor materials is widely investigated, the possibilities of complex oxides to grow in such a way are much less explored. However, self-assembled growth of oxides could allow functionalities not attainable with the common device fabrication technology of multilayer deposition and lateral patterning. An outstanding example is the fabrication by a self-assembly technique of arrays of ferrimagnetic spinel CoFe2O4 nanopillars embedded in a ferroelectric BaTiO3 matrix [1]. The nanostructured hybrid system displayed a remarkable coupling of magnetic and ferroelectric properties not observed in ordinary multilayer structures. Understanding the growth mechanisms of these nanocomposites is essential for its controlled fabrication and optimization of properties.Here we report on the self-assembled growth of three-dimensional pyramidal objects in ferrimagnetic spinel, CoCr2O4 and NiFe2O4, epitaxial films. The spinel islands [2] have a shape very similar to the SiGe pyramids and hut clusters, but detailed characterization reveals important dissimilarities. They are {111} faceted pyramids and hut clusters, and thus have a very high aspect ratio, and moreover, they maintain the shape during growth. The objects are perfectly oriented along the <110> directions and have a certain positional order along the same direction. The growth progression was investigated: at early stages strained (dislocation free) small nanometric pyramids form, which is followed by a structural relaxation and a spectacular growth of some of the objects. We demonstrate that the size of the objects (from the nanometer to the micrometer) and area coverage (from isolated pyramids to fully faceted surfaces) can be controlled by the deposition time, growth temperature and substrate used. The driving forces for the observed {111} faceted objects and surfaces, and bi-modal or single-modal size distribution are discussed.Our demonstration of self-organized growth of ferrimagnetic spinel pyramids and hut clusters of controlled size and on different surfaces may open the possibility of controlled fabrication of hybrid systems combining these ferrimagnetic structures with other functional materials. References[1] H. Zheng et al., Science 303, 601 (2004); F. Zavaliche et al., Nano Letters 5, 1793 (2005)[2] U. Lüders et al., Phys. Rev. B, 70, 045403 (2004); Nanotechnology 16, S190 (2005)
9:00 PM - Q3.12
Alignment-Sensitive Reversal Mechanisms of Epitaxial-FeF2/Polycrystalline-Ni Exchange Biased Thin Films
Justin Olamit 1 , Kai Liu 1 , Zhi-Pan Li 2 , Ivan Schuller 2
1 Physics Department, University of California, Davis, California, United States, 2 Physics Department, University of California - San Diego, La Jolla, California, United States
Show AbstractMagnetization reversal mechanisms of epitaxial-FeF2/polycrystalline-Ni exchange biased thin films have been investigated with vector magnetometry on a Vibrating Sample Magnetometer (VSM). The samples have been exchange biased by field cooling along the FeF2 spin axis, the [001] direction. The transverse hysteresis loops – sensitive to magnetic moments perpendicular to the applied field - show that when the applied field is misaligned with the FeF2 spin axis, the reversal is predominantly by rotation [1, 2]. When the applied field is aligned with the spin axis, the transverse loop is flat, indicating predominantly domain formation and motion. We have employed a First Order Reversal Curve (FORC) [3, 4] technique to further investigate the reversal mechanisms. When the applied field is aligned with the spin axis, FORC analysis shows that the magnetization switching is highly irreversible (82%), indicating that domain nucleation and motion is likely the reversal mechanism. With a misalignment of 5°, the FORC shows that majority of the magnetic switching (57%) was by a reversible mechanism, consistent with the rotation seen with vector magnetometry. These results show that the magnetization reversal mechanisms are extremely sensitive to the alignment of the applied field with the antiferromagnet spin axis and the exchange bias direction.*Work supported by ACS-PRF, Alfred P. Sloan Foundation, and DOE.1.Olamit, Arenholz, Li, Petracic, Roshchin, Morales, Batlle, Schuller, and Liu , Phys. Rev. B 72, 012408 (2005).2.Olamit, Li, Schuller, and Liu, Phys. Rev. B, in press.3. Davies, Hellwig, Fullerton, Denbeaux, Kortright and Liu, Phys. Rev. B 70, 224434 (2004). 4. Davies, Hellwig, Fullerton, Jiang, Bader, Zimanyi, and Liu, Appl. Phys. Lett. 86, 262503 (2005).
9:00 PM - Q3.13
Computational and Experimental Evidences for Asymmetric Interfacial Mixing of Co-Al system
Sang-Pil Kim 1 2 , Jae-Young Park 3 , Kwang-Ryeol Lee 1 , Chung-Nam Whang 3 , Yong-Chae Chung 2
1 Future Technology Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 CPRC, Department of Ceramic Engineering, Hanyang University, Seoul Korea (the Republic of), 3 Institute of Physics & Applied Physics, Yonsei University, Seoul Korea (the Republic of)
Show AbstractFor the development of thin film multiplayer devices such as MRAM or GMR/TMR sensors, atomic scale intermixing at the interface should be controlled in a systematic manner. In the present work, we employed classical molecular dynamics simulation to investigate the atomic scale intermixing at room temperature during thin film deposition in Co-Al binary system. When Co atom was deposited on Al(001) surface, atomic intermixing was easily achieved at the interface, and highly ordered CoAl compound phase of B2 structure was formed spontaneously. On the other hand, when Al atom was deposited on Co surface, atomically sharp interface was formed between the deposited Al layer and Co substrate. Using a coaxial impact collision ion scattering spectroscopy (CAICISS) and a magneto-optical Kerr effect (MOKE) measurement, these asymmetric intermixing phenomena were confirmed. Experimentally observed interfacial structures were in good agreement with the simulation results.
9:00 PM - Q3.14
Quadrupole Magnetic Force Microscopy Tip and its Imaging Performance.
Gang Han 2 1 , Yihong Wu 2 , Yuankai Zheng 1
2 Electrical and Computer Engineering, National University of Singapore, Singapore Singapore, 1 , Data storage Institute, Singapore Singapore
Show AbstractMagnetic force microscopy (MFM) has become a standard tool for investigating magnetic nanostructures by detecting the magnetic stray field distribution of a magnetic sample. The research on MFM has been centered on several aspects which include but are not limited to resolution enhancement, stabilization improvement, quantitative image interpretation and tip-sample interaction reduction. So far, many efforts had been made to improve the resolution of MFM through sharpening the tips using different approaches such as attaching carbon nanotubes to the original tips, trimming the tips by focus ion beam, electron beam lithography, and ion beam etching, selective deposition by self-field emission, electron beam irradiation, and focused electron beam decomposition and deposition. We have previously demonstrated that the resolution of the MFM tips can be further improved by using an FM/Ru/FM trilayer as the magnetic coating. The improvement was attributed to the formation of a point-dipole tip because of the antiferromagnetic coupling between the two FM layers, which form a relative smaller effective volume interacting with the sample than that of the single layer tip. In order to further improve the performance of the tip, in this work we reported on the fabrication and testing of a double synthetic tip in which a central thicker FM layer is antiferromagnetically coupled with two thinner FM layers at both sides via an ultrathin Ru layer. The tip was prepared by coating one side of a bare tip with a structure of Ta ( 10nm) / NiFe (2nm) / IrMn (10 nm) / CoFe (4nm) / Ru (0.8nm) / CoFe (10nm) / Ru (0.8nm) /CoFe (4nm) / IrMn (10nm) / Ta (10nm). In this design, it is expected that a quadrupole will be formed at the apex of the tip from the two synthetic structures. Theoretically this kind of tip should have a better resolution because the dipoles due to the two synthetic layers are oriented opposite with each other, leading to a reduction of DC response of the overall tip. Our preliminary results showed that the performance of the tip is comparable to that of the conventional tips. Further experiments are being carried out to optimize the structure of the tips so as to improve its performance.
9:00 PM - Q3.2
Electrodeposition and Characterisation of Fe-Based Oxides for Magnetic Tunnel Junctions.
Chien-Lung Teng 1 , Mary Ryan 1
1 Department of Materials, Imperial College London, London United Kingdom
Show AbstractMagnetite (Fe3O4) is of interest because of fascinating magnetic properties for use in spintronic devices. In this work, polycrystalline and epitaxial Fe3O4 have been electrochemically deposited at low growth temperature (<90o) upon nickel oxides formed on Single-phase rolling-assisted biaxial textured substrates (RABiTS) of pure nickel with {100}<001> crystallographic orientation. The formation of nickel oxides is carried out by means of air-formed and surface oxidation epitaxy (SOE), respectively. XRD pattern shows there is no prefered orientation as Fe3O4 thin films grow on native nickel oxide; Moreover, with SOE it becomes highly out-plane and in-plane oriented due to close lattice matching between epitaxial NiO (a=0.417nm) and Fe3O4 (a=0.840nm). Structurally, AFM and FEGSEM images reveal Fe3O4 thin films are grown continueously and very flat for both cases, apart from loose clusters due to nucleation effect. These clusters agglomerate and become bigger as higher current density applied to substrates, and EDS analysis indicateds stoichiometric composition of clusters is identical with Fe3O4 thin films. In order to form sandwich structure (FM/I/FM), here we propose Fe-based oxides, α-FeOOH (Néel Temperature= 400K) and α-Fe2O3, as insulating/spacer layers. Magnetic properties (TMR vs. H, Resisitivity vs. H and Ms vs. H) for these two magnetic tunnel junctions (MJT's) are examined.
9:00 PM - Q3.3
Thickness and Temperature Ddependent Mmagnetic Properties of Uultrathin Fe/Al Nanostructures.
Ranjeet Brajpuriya 1 , Shilpa Tripathi 1 , Anupam Sharma 1 , T. Shripathi 1 , S.M. Chaudhari 1
1 , UDCSR, Indore India
Show AbstractStudies of magnetic interactions between ferromagnetic films separated by non-magnetic metallic films have been a subject of extensive investigations from both theoretical and experimental point of view. These kinds of artificial structures are expected to provide understanding of surface magnetism and transport phenomena, such as interlayer magnetic-coupling, surface anisotropy, magneto-optical effect and giant magneto-resistance, etc. In this respect, recently Fe/Al multilayer systems have been studied extensively because of their attractive soft magnetic properties such as low coercivity, high remanence and low saturation field, shown to be a good candidate for the possible above applications. Therefore, in this paper, the magnetic properties of electron beam evaporated ultra thin Fe-Al multilayers are studied as a function of Fe layer thickness keeping Al layer thickness constant. To investigate layer structure and surface morphology Grazing incidence X-ray diffraction (GIXRD) and Atomic force microscopy (AFM) techniques have been employed. Structural measurements carried out on structures having lower Fe thickness (≤ 2 nm) show substantial intermixing between Fe and Al layers during deposition. The resulting structures in these cases show loss of periodicity and resemble mostly a single layer composite film consisting of Fe and Al clusters. The magnetic measurement carried out using Vibrating sample magnetometer (VSM) at 300K and100K indicates that all the multilayer samples exhibit soft magnetic properties, having in-plane easy direction of the magnetization. We have also found that coercivity increases at lower temperature for lower Fe thickness (≤ 2 nm) multilayer samples. The observed soft magnetic behaviour in these multilayer samples is explained in terms of a) weak magnetic interactions between crystal grains and their sizes b) morphological changes occurred due to Fe thickness variation in deposited bilayer films and c) formation of non- magnetic Fe-Al phase at the interfaces.
9:00 PM - Q3.4
Alloy Synthesis by Atomic Layer Laminations for Read Sensor Applications.
Chih-Ling Lee 1 , Adrian Devasahayam 2 , Ming Mao 1 , Chih-Ching Hu 2 , Vicent Ip 2 , Piero Sferlazzo 2
1 , Veeco Instruments Inc, Fremont, California, United States, 2 , Veeco Instruments Inc., Plainview, New York, United States
Show Abstract9:00 PM - Q3.5
How Does the Interface Structure Influence the Thickness Dependence of Tunneling Magnetoresistance?
Christian Heiliger 1 , Peter Zahn 1 , Bogdan Yavorsky 1 , Ingrid Mertig 1
1 Department of Physics, Martin-Luther-University Halle-Wittenberg, Halle Germany
Show AbstractNew experiments [1,2,3] based on epitaxially grown Fe/MgO/Fe samples obtained TMR ratios which exceed the predictions by Julliere's model [4]. In addition an oscillating behaviour of the tunneling magnetoresistance (TMR) depending on the barrier thickness was found [1]. The measured bias voltage characteristics show features which could be related to the electronic structure of the system. The aim of our work is to demonstrate the influence of different interface geometries on the thickness dependence of the TMR ratio. One of the considered interface structures was with a mixed Fe-oxide layer on both interfaces, experimentally proven at the bottom and the top Fe electrode [5,6]. A second configuration was an ideal one, where no intermixed layer occurs, which can be produced under oxygen deficiency [2]. To complete this investigation a junction with a mixed Fe-oxide layer and an ideal interface was considered.A screened Korringa-Kohn-Rostoker (KKR) method based on density functional theory was applied to calculate the electronic and magnetic structure of the different junctions self-consistently. The Landauer conductance of planar junctions was calculated using the Baranger-Stone scheme by means of Green's functions in the limit of coherent tunneling. Positive and negative TMR ratios are obtained as a function of interface structure, independent on barrier thickness.[1] S. Yuasa, T. Nagahama, A. Fukushima, Y. Suzuki, and K. Ando, Nature Materials 3, 868 (2004).[2] J. Faure-Vincent, C. Tiusan, E. Jouguelet, F. Canet, M. Sajieddine, C. Bellouard, E. Popova, M. Hehn, F. Montaigne, and A. Schuhl, Appl. Phys. Lett. 82, 4507 (2003).[3] S.S.P. Parkin, C. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, and S.-H. Yang, Nature Materials 3, 862 (2004).[4] M. Julliere, Phys. Lett. 54A, 225 (1975)[5] H. L. Meyerheim, R. Popescu, J. Kirschner, N. Jedrecy, M. Sauvage-Simkin, B. Heinrich, and R. Pinchaux, Phys. Rev. Lett. 87, 076102 (2001). [6] C. Tusche, H. L. Meyerheim, N. Jedrecy, G. Renaud, A. Ernst, J. Henk, P. Bruno, and J. Kirschner, Phys. Rev. Lett. 95, 176101 (2005)
9:00 PM - Q3.6
Epitaxial NiFe/GaAs via Electrochemistry.
Z.L. Bao 1 , A.A. Talin 2 , A. Arrott 1 , Karen Kavanagh 1
1 Physics, Simon Fraser University, Burnaby , British Columbia, Canada, 2 , Sandia National Laboratories, Livermore, California, United States
Show AbstractWe have recently shown that α-Fe films can be grown epitaxially by electrodeposition on (111), (110) as well as (001) GaAs substrates. In this work we report the epitaxial growth of single crystalline FexNi(1-x) films by electrodeposition on n-GaAs (001) oriented substrates from aqueous electrolytes. Iron, nickel, and ammonia sulphate (FeSO4, NiSO4 and (NH4)2SO4) electrolyte mixtures were used at room temperature, under galvanostatic conditions. The films nucleate as islands and relax the mismatch strain before coalescing into continuous layers. Pure Ni (f = 12 % when rotated about the normal by 45°) forms (011) oriented films that are also remarkable in their purity and narrow rocking curve widths. The film compositions, as determined by energy dispersive x-ray and Auger spectroscopies, are a function of the ratio of Fe++ to Ni++ in the electrolyte, consistent with previous literature reports on electrodeposited films. In the presence of FeSO4, Ni-rich FexNi(1-x), nucleates as face-centered cubic (fcc) (001) oriented films. The films remain single phase, fcc, up to an Fe composition of at least 17 at. %. With increasing additions of FeSO4 the films nucleate in the body-centered cubic (bcc) phase forming two-phase solid solutions. For Fe concentrations above 60 at. % the films are single phase, bcc Fe-rich FexNi(1-x) films, and (001) oriented. The resulting FexNi(1-x) /GaAs interfaces are atomically abrupt exhibit close to ideal diode behaviour (barrier height 0.82 eV, n = 1.10). The strain relaxed, electrodeposited Fe films are magnetic with 2-fold and 4-fold magnetic anisotropies aligned with in-plane <100> and <110> directions. Transmission electron microscopy, SQUID, and electrical investigations of the NiFe films will be reported. ZB and KK acknowledge funding support from the BC Advanced Systems Institute and NSERC Canada. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000
9:00 PM - Q3.7
Annealing Induced Changes at Co/GaAs (001) Interface.
Anupam Sharma 1 , R. Brajpuriya 1 , Shilpa Tripathi 1 , T. Shripathi 1 , S.M. Chaudhari 1
1 , UGC-DAE Consortium FOr Scientific Reserach, Indore, M.P., India
Show AbstractIn recent years, the study of magnetic thin films and their interfaces with semiconductor surface has been received considerable attention due to their potential technological applications as non-volatile memories and in the newly developing field of spintronics. In this respect, Co/GaAs system has attracted much attention of the researchers due to its applications in spintronics, Giant Magneto Resistive (GMR) devices because of the high spin polarization of the carriers at the Fermi level. However, all these properties are very susceptible to the degradation with temperature, resulting from the significant intermixing between the substrate and the overlayer. Therefore, understanding the growth and microstructural evolution of Co on GaAs substrate and how the annealing affects the structure, magnetic and electronic properties, is of great technological interest. Present study is therefore, aimed to investigate the structural, magnetic, electronic and transport properties of ion beam sputter deposited Co (40 nm) thin film on GaAs substrate followed by annealing at different temperatures. The X-ray diffraction measurements show oriented growth of as-deposited Co film in hcp (002) direction. However, the sample annealed at higher temperatures shows formation of ternary Co2GaAs phase at the interface. The associated magnetization and resistivity measurements show decrement in magnetization and resistivity with increasing annealing temperature. Corresponding, XPS measurements also show modifications in core level as well as in valence band spectrum further supporting the formation of ternary Co2GaAs phase at the interface. The observed results are mainly attributed to the changes in electronic structure at the Co/GaAs interface.
9:00 PM - Q3.8
Characterization of a Continuous Flow RESS Apparatus for Growth of Magnetic Thin Films
Silvia De Dea 1 , Dominic Graziani 2 , David Miller 1 , Robert Continetti 2
1 Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, California, United States, 2 Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, United States
Show AbstractA continuous flow Rapid Expansion of Supercritical Solution (RESS) apparatus has been developed to grow iron oxide thin films with particles in the range of 100nm to 1000nm. The magnetic thin film is produced by rapidly expanding a high pressure supercritical solution of ferric acetylacetonate (Fe(acac)3) and CO2 and directing the resulting supersonic jet onto both hot and cold silicon wafers. We have moved from a batch syringe pump type RESS source to a continuous source for better RESS/film control and better characterization of the impinging jet with a time-of-flight mass spectrometer (TOF). In the same RESS apparatus, the expansion can be made into ambient pressure conditions, passing through a shock wave, with controlled background composition, or into vacuum. While thermal decomposition of Fe(acac)3, with subsequent formation of α-Fe3O3, is expected to happen only at the hot surface, we observed a weak ferromagnetic phase in the cold sample which indicates an unexpected change in the magnetic properties of the original compound, likely occurring upon interaction with the solid surface. We present SQUID, Mossbauer, and SEM magnetic and structural data for these thin particle films and discuss the energetics of the decomposition of Fe(acac)3 on the hot and cold surfaces. We describe and characterize our continuous flow RESS apparatus, which allows us to control particle morphology and composition by adjusting process parameters such as pre-expansion temperature, pressure and composition of the initial solution, temperature of the substrate, nozzle to substrate distance, and expansion chamber pressure and composition. We also present some combined thermodynamic, kinetic, and fluid mechanic models to describe the properties of the RESS free jet expansion and of the solubility of Fe(acac)3 in the jet. We discuss the details of the interface of the RESS source to the TOF mass spectrometer, designed to directly probe the RESS expansion and to help elucidate the nature of the particles that strike the surface during film growth.
Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q4: Switching of Devices and Heterostructures
Session Chairs
Wednesday AM, April 19, 2006
Room 3020 (Moscone West)
9:30 AM - **Q4.1
Switching Issues for High Density MRAM
Taewan Kim 1 , Keewon Kim 1 , Youngjin Cho 2 , Injun Hwang 2 , Jangeun Lee 4 , Wonchul Jeong 3
1 Nano Devices Lab., Samsung Advanced Institute of Technology, Suwon Korea (the Republic of), 2 Nano Fabrication Center, Samsung Advanced Institute of Technology, Suwon Korea (the Republic of), 4 Process Development Team, Semiconductor R&D Division, Suwon Korea (the Republic of), 3 Advanced Technology Development Team, Semiconductor R&D Division, Suwon Korea (the Republic of)
Show AbstractSuccessful demonstrations by the previous studies ensure that MRAM technology is a strong candidate of universal memory among the other new memory technologies from the viewpoints of power consumption, speed, scalability, retention, endurance, and density. However there are still some fundamental issues to be solved to realize density requirement that is attributted to small switching margin in the core array consisting of sub-micron or deep submicron magnetic tunnel junction (MTJ) cells[1]. In a MRAM array, the conventional writing operation uses a half selection scheme that induces a magnetic field simultaneously by two orthogonal line currents on a specific cell. However some cells selected by only one current line, Digit line (D/L) or Bit line (B/L), are partially or fully switched, which cells acts as a fail bit in the array. This writing scheme is directly related to the low writing current margin due to asteroid distribution.In this study, we consider the technological issues to improve the writing margin in submicron MRAM array, such as low Ms free layer, Synthetic anti-ferromagnetic (SAF) free layer and ultra-smooth roughness of bottom electrode. A new switching architecture without digit line current, which uses a local magnetic field generated directly by the current flowing at bottom electrode, is also proposed. 1. Taewan Kim, et al, J. of Magnetism. and Magnetic. Materials. 282, 232-236 (2004)
10:00 AM - **Q4.2
Giant Magnetoresistance in Multilayer Magnetic Rings
Caroline Ross 1 , F.J. Castano 1 , D. Morecroft 1 , W. Jung 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractThin film magnetic multilayer rings exhibit a variety of interesting magnetic states and have applications in memory, logic or sensing devices. We have prepared circular and elliptical rings with diameters of 500 nm – 20 µm from from pseudo-spin-valve Co/Cu/NiFe films and spin-valve IrMn/Co/Cu/NiFe films. Each ring has 4 – 6 non-magnetic electrical contacts, and has been characterised by magnetoresistance measurements. Major loops show three well-defined resistance states corresponding to the NiFe and Co in parallel onion states (lowest resistance), the NiFe in an onion and the Co in a vortex state (intermediate resistance) and the NiFe and Co in antiparallel onion states (highest resistance). The range of stability of these configurations depends on the ring geometry and ellipticity and on the layer thicknesses. Minor loops, in which the NiFe is cycled without disturbing the Co, were also measured. The NiFe behavior differs depending on whether the Co is in a vortex state, in which case it weakly interacts with the NiFe, or whether the Co is in an onion state, in which case there are strong magnetostatic interactions between the layers. The details of reversal, for example the chirality of the vortex state in the Co, and the propagation of domains in the NiFe, can be deduced from these measurements. Moreover, measurements using more than one contact geometry enable the behavior of specific segments of the rings to be followed. This rich behavior makes the rings suitable for multiple-bit storage, and possibly for programmable logic devices.
10:30 AM - Q4.3
Control of Vortex Chirality in Magnetic Ring Elements*
Vitali Metlushko 1 , P. Vavassori 2 , M. Grimsditch 3 , U. Welp 3 , N. Zaluzec 3 , G. Crabtree 3 , J. Unguris 4 , B. Ilic 5 , A. Imre 6 , L. Ji 6 , W. Porod 6 , Xiaobin Zhu 7
1 , UIC, Chicago, Illinois, United States, 2 , University of Ferrara, Ferrara Italy, 3 , MSD ANL, Argonne, Illinois, United States, 4 , NIST, Gaithersburg, Maryland, United States, 5 , Cornel University, Ithaca, New York, United States, 6 , University of Notre Dame, Notre Dame, Indiana, United States, 7 , University of Alberta, Edmonton, Alberta, Canada
Show AbstractRecent studies on Co, Ni and permalloy rings have shown that a totally flux-closed magnetic vortex structure is stable at remanence. The two chiralities of the vortex, clock-wise and anti-clock-wise, have been proposed as the carriers for the stored information that could be read in a magneto-resistance-based device. To visualize the magnetization reversal process in individual rings we employed several different imaging techniques: magnetic force icroscopy (MFM), scanning Hall microscopy, magneto-optical (MO) imaging, Lorentz STEM (LSTEM) and scanning electron microscopy with polarization analysis (SEMPA). We found that MO, LSTEM and SEMPA allow a direct determination of magnetic vortex chiralities and that by controlling the shape of the nanoscale magnetic ring and the direction of applied magnetic field we can precisely tune the switching mechanism and reliably predict the chirality of the vortex states. The experimental results were compared with detailed micromagnetic simulations.*Supported by the US NSF under contract DMR-0210519. Work at ANL was supported by US Department of Energy, BES Materials Sciences under contract W-31-109-ENG-38.
10:45 AM - Q4.4
Domain Structures and Current-controlled Switching Characteristics of Micron Sized Permalloy Structures with Varying Aspect Ratios.
S. C. Seah 1 , Y. S. Soh 1 , V. Ng 1
1 Information Storage Materials Laboratory, Electrical and Computer Engineering Department, National University of Singapore, Singapore Singapore
Show AbstractIn this paper, we shall explore the different domain configurations observed during the switching process of micron sized permalloy structures with aspect ratios of 2, 3 and 4 as well as their current-controlled switching characteristics. The shape is a rectangle with a semicircle attached at each end. Permalloy 40nm thick round-ended rectangles of size 4μm x 1μm and 2μm x 1μm were fabricated in an array using electron beam lithography and deposited using thermal evaporation. The array was placed on a 50μm wide 200nm thick gold conductor fabricated using photolithography and thermal evaporation. Using magnetic force microscopy, we examined the remnant states of the 4x1um structures after attempting to switch the domain configuration of the structures with the magnetic field generated by passing a current through the conductors. The current value was varied to determine the switching current of the structures. Equivalent field strength was calculated using Finite Element Method Magnetics (FEMM) simulation of the 50um gold conductor. MFM images were used to observe the remnant state of saturation state as well as the remnant states of the intermediate fields of the switching process in both the easy and hard axis. M-H loops of the structures were obtained using the vibrating sample magnetometer. At the same time, using Object Oriented Micro-magnetic Framework (OOMMF), we simulated the M-H loops and the images of the domain configurations from -1T to 1T. We relaxed the various states observed at different fields to determine the switching field required. The MFM images as well as the M-H loops were compared with simulation results.The MFM images of the 4μm x 1μm structures showed a quasi-single domain configuration when saturated along the easy axis. Switching along the hard axis was unstable and the array showed different states even after attempted saturation at high current of 1A. From the simulations, the quasi-single domain was observed in the easy axis switching, confirming the MFM images. The 3-diamond state was dominant when saturated along the hard axis while 2-diamonds and multi-diamond states were also observed.The as-deposited images of 2μm x 1μm structures showed dominantly single-diamond domain configurations and some displaying chess-board configurations. OOMMF simulations showed that easy axis saturation resulted in 1-diamond structure when relaxed after applying a low field and chess-board structure after a high field application. Hard axis saturation resulted in single-vortex structure at low field, triangle structure at intermediate fields and 1-diamond structure at high fields. We will report on these sizes as well as the 3μm x 1μm structures to complete the characterization of different aspect ratios for this shape.
11:30 AM - **Q4.5
Fingerprinting Magnetization Reversal in Magnetic Nanostructures.
Kai Liu 1
1 Physics Department, University of California, Davis, California, United States
Show Abstract12:00 PM - Q4.6
Onset of Exchange Anisotropy in Tetragonal Distortedfcc-Cobalt(001)/γ-Manganese(001)Bilayers.
Harm Wieldraaijer 1 , Wim de Jonge 1 , Juergen Kohlhepp 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractWe use well-prepared ultrathin fct-Co(001) films as templates for the stabilization of tetragonal distorted γ-Mn(001) films by Molecular Beam Epitaxy (MBE) methods. The Mn overlayers can be grown with a very high crystalline quality and extremely flat in a layer-by-layer mode up to a thickness of roughly 20-25 monolayers (ML). Above this thickness, the Mn films slightly roughen but still keep their face-centered-tetragonal (fct) structure, until above roughly 60 ML a transformation to the complex body-centered-cubic α-Mn bulk structure occurs.X-ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), as well as Auger/Photoelectron Diffraction (AED/XPD) measurements show that extensive interdiffusion and/or CoMn alloy formation at the Co/Mn interface can be excluded. With a combination of LEED, AED/XPD, and X-ray Diffraction (XRD) results, the tetragonal distortion of the Mn films is determined to c/a = 1.06. Recent ab-initio calculations predict an in-plane (001) antiferromagnetic (AFM) state for such a c-axis expanded γ-Mn phase.Indeed, room temperature (RT) antiferromagnetism of the fct-Mn is evidenced by the observation of a shifted magnetization loop and an enhanced coercive field for fct-Mn covered ferromagnetic (FM) Co(001) buffer layers, which is explained by a unidirectional exchange anisotropy induced by an exchange interaction at the FM-Co/AFM-Mn interface. Temperature dependent magnetization measurements for samples with >20ML Mn indicate blocking temperatures around 400 K and Néel temperatures well above 450 K.A closer MOKE investigation of the thickness dependence of the coercitivity (HC) and the shift field (HE) as a function of the Mn thickness using wedge shaped sample structures, shows two interesting features: (1) An enhanced HC is observed for Mn thicknesses as small as 1.3 ML (at 10 K) and 3 ML (RT), respectively, clearly evidencing the AFM state in ultrathin Mn films. (2) At 10 K a small negative HE is already observed at a thickness lower than 2 ML Mn, keeping its negative sign for thicker films, whereas at RT a small positive HE first sets in around 3-4 ML Mn, which is thereupon changing at 5-6 ML Mn into a negative HE. This change of sign in HE is also observed as a function of temperature for Mn-thicknesses between 2 and 5 ML. In the course of the presentation, this striking behaviour will be explained by the interplay of the growth mode of the Mn on the Co(001)-buffer layer and the thickness dependent magnetic properties of the ultrathin Mn overlayers.
12:15 PM - Q4.7
Planar Domain Walls in Exchange Biased Bilayers.
Elke Arenholz 1
1 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show Abstract12:30 PM - Q4.8
Controlling Magnetism by Stacking Individual Atomic Monolayers of Magnetic- and Non-magnetic Materials.
Farid El Gabaly 1 , Silvia Gallego 2 , Christof Klein 4 , Carmen Munoz 2 , Laszlo Szunyogh 3 6 , Peter Weinberger 3 , Kevin McCarty 5 , Andreas Schmid 5 , Juan de la Figuera 1
1 Dpt. de Fisica de la Materia Condensada and Centro de Microanalisis de Materiales, Universidad Autonoma de Madrid, Madrid, Madrid, Spain, 2 , Instituto de Ciencia de Materiales de Madrid, Madrid, Madrid, Spain, 4 , Sandia National Laboratories, Livermore, California, United States, 3 Center for Computational Materials Sciencie, Technische Universitat Wien, Vienna Austria, 6 , Budapest University of Technology and Economics, Budapest Hungary, 5 , Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractMagnetic properties of films are often a function of film thickness; for quite some time this has been a fruitful topic for research as well as for device applications. In this work we show just how rich these phenomena are, when measurements are made using microscopes that allow us to study essentially defect-free film regions in which thickness is perfectly homogeneous on the atomic scale coupled to fully relativistic ab-initio calculations.Spin-polarized low-energy electron microscopy (SPLEEM) simultaneously shows the detailed atomic-layer structure of ultrathin films during their deposition and provides with a detailed picture of magnetic properties. When we deposit films of Co onto Ru(0001) substrates in the thickness range of up to 3 atomic monolayers, SPLEEM reveals that the easy axis of magnetization switches twice in this range: both one-monolayer and three-monolayer thick regions are magnetized in a direction within the film plane, while two-monolayer thick Co/Ru(0001) regions are magnetized perpendicular to the film plane. By measuring the thickness-dependent relaxation of epitaxial strain in the Co layers and combining the experimental information with ab-initio computations of the magnetic anisotropy energy, we show how the unusual layer-by-layer double-spin-reorientation transition results can be understood in detail. Moreover, we find rather curious, additional possibilities to induce dramatic changes of the magnetism by adding atomic monolayers of Cu on top of the Co films. When we add just one single atomic Cu layer on top of in-plane magnetized Co/Ru(0001) films of three or four monolayer thickness, the magnetization axis switches to the direction perpendicular to the film plane. Adding just one additional Cu atomic layer flips the magnetization again to an in-plane configuration. Fully relativistic ab-initio calculations show that these spin reorientation transitions are the result of effects at the Cu/Co interface.
Q5: Novel Materials: High Frequency and Spin Injection
Session Chairs
Wednesday PM, April 19, 2006
Room 3020 (Moscone West)
2:30 PM - **Q5.1
Magnetostrictive Materials for High Frequency Applications.
R van Dover 1 , Noble Woo 2 , Jonathan Petrie 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States
Show AbstractThe magnetostrictive properties of thin films at high frequencies (~ 100 MHz – 1 GHz) are of interest for a variety of applications, such as Magnetostrictively Transduced Surface Acoustic Wave Devices (MTSAWs) and strain-assisted switching in magnetic memory devices. We have studied MTSAW devices and demonstrated their feasibility, albeit with a high insertion loss due to incorporation of a magnetic layer with only a low magnetostriction. In order to improve performance we have employed various approaches to obtaining a larger magnetostrictive response. One approach involves engineering the Ferromagnetic Resonance (FMR) frequency of the magnetic material to coincide with an acoustic resonance, which leads to increased overall magnetoacoustic coupling at that resonance frequency. Experimental observation confirms the expected effect, although the response is complex. A second approach is to identify materials with a high response, dλ/dH, where λ(H) is the field-dependent magnetostriction. In order to evaluate the magnetostriction of a wide range of materials in an efficient manner, we prepare composition-spread films using a three-gun on-axis magnetron cosputtering system. Measurement of λ in uniform-composition thin films has typically been accomplished by measurement of (overall) substrate curvature as a function of field. We have developed a method to measure λ locally by depositing the composition-spread films on prefabricated 2D arrays (75 x 75) of small (typically 50 x 500 µm) cantilever beams prepared on a silicon substrate using MEMS techniques. A magnetic field is applied in two directions using orthogonal Helmholtz coils. The magnetostriction of the film results in differential strain of the thin film/cantilever system, and the resulting curvature is detected using an optical system and automated x-y scanning. The data are then analyzed to yield a quantitative measure of the quasistatic magnetostrictive response, λ(H), at each position. This “combinatorial” materials science study directly identifies promising thin film compositions, but the data must be interpreted with care, since incident-angle effects can lead to anomalous magnetic anisotropy in the films. In the past, the development of magnetostrictive materials has focused on development of bulk materials with high λ (thin film work has focused on finding materials with low λ) so this approach represents a novel strategy with unique potential.
3:00 PM - **Q5.2
Spin Waves and Dynamic Coupling in 2-D Magnetic Nanowire Arrays.
M. H. Kuok 1 , Z. K. Wang 1 , H. S. Lim 1 , S. C. Ng 1 , J. L. Goh 1 , H. L. Su 2 , S. L. Tang 2
1 Department of Physics, National University of Singapore, Singapore Singapore, 2 National Laboratory of Solid State Microstructure, Nanjing University, Nanjing, Jiangsu, China
Show Abstract3:30 PM - **Q5.3
Combinatorial Investigation of Magnetic Metallic Alloy Thin Films.
Ichiro Takeuchi 1 2
1 Materials Science and Engineering, University of Maryland, College Park, Maryland, United States, 2 Center for Superconductivity Research, University of Maryland, College Park, Maryland, United States
Show AbstractWe have developed a methodology for rapidly investigating large compositional phase space of magnetic metallic alloy thin films and heterostructures in order to optimize their properties for a variety of magnetic device applications. A number of deposition techniques have been implemented for synthesis of combinatorial thin film libraries of different designs. High vacuum three-gun magnetron co-sputtering systems are used to create composition spreads on 3-inch Si wafers, where natural compositional gradient across the wafers can cover large fractions of ternary phase diagrams. Wavelength dispersive spectroscopy is used to map the exact composition distribution of every wafer. A newly developed multi-gun combinatorial electron-beam deposition system has an automated 2-dimensional shutter system which allows fabrication of libraries with well-defined compositional variation across 1 cm x 1 cm chips. In order to quickly survey the composition-structure-magnetic property relationship across combinatorial samples and identify possible new compositions with enhanced physical properties, various rapid characterization techniques are employed together. Room-temperature scanning SQUID microscopy allows mapping of the magnetic field distribution. Obtained distribution is converted to quantitative remanent magnetization mapping using a numerical algorithm. Synchrotron x-ray magnetic circular dichroism and magneto-optical Kerr effect have been used to systematically measure hysteresis loops at different sites. Scanning x-ray microdiffraction is performed to map the structural phase distribution. I will discuss experiments where we used the co-sputtering technique to study ternary systems containing Heusler alloys and related compounds for screening of ferromagnetic shape memory alloys, magnetostrictive materials, and spintronic materials. Investigation of exchange coupling between hard and soft magnetic layers aimed at optimizing nanocomposite permanent magnets in bilayer libraries made by the electron-beam deposition system will also be discussed. This work is performed in collaboration with M. Wuttig, S. E. Lofland, F. C. Wellstood, L. Knauss, J. W. Freeland, M. Yu and J. P. Liu. This work is supported by ONR, NSF and DARPA.
4:30 PM - **Q5.4
Ferromagnetic Metal/Compound Semiconductor Heterostructures: Growth, Interfacial Reactions and Spin Transport.
C. Palmstrom 1 , C. Adelmann 1 , X. Lou 2 , X. Y. Dong 1 , B.D. Schultz 1 , J. L. Hilton 1 , J. Q. Xie 1 , J. Strand 2 , S. A. Crooker 3 , M. Furis 3 , D. L. Smith 4 , P. A. Crowell 2
1 Dept. of Chemical Eng. & Matl's Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 School of Physics & Astronomy, University of Minnesota, Minneapolis, Minnesota, United States, 3 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 4 Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractControl of the interfacial properties of ferromagnetic metals and semiconductors is important for optimizing magnetic properties and spin dependent transport across these interfaces. Interfacial reaction studies have been used to determine metal-Ga-As thin film phase diagrams and for predicting the thermodynamic stability of ferromagnetic metallic compounds on GaAs. Molecular beam epitaxial (MBE) growth in combination with in-situ STM, XPS, Auger, RHEED and LEED and ex-situ XRD, RBS, TEM, magnetotransport and magnetic characterization have been used to develop ferromagnetic elemental and metallic compound/compound semiconductor tunneling contacts for spin injection. These have included epitaxial Fe, Fe3Ga, Co2MnGe, and δ-MnGa ferromagnetic contacts on Ga1-xAlxAs. The efficiency of the spin polarized current injected from the ferromagnetic contact was determined by measuring the electroluminescence polarization of the light emitted from Al1-xGaxAs light emitting diodes. Interfacial reactions and the semiconductor device band structure were found to have a dramatic influence on the measured spin injection. Lateral spin-transport devices with epitaxial Fe source and drain tunnel-barrier contacts were fabricated. Kerr microscopy was used to image the electrical spin injection and accumulation in the n-doped GaAs channel. The emission of spins from the source was observed, and a region of spin accumulation was imaged near the ferromagnetic drain contact. Both injected and accumulated spins have the same orientation (antiparallel to the contact magnetization), indicating that electron spins are polarized by reflection from the ferromagnetic drain contact. The electrical conductance could be modulated by controlling the spin orientation of optically injected electrons flowing through the drain contact.
5:00 PM - **Q5.5
Current-induced Macrospin vs. Spin-wave Excitations in Spin Valves.
Arne Brataas 1 , Yaroslav Tserkovnyak 2 , Gerrit Bauer 3
1 Department of Physics, Norwegian University of Science and Technology, Trondheim Norway, 2 Physics Department , Harvard University, Cambridge, Massachusetts, United States, 3 Kavli Institute of NanoScience, Delft University of Technology, Delft Netherlands
Show AbstractWe will discuss current-induced magnetic excitations in spin valves. We have found that the spin transfer torque exerted on the magnetization by transverse spin currents as well as the Gilbert damping constant depend strongly on the wave length of the excitation (spin wave). The onset of macrospin (zero wavelength) vs. finite wavelength spin-wave instabilities depends on the device parameters and the current direction, in agreement with recent experimental findings.
5:30 PM - Q5.6
Properties of Epitaxial Films of the Heusler Alloy Co{2}Cr{1-x}Fe{x}Al
Rajesh Kelekar 1 , Bruce Clemens 1
1 , Stanford University, Stanford, California, United States
Show AbstractWe have grown epitaxial thin films of the Heusler alloy Co{2}Cr{1-x}Fe{x}Al. Compounds with small x -- the level of Fe doping -- have been predicted to be half-metallic, where the conductions electrons at low temperature all have the same spin. In addition, bulk polycrystalline compacts of Co{2}Cr{.6}Fe{.4}Al displayed negative magnetoresistances of -30% at room temperature in magnetic fields of less than 1000 Oe. The properties of the single crystalline thin films we have grown for the first time show a sharp dependence on the level of Fe doping. Films with low Fe doping show large semiconductor-like resistivities, large reductions in the magnetic moment as compared to theory, and large values of the anomalous Hall resistivity. X-ray magnetic circular dichroism of these thin films shows that the magnetic moment on the Cr atom is virtually extinguished. Anomalous x-ray diffraction indicates that the films across the whole range of compositions contain a significant amount of selective transition metal disorder, where the transition metal atoms are disordered on the lattice sites while the Al atom is positioned on a single lattice site. These features indicate that Cr is much more sensitive to the local environment than is Fe. We have incorporated these Heusler alloys into epitaxial superlattices with Cr and spin valve-type trilayers. Though superlattices do not show giant magnetoresistances or any evidence of antiferromagnetic coupling, current-in-plane spin valves show large giant magnetoresistances of up to 7% at room temperature, the largest for any Heusler alloy or proposed half-metal.
5:45 PM - Q5.7
CrSb/GaSb Heterojunction - a Possible Robust Spin Current Injector.
Yuan Ping Feng 1 , Rongqin Wu 1 , Guowen Peng 1 , Lei Liu 1
1 Physics, National University of Singapore, Singapore Singapore
Show Abstract
Symposium Organizers
Thomas Ambrose Seagate Research
William Bailey Columbia University
David Keavney Argonne National Laboratory
Y. Daniel Park Seoul National University
Q6: Magnetic Nanoparticles and Nanowires
Session Chairs
Thursday AM, April 20, 2006
Room 3020 (Moscone West)
9:30 AM - Q6.1
Submicron Spherical Hollow Magnets: Synthesis, Dispersibility, Coercivity and Glassiness
Kunio Awaga 1 , Yasuharu Kozuka 1 , Mototaka Ohnishi 1 , Hirofumi Yoshikawa 1 , Shunji Bandow 2 , Sumio Iijima 2 , Motoyasu Kobayashi 3 , Atsushi Takahara 3
1 , Nagoya University, Nagoya Japan, 2 , Meijo University, Nagoya Japan, 3 , Kyushu University, Fukuoka Japan
Show AbstractThe preparation of sub-micron magnetic hollow spheres is a promising approach towards developing novel magnetic materials. Their comparatively large size means that the critical temperatures of magnetic ordering or magnetization blocking would be high enough for practical application. In addition, the spherical hollow structure is advantageous for various surface modifications, which would enable further applications. Further, the magnetic domains that exist on the hollow sphere are of great interest to fundamental science; various domain patterns are energetically allowed in this highly-symmetric shape, in contrast to a needle-shape magnet, which always exhibits magnetic polarization parallel to the needle axis. The existence of these energetically-degenerated domain structures should lead to materials that change their magnetic property under certain conditions. In the present work, we prepared spherical hollow magnets of ccp- and hcp-Co, Co3O4, Fe, Fe3O4, α-Fe2O3, etc. with a diameter of ca. 500 nm and a shell thickness of ca. 50 nm, using polystyrene-bead templates. These particles were characterized by SEM, TEM, ED, XRD, etc. The magnetic measurements on the Fe3O4 particles revealed a temperature-dependent coercive field that showed a significant decrease with an increase in temperature in the wide range 2-300 K. By a surface modification with hydrophilic organic polymers, the Fe3O4 spheres obtained good dispersibility in water. The separate syntheses of hollow spheres of ccp- and hcp-Co were successful under nearly identical calcination conditions, but from different precursors.
9:45 AM - Q6.2
Low-temperature Synthesis, Assembly, and Properties, of Monodisperse FePt-silica core-shell Nanomagnets of Tunable Size, Composition and Thermal Stability.
Qingyu Yan 1 , Arup Purkayastha 1 , Taegyun Kim 1 , Roland Kroger 2 , Arijit Bose 3 , Theodorian Theodorian Borca-Tasciuc 4 , Ganapathiraman Ramanath 1
1 Matierials Science, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Institut fur Festkorperphysik, Universitat Bremen, Otto-Hahn-Allee , Bremen, Germany, 3 Chemical Engineering Department, University of Rhode Island, Kingston, Rhode Island, United States, 4 Mechanical Engineering Department, Rensselaer Polytechnic Institute, Troy, New York, United States
Show Abstract10:00 AM - Q6.3
L10 Order In FePt Thin Films And X-Ray Rapid Thermal Annealing (XRTA).
Rosa Alejandra Lukaszew 1 , Jonathan Skuza 1 , Cesar Clavero 2 , Alfonso Cebollada 2 , Eric Dufresne 3
1 Physics and Astronomy, University of Toledo, Toledo, Ohio, United States, 2 , IMM, Marid Spain, 3 MHATT, APS, ANL, Argonne, Illinois, United States
Show AbstractHighly ordered L10 FePt thin films and nano-structures are important for magneto-recording applications because this ordered phase exhibits very large magnetic anisotropy.1 One possibility to achieve high degree of L10 order in epitaxial but chemically disordered films is to perform annealing treatments.2 One interesting variation of such treatments is rapid thermal annealing (RTA). RTA is widely used for electronic materials processing, from the activation of dopants to the healing of lattice defects caused by ion implantation.Here we describe an innovative application of x-ray undulator radiation to simultaneously perform RTA and to probe structural changes that occur during annealing. This is made possible by the high power-density of undulator beams (~100 W/mm2), combined with their excellent properties as a probe of crystal structure (especially collimation and high-brightness). XRTA is similar to laser annealing, but there is a unique advantage in that the x-ray energy can be tuned to enhance the coupling into the absorption edge of a particular species, thereby permitting selective annealing of buried layers and nanostructures. In our studies at the MHATT-CAT beam line we have used XRTA to enhance the degree of L10 chemical order in epitaxial (001) FePt thin films. Using a (111) Silicon crystal in the diffractometer analyzer arm, we were able to monitor the x-ray diffraction pattern in real time as the samples were heated by the undulator x-ray beam. The undulator beam (1st harmonic ~10 keV) heated a 1 mm2 spot in the sample to ~1000 C in a few seconds. We observed the enhancement of the fcc-fct transition with 30 msec temporal resolution in Bragg geometry. The results demonstrate that undulator radiation offers a unique possibilities for materials processing, particularly the ability to use the same beam for heating and as a structural probe. This work was partially supported by NSF-DMR (Grant #0355171), the American Chemical Society (PRF grant # 41319-AC) and the Research Corporation Cottrell Scholar award.1 A. Cebollada, R. F. C. Farrow, and M. F. Toney, Magnetic Nanostructures, edited by H. S. Nalwa (American Scientific Publishers, 2002), pp. 93 – 118.2 Xiao-Hong Xu, Hai-Shun Wu, Xiao-Li Li, Fang Wang and Jing-Fang Duan, Physica B 348, 436 (2004).
10:15 AM - Q6.4
Reconstruction of the Magnetization Distribution of Small CoPt Patterns via Magneto-optic Imaging and Magnetic Force Microscopy.
Christian Jooss 1 , Sebastian Dreyer 1 , Sven Schnittger 1 , Jonas Norpoth 1 , Sibylle Sievers 2 , Martin Albrecht 2 , Uwe Siegner 2
1 Institute of Materials physics, University of Goettingen, Goettingen Germany, 2 , Physikalisch-Technische Bundesanstalt, Braunschweig Germany
Show Abstract10:30 AM - Q6.5
c-Axis Oriented, Isolated L10-FePt Nanoparticle Monolayers on TiN Underlayers with Controlled Grain Sizes.
Suguru Noda 1 , Yoshiko Tsuji 1
1 Department of Chemical System Engineering, The University of Tokyo, Tokyo Japan
Show Abstract10:45 AM - Q6.6
Indirect Exchange Interaction in Granular Permalloy Films.
Anatoli Pogorily 2 1 , Anatoli Kravets 2 , Yurii Dzedzerja 2 , Olena Shypil 1 , Chester Alexander 1
2 Physics of Thin Films, Institute of Magnetism, Kyiv, 03142 Ukraine, 1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States
Show Abstract11:30 AM - Q6.7
Ferromagnetism from a Co-porphyrin Supramolecular Assembly.
Anthony Caruso 1 , Doug Schulz 1 , Pam Jeppson 1 , Laura Jarabek 1 , Douglas Chrisey 1
1 , North Dakota State University, Fargo, North Dakota, United States
Show AbstractFerromagnetism from an air-stable supramolecular assembly of a cobalt porphyrin-based system has been observed above room temperature with saturation not observed up to 7 Tesla. Low temperature (5K) measurements indicate an extremely strong exchange mechanism from the ligand fields and this observation will be a focal point of this talk. We hypothesize that the organic assembly provides a scaffolding to hold the transition metal centers in place for direct exchange. In addition, two rather strange responses are observed for this system: (1) a metamagnetic transition occurs at 130K; and, (2) classical exchange bias is observed and is enhanced for field cooled samples. Thin films and chemical variations will also be discussed in the context of advantages for future spintronic devices that will use organic materials.
11:45 AM - Q6.8
Fabrication of Ordered Mesoporous Silica with Encapsulated Iron Oxide Particles via Doped Block Copolymer Templates in Supercritical Carbon Dioxide
David Hess 1 , James Watkins 1 , Rajesh Naik 2 , Melanie Tomczak 2
1 chemical engineering, University of Massachusetts-Amherst, Amherst, Massachusetts, United States, 2 Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright Patterson Air Force Base, Ohio, United States
Show AbstractEncapsulation of ferritin within mesoporous silica provides a potential route to devices that rely on isolated anti-ferromagnetic metal oxide clusters. Ferritin is a ubiquitous iron storage protein that is made up of a self-assembling protein shell 12 nm in diameter and with an 8 nm cavity that contains an iron oxide particle. Recently ferritin has been used to form ordered arrays of iron nanoparticles. Upon dipping a substrate into a solution, ferritin aligns in a hexagonal type monolayer arrangement. Upon heating, the shell is removed and the spherical iron oxide nanodot remains. Recently, our group has reported a new approach for fabricating mesoporous silicates involving the selective condensation of a silica precursor in the hydrophilic domain of a highly ordered amphiphilic block copolymer using supercritical carbon dioxide as a delivery medium. In this approach template organization silica network formation occur in discreet steps. Consequently the template can easily be doped with active components in specific sub-domains prior to silica infusion and removal of the template by calcination. In this work, a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer was doped with horse spleen ferritin and a low concentration of a p-toluenesulfonic acid catalyst. The template was then spin-casted onto a silicon wafer. Upon drying the block copolymer undergoes microphase segregation and both the acid catalyst and ferritin partition into the hydrophilic domain. The doped template was then exposed to a solution of tetraethyl orthosilicate (TEOS) in supercritical carbon dioxide at 60 oC and 125 bar. TEOS selectively condensed in the PEO domain to yield a nanocomposite material, which upon calcination yielded an iron oxide-doped, well-ordered, mesoporous silica film. TEM and XRD analysis indicated that the structure of the mesoporous silica matrix was dependent upon the concentration of ferritin dopant. XRD revealed the presence of crystalline iron oxide within the mesoporous support. Samples were also characterized using a semiconducting quantum interference device (SQUID) and were found to possess interesting magnetic properties.
12:00 PM - Q6.9
A Novel Polymer Mediated Approach for the Synthesis of Magnetic Nanoparticles.
Suresh Valiyaveettil 2 3 , Swaminathan Sindhu 3 , Subbiah Jagadesan 2
2 Deaprtment of Chemistry, National University of Singapore, Singapore Singapore, 3 NUSNNI, National University of Singapore, Singapore Singapore
Show Abstract12:15 PM - Q6.10
Development of Block Co-Polymers as Self-Assembling Templates for Magnetic Media and Spin-Valves
Martin Bakker 1 2 , Vishal Warke 1 2 , David Nikles 1 2 , Jimmy Mays 3 , Phillip Britt 3
1 Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama, United States, 2 Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, United States, 3 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractPoly(styrene)-Poly(methylmethacrylate) block co-polymers (PS-b-PMMA) of appropriate block length and PS to PMMA ratio self-assemble into a 2-D hexagonal phase in which the PS majority phase is continuous and surrounds cylinders of the minority, PMMA phase. By UV irradiation and washing with acetic acid it is possible to remove the minority phase to leave empty channels. It is also possible to rearrange the PMMA phase with acetic acid to leave somewhat smaller pores. For most substrates the interactions between the polymer and the substrate surface are such that one block is preferentially adsorbed to the substrate resulting in alignment of the PMMA domains parallel to the substrate surface. It is possible to orient the polymer perpendicular to the surface by first adding a thin film of a random PS-PMMA co-polymer before applying the PS-b-PMMA block co-polymer. However thin films of the random PS-PMMA do not give good surface coatings, and thicker films are generally too thick for the pores in the PS-b-PMMA block co-polymer to be propagated to the substrate surface. For a few substrates, thin PS-b-PMMA films naturally adopt a perpendicular orientation after annealing, washing with acetic acid produces arrays of pores of diameter as small as 3 nm. For a number of other substrates the interaction between the polymer blocks and the surface is such that upon annealing the polymer rearranges to form micron sized domains which are not polymer coated, surrounded a areas which have a thicker polymer coating. We have observed this behavior with both carbon coated substrates and with ITO glass substrates. In both cases the areas of polymer are perpendicularly oriented, and upon washing with acetic acid give rise to pores that extend completely through the polymer film. In some cases films on ITO glass are continuous even after annealing. After washing with acetic acid it was possible to electrodeposit nickel into the pores to give nickel nano-pillars of 18 nm diameter.
12:30 PM - Q6.11
Room Temperature Ferromagnetism in Ge-based Nanowires.
Olga Kazakova 1 , Jaideep Kulkarni 2 , Justin Holmes 2 , Sergej Demokritov 3
1 , NPL, Teddington, Middlesex, United Kingdom, 2 , UCC, Cork Ireland, 3 , WWU, Munster Germany
Show AbstractSpintronics requires fabrication of ferromagnetic nanostructures that can transport spin-polarized carriers at room temperature, be electrically tunable and easily compatible with existing silicon manufacturing technologies. The most direct method to induce spin-polarized electrons into a semiconductor is by introducing transition metal dopants, producing a dilute magnetic semiconductor (DMS) [1, 2]. Extensive research has been carried out in order to create DMS materials with well-established room temperature ferromagnetism. Despite several encouraging results, DMS materials are still struggling to reliably achieve the desired high Curie temperature.Here we report the first time observation of room temperature ferromagnetism in group-IV DMS materials in the form of 1D structures. We investigate magnetic properties of Ge nanowires (NWs) having a smallest diameter of 35 nm and length of 60 μm, synthesised within the pores of anodised aluminium oxide (AAO) templates [3] and doped with various transition metals (Mn, Co or Cr). Structural analysis of the NWs showed the existence of a highly crystalline germanium host lattice containing discrete dopant atoms. The dependences of the magnetic characteristics of the NWs on their diameter, transition metal concentration and preparation details were investigated by means of SQUID magnetometry. Of the all studied magnetic impurities, Mn-doped NWs are of particular interest, as these structures display well-pronounced ferromagnetic properties at room temperature [4], whereas a typical transition temperature for GeMn thin films is only about 110 K [5]. Ferromagnetic ordering reaches a maximum at intermediate Mn concentrations followed by a decay in the magnetic properties at x = 5 %. The effect of interface related anisotropy in NWs of different diameters was also studied. By eliminating the surface strain we were able to improve ferromagnetic performance of NWs at room temperature. We also show that room temperature ferromagnetism was preserved after post-annealing, meaning GeMn NWs are compatible with present CMOS technology. The magnetic properties of the NWs can be understood by considering the influence of co-dopant non-magnetic impurities, i.e. oxygen and carbon, as well as confinement effect at the interfaces.The observation of ferromagnetic properties in Mn-doped Ge NWs at 300 K as well as compatibility of germanium and silicon allow straightforward integration of Ge NWs into mainstream electronics and open the way for room-temperature spintronic devices. 1. A. H. Macdonald, et al., Nature Materials 4, 195 (2005). 2. S. J. Pearton, et al., Physica B 340-342, 39 (2003). 3. J. S. Kulkarni, et al., Chem. Mat. J. Mater. Chem. 17, 3615 (2005). 4. O. Kazakova, et al., Phys. Rev. B. 72, 0944415 (2005). 5. Y. D. Park, et al., Science 295, 651 (2002).
12:45 PM - Q6.12
Diluted Magnetic Semiconductor Zn(Mn)O Nanowire Array From A Self-Formed ZnO Substrate.
Jingjing Liu 1 , Minhui Yu 1 , Weilie Zhou 1
1 AMRI/Chemistry, University of New Orleans, New Orleans, Louisiana, United States
Show AbstractWell-aligned diluted magnetic semiconductor (DMS) Zn1-xMnxO nanowires have been successfully fabricated from a self-formed ZnO substrate using chemical vapor deposition (CVD) method. The as-synthesized Mn-doped ZnO nanowires were characterized by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The nanowires are well-aligned and are perpendicularly-grown along the c-aixs having single crystalline structure. Electron energy X-ray dispersive (EDS) analysis, X-ray diffraction (XRD) spectrometry and TEM analysis clearly showed that Mn was successfully doped in the ZnO nanowires and substrate. Ferromagnetic ordering of the as-synthesized Zn1-xMnxO nanowire arrays was observed at 5 K with Curie temperature of 44 K and room temperature by superconducting quantum interference device (SQUID) measurement due to different synthesis condition. The DMS nanowire arrays with room temperature ferromagnetic ordering have strong applications in spintronic nanodevices.
Q7: Dilute Magnetic Semiconductores, Oxide Heterostructures, and Multiferroics
Session Chairs
Thursday PM, April 20, 2006
Room 3020 (Moscone West)
2:30 PM - Q7.1
Exchange Biasing with Magnetoelectric YMnO3 Epitaxial Films
Xavier Marti 1 , Florencio Sanchez 1 , Nico Dix 1 , David Hrabovsky 1 , Vassil Skumryev 2 , Maria-Victoria Garcia-Cuenca 3 , Cesar Ferrater 3 , Manuel Varela 3 , Ulrike Luders 4 , Jean Francois Bobo 4 , Josep Fontcuberta 1
1 , ICMAB-CSIC, Bellaterra Spain, 2 , Institut Catala de Recerca i Estudis Avancats, Barcelona Spain, 3 Fisica Aplicada i Optica, Universitat de Barcelona, Barcelona Spain, 4 , Onera, Toulouse France
Show AbstractBiferroic materials are currently receiving much attention as they show the existence of ferroelectricity and magnetic order. Among oxides, the intriguing coexistence of ferroelectricity and antiferromagnetism in hexagonal YMnO3 has been much discussed. However, recently research has been also driven on the orthorhombic phase of this oxide (o-YMnO3) as it displays substantial changes on the dielectric constant close to the antiferromagnetic order temperature which make it very appealing for a new generation of magnetoelectric devices. Unfortunately, in bulk form o-YMnO3 can only be prepared under very high pressure thus facing severe problems of integration.Here, we report on growth of YMnO3 films. We will show first that by appropriate selection of substrates, the o-YMnO3 epitaxial films can be stabilized. Moreover, we will show that by appropriate choice of the substrate orientation, the texture of the YMnO3 can be tuned. Magnetic characterization of these films reveals the presence of a magnetic transition at about 40K from a paramagnetic state to a canted antiferromagnet. Aiming to integrate the magnetoelectric YMnO3 with ferromagnetic oxides in a new generation of magnetic devices, we have successfully grown epitaxial o-YMnO3 films on the SrTiO3 substrates buffered with either Pt or ferromagnetic SrRuO3 epitaxial film or capped with Permaloy. When measuring YMnO3/SrRuO3 bilayers a well defined exchange bias field appears. The exchange bias field decreases with temperature and vanishes at around 40 K, the Néel temperature of the o-YMnO3; the magnetoelectric character of YMnO3 offer the possibility of controlling the exchange bias and thus magnetic switching, by an electric field.
2:45 PM - Q7.2
Magneto-Electric Coupling in Multiferroic Oxide Films by Photoemission Electron Microscopy
Tong Zhao 1 , A. Scholl 2 , F. Zavaliche 1 , H. Zheng 1 , M. Barry 1 , M. Cruz 1 , Q. Zhan 1 , Y. H. Chu 1 , A. Doran 2 , L. Martin 1 , R. Ramesh 1
1 Department of Physics and Department of Materials Science and Engineering, University of California, Berkeley, California, United States, 2 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractMultiferroic materials, showing simultaneous (anti-)ferromagnetic and ferroelectric ordering, have attracted much attention. The coupling between these two ordering parameters has great potential for novel electronic device applications. On the other hand, the new physics behind the materials and phenomena is interesting itself from the fundamental science point of view. To probe the magneto-electric coupling, therefore, is with great interests. In this work, a unique technique, namely Photoemission Electron Microscopy (PEEM) at the Advanced Light Source of the Lawrence Berkeley National Laboratory was utilized to probe the magneto-electric coupling in a multiferroic nanostructure comprised of ferrimagnetic CoFe2O4 pillars embedded in ferroelectric BiFeO3 matrix which was prepared by pulsed laser deposition through a self-assembly process. Piezoelectric force microscopy (PFM) was used to study the ferroelectric property, and to switch the ferroelectric polarization in the BiFeO3 matrix of the nanostructure by applying an electrical voltage through the PFM tip. The magnetic structure of the CoFe2O4 pillars in the nanostructure was measured by PEEM with a circularly-polarized X-Ray. The nanostructure was magnetized in a magnetic field to align the magnetization of the CoFe2O4 pillars. Then an electrical voltage was applied to a selected area to switch the ferroelectric polarization of the BiFeO3 matrix. A clear magnetic circular dirchroism was observed at both Co and Fe L edges of the CoFe2O4 pillars between the poled and unpoled areas. This is a strong evidence that the magnetization of the CoFe2O4 pillars were switched by switching the ferroelectric polarization of the BiFeO3 matrix. The magneto-electric coupling as a function of electric voltage and CoFe2O4 pillar size was studied. X-Ray absorption spectrum was also measured to study the cation site occupancy and valence state in the CoFe2O4 spinel structure. Linearly polarized X-Ray was used to probe the antiferromagnetic-ferroelectric coupling in a BiFeO3 thin film. A clear linear dichroism and a similar domain structure to the PFM measurement were observed before and after electrical poling on the BiFeO3 film, implying an antiferromagnetic-ferroelectric coupling in this material.This work is supported by an ONR-MURI and a LBL-LDRD.
3:00 PM - Q7.3
Multiferroic Characteristics of Highly Oriented ferrite-ferroelectric Multilayered and Composite Films Deposited by Laser Ablation.
Sarath Witanachchi 1 , H Nagaraja 1 , R Heindl 1 , H Srikanth 1 , Pritish Mukherjee 1
1 Department of Physics, University of South Florida, Tampa, Florida, United States
Show AbstractCoupling of ferroelectricity and ferromagnetism in a single system presents interesting applications in spintronics and sensors. Ferrimagnetic BaFe12O19 (BaM) and ferroelectric Ba0.5Sr0.5TiO3 (BSTO) are reasonably compatible materials to fabricate multilayered structures to investigate such coupling. In this study we have used a dual-laser ablation process to deposit single and multilayered films on various substrates that included A-plane and C-plane sapphire, 96% polycrystalline Al2O3 and MgO. Single and multilayer films deposited under optimum growth temperature and oxygen pressure showed high degree of orientation without any post deposition treatment. Low-angle x-ray diffraction and rocking curve measurements confirmed the epitaxial-like growth on sapphire substrates. While BST films were observed to grow epitaxially on MgO, films deposited from an ablation target that contained 50-50 mixture of BaM and BSTO showed the formation of separate phases with random orientation. However, when the composite film was deposited on MgO with a thin BSTO layer, the BaM and BSTO grains in the film became highly oriented. A versatile microwave probe station has been used to investigate the high-frequency characteristics of the films with applied parallel and perpendicular magnetic as well as electric fields. Magnetic hysteresis studies of multi layered structures at various field orientations showed large coercivities. Correlation between the microstructure of the films and the ferromagnetic and ferroelectric properties will be presented.
3:15 PM - Q7.4
Self-Assembled BiFeO3-CoFe2O4 Thin Film Nanostructure Growth Evolution
S. Y. Young 1 , L. Salamanca-Riba 1 , H. Zheng 2
1 M