E1: Perovskite Manganites
Monday AM, November 30, 2009
Room 103 (Hynes)
9:30 AM - **E1.1
Sr2FeMoO6 — A Fascinating Compound.
D. D. Sarma 1 Show Abstract
1 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka, India
Sr2FeMoO6 is a magnetic metal with an unusually high Curie temperature (~ 420 K) and belongs to the double perovskite family of compounds. It shot in to fame a few years ago due to its remarkable magnetoresistive properties.  We discuss the origin of ferromagnetism in this and related compounds based on a mechanism driven by the kinetic energy, [2-4] establishing these as members of a new class of magnetic materials; this mechanism also explains [4,5] the ferromagnetic coupling of transition metal ions in dilute magnetic semiconductors, such as Mn-doped GaAs and in pyrochlore, Tl2Mn2O7.  We also show  that the magnetoresistance in Sr2FeMoO6 arises from a magnetically triggered nearly-resonant tunnelling condition in contrast to other mechanisms discussed so far in the context of CMR and GMR materials. Finally, I shall present our very recent and unpublished results that are relevant to the nature of disorder in this material, which has drastic influence on not only magnetic but also electronic structure of these compounds , providing a critical test-bed for Altshuler-Aronov theory.This work has been contributed by S. Ray, D. Topwal, P. Mahadevan, T. Saha-Dasgupta, M. De Raychaudhury, A. Kumar, P. Sanyal, H.R. Krishnamurthy, C. Dasgupta, R. Cimino, S. Turchini, S. Zennaro and N. Zema, A. Zunger, K. Tanaka, M. Kobayashi, A. Fujimori, H. Kato, Y. Tokura, M. Avignon, C. Meneghini, F. Liscio, F. Bardelli, and S. Mobilio.References:1. K. I. Kobayashi et al., Nature (London) 395, 677 (1998).2. D.D. Sarma et al., Phys. Rev. Lett. 85, 2549 (2000).3. S. Ray et al., Phys. Rev. Lett. 87, 097204 (2001).4. D.D. Sarma, Current Opinion in Solid State & Material Sciences 5, 261 (2001).5. P. Mahadevan, A. Zunger and D.D. Sarma, Phys. Rev. Lett. 93, 177201 (2004).6. T. Saha-Dasgupta, M. De Raychaudhury and D. D. Sarma, Phys. Rev. Lett. 96, 087205 (2006).7. D.D. Sarma et al., Phys. Rev. Lett. 98, 157205 (2007).8. M. Kobayashi, K. Tanaka, A. Fujimori, S. Ray, and D.D. Sarma, Phys. Rev. Lett. 98, 246401 (2007).
10:00 AM - E1.2
Controlled Fabrication of Epitaxial Ferromagnetic Oxide Artificial Nano-Constriction Structures and their Giant Magnetoresistive Properties at Room Temperature.
Hidekazu Tanaka 1 , Kazuya Goto 1 , Tomoji Kawai 1 Show Abstract
1 ISIR-Sanken, Osaka University, Osaka Japan
Epitaxial (Fe,Mn)3O4 (FMO) ferromagnetic oxide artificial nanowire (NW) structures were deposited across Pt/Cr bi-layer electrodes with a high controllability on their shape and positioning by using Atomic Force Microscope (AFM) lithography with Molybdenum (Mo) lift-off in combination with a Pulsed Laser Deposition (PLD) technique . The oxide wire widths were systematically controlled from 5µm down to 100nm by controlling AFM lithography current for Mo-mask. The resistivity of FMO-NW structures was increased at below around 400nm in width. Magnetic Force Microscope (MFM) revealed that FMO-NW with 120nm showed single line of aligned ferromagnetic domains which caused the resistivity increase . As further development, we have fabricated ferromagnetic oxide FMO nano-constriction structures within the artificial NW-structures by using two different steps of AFM lithography and PLD technique. The sizes of three FMO nano-constriction structures were 150nm, 100nm and 50nm. The I-V characteristic of the 50nm width FMO constriction structure was dramatically changed from linear to non-linear property after making the nano-constriction structure. The MR ratio was also dramatically increased from -0.2% to 150% at room temperature. Because the length of spin domain wall and the size of nano-constriction structure become comparable, it is considered that large spin polarization [3, 4] of (Fe,Mn)3O4 and compression of domain wall bring huge MR at room temperature. We also discuss this mechanism.Our results offer well-defined epitaxial transition metal oxide nanostructures with a remarkable flexibility toward nano-scale oxide spintronics.References H. Tanaka et al,  Adv. Mater, 20 (2008) 909-913,  Nano Lett., 9 (2009) 1962–1966,  Phys. Rev. B, 76 (2007) 205108,  Solid State Commun., 147 (2008) 397-400
10:15 AM - E1.3
(S)Tem-Eels Investigation of Composition Profiles in (001) and (110) LCMO Layers as a Function of Layer Thickness.
Sonia Estrade 1 , Josep Rebled 1 , Ingrid Infante 2 , Gervasi Herranz 2 , Florencio Sanchez 2 , Josep Fontcuberta 2 , Rosa Cordoba 3 , Francisco de la Pena 4 , Michael Walls 4 , Christian Colliex 4 , Jordi Arbiol 1 , Francesca Peiro 1 Show Abstract
1 MIND-IN2UB, Dept. Electrònica, Universitat de Barcelona, Barcelona, Catalunya, Spain, 2 , Institut de Ciència de Materials de Barcelona-CSIC, Barcelona, Catalunya, Spain, 3 , INA: Instituto Universitario de Investigación en Nanociencia de Aragón, Zaragoza Spain, 4 , Laboratoire de Physique des Solides, UMR CNRS, Orsay France
Although the microstructure, chemistry and electronic structure of mixed-valence ferromagnetic manganite films, as LCMO (La2/3Ca1/3MnO3), have been the object of very detailed (S)TEM-EELS studies in recent years [1,2], little attention has been paid, so far, to (110) LCMO films, reported to display enhanced magnetic properties when compared to their (001) counterparts . Besides, very thin LCMO films seem to present an anomalous behaviour when compared to thicker films , and, thus, a systematic comparative characterization of (001) and (110) LCMO films is of the utmost importance to understand the overall behaviour of these films.In the present work, we address the detailed (S)TEM-EELS characterization of (001) and (110) LCMO / STO (SrTiO3) thin films of a wide rage of thicknesses. Microstructural and chemical characterization of moderately thin LCMO layers grown on STO reveals that different stress relieving mechanisms exist for the two considered orientations, with defects found in more relaxed (110) films, and a Ca2+ ion migration towards free surface, and concomitant Mn oxidation state variation, in more stressed (001) films. Yet, for very thin films (thickness below some 20 nm), a La3+ ion migration towards free surface, and concomitant Mn oxidation state variation, is found, irrespective of layer orientation. In this thickness range, cation incorporation rate instead of elastic energy seems to determine the chemistry of the layers.References: Varela, M., Oxley, M. P., Luo, W., Tao, J., Watanabe, M., Lupini, A. R., Pantelides, A. R., Pennycook, S. J., Physical Review B, 79, 085117 (2009) Simon, J., Walther, T., Mader, W., Klein, J., Reisinger, D., Alff, L., Gross, R., Appl. Phys. Lett. 84, 3882 (2004) I. C. Infante, F. Sánchez, J. Fontcuberta, M. Wojcik, E. Jedryka, S. Estradé, F. Peiró, J. Arbiol, V. Laukhin, and J. P. Espinós, Phys. Rev. B 76, 224415 (2007) YL Qin, HW Zandbergen, ZQ Yang, J Aarts, Phil. Mag., 85, 4465 (2005)
10:30 AM - E1.4
Tailoring Anisotropy in Half Metallic La0.7Sr0.3MnO3 Thin Films.
Paolo Perna 1 2 , Francisco Teran 1 , Erika Jimenez 1 , Julio Camarero 1 , Laurence Mechin 2 , Rodolfo Miranda 1 Show Abstract
1 , IMDEA Nanociencia, Madrid, Madrid, Spain, 2 , GREYC - ENSICAEN, Caen France
In the past decades many improvements in the fabrication of artificial magnetic nanostructures, thin films , superlattices have been made tailoring the properties of a large class of materials exploiting to advanced techniques of patterning and stress relaxation mechanisms. Planar devices making use of magnetic domain walls (DW) could represent a valuable alternative to the common read heads composed of multilayered ferromagnetic heterostructures exploiting the out-of-plane uniaxial magnetic anisotropy originating from the interfaces. An interesting class a materials based on the mixed-valence manganese oxide exhibiting a metal-insulator transition accompanied by so-called colossal magnetoresistance (CMR) effects have attracted high interest in the condensed matter physics community. Thanks to their almost 100 % spin polarization , they appear as potential candidates for spintronics applications such as read-heads for magnetic hard disks and non-volatile magnetic memories. The manganite of composition La0.7Sr0.3MnO3 (LSMO) showing a Curie temperature above 300 K is of particular interest since it can potentially lead to devices operated at room temperature.We present the high-resolution angle resolved vectorial Kerr magnetometry characterizations at room temperature of epitaxial La0.7Sr0.3MnO3 thin films grown by pulsed laser deposition onto SrTiO3 in different crystallographic directions and on vicinal substrates. The symmetry breaking effects due to the strain induced by the substrate is depicted within the framework of the current theory involving four-fold and two-fold magnetic anisotropy competition . La0.7Sr0.3MnO3 films grown on vicinal (001) SrTiO3 show a pure uniaxial anisotropy with the easy axis lying along the steps. In films grown onto (110) SrTiO3 and onto (001) SrTiO3 a more complicated picture can be described in term of biaxial and uniaxial anisotropy contributions, related to the magnetocrystalline and substrate induced strain effects, respectively. In particular, in vicinal films the symmetry breaking at atomic steps is at the origin of the in-plane uniaxial magnetic anisotropy contribution, whereas in flat (001) and (110) films the biaxial magnetocrystalline anisotropy is balanced by the contribution of the uniaxial anisotropy due to the strain induced by the substrate, resulting in a butterfly-like behavior of the remanence magnetization polar plot rather than a four-leaves one that is expected for a system with cubic symmetry .  A. Ruotolo, et al., Appl. Phys. Lett. 91, 132502 (2007) M. Mathews, et al., Appl. Phys. Lett. 87, 242507 (2005) M. Bowen, et al., Appl. Phys. Lett. 82 (2), 233 (2003) J. Camarero et al., Phys. Rev. B 77, 024426 (2008)
10:45 AM - E1.5
Substrate Effects on Magnetotransport Properties of the Pr0.5Ca0.5MnO3 Thin Films.
Margo Staruch 1 , Joseph Budnick 1 , Menka Jain 1 2 , Haiyan Wang 3 Show Abstract
1 Physics , University of Connecticut, Storrs, Connecticut, United States, 2 Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 3 Electrical and Computer Engineering, Texas A&M University, College Station, Texas, United States
Thin films of Pr0.5Ca0.5MnO3 were deposited on (001) SrLaAlO4, (001) LaAlO3, (001) NdGaO3, and (001) SrTiO3 substrates using a chemical solution deposition technique. X-ray diffraction and surface area diffraction patterns confirmed that the films were c-axis oriented. Films were also strained as expected due to lattice mismatch between film and substrate. The extent and type of strain varied with thickness of the film and substrate, respectively. The zero field resistivity of the films showed semiconducting behavior. The magnetotransport behavior of these films has been compared with the bulk behavior, which exhibits charge ordering and very high melting magnetic fields (27T). For films of different thickness on SrTiO3 substrate, it was found that the thinnest film had maximum strain and maximum magnetic field was required to melt the charge ordering in the film. With the increase in the thickness of the film the strain is reduced but the melting field drops considerably leading to colossal magentoresistance at low temperatures. The change in melting field due to the thickness of the film is most likely related to the defects produced by the relaxation of the stress in the film. More details of the structural, microstructural, and magnetotransport properties of the Pr0.5Ca0.5MnO3 films with both compressive and tensile strains will be presented.
11:30 AM - **E1.6
Half-metallic Manganite Surfaces.
Neil Mathur 1 , Lee Phillips 1 , Xavier Moya 1 , Alessandro Potenza 2 , Stuart Cavill 2 , Sarnjeet Dhesi 2 Show Abstract
1 Materials Science, University of Cambridge, Cambridge United Kingdom, 2 , Diamond Light Source, Didcot United Kingdom
Carbon nanotubes are good for spin transport as the carriers move quickly and depolarize slowly . Half-metallic manganites make suitable electrodes for these non-magnetic channels, but there is scope for optimizing the manganite surface, cf. the optimization of buried oxide interfaces in magnetic tunnel junctions . I will discuss epitaxial manganite films that were grown by pulsed laser deposition, and studied at various temperatures on beamline I06 of the Diamond Light Source using PhotoEmission Electron Microscopy (PEEM) and X-Ray Magnetic Circular Dichroism (XMCD).  LE Hueso, JM Pruneda, V Ferrari, G Burnell, JP Valdés-Herrera, BD Simons, PB Littlewood, E Artacho, A Fert and ND Mathur, Nature 445 (2007) 410.  Y Ishii, H Yamada, H Sato and H Akoh, Appl. Phys. Lett. 89 (2006) 042509
12:00 PM - E1.7
The Origin of Spin-valve-type MR (SVMR) in Sr2FeMoO6.
Sugata Ray 1 2 , Srimanta Middey 2 , Somnath Jana 2 , Alok Banerjee 3 , Rajeev Rawat 3 , Prabuddha Sanyal 4 , Dipankar Das Sarma 5 Show Abstract
1 Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India, 2 Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata, West Bengal, India, 3 , UGC-DAE CSR, Indore, Madhya Pradesh, India, 4 , S. N. Bose National Centre for Basic Sciences, Kolkata, West Bengal, India, 5 Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore, Karnataka, India
Understanding the spin-dependent tunneling of conduction electrons through an insulating barrier, placed between two ferromagnetic metallic electrodes, as a result of the application of external magnetic field is becoming a key requirement in the emerging field of spintronics . This exciting electronic process leads to drastic changes in electrical conductivity of a material as a function of applied magnetic field and is termed as tunneling magnetoresistance (TMR). The simplistic theoretical model of TMR predicts that the electrical resistance should be lowest in case of perfect parallel spin alignment of the neighboring ferro-/ferrimagnetic grains, enabling maximum electron tunneling, while the highest resistive state of the material, at a given temperature, should occur at the zero magnetization point (the magnetic coercivity, Hc) . This prediction has been proved to be true for most of the TMR materials, while only recently an exception was observed  in cold-pressed polycrystalline pellets of a room temperature TMR system, namely Sr2FeMoO6 . This unusual phenomenon immediately opened up an avenue of interest with its valve-like control on TMR, and was duly named as spin-valve-type MR (SVMR). However, given the difficulty, the real mechanism behind SVMR was not experimentally investigated in the beginning, although an appealing theoretical picture was proposed . Consequently, we tried to probe SVMR using magnetic measurement techniques like ac and dc susceptibilities as well as magnetoresistance measurements, in great detail, which has revealed the true origin of SVMR. Our experimental investigation proves the spontaneous formation of a spin-glass (SG) like surface layer around each soft ferrimagnetic (FiM) grain of Sr2FeMoO6 and also the existence of a very strong exchange coupling between the glassy skin and the soft ferrimagnetic core, generating the well-known ‘exchange bias’ effect . Our results reveal that the pinned ferrimagnetic spins at the FiM/SG interface act as the ‘valve’, which is the central point of the SVMR effect. The concurrence of the temperature at which SVMR appears and the Tg of the glassy surface provides the most revealing and conclusive proof for the proposed mechanism. In support of these experimental results, detailed simulation studies, based on model Hamiltonian, are currently underway on an extended version of our earlier proposed model. S. A. Wolf et al., Science 294, 1488 (2001). S. Perkin, H. Yang, S.-H. Yang, and M. Hayashi, Handbook of Magnetism and Advanced Magnetic Materials, edited by H. Kronmüller and S. Perkin (Wiley 2007). D. D. Sarma, S. Ray, K. Tanaka, M. Kobayashi, A. Fujimori, P.Sanyal, H.R. Krishnamurthy, and C. Dasgupta, Phys. Rev. Lett. 98, 157205 (2007). K. -I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, and Y. Tokura, Nature 395, 677 (1998). M. Ali et al., Nature Materials 6, 70 (2007).
12:15 PM - E1.8
Growth and Structure-Property Correlations in La0.67Sr0.33MnO3/CeO2/YSZ/Si(001) Heterostructures.
Ravi Aggarwal 1 , Sudhakar Nori 1 , Punam Pant 1 , Roger Narayan 1 2 , Jagdish Narayan 1 Show Abstract
1 Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Joint Department of Biomedical Engineering, University of North Carolina, Chapel Hill, North Carolina, United States
The half-metallic perovskite La0.67Sr0.33MnO3 (LSMO) is an ideal candidate for multifunctional epitaxial heterostructures which have potential applications in spintronics and other memory devices . The important physical phenomena exhibited by manganites, such as metal-insulator transition (MIT), colossal magnetoresistance (CMR) and para to ferromagnetic phase transition are very sensitive to the choice of substrate material, growth conditions and buffer layers used . Growth of high quality epitaxial films of LSMO on Si(001) substrates is important for technological applications of LSMO. In the present work, we report growth and characterization of epitaxial LSMO/CeO2/YSZ/Si(001) heterostructures deposited by pulsed laser deposition. The growth of LSMO on CeO2 buffer layer in the present study is in contrast to the earlier reports in the literature where a Bi4Ti3O12 (BTO) buffer layer (along with YSZ buffer layer) has been used to integrate LSMO with Si(001). The detailed x-ray diffraction characterization confirmed that LSMO films were (001) oriented and epitaxial. The crystalline quality of LSMO films was assessed by rocking curve measurements. Further structural characterization was done with high resolution transmission electron microscopy which showed that interfaces in LSMO/CeO2/YSZ/Si(001) heterostructure were sharp. The electrical resistivity measurements showed that the resistivity of LSMO films peaks at ~260 K. Isothermal magnetization was measured at different temperatures in the range 10 - 350 K, with magnetic field strengths of up to 20 kOe. It was observed that below 320 K the LSMO films showed perfect ferromagnetic (FM) behavior, with paramagnetic (PM) behavior becoming prominent at higher temperatures. The temperature dependent magnetization data also corroborated well with M(H) data and indicated a FM-PM transition at ~340 K. It was found that Bloch’s T3/2 law fits fairly well to the saturation magnetization data. References:1. A. Tiwari, C. Jin, D. Kumar and J. Narayan, Rectifying electrical characteristics of La0.7Sr0.3MnO3/ZnO heterostructure, Applied Physics Letters 2003;83:1773.2. V. Bhosle, J.T. Prater and J. Narayan, Anisotropic magnetic properties in  oriented epitaxial La0.7Sr0.3MnO3 films on (0001) sapphire, Journal of Applied Physics 2007;102.
E2: Spin Transport, Magnetoresistance
Monday PM, November 30, 2009
Room 103 (Hynes)
2:30 PM - **E2.1
Electric-field Manipulation of Magnetization Vector.
Hideo Ohno 1 Show Abstract
1 Laboratory for Nanoelectronics and Spintronics Research Institute of Electrical Communication, Tohoku University, Sendai Japan
Ferromagnetism and magnetization in Mn-doped III-V semiconductors can be manipulated by various means; for example, by changing its carrier concentration by electric fields or by spin-current flowing along with the electric current. In addition, because of the anisotropic spin-orbit split valence bands, one can expect to see modulation of the magnetic anisotropy potential landscape by changing the carrier concentration. Here, we report electrical control of magnetization direction through manipulating magnetic anisotropy by electric-fields . In order to measure the magnetic anisotropies under a gate that applies the electric-field to the channel, we measured the planar Hall effect as a function of the angle of in-plane external magnetic-fields. Analyses showed that there are biaxial as well as uniaxial anisotropies. As the sheet carrier concentration is reduced by applying electric-field to the channel, the uniaxial anisotropy field reduced its magnitude and eventually changed its sign, whereas no significant change was apparent in the biaxial anisotropy field. From the electric-field dependent anisotropy fields, one can show that the angle of the magnetization direction in the absence of magnetic fields can be modulated by electric-fields. This opens up a unique route for switching magnetization direction solely by electronic means, not resorting to magnetic-field, spin-current, mechanical stress, nor multiferroics. References:D. Chiba, M. Sawicki, Y. Nishitani, Y. Nakatani, F. Matsukura, and H. Ohno, Nature, 455 (2008).Acknowledgement:We acknowledge D. Chiba, F. Matsukura, M. Sawicki, Y. Nishitani, Y. Nakatani and T. Dietl for fruitful collaboration and discussion. This work was supported in part by the "Research and Development for Next-Generation Information Technology" program from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
3:00 PM - E2.2
Multiferroic Tunnel Junctions: Prediction of Four Resistance States from First-principles.
Julian Velev 1 , Chun-Gang Duan 3 , J. Burton 2 , Alexander Smogunov 4 , Erio Tosatti 4 , Sitaram Jaswal 2 , Evgeny Tsymbal 2 Show Abstract
1 Department of Physics, University of Puerto Rico, San Juan, Puerto Rico, United States, 3 Key Laboratory of Polarized Materials and Devices, East China Normal University, Shanghai China, 2 Department of Physics, University of Nebraska, Lincoln, Nebraska, United States, 4 , International Centre for Theoretical Physics International Centre for Theoretical Physics , Trieste Italy
Electron tunneling and ferroelectricity have had long but separate histories. In the past decade both attracted significant interest due to application in electronic devices such as magnetic tunnel junctions (tunneling) and ferroelectric capacitors (ferroelectricity) relevant to non-volatile random-access memories. Recently, driven by demonstrations of ferroelectricity in ultrathin films, it was proposed to combine these two phenomena in a multiferroic tunnel junction (MFTJ) utilizing a ferroelectric barrier between two magnetic electrodes . Due to sensitivity of the conductance to both the magnetization alignment of the electrodes (magnetoresistance) and orientation of the polarization in the ferroelectric barrier (electroresistance), this junction can serve as a four-state resistance device. Here based on first-principles calculations we demonstrate the existence of the four resistance states in SrRuO3/BaTiO3/SrRuO3 MFTJs with asymmetric interfaces. We find that the resistance of such a MFTJ is significantly changed when the electric polarization of the barrier is reversed and/or when the magnetizations of the electrodes are switched from parallel to antiparallel. These results reveal exciting prospects of MFTJs for application in multifunctional electronic devices.
1. E. Y. Tsymbal and H. Kohlstedt, “Tunneling across a ferroelectric”, Science 313, 181 (2006).
3:15 PM - E2.3
Modification of The Interface Between Ferromagnetic Metal Electrodes and Organic Substances for Improvement of Spin-valve Characteristics.
Ryuta Adachi 1 , Ryota Sasaki 1 , Hirokazu Tada 1 Show Abstract
1 Division of Materials Physics, Osaka University, Toyonaka Japan
Considerable attention has recently been paid to the study of spin dependent carrier tarnsport in sandwich structures composed of organic materials (OMs) and ferromagnetic (FM) metal electrodes. It has been reported, in the research field of semiconductor-based spintronics, that the conductivity mismatch between FM metals and semiconductors prevents spins from the efficient injection. It seems also the case for organic spintronic devices. Most works reported so far on organic spintronics used half metallic La0.67Sr0.33MnO3 (LSMO) as an electrode of sandwich structures. Clear spin-valve characteristics were not demonstrated for the devices composed of OMs and ordinal FM metals such as Co, Fe and NiFe. In the present study, we modified the FM metal-organic interface in layered sandwich structures by inserting thin oxide layers, and nanocomosites to improve the spin injection properties.The devices were prepared on thermally grown SiO2 surfaces. The bottom electrodes with a dimension of 1 mm x 5mm x 30 nm were fabricated on SiO2 by photolithogaraphy and lift-off techniques. The electrodes were coated with Al2O3 or nanocomposites of pentacene/Co. Al2O3 was prepared by oxidation of thin aluminum films deposited on the NiFe elecrodes. Nanocomposite layers were prepared by vacuum evaporation of Co metals onto thin pentacene layers. Organic films with the thickness of approximately 200 nm were deposited on the modified electrodes. The upper electrodes were prepared by evaporation of Co, on which a thin aluminum layer was prepared to protect the Co layer from the oxidation. Spin-valve characteristics with a mageto-resistance ratio of 2 – 10 % were observed for the devices with modified electrodes in the temperature range from 5 K to 100 K. It was found that the modification of the FM metals-organic interfaces improved the spin injection characteristics.
3:30 PM - E2.4
Growth and Carrier Dynamics of Molecular Superlattice Toward Molecular Spintronics Device.
Nobuya Hiroshiba 1 2 , Ryoma Hayakawa 1 , Petit Matthieu 1 , Toyohiro Chikyow 1 , Kiyoto Matsuishi 2 , Yutaka Wakayama 1 Show Abstract
1 Advanced Electronic Materials Center, NIMS, Tsukuba Japan, 2 Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba Japan
The aims of this study are to clarify the carrier dynamics properties such as charge transfer (CT) and optical carrier generation, recombination at hetero molecular interface and to explore the possibility of application of the molecular superlattice for spintronics. Organic molecules are one of major candidate materials for spintronics because they have weak spin-orbital (SO) interactions, long spin relaxation time and a variety of choice of materials. Moreover, the organic molecules have various functions related with spin degree of freedom (SDF) such as ferromagnetism, magnetic-resistance (MR) and superconductivity. These functions related with SDF are mainly caused by CT and/or optical carrier doping at the hetero molecular interface in superlattice structure . However, the device applications using these functions have not been successful so far and challenges still remain for basic studies of molecular single-crystals. To solve these issues, we propose molecular superlattice as a thin-film device, where p- and n-type semiconducting molecules are alternately stacked with well-defined order. In this study, the molecular superlattice consisting of N,N’-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) and quaterrylene (QT) layers were prepared by using an ultra-slow deposition technique [2,3]. The morphology of the films and the orientation of the molecules in each layer were analyzed by atomic force microscopy (AFM) and an x-ray reflection (XRR) method. To examine the carrier transport, the transistor properties of hetero molecular devices were measured. In addition, the displacement current measurements (DCM) were carried out to clarify the process of carrier injection and accumulation. Photoluminescence (PL) was measured to investigate optical properties and carrier dynamics.Ambipolar properties were observed in transistor properties, indicating that the hole and electron carriers were transported in QT and PTCDI-C8 respective layers without annihilation by scattering at the hetero interface. This result clearly demonstrates that the molecular superlattice has a long mean free path, which is advantageous for spin carrier transport. On the other hand, we observed unique photoluminescence excitation (PLE) spectra, which are different from those of single-component molecular films. The result shows the possibility of CT between QT and PTCDI-C8 layers.These results demonstrate that the molecular superlattice possesses a potential for controlling spin carrier injection and transport. N. Toyota, M. Lang and J. Müller, Low-Dimensional Molecular Metals, Springer Series in Solid-State Sciences vol. 154, Springer-Verlag, Berlin Heidelberg (2007). H. Sasaki et. al., Appl. Phys. Lett. 88 (2006), 081907 N. Hiroshiba et. al., Org. Electronics, (in press).
3:45 PM - E2.5
Bias Dependence of Spin Filter Tunneling in Magnetic Insulators.
Martina Mueller 1 2 , Guo-Xing Miao 2 , Jagadeesh Moodera 2 , Claus Schneider 1 Show Abstract
1 Institute of Solid State Research, Research Centre Juelich, Juelich Germany, 2 Francis Bitter Magnet Lab, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Spin filtering (SF) is an efficient route to produce highly spin-polarized currents which are essential for spintronics applications. SF relies on electron tunneling through a magnetic tunnel barrier whose exchange-split conduction band leads to different tunneling probabilities for each spin type. The SF efficiency is directly connected to the magnetic and transport properties of the barrier, while its quantification is usually conducted by indirect techniques, i.e. magnetoresistance measurements or using superconducting counterelectrodes.We present a study of the characteristic features of spin-filter tunneling in Europium Oxide (EuO)-based tunnel junctions without using magnetic electrodes or external magnetic fields for spin detection. Current-voltage (I-V) curves show a pronounced bias voltage dependence which is caused by the superposition of direct- and Fowler-Nordheim (FN) tunneling paths. Direct evidence for spin filtering is given by successive onsets of FN tunneling at high bias voltages. From temperature-dependent I-V curves we extracted the variation of effective tunnel barrier height below the Curie temperature, which systematically correlates with the spontaneous magnetization of EuO. We deduced an exchange splitting of EuO of 0.48 eV and thus could determine the resulting spin filter efficiency of the magnetic tunnel barrier by fully electrical means.
4:30 PM - **E2.6
Magnetic Field Effects from Excited States in Organic Semiconductor Devices.
Bin Hu 1 Show Abstract
1 , University of Tennessee, Knoxville, Tennessee, United States
It has been found that non-magnetic organic semiconductors can exhibit magnetic responses in electrical current, electroluminescence, photoluminescence, and photocurrent when an external magnetic field is applied. These intrinsic magnetic responses have been named as magnetic field effects. Especially, the magnetic field effects indicate that an external magnetic field can be used as a non-contact approach to control the light-emitting, photovoltaic, and charge transport processes in organic semiconductors. Furthermore, the magnetic field effects can be used as effective tools to visualize spin-dependent excited states and charge transport in electroluminescence, photoluminescence, photocurrent, and electric current in organic semiconductors. In particular, the magnetic field effects have opened a new direction to develop organic spintronics by using all non-magnetic organic semiconductors.In general, the magnetic field effects can be naturally attributed to (i) magnetic field-dependent singlet/triplet ratio and (ii) singlet/triplet ratio-dependent excited processes . There are two critical issues: tuning spin-orbital coupling and controlling spin-spin interaction, that can determine the amplitude and sign of magnetic field effects. Specifically, tuning spin-orbital coupling can modify the dependence of singlet/triplet ratio on magnetic field and consequently changes the amplitude of magnetic field effects. On the other hand, controlling spin-spin interaction can directly change singlet and triplet involvements in spin-dependent excited processes, leading to positive and negative tuning on magnetic field effects. Based on recent research progress, this presentation will discuss (i) how spin-orbital coupling can be tuned by using inter-molecular interaction in controlling the amplitude of magnetic field effects , (ii) how singlets and triplets are involved in magnetic field effects in understanding the mechanism of magnetic field effects , and (iii) how the spin-spin interaction can be changed by controlling electron-hole pairing distance in tuning magnetic field effects with positive and negative signs towards the development of magnetic field effects-based organic spintronics.
5:00 PM - E2.7
Hysteretic Magnetoresistance in Organic Diodes.
Sayani Majumdar 1 2 , Himadri Majumdar 1 , Harri Aarnio 1 , Ronald Osterbacka 1 Show Abstract
1 Dept. of Physics and Center for Functional Materials, Åbo Akademi University, Åbo, Åbo, Finland, 2 Wihuri Physical Laboratory, Department of Physics and Astronomy, University of Turku, Turku, Turku, Finland
Over the past two decades, the growth of organic electronics has been phenomenal. All research has been predominantly focused towards the understanding of charge transport in organic semiconductors (OS) small molecules and pi-conjugated polymers (PCP)s and its implications in devices. Study of spin in organics is a relatively unexplored territory. It provides a whole new scope of physics along with immense potential for application. Major challenge in the field of spintronics is the optimization of electron spin lifetimes across relevant length scales and hetero-interfaces. In this respect, OS is envisioned to be better than their inorganic counterparts. OS comprises of light molecules like carbon, hydrogen etc. and possess low spin-orbit interaction and hyperfine interaction; leading to longer spin-correlation length. PCPs are especially the better choice as their conjugation length is much higher than the small molecules and oligomers, leading to better transport properties and their solution processability lead to simpler fabrication process. Magnetic field effects on OS were demonstrated already in the early ‘90s. However, demonstration of sizable room temperature magnetoresistance under small applied magnetic fields in organic diodes  (OMAR) caused resurgence of interest. To explain this effect several models have been proposed but none of them could fully explain all the experimental observations. Earlier, we reported that for electron-hole pair formation is important for observation of OMAR in PCP diodes . Here, we report another important aspect of this effect - hysteretic OMAR in polymeric diodes. We found that magnitude and lineshape of OMAR depends strongly on the scan speed of the magnetic field and on the time delay between two successive measurements. The width and magnitude of OMAR varied with the polymeric material. These observations in organic diodes are in addition to the weak ferromagnetic ordering in the PCP materials in the ground state . The ferromagnetism could not be explained by the impurity content in the material alone. All these experimental observations will be discussed during the presentation and signature of similarity of magnetotransport in other inorganic materials will be provided. References:T. L. Francis, Ö. Mermer, G. Veeraraghavan and M. Wohlgenannt, New J. Phys. 6, 185 (2004).S. Majumdar, H. S. Majumdar, H. Aarnio and R. Österbacka, Phys. Rev. B 79, 201202 (2009).S. Majumdar, H. S. Majumdar, J. O. Lill, J. Rajander, R. Laiho and R. Österbacka, Synth. Met. (submitted, arXiv:0905.2021).
5:15 PM - E2.8
Magnetic Tunnel Junctions with Co-based Perpendicular Magnetic Anisotropy Multilayers.
Zeenath Tadisina 1 2 , Anusha Natarajarathinam 2 , Subhadra Gupta 1 2 , Tim Mewes 2 , Patrick LeClair 2 , Eugene Chen 3 , Shengyuan Wang 3 Show Abstract
1 Metallurgical & Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, United States, 2 MINT Center, The University of Alabama, Tuscaloosa, Alabama, United States, 3 , Grandis, Inc.,, Fremont, California, United States
CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJ) with perpendicular magnetic anisotropy (PMA) free and reference layers have been optimized for high PMA and high tunneling magnetoresistance (TMR). The effect of Co thickness, Pd thickness and number of Co/Pd bilayers on the anisotropy and coercivity of the [Co/Pd]*n multilayer films have been studied for both free and reference layers. The best results were obtained for [Co(0.2nm)/Pd(1nm)]*9 bilayers for a reference layer with a coercivity of 5000 Oe after annealing, and [Co(0.3nm/Pd(1nm)]*4 bilayers for the free layer with a coercivity of 1200 Oe after annealing. The magnetic behavior of these PMA systems was studied by alternating gradient magnetometry. A Pd/Co ratio of 5 resulted in high perpendicular anisotropy and perfectly square loops, whereas decreasing the Pd/Co ratio to 1 at a Co thickness of 0.5 nm resulted in bow-tie shaped out-of-plane anisotropy loops indicating a multi-domain structure. X-ray diffraction, transmission electron microscopy (TEM) and local electrode atom probe (LEAP) studies were carried out to investigate the structure of the multilayers, interface smoothness, and growth of (111) texture as a function of deposition conditions and post-deposition annealing. CoFeB/MgO/CoFeB trilayers sandwiched between the PMA multilayer material systems were studied as a function of CoFeB thickness and MgO thickness. The resistance-area (RA) product and tunneling magnetoresistance (TMR) of the unpatterned MTJ stacks were tested by current-in-plane tunneling (CIPT) measurements to optimize the MgO barrier and PMA stacks prior to actual device fabrication. After initial matrix experiments were carried out to determine the experimental parameter space, a statistical Design of Experiments (DOE) was conducted to optimize the film structure that would maintain stable perpendicular anisotropy for a reasonable thickness of CoFeB and MgO. The CoFeB thickness was varied from 0.4 nm to 2 nm and the MgO thickness was varied between 1 to 2 nm. The transport properties of the patterned MTJ stacks were measured in a PPMS system from 10K to 400 K. A TMR of 10% at 10K was obtained for these perpendicular MTJ’s (pMTJ) even before optimization of the MgO and CoFeB with a combination of in-situ lamp annealing and post-deposition furnace annealing and field cooling.
5:30 PM - **E2.9
Interfacial Magnetoelectric Coupling for Electrically Controllable Spin-based Properties.
Vincent Garcia 1 2 , Manuel Bibes 1 , Stephane Fusil 1 , Karim Bouzehouane 1 , Shaima Enouz-Vedrenne 4 , Cyril Deranlot 1 , Laura Bocher 3 , Dominique Imhoff 3 , Neil Mathur 2 , Agnes Barthelemy 1 Show Abstract
1 , Unité Mixte de Physique CNRS/Thales, Palaiseau France, 2 Dept. Materials Science, University of Cambridge, Cambridge United Kingdom, 4 , Thales Research & Technology, Palaiseau France, 3 Laboratoire de Physique des Solides, Université Paris Sud, Orsay France
Spintronics focuses on physical effects that take advantage of the electron spin in addition to its charge . A current drawback of spintronics is the large power that is usually required for magnetic writing, in contrast with nanoelectronics that relies on zero-current, gate-controlled operations. Efforts have been made to control spins with electric fields , for instance by varying the spin-relaxation rate, the Curie temperature, or the magnetic anisotropy with a gate voltage, but these effects are usually small and volatile. Direct or strain-mediated magnetoelectric coupling may be used to manipulate magnetization , and thus possibly the spintronic response of spin-valves or magnetic tunnel junctions . Often, the key magnitude controlling the spintronic response is not the magnetization but the carrier spin polarization. Motivated by this observation we have defined and studied magnetic tunnel junctions integrating a ferroelectric barrier. At room temperature, we use piezoresponse force microscopy to show robust ferroelectricity down to 1 nm in highly strained BaTiO3 films, and conductive atomic force microscopy to demonstrate the resistive readout of the polarization state via its influence on the tunnel current. The resulting electroresistance effect scales exponentially with the ferroelectric film thickness, reaching ~75000 % at 3 nm . Our approach exploits the otherwise undesirable leakage current - dominated by tunneling at these very low thicknesses - to read the polarization state without destroying it.We also find a reversible variation of the tunnel magnetoresistance depending on the polarization direction in the ferroelectric. This experimentally demonstrates the predicted local, large and non-volatile control of spin polarization using a ferroelectric . Our results may be viewed as the signature of a giant interfacial magnetoelectric coupling and a step towards the ferroelectric control of other spin-based properties like magnetic anisotropy and exchange interactions. More practically, they suggest a new low-power approach for spin-based information control.  C. Chappert et al, Nature Mater. 6, 813 (2007). H. Ohno, et al, NATURE 408, 944 (2000) Y.-H. Chu, et al. Nature Mat. 7, 478 (2008) ; W. Eerenstein et al, Nature Mater. 6, 348 (2007) M. Bibes & A. Barthélémy, Nature Mater. 7, 425 (2008) ; C. Binek & B. Doudin, J. Phys.: Condens. Matter. 17, L39 (2005) V. Garcia et al.; Nature doi : 10.1038/nature08128 C.-G. Duan et al, Phys. Rev. Lett. 97, 047201 (2006)
E3: Magnetoresistance and X-ray Spectroscopy in Organic and Molecular Systems
Tuesday AM, December 01, 2009
Room 103 (Hynes)
9:30 AM - **E3.1
Carbon Nanotubes, Graphene, Molecules : Promising Materials for Spintronics.
Albert Fert 1 , Jean-Marie George 1 , Luis, E. Hueso 2 , Henri Jaffres 1 , Neil, D. Mathur 2 , Richard Mattana 1 , Michael Tran 1 Show Abstract
1 CNRS/Thales, Universite Paris-Sud, Palaiseau France, 2 Department of Materials Science, University of Cambridge, Cambridge United Kingdom
The usual materials of classical spintronics are magnetic and nonmagnetic metals, magnetic and nonmagnetic semiconductors and, for tunnel junctions, insulating materials like MgO or alumina. However, nowadays, promising results begin to be obtained with a new family of materials which includes carbon nanotubes, graphene and several types of magnetic or non-magnetic molecules. The general advantage of carbon-based materials is mainly their long spin lifetime related to the small spin-orbit coupling of carbon, but, as we will see, the very high electron velocity of some of them is also of great interest for spintronics.The first part of the talk will be an introduction on classical spintronics and a review of what can be done with molecular materials for TMR, spin transport in lateral structures, magnetic switching or microwave generation by spin transfer. In the second part of the lecture I will focus on the general problem of spin transport in anonmagnetic lateral channel between a spin-polarized source and a spin-polarized drain, a structure which is at the basis of several concepts of logic devices or spin transistors. The main difficulty is related to the transformation of the spin information – related to the magnetic configuration of the electrodes- into a large electrical signal, ideally ΔV/V ≈ 1 or larger, if V is the bias voltage and ΔV some voltage variation induced by a change of the magnetic configuration. In experiments on structures in which the lateral channel is a metal or a semiconductor, ΔV/V does not exceed a few 1% and the electrical signal ΔV is generally in the μV range . In contrast, in the experiments on carbon nanotubes between ferromagnetic contacts we will present, high values of ΔV/V ( above 70%) and large ΔV (of the order of 100 mV) can be obtained . After a description of the theoretical background, we will discuss the origin of the difficulties for semiconductors and explain why large values of ΔV/V and ΔV can be easily obtained with carbon nanotubes. We will emphasize the potential of carbon nanotubes, graphene and other molecules for spintronics, and conclude by presenting some next challenges for molecular spintronics. Jonker, B.T. and Flatté, M.E.F. Electrical spin injection and transport in semiconductors, in Nanomagnetism (eds. Mills D.L. & Bland J.A.C.) (Elsevier, 2006). Hueso, L. E., Pruneda J-M., Ferrari V., Burnell G., Valdés-Herrera J.P., Simons B.D., Littlewood P.B., Artacho E., Fert A. and Mathur N.D.. Transformation of spin information into large electrical signals via carbon nanotubes, Nature 445, 410 (2007).
10:00 AM - E3.2
Single Molecular Magnet - Gold Nanoparticle Composites.
Balaji Gopalan 1 , Gudrun Bovenkamp 1 , Vadim Palshi 1 , Challa Kumar 1 Show Abstract
1 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States
The past decade has witnessed a great interest in single molecule magnets (SMM) especially those based on manganese such as Mn12-acetate due to their potential applications in ultrahigh density data storage and quantum computing devices. The focus so far has been in finding different derivatives of Mn or other complexes based on transition elements in order to raise the blocking temperature of single-molecule magnets (SMMs). Herein we propose a fundamentally new approach of generating new category of SMMs through formation of composite structures by binding SMMs on to nanoparticles. In this report, we present the synthesis and characterization of single molecular magnet - gold nanoparticle composite- Mn12-cysteinate functionalized gold nanoparticles. The synthesis involved first functionalization of Mn12-acetate using cysteine ligand to obtain Mn12-cysteinate. This is followed by a ligand exchange reaction between dodecanethiol capped Au nanoparticles and Mn12-cysteinate ligands resulting in Mn12-cysteinate-Gold nanoparticle composite. The composite was characterized using several methods, primarily synchrotron radiation-based X-ray absorption spectroscopy and high resolution transmission electron microscopy (HRTEM).
10:15 AM - **E3.3
A Site-specific Kondo Effect at Ambient Temperatures in Iron-based Molecules and the Modulation.
Hongjun Gao 1 , L. Gao 1 , Q. Liu 1 , Y. Zhang 1 , N. Jiang 1 , H. Zhang 1 , S. Du 1 , W. Hofer 2 Show Abstract
1 , Institute of Physics, CAS, Beijing China, 2 , University of Liverpool, Liverpool United Kingdom
Due to their importance in the emerging field of molecular electronics, the transport properties of single molecules have been the focus of intensive research. Magnetic properties of transition metal atoms in a host molecule can be detected by a Kondo resonance in cryogenic scanning tunneling microscopes. These properties are important variables in the fabrication of single molecule devices. In this talk, I will present measurements of the Kondo effect of iron phthalocyanine (FePc) molecules on an Au(111) surface. Our results indicate a high Kondo temperature, well above room temperature, for the FePc molecule adsorbed on Au(111) surface. It is further revealed that a substantial change of the effect is with the adsorption configuration of the molecule. Furthermore, magnetic properties of an interface can be controlled at the molecular level by site-specific adsorption and modification of molecular structures. We verify by first-principles calculations that the molecular environment alters the hybridization of spin-polarized states of the molecule with states of the metal substrate, and that this alteration depends on the adsorption site of the molecule. The finding opens up the possibility to tailor magnetic properties of an organic interface to the desired specifications.
10:45 AM - E3.4
Experimental and Device Modeling Approaches to Understanding the Sign Change in Organic Magnetoresistance.
Francisco Bloom 1 , Martijn Kemerink 1 , Wiebe Wagemans 1 , Bert Koopmans 1 Show Abstract
1 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
The organic magnetoresistance effect (OMAR) has been a scientific puzzle since no traditional magnetoresistance mechanisms can explain a room temperature low magnetic field (~20mT) effect with such a large magnitude (up to 25%) without the use of any magnetic materials. Another puzzling feature of OMAR is that the sign of the magnetoresistance (MR) can be changed by varying the temperature or the applied voltage. This sign change has been the topic of much recent research and resolving the origin of the sign change should be a major step in understanding the microscopic origin of OMAR.Previously, we have explored the properties of the sign change experimentally with Alq3 based devices. These devices showed a strong correlation between the sign change and the onset of minority charge carrier (hole) injection and we could describe the lineshape and MR(V) behavior as a superposition of two MR effects of opposite sign . From this work we concluded the separate MR effects were from the mobilities of holes and electrons having different responses to magnetic fields, which is best described by the bipolaron model for OMAR . To test this conclusion, we employed analytical and numerical device models assigning separate magnetomobilities to holes and electrons . The models show, counterintuitively, that in the case when the minority charge carrier contact is injection limited, a decrease in minority charge carrier mobility increases the current. This is a result of the minority carrier contact acting like a constant current source, and of the compensation of the majority carrier space charge by the oppositely charged minority carriers. The models show that lowering the space charge in the device by increasing the recombination decreases the strength of this sign inversion, this is verified experimentally in Alq3 devices doped with recombination centers . We show that these models describe the observed MR(V) behavior very well. If one assumes the magnetic field acts to reduce the mobility of electrons and holes, we observe that our models can reproduce all the sign changes observed in literature.The fact that the current can increase when the minority carrier mobility decreases may explain the fact that in experiments the magnitude of the negative MR features has been much larger than the positive MR features, even though, microscopically, the bipolaron model predicts the opposite. Therefore, the presence of both signs of magnetoresistance may be related only to the device physics and not to the microscopic mechanism which causes OMAR. F. L. Bloom, W. Wagemans, M. Kemerink, and B. Koopmans, Phys. Rev. Lett. 99 257201 (2008).P. A. Bobbert, T. D. Nguyen, F. W. A. van Oost, B. Koopmans, and M. Wohlgenannt , Phys. Rev. Lett. 99, 216801 (2007).F. L. Bloom,M. Kemerink, W. Wagemans, and B. Koopmans (submitted)F. L. Bloom, J. M. Veerhoek, W. Wagemans, and B. Koopmans (in preparation)
11:30 AM - **E3.5
Controlling the Magnetic Coupling of Fe-Porphyrin Molecules to Ferromagnetic Films.
Wolfgang Kuch 1 Show Abstract
1 Institut für Experimentalphysik, Freie Universität Berlin, Berlin Germany
Organic molecules as building blocks of surface-mounted nanoscale systems have reached tremendous impact in solid state physics. Metallo porphyrins display a quasi-planar geometry, allowing for a two-dimensional assembly as electronic circuits or devices, while the fourfold coordinated metal center can be accessed by two additional adsorption sites. This can be used to gain control on the metal center spin. Manipulating the size or direction of the spin of the metal ion by tuning the molecule–substrate interaction would be an important step towards the realization of a surface-mounted molecular spintronic device.X-ray magnetic circular dichroism (XMCD) spectra of paramagnetic Fe-octaethylporphyrin molecules adsorbed on ferromagnetic epitaxial ultrathin Ni and Co films on Cu(001) reveal a ferromagnetic coupling of the Fe atom to the Ni and Co films. From the temperature dependence of the Fe and substrate XMCD signals, the magnetic coupling strength can be estimated for the two magnetic substrates, being weaker in the case of Ni. Interestingly, the sign of the magnetic coupling can be reversed when placing oxygen atoms between the porphyrin molecules and the substrate. The insertion of oxygen is achieved by growing the Ni and Co films on a preoxidized Cu(100) surface. The oxygen atoms act as a surfactant for the growth of the ferromagnetic films, floating on top of the surface. This results in a well-characterized c(2×2) superstructure of 0.5 monolayer of atomic oxygen on top of the ferromagnetic film. XMCD reveals an antiferromagnetic coupling between the metal centers of the Fe-octaethylporphyrin molecules and the Ni and Co films, the coupling energy of which is comparable to the ferromagnetic interaction found without oxygen in the case of the Ni substrate, but weaker in the case of the Co substrate. The antiferromagnetic coupling resembles the situation in systems like, for example, transition metal monoxides, in which a super-exchange interaction across oxygen atoms is responsible for the antiferromagnetism,Results were obtained in collaboration with M. Bernien, H. Wende, J. Miguel, C. Weis, M. Piantek, X. Xu, Ph. Eckhold, J. Kurde, K. Baberschke, and B. Krumme.
12:00 PM - E3.6
Magnetic Couplings in Molecular Semiconductor Thin Films.
Sandrine Heutz 1 , Zhenlin Wu 1 , Salahud Din 1 , Marc Warner 2 , Gabriel Aeppli 2 , Tim Jones 3 , Wei Wu 2 , Andrew Fisher 2 Show Abstract
1 Department of Materials and London Centre for Nanotechnology, Imperial College London, London United Kingdom, 2 Department of Physics and Astronomy and London Centre for Nanotechnology, University College London, London United Kingdom, 3 Department of Chemistry, Warwick University, Warwick United Kingdom
Molecular thin films can be used as alternatives to inorganic semiconductors in optoelectronic applications, such as photovoltaics, organic light emitting devices and field-effect transistors, due to the delocalised π -electrons on the organic backbone. In addition, some macrocycles, such as phthalocyanines (Pcs) or porphyrins, can also contain transition metal ions with unpaired spins, as in the case of CuPc (S=1/2). Pcs crystallise in a variety of structures, and magnetic coupling in the metal backbones has been observed. However, to exploit the combination of magnetism and semiconducting properties that render those materials particularly interesting for spintronic applications, their thin film magnetic properties need to be assessed and controlled. In this contribution, we present the growth and magnetic characterisation of molecular thin films deposited using sublimation techniques in the high and low vacuum regime, namely organic molecular beam deposition (OMBD) and organic vapour phase deposition (OVPD). These techniques are compatible with current optoelectronic device fabrication and can produce a range of morphologies and structures in Pcs, all corresponding to stacked molecules, but with different molecular shifts. We show that in the case of MPc films, magnetic coupling is indeed observed, enabling the transfer of interesting single crystal properties onto flexible substrates [1, 2]. Crucially, this coupling depends on the structure of the films, and can be switched between ferro and antiferro-magnetic during polymorphic phase transition. This work is extended to structures containing a combination of polymorphs, which can be fabricated by controlled nucleation or self-assembly in OVPD, and highlight the versatility of molecular films for information storage and logic.  Heutz et al., Advanced Materials 19 (2007)3618. J. van den Brink and A. F. Morpurgo, Nature 450 (2007) 177.
12:15 PM - E3.7
Laterally Resolved Picosecond Magnetodynamics in Spin-Valve-Type Thin Film Elements.
Claus Schneider 1 , Alexander Kaiser 1 , Carsten Wiemann 1 , Stefan Cramm 1 , Carsten Tieg 2 Show Abstract
1 Institute of Solid State Research IFF-9, Research Center Juelich, Juelich Germany, 2 , ESRF, Grenoble France
Exploring the ultimate time scales of magnetic switching processes is an important issue in spin electronics. In particular, in spin valves or magnetic tunneling junctions magnetocrystalline anisotropies and interlayer exchange coupling are going to change the magnetodynamic response of the entire system as compared to the dynamics of the individual layers. Understanding the role of these interactions is a key to the design of optimized devices.We have thus employed time-resolved x-ray photoemission microscopy to address the magnetization dynamics in spin-valve type model systems in the nano- and picosecond regime. In Co/Cr/Fe(001) single crystalline elements we find a strong influence of the magnetocrystalline anisotropy, which tends to suppress rotation processes. On the nanosecond time scale, the dynamic response of top and bottom magnetic layer is the same, whereas on the picosecond time scale there is an indication for a dynamic “decoupling” of the layers. In polycrystalline FeNi/Cr/FeCo trilayer elements, the interlayer coupling character is found to determine the dynamic response. In particular, the rotational processes in the FeNi and FeCo layers differ significantly in the precession amplitude, which can be related to the different coercivity of the individual layers. By contrast, the motion of domain walls in both layers closely agrees, which can be explained by an enhancement of the interlayer coupling due to the domain wall stray fields. Our examples demonstrate that the detailed magnetodynamics in coupled magnetic layers is quite complex and depends strongly on the time scale under consideration. This has consequences for the performance of GMR-based spintronics elements.
12:30 PM - E3.8
Magnetic Responses of Ferromagnetic Ultrathin Film Upon the Presence of Adjacent Organic Layer.
Yuet-Loy Chan 1 , Ya-Jyuan Hung 1 2 , Jia-Hao Wang 1 2 , Hsu-Ting Chang 1 2 , Ying-Chang Lin 1 , Chih-Hao Lee 2 , Yao-Jane Hsu 1 , Der-Hsin Wei 1 Show Abstract
1 , National Synchrotron Radiation Research Center, Hsinchu Taiwan, 2 Department of Engineering and System Science, National Tsing Hua University, Hsinchu Taiwan
One recent development in spintronics is the incorporation of organic materials to take advantage of its weak spin de-coherence process that low-Z materials generally possess. Intuitively, a hybrid structure consisted of an organic layer sandwiched by two ferromagnetic films should be able to enjoy less constraint during structure fabrication while maintains its magnetic properties comparable with the all solid-state ones. However, for spin to travel through a ferromagnet/organic semiconductor/ferromagnet (FM/OSC/FM) trilayers, it involves not only the spin transport within organic film but also the spin injection and reception across FM/OSC interfaces. Therefore, it is imperative to explore what chemical and magnetic properties that interfaces are exhibited so that improved understanding on the electron/spin transport within organic-inorganic hybrid structures could be established. In this study, we utilized the in-situ prepared Cobalt/Pentacence/Cobalt (Co/Pc/Co) trilayer to examine how a magnetic (organic) thin film would response to the presence of an adjacent organic (magnetic) layer at ultrathin film regime. Employing the X-ray photoelectron spectroscopy (XPS) at various deposition stages, we found the Co layer is chemically inert upon the addition of Pc, but becomes chemically reactive when depositing on Pc layer. Unexpected disparity was also found in the magneto-optic Kerr effect (MOKE) measurements between Co/Pc and Pc/Co bilayers, in which a relatively high Co coverage was needed to have the magnetic order become observable. Employing the X-ray photoemission electron microscope (XPEEM), we found the morphology of Pc layer had an apparent impact on the domain configuration of Co top layer.
12:45 PM - E3.9
Investigations on the MTJ Interface Fe-MgO using Electron Spectroscopy with X-ray Standing Waves.
Benjamin Balke 1 2 5 , Christian Papp 1 2 , See-Hun Yang 3 , Stuart S. P. Parkin 3 , Sven Doering 4 , Ulf Berges 4 , Carsten Westphal 4 , Charles Fadley 1 2 Show Abstract
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 Dept. of Physics, Univ. of California Davis, Davis, California, United States, 5 , Univ. of Mainz, Mainz Germany, 3 , IBM Almaden Research Center, San Jose, California, United States, 4 Physics Dept., Tech. Univ. of Dortmund, Dortmund Germany
Multilayer nanometer-scale structures are ubiquitousin current magnetic devices, and the detailed characteristics of the layersmaking them up, including the interfaces between layers, are often decisive asto ultimate functional properties. Buried layers and interfaces are thus crucialelements in such devices, as well as many other nanoscale structures ofpotential interest in technology, but characterizing them fully presents uniquechallenges.We have used x-ray standing wave excitation of photoelectrons to study buried layersand interfaces between Fe and MgO as used in MTJs at the ALS in Berkeley.The samples were grown on synthetic multilayer mirrors and the x-ray incidence anglewas tuned to the 1st order Bragg reflection. This technique permits scanning the resultant standing wave field through nm-scale structures and analyzing the depth distribution of their structural, chemical, electronic, and magnetic properties.We were able to investigate the structural interfaces in the MTJ stack as well as the magnetic Fe-MgO interface performing depth-resolved XMCD measurements. Furthermore, it was possible to extract the matrix-element weighted depth-resolved DOS from the measurements of the valence band using the standing wave technique.BB and CP gratefully acknowledge the Feodor-Lynen fellowship of the Humboldt foundation.
E4: Heusler Alloy and DMS
Tuesday PM, December 01, 2009
Room 103 (Hynes)
2:30 PM - **E4.1
Nuclear Magnetic Resonance Studies of Highly Spinpolarized Heusler Compounds.
Sabine Wurmehl 1 , Jürgen Kohlhepp 2 , Henk Swagten 2 , Bert Koopmans 2 , Christian Blum 3 1 , Vadim Ksenofontov 3 , Horst Schneider 3 , Gerhard Jakob 3 , Daniel Ebke 4 , Günter Reiss 4 , Bernd Büchner 1 Show Abstract
1 Institute of Solid State Research, IFW Dresden, Dresden Germany, 2 Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven Netherlands, 3 , Johannes Gutenberg - Universität, D-55128 Mainz Germany, 4 Thin Films and Nano Structures, Department of Physics, Bielefeld University, D-33501 Bielefeld Germany
A successful application of highly spin-polarized materials in spintronic devices requires a detailed knowledge of the interplay between the structure and the magnetic and electronic properties. This is achieved by gaining knowledge of the local structure by means of Nuclear Magnetic Resonance (NMR). NMR directly probes the local environments of the active atoms and is thus able to resolve neighboring shells providing a unique tool to study the (local) structural properties of spin polarized materials [1,2].This talk will focus on recent results of structural characterization of highly spin polarized Heusler compounds by means of NMR, in particular focusing on thin films of Co2FeSi. Co2FeSi is predicted to be a half-metallic ferromagnet with an extraordinary high magnetic moment and Curie temperature . However, a low tunnel magneto-resistance ratio, a lower spin polarization than predicted, and a lower magnetic moment were experimentally observed in thin film samples. The NMR study shows the main resonance line corresponding to 59CoCo nuclei in the L21 environment but also additional resonance lines at the high frequency side of the main line with mutual spacing of 32 MHz. The additional resonance lines correspond to 59Co with more Fe next neighbours than expected for the L21 type ordering, which is interpreted as the formation of an off-stoichiometric film yielded by sputtering from a stoichiometric target. This off-stoichiometry might explain the observed deviations from the expected half-metallic behaviour . S. Wurmehl et al., Appl. Phys. Lett. 91, 052506 (2007). S. Wurmehl, J. T. Kohlhepp, Topical review in J. Phys. D: Appl. Phys. 41, 173002 (2008). S. Wurmehl et al., Appl. Phys. Lett. 88, 032503 (2006). S. Wurmehl et al., J. Phys. D: Appl. Phys. 42, 084017 (2009).
3:00 PM - E4.2
Doping the Half-Heusler Alloy NiMnSb With 3d-Metals.
Igor Abrikosov 1 , Bjoern Alling 1 , Marcus Ekholm 1 , Andrei Ruban 2 Show Abstract
1 Department of Physics and Measurement Technology (IFM), Linkoping University, Linkoping Sweden, 2 Department of Material Science and Engineering, Royal Institute of Technology, Stockholm Sweden
The crystal structure has a pronounced effect on the magnetic properties of materials, and therefore its modification allows one to tune the magnetic properties of the alloys. We illustrate this idea by results of a theoretical study of the effect of doping the half-Heusler alloy NiMnSb with the magnetic 3d-metals Cr, Mn, Fe, Co, and Ni both with respect to energetics and magnetic properties [1,2]. The half-Heusler alloy NiMnSb is believed to be half-metallic at 0 K. This would make it ideal for spintronics applications. However, experiments have failed to reproduce full spin polarization at elevated temperatures. Starting from the formation energies we discuss the possibility of placing the 3d- dopants on different crystallografic positions in the alloy. We calculate total and local magnetic moments, effective exchange interactions as well as the density of states and outline strategies to tune the magnetic properties of the alloy. Doping of NiMnSb with Cr as well as substituting some Ni with extra Mn has the largest impact on magnetic interactions in the system while preserving the half metallic property. In particular, we predict that there is very large exchange coupling between the anti-site Mn and the atoms of the original Mn sublattice. Moreover, we have investigated the effect of replacing the Ni with Mn on the structural stability of this compound. Our calculations show that the disordered alloy can be stable at and above room temperature. Therefore, we suggest the possibility that Mn and Cr dopants increase the thermal stability of half-metallicity in NiMnSb with implications for its possible usage in spintronics applications. We also demonstrate the importance of thermal effects such as temperature-induced electronic, magnetic and vibrational excitations, as well as structural defects in the first-principles calculations of the magnetic critical temperature of complex alloys using half-Heusler Ni1−xCuxMnSb alloys as a case study . The thermal lattice expansion and one-electron excitations have been accounted for self-consistently in the Curie temperature calculations. In the Ni-rich region, electronic excitations, thermal expansion, and structural defects substantially decrease the calculated Curie temperature. At the same time, some defects are shown to increase the Curie temperature in Cu-rich samples. B. Alling, S. Shallcross, and I. A. Abrikosov, Phys. Rev. B 73, 064418 (2006). B. Alling, M. Ekholm, and I. A. Abrikosov, Phys. Rev. B 77, 144414 (2008). B. Alling, A. V. Ruban, and I. A. Abrikosov, Phys. Rev. B 79, 134417 (2009).
3:15 PM - E4.3
Epitaxial growth of a Full-Heusler Alloy Co2FeSi on Silicon.for Si-based Semiconductor Spintronics
Shinya Yamada 1 , Yuji Enomoto 1 , Kenji Kasahara 1 , Kazutaka Yamane 1 , Kenji Yamamoto 1 , Yuichiro Ando 1 , Kohei Hamaya 1 2 , Masanobu Miyao 1 Show Abstract
1 Department of Electronics, Kyushu University, Fukuoka Japan, 2 PRESTO, Japan Science and Technology Agency, Kawaguchi Japan
Electrical spin injection and detection in ferromagnet/semiconductor devices are key technologies for semiconductor spintronic applications. Silicon (Si) has been regarded as an ideal material because it has a long spin relaxation time and compatibilities with Si-ULSI technologies. Recently, we explored highly epitaxial growth of a ferromagnetic binary-Heusler alloy Fe3Si on Si by low-temperature molecular beam epitaxy (LT-MBE) and obtained high-quality heterointerfaces at the growth temperature (TG) of 130 oC . Using the high-quality interfaces, we also demonstrated electrical spin injection and detection through Fe3Si/Si Schottky tunnel barriers . However, the spin transport in Si has been limited in low temperature regime (< 273 K). To realize room-temperature demonstration of the spin transport in Si, we need to use half-metallic source-drain contacts which can realize high efficiency of the spin injection and detection. Considering these, we hope that full-Heusler alloys are promising materials for its half-metallic source-drain contacts. Expanding our LT-MBE techniques for Fe3Si/Si, we have explored highly epitaxial growth of a full-Heusler alloy Co2FeSi on Si(111) at various growth temperatures (TG). Co, Fe and Si were co-evaporated using Knudsen cells. During the growth, we observed in-situ reflection high energy electron diffraction (RHEED) patterns. At TG = 60 - 200 oC, the RHEED patterns exhibited symmetrical streaks, implying single crystal and good two-dimensional epitaxial growth of the Co2FeSi layers. To evaluate the Co2FeSi/Si interfaces, we measured cross-sectional transmission electron micrograph (TEM). For TG = 200 oC, ~ 4-nm-thick interlayers can be identified at the Co2FeSi/Si interface. Although the interlayer materials were decreased for TG = 130 oC, they can be seen yet. For TG = 60 oC, on the other hand, a very flat interface within the fluctuation of a few monolayers was observed. We also evaluated the presence of L21 ordered structures near the interface. Superlattice reflections of nano electron diffraction patterns were very weak for TG = 60 oC unfortunately, but we can enhance them by optimizing post-annealing conditions. For the Co2FeSi/Si layers annealed at 200 oC for 30 min, L21 ordered structures were observed with an abrupt heterointerface. Furthermore, magnetic properties of the Co2FeSi/Si(111) were similar to those of the high-quality Fe3Si/Ge(111) and Fe3Si/Si(111) which we observed [1,3]. From these facts, we made ready to demonstrate electrical spin injection and detection in silicon through a half-metallic Co2FeSi/Si(111) interface.  K. Hamaya et al ., Appl. Phys. Lett. 93, 132117 (2008).  Y. Ando et al ., Appl. Phys. Lett. 94, 182105 (2009). Y. Ando et al ., J. Appl. Phys. 105, 07B102 (2009).
3:30 PM - **E4.4
Spin-resolved Unoccupied Density of States in Heusler Alloys.
Hans-Joachim Elmers 1 , Michael Kallmayer 1 , Peter Klaer 1 , Horst Schneider 1 , Elena Arbelo Jorge 1 , Christian Herbort 1 , Gerhard Jakob 1 , Martin Jourdan 1 Show Abstract
1 , University Mainz, Mainz Germany
Within the field of spintronics half-metallic ferromagnetism (HMF) plays a major role. HMF stands for a metallic character of e.g. the majority-spin states while the minority-spin states comprise an energy gap at the Fermi level. Thus, the electrical current is carried exclusively by majority-spin states making HMF materials very attractive for the fabrication of spintronic devices. Co-based Heusler alloys Co2YZ (transition metal Y and main group element Z) have attracted much attention in this field because ab-initio theory has predicted HMF and a high Curie temperature for many of these compounds. Moreover, in order to overcome the thermally induced suppression of high spin polarization in Heusler alloys a tailoring of the band structure through doping of the ordered alloys has been proposed. In contrast to the numerous theoretical predictions, only a few experimental verifications of the calculated band structures exist. A direct study of the band gap is however of particular importance for further improvement of these alloys with respect to spintronic applications. Although spin-resolved photoemission or scanning tunneling spectroscopy can directly probe the spin polarization at a half-metal surface, these methods have no access to the crucial buried interfaces in spintronic devices. We investigate the electronic properties of epitaxial Heusler films and polycrystalline bulk samples using circular dichroism in X-ray absorption spectroscopy (XMCD). Considering final state electron correlations, we are able to extract the spin-resolved partial density of states at the Co atom from XMCD data. Our experimental results corroborate the predicted half-metallic ferromagnetic properties of these alloys and reveal a compositional dependence of the Fermi energy position within the minority band gap.
4:30 PM - E4.5
Negative Magnetoresistance in Solid State Materials. The One Effect – The One Physical Model?
Sergei Obukhov 1 Show Abstract
1 , A.F.Ioffe Institute of Physics and Technology, Saint-Petersburg Russian Federation
Effect of resistance decrease in magnetic field, Negative Magnetoresistance (NM), has been an important issue in semiconductors, ferromagnetic metals, superconductors and graphen studies over the past 50 years. But despite the considerable attention to the physical nature of this phenomenon and perspective to use NM in magneto-electronics the physical nature of NM still remains the matter of discussion. Giant Negative Magnetoresistance (ρB=0/ρB~105÷107) observed in Diluted Magnetic Semiconductors (DMS), for example in Hg1-xMnxTe and Magnetic Semiconductors (MS) such as Gd1-xvxS4 and EuSe, has always been explained in the framework of spin-polaron model [1,2]. In case of ferromagnetic metals a relatively small NM effect, (ρB=0/ρB)max< 1,2, was described as s-d scattering decrease in magnetic field. For NM effect explanation in materials which did not contain transition or rare earth metals ( nonmagnetic semiconductors, graphen, superconductors and etc.) the quantum correction  and weak localization models  have been used.Our experiments have demonstrated that gigantic value of NM (ρB=0/ρB=5T~104) can also be observed in nonmagnetic semiconductors - uniaxially stressed doped single crystals- p-Ge(Ga), p-InSb(Ge) and p-InSb(Mn) at low temperature T~1,2K and acceptor concentration Na within the critical concentration of Metal-Insulator Transition (MIT) Ncr =Na=NGa=NGe =NMn~1017cm-3. These facts support a new view on the nature of NM in solid state materials. We propose that the field of elastic stress around impurities or vacancies can restructure energy band and form antiferromagnetic spin-ordered sub-bands in solid state materials. These sub-bands are separated by energy gap Δ with the maximal value Δ~1meV in the case of InSb(Mn)  and by energy gap Δ in the range 1÷10meV in MS and DMS. External magnetic field aligns spins, closes the gap Δ between sub-bands and causes MIT, e.g. NM in semiconductors. In solid state materials with high impurity concentration like ferromagnetic metals and graphen these sub-bands could overlap in the absence of magnetic field. In the framework of above model external magnetic filed enhances sub-bands overlapping and thus conductivity increases. In superconductor cuprates the energy gap Δ could be associated with pseudogap reduced by magnetic field. References J.K.Furdyna and J.Kossut. Diluted Magnetic Semiconductors , Vol. 25, Semiconductors and Semimetals, Academic Press, Inc. (1988). E.L.Nagaev. Physics of Magnetic Semiconductors, Moscow, Mir. (1983).  B.L.Altshuler and A.G.Aronov, in Electron-Electron Interaction in Disordered Systems, edited by A.L.Efros and M.Pollak. North-Holland, Amsterdam. (1985). X. Zhang, Q.Z. Xue, D.D. Zhu. Physics Letters A, 320, 471(2004). S.A.Obukhov.Phys. Stat. Sol. (b) 242, No. 6, 1298–1306 (2005).
4:45 PM - E4.6
CeO2 based Diluted Magnetic Dielectrics for Spin Filter Application.
Paul Slusser 1 , Dhananjay Kumar 2 , Ashutosh Tiwari 1 Show Abstract
1 Materials Science and Engineering, university of utah, Salt Lake City, Utah, United States, 2 Mechanical Engineering, NC A& T University, Greensborro, North Carolina, United States
The ongoing search for room temperature spin filters has uncovered several candidate materials recently. Highly notable among these materials are the cerium oxide based Diluted Magnetic Dielectrics (DMDs). Few years ago our group showed that CeO2:Co films exhibit high temperature ferromagnetism with a giant magnetic moment. That report created a lot of excitement in the field. This was mostly because apart from being ferromagnetic CeO2:Co posses a cubic structure and is closely lattice matched with silicon. Moreover CeO2 has a high dielectric constant (26) and is transparent over the entire visible spectrum with a bandgap of 3.4 eV. Now in this report we are describing the synthesis and magnetic characterization of three new CeO2 based DMD systems. These systems are: copper, manganese, and zinc doped cerium oxide [Ce1-xCuxO2-δ, Ce1-xMnxO2-δ, and Ce1-xZnxO2-δ]. High quality epitaxial thin films of these materials were grown on LaAlO3(001) substrates by pulsed laser deposition (PLD) and were characterized using several state-of-the-art characterization techniques. Magnetization measurements showed that in the case of Ce1-xCuxO2-δ (x=0.03), films exhibit room temperature ferromagnetism with a saturated magnetization of ~1.0 μB/Cu atom. However, when additional copper ions were introduced into the system (x=0.15) superparamagnetic behavior was observed. In the case of Ce1-xMnxO2-δ films ferromagnetic behavior was observed in both the x=0.03 as well as x=0.15 samples. But, in the case of x=0.15 film evidence of phase separation was observed. In contrast to the above two cases, the Ce1-xZnxO2-δ and undoped CeO2 films showed a diamagnetic response. An in-depth structure-property correlation and analysis was performed to understand the observed magnetic behavior of the materials.
5:00 PM - E4.7
Samarium Oxide Thin Film Dilute Magnetic Dielectrics.
Nathan Gray 1 , Ashutosh Tiwari 1 , Dhananjay Kumar 2 Show Abstract
1 Materials Science and Engineering, University of Utah, Salt Lake City, Utah, United States, 2 Mechanical Engineering, North Carolina A&T, Greensborro, North Carolina, United States
Spin tunnel filters are a very promising method of spin injection into semiconductors, which is required to achieve a functional spin transistor. EuSe, a magnetic insulating material, has been shown to achieve nearly 100% spin polarized injection current by Moodera. Dilute magnetic dielectrics also have very good potential for high efficiency spin injection as tunnel filters. In 2006 Co doped CeO2 was discovered to exhibit room temperature ferromagnetism with a giant magnetic moment. Motivated by these results, we have explored another dilute magnetic dielectric based on a rare earth oxide system, Co doped Sm2O3, and the type and origin of its magnetic properties.Samarium oxide is already under investigation as an optoelectronic material and a high-k dielectric replacement for SiO2 in complementary metal oxide semiconductors (CMOS). It has been shown to have a high dielectric constant (30.5 for epitaxial cubic phase). It is also chemically and thermodynamically stable with Si.We have doped Sm2O3 with varying amounts of Co, and fabricated thin films using pulsed laser deposition. Several deposition parameters were varied, and the effects on final crystal structure were determined. The films were extensively characterized to determine crystalline phase properties and magnetic characteristics. They show super paramagnetism with high magnetic moment per Co ion, and very large hysteretic response below blocking temperature. The origin of this magnetism is investigated, and the presence of secondary magnetic phases of cobalt and cobalt oxide are ruled out through x-ray diffractometry and high resolution transmission electron microscopy. A relationship between defect density and magnetic moment is found and discussed.
5:15 PM - E4.8
Intrinsic Exchange Bias in a Single Phase Ferrimagnet.
Kevin West 1 2 , Dao N. Nam 3 , Jiwei Lu 3 , Ryan Comes 2 , Stuart Wolf 3 Show Abstract
1 Physics , University of California at San Diego, La Jolla, California, United States, 2 Engineering Physics, University of Virginia, Charlottesville , Virginia, United States, 3 Material Science and Engineering, University of Virginia , Charlottesville , Virginia, United States
Exchange bias (EB) commonly presents itself as a shift of the hysteresis loop along the field axis in systems that have a ferromagnetic-antiferromagnetic interface. The EB phenomena has generally been considered an interface effect. Here we report on very peculiar magnetic hysteresis behavior of Ru0.25Cr0.75O2/TiO2 (001) thin films. We describe three previously unreported magnetic phenomenon inherent to this material; (i) intrinsic positive exchange bias in a single phase epitaxial thin film ferrimagnet with the absence of an exchange interface (ii) field modulation of the shifted M(H) loop along the field axis direction and (iii) inverted hysteresis (IH) loops in which the overall area of the loop is negative. We argue that some of these results can be understood within the context of a very unusual interface of the epitaxial film with the TiO2 (001) substrate.
5:30 PM - E4.9
Electronic and Magnetic Properties of Small Co-O Nanowires.
Liudmila Pozhar 1 Show Abstract
1 Physics, University of Idaho, Moscow, Idaho, United States
Electron spin states of exchange-biased, core-shell quantum dots (QD) and wires (QW) satisfy major requirements applied to elements of quantum information processing and communication systems that are expected to support integrated information storage-processing, enable secure communications, and provide for a significant increase in efficacy and reliability of electronic devices and computational equipment of the future. In particular, typical electron spin relaxation time exceeding a nanosecond and reaching milliseconds has been experimentally observed for such systems, and a possibility to control single electron spins localized on an individual QD or QW has been demonstrated. In the work reported here electronic and magnetic properties of exchange-biased, core (Co)-shell (Co-O) QWs have been studied using the first principle, quantum many-body theoretical methods, including the Hartree-Fock (HF), restricted open shell HF, configuration interaction (CI) and multiconfiguration self-consistent field (CASSCF/MCSCF) approximations. The initial structure of the Co cores of the studied QWs have been derived from that of the corresponding symmetry elements of Co bulk lattices, and the minimum number of oxygen atoms has been added to the “surface” of the cores forming the corresponding shells to stabilize the molecules. The final geometry and composition of the studied QWs have been obtained by the total energy minimization procedure. In the case of the “quantum-confined” QWs the total energy has been minimized while applying spatial constraints to the center-of-mass positions of the QW atoms, to reflect QW synthesis in quantum confinement. The geometry of the corresponding “vacuum” QWs have been obtained by the unconditional total energy minimization, when the spatial constraints applied to the quantum-confined QWs were relaxed. The studies have been focused on the electronic energy level structure, direct optical transition energy (OTE), and charge and spin density distributions (CDD and SDD, respectively). The obtained results indicate that both vacuum and confined Co/C-O QWs are stable being molecules from a chemical standpoint, and possess significant SDDs that are about an order of magnitude larger than those of the previously studied semiconductor compound QDs. A typical ground state of these QWs is a HF triplet. The OTE of these QWs can be manipulated in a wide interval of values from below 1 eV to over 5 eV by adjusting the number of oxygen atoms in the shells, Co atoms in the cores, and the structure and composition of the quantum confinement. The obtained results closely correlate with experimental data available in literature and confirm that the studied QWs models can be used to guide nanomaterials development for integrated quantum information storage-processing.
5:45 PM - E4.10
Size-selected Magnetic MnAs Nanoparticles by Nanoparticle Conversion.
Michael Wolff 1 , Detlef Goerlitz 1 , Kornelius Nielsch 1 , Maria Messing 2 , Knut Deppert 2 Show Abstract
1 Institute of Applied Physics, University Hamburg, Hamburg Germany, 2 Solid State Physics, University Lund, Lund Sweden
Nanostructured manganese arsenide (MnAs) has been studied extensively in the past years as it is an attractive material for spintronic applications. Typically, MnAs nanoparticles are synthesized by post-growth annealing of GaAs:Mn structures at elevated temperatures or by self-organized growth. In both cases, control over particle diameter and number concentration is difficult to achieve, because the formation of MnAs nanoparticles depends on various parameters such as temperature and Mn content. We report on the synthesis of ferromagnetic manganese arsenide nanoparticles via conversion of primary Mn particles which are generated in an aerosol process using a spark discharge generator. After sintering and size-selection in an aerosol setup, the particles are deposited on GaAs(100)B and Si(111) substrates. Subsequent conversion to MnAs particles takes place in an annealing process under hydrogen atmosphere with an arsine background pressure. No protection against oxidation or other kinds of degradation is applied before or after the annealing process. The annealed MnAs particles exhibit hexagonal facets on GaAs(100)B substrates, whereas on Si(111) they remain spherical. Remanence curves with a Curie temperature slightly above room temperature and hysteretic behavior of the annealed particles are verified using a SQUID magnetometer. The in-plane and out-of-plane coercivities of the particles were found to be different for the two studied substrates. While the coercivities were found to be similar on Si substrates, particles deposited on GaAs showed an anisotropic ferromagnetic behavior with distinguished differences between the two directions, which can be attributed to an epitaxial relationship between the MnAs particles and the GaAs as illustrated already by the hexagonal facets. The saturation magnetization was found to be of the order of 150 to 300 emu/cm3. The lower value as compared to bulk may be attributed to oxygen in the particles. Our work shows that the concept of nanoparticle conversion can yield size-selected magnetic nanoparticles with the possibility to tune size, density and material independently. This opens up the creation of other materials such as MnN, MnP or MnSb.
E5: Spinel, Rutile, DMS
Wednesday AM, December 02, 2009
Room 103 (Hynes)
9:30 AM - **E5.1
Spineltronics: Manipulating Spins in Spinel Compounds.
Brent Melot 1 , Daniel Shoemaker 1 , Ram Seshadri 1 Show Abstract
1 Materials, University of California, Santa Barbara, California, United States
The spinel crystal structure is seeing resurgent interest because of the possibility of exotic magnetic ground states associated with magnetic frustration and complex magnetic ordering. We describe work on spinel oxides with Cr and Mn on the B site, and magnetic tuning of the A site, from the dilute limit of no magnetism on the A site, to fully magnetic A sites. Starting with the parent compounds, CoCr2O4 and Mn3O4, both of which have conical ordering and hence multiferroic ground states, we show that magnetic tuning on the A site (substituting Co2+ and Mn2+ by Zn2+) yields rich magnetic phase behavior on the one hand, and intrinsic exchange bias on the other.
10:00 AM - E5.2
Atomic and Electronic Structure of Fe3O4(111)/MgO(111) and Fe3O4(111)/Al2O3(0001) Interfaces.
Vlado Lazarov 1 , Andras Kovacs 3 , Pedro Galindo 4 , Michael Weinert 2 Show Abstract
1 Physics, University of York, York United Kingdom, 3 Materials, University of Oxford, Oxford United Kingdom, 4 Computer Science, University of Cadiz, Cadiz Spain, 2 Physics, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin, United States
Magnetite is half metal oxide that is attracting a lot of attention in spintronics due to its high Curie temperature, which is well above the room temperature (850 K). Recent efforts to incorporate magnetite in a device type of structures such as spin valves and magnetic tunnel junctions (MTJ) were not very successful. The reports on tunnelling magneto resistance ratio on Fe3O4(100)/MgO(100) based MTJs were less than 20 %. In addition questions about 100 % spin-polarization of the thin magnetite(100) films were raised in the light of the recent spin polarized XPS data, that is showing only 50-60% spin polarization of (100) oriented magnetite films. However subsequent experiments on (111) oriented magnetite thin films have shown that spin-polarization in these films can be as high as 85%. In this work we studied the atomic and electronic structure of Fe3O4(111)/MgO(111) and Fe3O4(111)/Al2O3(0001) interfaces and their effect on the spin transport from the half metal electrode through the oxide (barrier). The films were grown by Molecular Beam Epitaxy on single crystal MgO(111) and Al2O3(0001) substrates. Aberration corrected HRTEM coupled with multislice image simulations were used for determining the atomic structure of the interfaces. The best fit atomic model structures to experimental data were found to be as following: ../Mg/O/Fe/.., and ../Al/O/Fe/… In both type of the interfaces Fe is in octahedral position. Density Functional Theory (DFT) modelling confirmed that these structure have the lowest total energy. Moreover the DFT calculated electronic structure of the experimentally found structures for both interfaces is similar. This particular interface atomic arrangement creates interface states at the Fermi level which can potentially induce spin flipping as a result of spin-orbit coupling of the tunnelling electrons. In addition we consider several other atomic interface models between magnetite and MgO(Al2O3) and discuss how their electronic structure would influence the spin transport across them.
10:15 AM - E5.3
Electrodeposited Superlattices in the Magnetite/Zinc Ferrite System.
Jay Switzer 1 , Rakesh Gudavarthy 1 , Elizabeth Kulp 1 , Andrew Wessel 1 , Samantha Matthews 1 Show Abstract
1 Department of Chemistry, Missouri University of Science and Technology, Rolla, Missouri, United States
In this work, both defect-chemistry and compositional superlattices in the Fe3O4/ZnFe2O4 system are electrodeposited. There is interest in depositing spintronic superlattices based on Fe3O4 because the ferrimagnetic material is purported to have 100% spin polarization at the Fermi level. Also, below the Verwey transition at 120 K, the material becomes ferroelectric. Hence, the material has multiferroic properties. In ZnFe2O4, the Zn(II) substitutes for Fe(II) producing a material that is antiferromagnetic below the Neél temperature. In the deposition of superlattices we use the fact that the departure from equilibrium is controlled through the applied overpotential. Films of Fe3O4 can be deposited with stoichiometries that depend on the applied potential. We prepare the films by electrochemical reduction of a Fe(III)-TEA (triethanolamine) complex at 80 oC in strongly alkaline solution. The deposition is believed to occur by an electrochemical-chemical (EC) mechanism. Because of the EC nature of the deposition reaction, it is possible to control the composition of the film through the applied potential. At low overpotentials at which j = 0, the surface concentration of Fe(TEA)3+ should be equal to the bulk concentration, whereas at high overpotential at which j = jL the surface concentration of Fe(TEA)3+ should approach zero. Hence the material should have an excess of Fe(III) at low overpotential and an excess of Fe(II) at high overpotential. Stoichiometric Fe3O4 should deposit at an applied potential of -1.065 V vs. Ag/AgCl. Superlattices of the material are electrodeposited by pulsing the applied potential between -1.01 and -1.065 V vs. Ag/AgCl. Compositional superlattices can be produced by adding Zn(II) to the deposition bath. Superlattices with modulation wavelengths ranging from 10 to 30 nm were produced. The superlattices were characterized by X-ray diffraction and focused ion beam (FIB) microscopy.Magnetoelectronic properties of the electrodeposited superlattices will be reported. A particularly interesting feature that the superlattices exhibit is resistance switching during perpendicular transport measurements.
10:30 AM - **E5.4
Prediction of False Ferromagnetism by LDA et al in Defected Oxides and 3d Impurities in Insulators.
Alex Zunger 1 Show Abstract
1 , NREL, Golden, Colorado, United States
Theorists have been predicting ferromagnetism in either dilute 3d doped insulators or in defected insulators using various derivatives of Local Density Approximation methods. Experimentalists often cite such predictions in support of generally vague evidence of magnetism in such systems. I discuss here a number of ways in which LDA and related methods, as currently practiced by many, can lead to "false positive" for predicting magnetism. These misuses include: (1)Level placed by LDA in continuum instead of in band gap ( Ref 1,2): Cases where LDA and LDA +U erroneously predict that a 3d level resides inside the host conduction band ( having thus extended wavefunctions) wheras better approximations place it inside the host band gap ( having localized wavefunctions). In this case LDA leads to the illusion of long range magnetic interactions. We show how to fix this error via the " Nonlocal external potential" (NLEP) method. (2)Forgetting Percolation (Ref 3,4): Cases where theorists introduce vacancies in their supercell to examine their effect on magnetism, but miss the point that the concentration of such vacancies is related to the (calculable) vacancy formation energy. Once this is calculated, one often finds out that the vacancy concentration is lower by many orders of magnitude then what is required for percolation of the magnetic interactions. We show how this problem can be fixed by self consistently linking the vacancy concentration to the formation energies. This shows that to achieve percolation one will need to enhance the vacancy concentration by 3-5 orders of magnitude relative to what equilibrium grants. Brutal growth conditions would be needed to accomplish such deviations from equilibrium . This may explain why often “good" samples show no magnetism whereas "poor “ ones do. (3)Forgetting Polaronic localization of ligand orbitals (Ref 5): In many cases LDA calculations of cation vacancies predict the ensuing acceptor wavefunction to be distributed equally on all anion ligands (thus, leading to rather delocalized magnetic interactions), yet in reality the acceptor wavefunction has a (polaron-like) localization on just a subset of anion ligands, leading, in reality, to vanishing magnetic interactions. We show how the polaron can be correctly descirbed by extending LDA to include self interaction effects. This, however, reveals virtually no magnetic interaction between cation vacancies in common oxides because the hole now resides in localized ligand orbitals.(1) S. Lany, H. Raebiger, and A. Zunger, Phys. Rev. B Rapid Communication 77, 241201 (2008).(2) H. Raebiger, S. Lany, and A. Zunger, Nature 453, 763-766 (2008).(3) J. Osorio-Guillen, S. Lany, S. V. Barabash, and A. Zunger, Phys. Rev. B 75, 184,421 (2007).(4) J. M. Osorio, S. Barabash, S. Lany and A. Zunger, Phy.s Rev. Letters 96, 107203 (2006).(5) J. A. Chan, S. Lany, and Alex Zunger, Physical Review Letters (In Press, 2009).* Work done in collaboration with S.Lany, J. Osorio, H. Raebiger and J.Chan and supported by DOE BES.
11:30 AM - E5.5
Robust Magnetism of (110) CrO2 Films.
Manjit Pathak 1 2 , Krishna Chetry 1 2 , Dipanjan Mazumdar 1 , Patrick LeClair 1 2 , Gary Mankey 1 2 , Arunava Gupta 1 Show Abstract
1 MINT Center, University of Alabama, Tuscaloosa, Alabama, United States, 2 Department of Physics and Astronomy, University of Alabama, Tuscaloosa, Alabama, United States
CrO2 is the only known half metal oxide with spin polarization of 98% and Curie temperature close to 395K. However, progress on CrO2-based structures such as magnetic tunnel junctions has been limited to date, perhaps in part due to the significantly strained CrO2 (100) films used in previous studies.In this study, CrO2 films of (100) and (110) orientations were epitaxially grown on TiO2 substrates with chemical vapor deposition (CVD) technique. Films on TiO2 (100) follow a layer-by-layer growth mode, with atomically smooth surfaces but significant out-of-plane compressive stress. In contrast, films on TiO2 (110) follow an island-like growth mode, and are found to be essentially strain-free for even the thinnest films studied (~35nm). Both orientations of CrO2 were studied with X-ray magnetic circular dichroism (XMCD). Analysis of our data with the conventional sum rules for 3d elements show almost 50% higher XMCD on (110) CrO2 films compared to (100) CrO2 films at the Cr-L2,3 edge at room temperature, consistent with SQUID magnetometry on the same samples. The orbital moment is found to be similar for both films and no significant XMCD was observed on the O-2p edge. This robust magnetism is attributed to the colinearity of Cr spins in (110) films which grows strain free on TiO2 substrates of same orientation, compared to canted Cr spins in strained (100) CrO2 films. This suggests (110)-oriented films may be more amenable to successful device fabrication.
11:45 AM - E5.6
Defect Controlled Ferromagnetic Properties of Vanadium Dioxide Thin Films: Structure-property Correlations.
Tsung-Han Yang 1 , Sudhakar Nori 1 , Honghui Zhou 1 , Jagdish Narayan 1 Show Abstract
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States
Inducing room temperature ferromagnetism (RTFM) in oxide dilute magnetic semiconductors (DMS) and insulators, such as ZnO, SnO2, In2O3 and MgO, has been a subject of intense investigation recently. The spins of the unpaired 3d electrons associated with the host atoms or the magnetic dopants or impurities in DMS materials interact with both defects and the oxygen atoms in a complex way altering the physical properties significantly. Defects arising out of oxygen vacancies play a critical role in determining the physical properties. Here, we present the control of defects in vanadium dioxide thin films by the stoichiometry adjustment route, owing to the narrow stability range in different phases, where the ratio of oxygen and vanadium varies from 1.5 to 2.5 such as V2O3, V3O5, V4O7, V5O9, VO2, and V2O5. Vanadium dioxide (VO2) exhibits an extremely sharp and ultra-fast phase transformation from low temperature monoclinic phase to high temperature tetragonal phase at 68 °C. The characteristics of phase transformation can be also controlled by defects and both coercivity and magnetization have been found to decrease with increasing temperature. In this study, we report ferromagnetism for the first time in VO2 thin film with Curie temperature around 500 K. By controlling the processing parameters, we are able to tune the defect contents and microstructure and consequently the magnetic properties. We also discuss correlations between the characteristics of phase transformation and ferromagnetic properties with the microstructure and the valence charge of undoped VO2 thin films, which are consistent with the analysis of transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). We believe that VO2 has high potential in multifunctional device applications related to spintronics, switching and magnetic recording.
12:00 PM - E5.7
Correlation Effects in the P-electron Magnet RbO2.
Claude Ederer 1 , Roman Kovacik 1 Show Abstract
1 School of Physics, Trinity College Dublin, Dublin, Co. Dublin, Ireland
Rubidium superoxide, RbO2, is an antiferromagnetic insulator with a Neel temperature around 15 K. It is thus a rare example of a bulk material with magnetic order resulting from partially filled p states. Here we present results of GGA+U calculations which show that the electron-electron interaction between p electrons on the oxygen sites leads to a strong tendency towards the formation of an orbitally polarized insulating state. This is in contrast to standard first principles calculations using the pure local density or generalized gradient approximations, which lead to a half-metallic ground state for this class of compounds. The energy differences calculated within the GGA+U approach between different orbitally ordered configurations are sizeable, indicating an orbital ordering temperature higher than the antiferromagnetic Neel temperature of ~15 K. Our results demonstrate the importance of correlation effects in p electron magnets such as RbO2, and suggest that this class of materials can provide important benchmark results for the future study of more complicated "d-0 magnets" where holes in the p electron states are induced by defects.
12:15 PM - E5.8
The Electronic Structure of GdN from X-ray Spectroscopy.
Andrew Preston 1 2 3 , Joe Trodahl 2 3 , Ben Ruck 2 3 , Louis Piper 1 , Alex DeMasi 1 , Kevin Smith 1 , James Downes 4 , Walter Lambrecht 5 Show Abstract
1 Department of Physics, Boston University, Boston, New York, United States, 2 , The MacDiarmid Institute for Advanced Materials and Nanotechnology, Wellington New Zealand, 3 School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand, 4 Department of Physics, Macquarie University, Sydney, New South Wales, Australia, 5 Department of Physics, Case Western Reserve University, Cleveland, Ohio, United States