Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S8: Focus Session: Spin-Dependent Transport, Tunneling, and Spin Torque |
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Sponsoring Units: GMAG Chair: Kui Gong, McGill University Room: 104 |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S8.00001: Novel Spin-dependent tunneling magnetoresistance of Fe/O/NaCl/O/Fe Kui Gong, Lei Zhang, Dongping Liu, Hong Guo We propose and theoretically investigate an very attractive novel magnetic tunnel junction (MTJ) Fe(001)/O/NaCl(001)/O/Fe(001) for spintronics. Due to the presence of the single p(1$\times$1)O layer between Fe electrode and NaCl insulator, the interfacial strain can be full released. Therefore, area perfectly ordered NaCl can be grow on top of Fe electrode. Since the unit cell of Fe crystal in [001] direction has two layers, there are two different kinds of contact interface between Fe electrode and NaCl insulator, i.e., the translational and mirror symmetry configurations. According to our ab initio total energy calculation, both of them are experimentally accessible. For the translational symmetry configuration, the tunneling magnetoresistance (TMR) ratio of Fermi energy is in the magnitude of 500\%. More interestingly, for the mirror symmetry configuration, the TMR ration will drastically increase to 5500\%. Different from the role of MgO barrier in well known Fe/MgO/Fe MTJ, the evanescent state with the $\Delta_5$ symmetry dominates the transmission of the majority spin electrons through the NaCl barrier. By studying the scattering states and the complex band structure of NaCl insulator, we systematically understand the transport properties of Fe/O/NaCl/O/Fe MTJ. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S8.00002: Interface structure of CoFeB/MgO magnetic tunnel junctions from hard x-ray photoelectron spectroscopy S. Mukherjee, D.D. Sarma, B. Pal, R. Knut, J. {\AA}kerman, S. Thiess, W. Drube, M. Gorgoi, A. Sahoo, P. Anilkumar, J. Perssons, O. Karis Present sensors in hard drives rely on tunnel magnetoresistance (TMR) in CoFeB/MgO/CoFeB structures. The device fabrication has been refined to meet strict demands. Despite this, fundamental understanding of the optimization process, i.e. post-annealing, is missing. In particular, boron diffusion has been suggested to be integral to the creation of a textured CoFe alloy with boron diffused either into the MgO tunnel barrier, forming boron oxides, or into a seed layer. Such diffusion would thus indirectly be essential for a large MR in the device. We have used hard x-ray photoelectron spectroscopy (HAXPES), to investigate a series of CoFeB/MgO/CoFeB structures. By systematically studying the modifications of chemical state of various constituents for different structures and post-annealing conditions, we are able to provide a detailed geometric interpretation of how elements diffuse and modify the structure. In particular we show that at the annealing temperatures required for achieving optimal MR, boron diffusion is limited to a very thin (sub-nm) region at the interface and does not progress beyond this point. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S8.00003: Recent Advances in Magnetic Tunnel Junction Materials and Stack for Thermally-Assisted Magnetic Random Access Memory Anthony Annunziata, Philip Trouilloud, Sebastien Bandiera, Stephen Brown, Michael Gaidis, Erwan Gapihan, Eugene O'Sullivan, Nathan Marchack, Daniel Worledge We report magnetic and electrical characterization measurements of sheet films and nanopillar magnetic tunnel junction devices useful for a new type of Magnetic Random Access Memory that is capable of operating at high ambient temperatures (greater than 125 C) and of surviving the high process temperatures used in silicon chip manufacturing. For unpatterned sheet film stacks, we report measurements of the magnetization versus applied field and temperature, antiferromagnet blocking temperature, and tunneling magnetoresistance. For patterned nanopillar devices in the size range of 80 - 200 nm, we report measurements of the tunneling magnetoresistance, depinning voltage and temperature, and sense and storage layer reversal fields. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S8.00004: Magnified Spin-Motive Forces in MRAM Magnetic Tunnel Junctions Stewart Barnes In the Slonczewski 2005 theory [1] for spin-torque-transfer (STT) of a magnetic tunnel junction (MTJ) the tunnelling magneto resistance (TMR) and Gilbert damping parameter $\alpha$ are of key importance. However the observed critical voltage from the switching of STT-MRAM implies a $\alpha$ ten times that measured by ferromagnetic resonance (FMR). In addition the TMR is strongly voltage dependent while the STT effect is not. This along with the weak dependence of the critical current on switching direction are inconsistent with the tunnelling model and have never been properly explained. Here will be described a circuit model based upon SU(2) theory for a MTJ for which the basic SMF of about 10$\mu$V is magnified to a 200mV shift between the parallel P and anti-parallel AP branches of the IV characteristic. It is implied that the TMR has for origin an SMF. [1] J. C. Slonczewski Phys, Rev. B71, 024411 (2005) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S8.00005: Spin Transfer Torque in Spin Filter Tunnel Junctions Christian Ortiz Pauyac, Alan Kalitsov, Aurelien Manchon, Mair Chshiev STT in MTJs is well known for its potential spin electronic applications. However, recently a new class of MTJs based on spin filtering across magnetic insulators (SFTJ) has been attracting much attention since in such MTJs electrons with a certain spin orientation tunnel much more efficiently. In this structure, STT remains to be addressed and clarified. Here we present a systematic study of its angular and voltage bias dependences consisting of one or two FM layers separated by a magnetic insulator (MI). The calculations were performed within the tight-binding model using NEGF technique in the framework of Keldysh formalism. We predict that STT is higher in magnitude compared to regular MTJs, which strongly depends in the relative directions of the magnetic states of the free layer (FM2) and MI. Namely, in case of parallel orientation of MI and FM2 moments in a FM1|MI|FM2 structure, the system behaves as a regular MTJ with a modest increase of STT magnitude. However, as the angle between MI and FM2 moments increases, the field-like torque becomes three orders of magnitude higher than the Slonczewski component and oscillates with bias as band-filling increases. This may have practical implications. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S8.00006: Spin-transfer torque in antiferromagnetic and ferrimagnetic tunnel junctions Pablo Merodio, Alan Kalitsov, Helene Bea, Vincent Baltz, Mairbek Chshiev Spin Transfer Torque (STT) and Giant-Magnetoresistance in ferromagnets (F) are the two essential underlying phenomena in modern spintronics. These effects have also been predicted to occur in nanostructures comprising only normal and antiferromagnetic materials. Therefore, antiferromagnets (AF) could potentially be used in place of F in future spintronic applications. We present a theoretical study of STT and Tunnelling Magnetoresistance (TMR) in AF and ferrimagnet (FI) based tunnel junctions, where two magnetic metal electrodes with at least one of them being FI or AF are separated by a thin nonmagnetic insulating barrier. We found that electronic structure parameters such as Fermi energy and exchange splitting of the FI and AF leads strongly influence STT and TMR properties including their bias dependence. In particular, STT spatial distribution within the leads shows nontrivial behavior which can be explained in terms of interplay between exchange splittings of the two AF or FI sublattices. Such insights will be of importance for optimizing current induced magnetization reversal phenomena. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S8.00007: Spin torques between ferromagnetic and compensated antiferromagnetic layers Adrian Popescu, Khartik Prakia, Paul Haney The current induced torques between a ferromagnetic layer and a compensated antiferromagnetic layer of various symmetries are considered. The general conditions under which these current induced torques can stabilize the out-of-plane configuration of the ferromagnet are provided, along with numerical results for specific models. The effects of phase breaking scattering and their experimental implications are also discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S8.00008: Theory of Intrinsic Spin Torque Due to Interface Spin-Orbit Coupling Alan Kalitsov, Mairbek Chshiev, William Butler, Oleg Mryasov The effect of intrinsic spin torque due to spin-orbit coupling (SOC) at the interface between thin ferromagnetic film and non-magnetic metal has attracted significant fundamental and applied research interest [1]. We report quantum theory of SOC driven spin torque (SOT) within the Rashba model of SOC and two-band tight binding (TB) Hamiltonian including s-d exchange interactions ($J$). We employ the non-equilibrium Green Function formalism and find that SOT to the first order in SOC has symmetry consistent with the earlier quasi-classical diffusive theory [2]. An obvious benefit of the proposed approach is the expression for the SOT given in terms of TB parameters which enables a physically transparent analysis of the dependencies of SOT on material specific parameters such as Rashba SOC constant, hopping integral, Fermi level and $J$. On the basis of analytical and numerical results we discuss trends in strength of SOT and its correlation with the Spin Hall conductivity.\\[4pt] [1] I. M. Miron \textit{et al}., Nature \textbf{476}, 189 (2011).\\[0pt] [2] A. Manchon and S. Zhang, Phys. Rev. B \textbf{78}, 212405 (2008). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S8.00009: Current-induced magnetization switching of a three terminal perpendicular magnetic tunnel junction by spin-orbit torque Murat Cubukcu, Marc Drouard, Olivier Boulle, Kevin Garello, Ioan Mihai Miron, Juergen Langer, Berthold Ocker, Pietro Gambardella, Gilles Gaudin A current flowing in the plane of a magnetic multilayer with structural inversion asymmetry, such as Pt/Co/AlO$_{\mathrm{x}}$, creates a torque on the magnetization [1]. This torque is due to the strong spin-orbit interaction present in such multilayers and can lead to fast magnetization reversal with a low writing energy [2]\textbf{. }We will present the first proof of concept of a perpendicular spin-orbit torque magnetic random access memory (SOT-MRAM) cell composed of a Ta/FeCoB/MgO/FeCoB magnetic tunnel junction. The basic write and read operations, i.e., the magnetization reversal by current injection in the Ta track and its detection using the high TMR signal, are demonstrated [3]. Our results open a path for the development of a novel class of three terminal MRAM combining fast, reliable and low energy writing. [1] I. M. Miron et al. Nature 476, 189 (2011) [2] K. Garello et al \underline {arXiv:1310.5586 }(2013) [3] M. Cubukcu et al., \underline {arXiv:1310.8235} (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S8.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S8.00011: Magnetoresistive anomaly in amorphous GdFeCo thin films Nattawut Anuniwat, Xiaopu Li, Joseph Poon, Jiwei Lu Spin valves generally consist of two ferromagnetic layers sandwiching a thin non-magnetic layer. High and Low resistance states can be obtained depending upon the relative magnetization alignment of the ferromagnetic layers. Recently, unexpected spin-valves like magneto-resistance has been observed in disordered ferrimagnetic crystalline material [1] and antiferromagnetic-based tunnel junction [2]. Here, we demonstrate spin-valve-like magnetoresistance in amorphous ferrimagnetic thin films. The amorphous GdFeCo films were deposited by rf magneton sputtering with the thickness $\sim$ 60 nm. The as-deposited film exhibited low saturation moment as $M_{S}$ $\sim$ 80 emu/cm$^{3}$. The compensation temperature is also observed near room temperature. The magneto-transport properties are performed on patterned Hall bar as a function of temperature ranging from 50K to 400K. The anomalous Hall resistance exhibits the same compensation temperature as magnetic moment. The asymmetric magnetoresistance reverse polarity at temperature below the compensation point indicating the different scattering mechanism than the anomalous Hall effect. We also demonstrate the bi-stable magneto-resistance with the absence of external H fields as a function of temperature up to 400K. The possible origins of this asymmetric MR will also be discussed.\\[4pt] [1] S. Singh, et al., Phys Rev Lett \textbf{109}, 246601 (2012).\\[0pt] [2] B. G. Park, et al., Nat Mater \textbf{10}, 347 (2011). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S8.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S8.00013: Role of spin polarization in FM/Al/FM trilayer film at low temperature Ning Lu, Richard Webb Measurements of electronic transport in diffusive FM/normal metal/FM trilayer film are performed at temperature ranging from 2K to 300K to determine the behavior of the spin polarized current in normal metal under the influence of quantum phase coherence and spin-orbital interaction. Ten samples of Hall bar with length of 200 micron and width of 20 micron are fabricated through e-beam lithography followed by e-gun evaporation of Ni$_{0.8}$Fe$_{0.2}$, aluminum and Ni$_{0.8}$Fe$_{0.2}$ with different thickness (5nm to 45nm) in vacuum. At low temperature of 4.2K, coherent backscattering, Rashba spin-orbital interaction and spin flip scattering of conduction electrons contribute to magnetoresistance at low field. Quantitative analysis of magnetoresistance shows transition between weak localization and weak anti-localization for samples with different thickness ratio, which indicates the spin polarization actually affects the phase coherence length and spin-orbital scattering length. However, at temperature between 50K and 300K, only the spin polarization dominates the magnetoresistance. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S8.00014: Ferroelectrically controlled organic spin valves with tunable magnetoresistance Mei Fang, Dali Sun, Xiaoshan Xu, Lu Jiang, Hangwen Guo, Ho Nyung Lee, Paul C. Snijders, T.Z. Ward, Zheng Gai, X.-G Zhang, Jian Shen, Lifeng Yin, Yanmei Wang, Wenting Yang Organic spin valves (OSV) with tunable magnetoresistance (MR) will promote organic spintronics for many potential applications. In this work, a novel ferroelectrically (FE) controlled organic spin valve (FE-OSV) was successfully fabricated by inserting a thin PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(PZT) buffer layer between the ferromagnetic bottom La$_{0.67}$Sr$_{0.33}$MnO$_{3}$(LSMO) electrode and the organic Alq$_{3}$ layer. The magnitude of MR values in these FE-OSV strongly depends on the history of the bias voltage applied, giving rise to a strong hysteretic behavior of MR vs. V. Moreover, the sign of MR in the FE-OSV can be electrically switched when the electric polarization of PZT layer is reversed. Both behaviors are not observed in the devices without the FE layer. These new findings are attributed to the tunability of the electric dipole moment of the PZT layer, which can actively shift the relative energy level between Alq$_{3}$ and LSMO and thence alter the spin injection. [Preview Abstract] |
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