Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VIII |
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Sponsoring Units: GMAG Chair: Eric Fullerton, University of California, San Diego Room: D170 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V19.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V19.00002: Ferromagnetic resonance studies of CoFeB-MgO Erik Shipton, Ken Lee, Jonathon Sapan, Jimmy Kan, Keith Chan, Eric Fullerton There has been much interest in ferromagnetic magnetic tunnel junctions (MTJs) as a potential candidate for spin transfer torque memories. Many parameters are important in order to optimize the spin transfer torque effect to minimize the critical switching current density (J$_{c})$ without compromising an energy barrier (E$_{B})$ between stable states. CoFeB/MgO systems have many desirable properties including high spin polarization and, thereby high tunnel magnetoresistance. Recently, Ikeda et al. reported that Fe-rich CoFeB/MgO MTJs can induce perpendicular anisotropy that is strong enough to overcome the in-plane shape anisotropy, demonstrating CoFeB-based perpendicular MTJs [1]. In this work, we have performed FMR studies as a function of alloy composition, layer thickness, pre and post annealing of CoFeB/MgO systems. Coplanar waveguide method VNA FMR experiments were performed [2]. From the FMR resonance frequency and linewidth we were able to extract the Gilbert damping as well as the effective magnetization. Experimental details as well as results will be presented. \\[4pt] [1] S. Ikeda et al, \textit{Nature Materials}~\textbf{9}, 721 - 724 (2010)\\[0pt] [2] J.M. Beaujour et al, Eur. Phys. J. B \textbf{59}, 475--483 (2007) [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V19.00003: Energy distributions of defects causing barrier-resistance noise in CoFeB/MgO/CoFeB tunnel junctions Ryan Stearrett, Xiaoming Kou, John Xiao, Ed Nowak Magnetic tunnel junction devices, such as field sensors, are well known to exhibit low frequency resistance noise having a 1/$f$ spectrum. This noise has its origins in a combination of electrical and magnetic mechanisms. Previously, we have shown that the resistance noise can be reduced significantly through thermal annealing. Here, we report on the energy distribution of the defects causing tunnel-barrier-resistance noise. The distributions are determined from a Dutta-Horn model for thermally activated charge trapping and detrapping kinetics. We also discuss how the distribution changes as a function of annealing time and its relation to the current-voltage characteristics and the voltage bias dependence of the 1/$f $noise. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V19.00004: Ultrafast Switching in Magnetic Tunnel Junction based Orthogonal Spin Transfer Devices Huanlong Liu, Daniel Bedau, Dirk Backes, Jordan Katine, J\"urgen Langer, Andrew Kent Orthogonal spin-transfer magnetic random access memory (OST-MRAM) uses a spin-polarizing layer magnetized perpendicularly to the free layer to achieve large spin-transfer torques and ultrafast energy efficient switching. We have fabricated and studied OST-MRAM devices that incorporate a perpendicularly magnetized polarizer and a magnetic tunnel junction, which consists of an in-plane magnetized free layer and synthetic antiferromagnetic reference layer. A switching probability of 100{\%} is observed for 500 ps pulses, requiring an energy of 250 fJ. The fast switching process indicates there is no incubation delay of several nanoseconds as observed in conventional collinear magnetized devices. Due to the perpendicular polarizer switching is possible for both pulse polarities. There is also evidence for precessional switching in the non-monotonic dependence of the switching probability versus pulse amplitude. This work was supported by Spin Transfer Technologies. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V19.00005: The effect of annealing on the spin-transfer torques of MgO MTJ nanopillars Yun Li, Hsin-wei Tseng, Pinshane Huang, John Read, Dan Ralph, Robert Buhrman Thermal annealing is essential for enhancing the tunneling magnetoresistane (TMR) of magnetic tunnel junctions, and many studies have focused on the effect of annealing on MTJ chemical, structural, and electrical transport properties. Here, we report the magnetic, electronic properties and the in-plane and field-like spin-transfer torques (STT) in both as-grown and post-annealed FeCoB/MgO/FeCoB MTJs nanopillars. We find that the 350 \r{ }C vacuum annealing breaks the symmetry of the bias dependence of the TMR, conductivity, and switching phase diagram (SPD). Moreover STT-FMR measurements indicate that annealing substantially increases the in-plane torque asymmetry with bias voltage direction, as well as affecting the field-like torque magnitude, with the latter indicating a very significant enhancement of interlayer exchange coupling across the barrier. This STT change is consistent with the change in chemical composition and structural coherency of the MTJ interfaces and electrodes, indicated by XRD and analytical STEM analyzes. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V19.00006: Bias voltage dependence of the total magnetic field in CoFeB magnetic tunnel junctions M.D. Riemer, J.Z. Sun, A.D. Kent We report experimental thermal-noise spectrum-based ferromagnetic resonance (T-FMR) measurements on CoFeB magnetic tunnel junctions in magnetic fields perpendicular to the film plane. The junctions tested have lateral sizes of $45 \times 80$ nm$^2$. In a simple model a dc junction bias voltage should affect both the slope and the intercept of the T-FMR frequency's dependence on applied magnetic field. The intercept would vary linearly with changes in bias voltage due to an electric field-induced change in uniaxial anisotropy [1]. The slope would have a quadratic dependence on changes in bias voltage based on the existence of a perpendicular spin-torque as discussed by Sankey {\it et al.} [2]. In this experiment we attempt to de-construct the contribution from these two mechanisms. This is done by a careful analysis of the magnetic field dependence of the T-FMR spectra [3]. \\[4pt] [1] Suzuki et al, Appl. Phys. Lett. {\bf 96}, 022506 (2010). \newline [2] Sankey et al, Nature Physics {\bf 4}, 67 (2008). \newline [3] Mascaro et al, Intermag/MMM paper FB-11. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V19.00007: Confirm existence of 90\r{ }-type coupling in Fe/MgO/Fe junction by investigating magnetic components perpendicular to the plane of incidence Xiaojing Tan We study 90\r{ }-type interlayer exchange coupling (IEC) in a Fe/MgO/Fe junction by linear magneto-optical Kerr effect (MOKE) in $p_{in}-p_{out}$ configuration, in which only in-plane magnetization perpendicular to the external field $H$is detected. By investigating the switching processes of the ferromagnetic vectors from parallel with- to perpendicular to $H$, we find there is a switching correlation between them: the ferromagnetic vector in the bottom layer always follows the switching direction of that in the top layer. Further analysis shows this kind of switching sequence is the direct consequence of 90\r{ }-type coupling between the two magnetic vectors, i.e., 90\r{ }-type coupling is indeed exists in Fe/MgO/Fe junction. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V19.00008: Influence of Interface on Conductance in AlO$_{x}$ Based Magnetic Tunnel Junctions Feng Guo, E. Dan Dahlberg A surprising minimum in the differential conductance at nonzero bias is observed in some magnetic tunnel junctions consisting of CoFe/AlO$_{x}$/CoFe; this pronounced conductance feature occurred for electrons tunneling from the bottom to top electrode. The presence of this conductance feature depends upon the oxidation time for creating the barrier from a thin Al layer; for short and moderate oxidation times the feature was present while for long oxidation times the conductance was found to be both symmetric about zero bias and monotonic with increasing bias voltage. To determine the origin of this feature samples were prepared where the oxidation states of the CoFe on each side of the barrier were studied by X-ray photoelectron spectroscopy: the conductance feature is observed only when the top CoFe layer is partially oxidized and it disappears when the CoFe on both sides of the junction has some oxidation present. More interestingly, the bias voltage of the conductance feature decreases with oxidation time. We attribute the differential conductance feature to the electronic structure and the chemical bonding at the bottom CoFe/AlO$_{x}$ interface. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V19.00009: Spin transfer torque in magnetic tunnel junctions with a perpendicularly magnetized polarizer Takahiro Moriyama, Theodore Gudmundsen, Luqiao Liu, R.A. Buhrman, D.C. Ralph Spin-torque devices containing magnetic layers with perpendicular magnetic anisotropy are of interest for strategies to reduce the switching currents in memory applications. We report spin-torque-driven ferromagnetic resonance (ST-FMR) measurements of the bias-dependent torque in magnetic tunnel junctions containing [Co/Ni]$_{x}$ multilayers possessing perpendicular anisotropy, acting as the polarizer layer providing spin-polarized current. We observe unusual dependence of the bias-dependent torque as a function of the magnetic orientation of the [Co/Ni]$_{x}$ multilayer. We speculate that this sensitivity to the magnetic orientation may originate from changes in the occupation of spin-polarized states at the Co/Ni interfaces associated with the perpendicular magnetic anisotropy. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V19.00010: Spin transfer induced domain wall motion by perpendicular current injection in MgO-based magnetic tunnel junctions A. Chanthbouala, R. Matsumoto, J. Grollier, V. Cros, A. Anane, A. Fert, A. V. Khvalkovskiy, K.A. Zvezdin, A. Fukushima, S. Yuasa The spin transfer effect allows to manipulate magnetic domain walls in ferromagnetic wires by current injection. Most experiments use the lateral configuration in which the current is injected directly through the wire where the domain wall (DW) propagates. In this geometry the critical current densities are of the order of 10$^8$ A.cm$^{-2}$. Here we show that by using the current-perpendicular to plane geometry, the current densities can be decreased by two orders of magnitude. Depending of the current sign, the DW propagates in the free layer of a magnetic tunnel junction in both directions, inducing large resistance variations. By investigating the physical origin of DW motion, we find that the field-like torque has a large contribution to the effect, as recently predicted by Khvalkovskiy \textit{et al.} (Phys. Rev. Lett. 2009). This result paves the way towards a new type of domain wall based magnetic memories. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V19.00011: High Voltage Pulse Measurements of Microwave Emission and Spin-Torque Effects in Magnetic Tunnel Junction H.W. Tseng, Y. Li, J.A. Katine, P.G. Gowtham, D.C. Ralph, R.A. Buhrman The character and strength of the in-plane and field-like spin transfer torque (STT) components in magnetic tunnel junctions at high bias voltages are crucial to the successful utilization of MTJs in STT MRAM. If the field-like torque (FLT), which is generally found to be symmetric with respect to bias direction for moderate voltages, $< \quad \pm $ 0.5 V, is too large it could result in unreliable switching (back-hopping) for negative bias voltage pulses (anti-parallel to parallel switching). Here we discuss pulse measurements of MgO MTJs at high bias that yield important information about the FLT component in the $\pm $ 0.5 to 1.0 V regime through analysis of both the thermally-excited FMR behavior and spin torque driven oscillations. In the structures studied we find a strong and highly asymmetric voltage-dependent FLT at high bias that under some field and voltage conditions can result in large amplitude, incoherent microwave dynamics that could have a strong effect in enhancing back-hopping. We will analyze possible mechanisms, including junction asymmetries and inelastic tunneling. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V19.00012: Microwave phase detection with a magnetic tunnel junction Xin Fan, Sangcheol Kim, Xiaoming Kou, James Kolodzey, Huaiwu Zhang, John Xiao A magnetic tunnel junction (MTJ) can detect microwave magnetic field due to the interplay between the ferromagnetic resonance and tunneling magneto resistance. Based on the fact that the tunneling resistance change is quadratically proportional to the rf magnetic field, we have designed a mixing circuit in which two microwaves interfere, giving rising to a dc voltage containing the phase difference between the two microwaves. With ability to detect microwave intensity and phase, the MTJ-based device may be used for on-chip microwave network analyzer and spectrum analyzer. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V19.00013: Time-resolved detection of spin-transfer-driven ferromagnetic resonance and spin torque measurement in magnetic tunnel junctions Chen Wang, Y.-T. Cui, R.A. Buhrman, D.C. Ralph Several experimental techniques have been introduced in recent years in attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs). The dependence of spin torque on bias is important for understanding fundamental spin physics in magnetic devices and for applications. However, previous techniques have provided only indirect measures of the torque and their results to date for the bias dependence are qualitatively and quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can be measured directly by using time-domain techniques to detect resonant magnetic precession in response to an oscillating spin torque. The technique is accurate in the high-bias regime relevant for applications, and because it detects directly small-angle linear-response magnetic dynamics caused by spin torque it is relatively immune to artifacts affecting competing techniques. At high bias we find that the spin torque vector differs markedly from the simple lowest-order Taylor series approximations commonly assumed. [Preview Abstract] |
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