Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session J37: Focus Session: Novel Magnetic Devices - Spin Torque I |
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Sponsoring Units: DMP GMAG Chair: Ursula Ebels, Spintec/CEA Room: E147-E148 |
Tuesday, March 16, 2010 11:15AM - 11:27AM |
J37.00001: Spin torque switching of magnetic tunnel junctions with perpendicularly magnetized Co/Ni and Co/Pd electrodes Xin Jiang, Brian Hughes, Stuart Parkin Spin transfer torque switching of magnetic tunnel junctions with perpendicularly magnetized electrodes are promising for reducing switching current and improving thermal stability of magnetic random access memory devices. In this presentation, we report studies of current induced switching of magnetic tunnel junctions with barrier formed from MgO and electrodes formed from Co/Ni and Co/Pd multilayers. The switching current density and thermal stability of the device are measured using voltage pulses with different widths. We show that the large damping constant of the Co/Pd multilayer gives rise to high switching current density. [Preview Abstract] |
Tuesday, March 16, 2010 11:27AM - 11:39AM |
J37.00002: Spin-Transfer switching with short current pulses in all perpendicular spin valve nanopillars Huanlong Liu, Daniel Bedau, Jean-Jacques Bouzaglou, Andrew Kent, Jonathan Sun, Jordan Katine, Eric Fullerton, Stephane Mangin Spin-transfer switching has been studied in spin-valves with perpendicularly magnetized free and reference layers and a lateral size of 100 nm x 100 nm. We demonstrate 100\% switching probability with high efficiency for current pulses as short as 300 ps, comparable to the magnetization precession time. Experimentally we find that the inverse reversal time $1/\tau= A (I-I_{c0})$ depends linearly on the current overdrive, where $I_ {c0}$ is the zero temperature critical current. The dynamic parameter A depends linearly on the applied easy-axis field, as predicted by a $T=0$ macrospin model for a magnet with uniaxial anisotropy. However, A depends on the applied field much more strongly than predicted by the model. To fit the data we must assume a large initial angle of the magnetization of the free layer. Possible explanations for the discrepancies are a breakdown of the macrospin picture because of domain wall nucleation or propagation. Supported by USARO (Grant No. W911NF0710643). [Preview Abstract] |
Tuesday, March 16, 2010 11:39AM - 11:51AM |
J37.00003: Spin-torque switching in magnetic tunnel junctions with a Co/Ni multilayer electrode having a reduced demagnetizing field Takahiro Moriyama, Theodore Gudmundsen, Pinshane Huang, Luqiao Liu, David Muller, Daniel Ralph, Robert Buhrman The critical current for spin-torque-driven switching of an in-plane magnetized free layer is approximately proportional to the perpendicular demagnetizing field of the free layer. In previous work, we demonstrated experimentally that by reducing the demagnetizing field the switching current can be decreased significantly (at least a factor of 5) without compromising the thermal stability of the free layer [1]. This work was carried out using all-metal spin valve devices having a small electrical resistance unsuitable for applications. Here we report the fabrication of magnetic tunnel junctions with in-plane magnetized Co/Ni-based magnetic free layers having a reduced demagnetizing field compared to conventional CoFeB layers. We describe the magnetoresistance properties of these junctions and their spin-transfer switching characteristics.\\[4pt] [1] L. Liu et al., Appl. Phys. Lett. 94, 122508 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 11:51AM - 12:27PM |
J37.00004: A Three terminal spin-torque driven magnetic switch Invited Speaker: A three-terminal spin-torque-driven magnetic switch is experimentally demonstrated. The device uses non-local spin-current and spin-accumulation as the main mechanism for current-driven magnetic switching. It separates the current-induced write operation from that of a magnetic tunnel junction based read. The write current only passes through metallic structures, improving device reliability. The device structure makes efficient use of lithography capabilities, important for robust process integration. I will give a review of the basic device structure and its characterstic transport properties. [Preview Abstract] |
Tuesday, March 16, 2010 12:27PM - 12:39PM |
J37.00005: Magnetic-field-assisted spin-transfer switching in nonlocal spin valves Han Zou, Xiaojun Wang, Yi Ji Nonlocal spin valve (NLSV) is a spin injection and detection device consisting of a nonmagnetic metal N connected to ferromagnetic spin injector F$_{1}$ and detector F$_{2}$. We fabricate NLSV devices, using 100 nm thick Cu as N, 10 -- 20 nm thick Co as F$_{1}$, and 3 nm thick Co as F$_{2}$. The widths of the Cu and Co wires are $\sim $ 150 nm. To ensure the electrical continuity of the F$_{2}$ electrode, a 5 nm thick Cu layer is placed underneath. The center-to-center separations between F$_{1}$ and F$_{2}$ are 200 - 350 nm. The nonlocal spin signals at 4.2 K are 2 - 4 milliohms. The spin-transfer switching has been achieved with the assistance of a magnetic field. The F$_{1}$ and F$_{2}$ electrodes are set in an anti-parallel configuration, and the magnetic field is set to a value smaller but close to the switching field of F$_{2}$. A d.c. current pulse with appropriate polarity is injected through F$_{1}$ to induce the spin-transfer. A small d.c. current ($<$ 0.5 mA) is sufficient to switch F$_{2}$ into being parallel with F$_{1}$. Analysis has been done to rule out possible artifacts due to Oersted fields. [Preview Abstract] |
Tuesday, March 16, 2010 12:39PM - 12:51PM |
J37.00006: Spin Torque Switching in Asymmetric Fe$_{60}$Co$_{20}$B$_{20}$/MgO/ Ni$_{60}$Fe$_{20}$B$_{20}$ Magnetic Tunnel Junctions Hinwei Tseng, Y. Li, J.C. Read, C. Wang, O.L. Lee, P.G. Gowtham, P.M. Braganca, D.C. Ralph, R.A. Buhrman Spin-torque (ST) studies involving MgO-based magnetic tunnel junctions (MTJs) generally have utilized junctions with fixed and free electrodes of similar or identical CoFe-based composition. Here we report the bias-dependent conductance, TMR, and ST behavior of nanopillar MTJs with asymmetric Fe$_{60}$Co$_{20}$B$_{20}$/MgO/ Ni$_{60}$Fe$_{20}$B$_{20}$ electrodes (AMTJ) having good performance (TMR$\sim $100{\%}, RA=10$\Omega \mu $m$^{2})$.We compare to symmetric (SMTJ) Fe$_{60}$Co$_{20}$B$_{20}$/MgO/Fe$_{60}$Co$_{20}$B$_{20}$ junctions (TMR $\sim $ \textbf{120{\%}}, RA = \textbf{10}$\Omega \mu $m$^{2})$. The magnitude of the bias dependence of TMR for the AMTJ is markedly less than for the SMTJ. In the AMTJs, using bias and field dependent measurements of thermally-activated ST-assisted reversal, we have observed a very significant asymmetry in the polarity-dependent ST critical currents and thermal activation barriers for reversal. Accordingly, the ST phase diagram, measured under high bias, shows a strong asymmetry with bias polarity, which we attribute to a polarity-dependent asymmetry in the field-like ST term at high bias. We will discuss the possible causes of these results based on the different band structures and work functions of FeCo and NiFe. [Preview Abstract] |
Tuesday, March 16, 2010 12:51PM - 1:03PM |
J37.00007: Mechanisms of Spin Torque Driven Ballistic Precessional Switching Oukjae Lee, Vlad Pribiag, Praveen Gowtham, Taka Moriyama, Dan Ralph, Robert Buhrman Spin-torque-driven ballistic precessional switching is a very fast and energy-efficient write operation in which the magnetization of a nanomagnet rotates from one stable state to the other without any preceding small-angle oscillation. This reversal scheme can be implemented with a non-collinear spin-valve device incorporating both a perpendicular polarizer to quickly excite a free layer and an in-plane analyzer to read its state via the GMR effect. Recent experiments utilizing such devices have shown significantly higher reversal speeds and a much narrower distribution of reversal probability with current, in comparison to conventional collinear devices. More interestingly, there was a pronounced asymmetry in the threshold currents for reversal as function of both the initial state and pulse current direction. This asymmetry can provide a way of achieving the desired state with a simple unipolar pulse current. We will discuss the details of the short-pulse reversal behavior, the physical origins of the asymmetry and the optimization of this structure for high-speed magnetic memory. [Preview Abstract] |
Tuesday, March 16, 2010 1:03PM - 1:39PM |
J37.00008: Magnetodynamics of spin torque switching in nanometer sized magnetic devices Invited Speaker: Since the theoretical predictions of by Berger and Slonczewski in 1996, spin-transfer dynamics in spin valves and tunnel junctions have been of great interest for the development of magnetic RAM, spin-torque nano-oscillators, and spin diodes. Spin transfer devices use spin polarized currents flowing through a nanoscale magnetic structure to induce torques on the local moments, causing a change in the direction of magnetization of the film. The magnetization can be switched more efficiently using spin transfer than through the use of magnetic fields and unique dynamical magnetization states can be stabilized. In order to understand the magnetization dynamics and the switching behaviors in nanoscale magnetic devices, it is important to characterize their normal modes of oscillation, thermal fluctuations, and the statistical switching current distributions. In conventional field switching, the switching event evolves from a normal mode whose frequency is driven to zero. Spin torque switching, in contrast, evolves from a mode whose linewidth is driven to zero. Nanoscale devices have many active magnetization modes each with different spatial distributions, frequencies, and effective activation volumes. Depending of the device shape and the profile of the excitation pulse different modes can contribute to the switching process. In this talk we describe our measurements of spin torque induced magnetization dynamics and switching in nanometer scale magnetic spin valves and MgO tunnel junctions. Switching probabilities were measured for pulses with durations from 0.1 ns up to 1 s and correlated with device level ferromagnetic resonance (FMR). Thermal and spin torque FMR data show complex spectra and evolution of the modes with applied fields and currents. The nanoscale FMR spectra can highlight device-to-device variations, be used to identify the modes important in the switching process, and measure the mode damping that determines the critical currents for switching. [Preview Abstract] |
Tuesday, March 16, 2010 1:39PM - 1:51PM |
J37.00009: ST-FMR Study of Spin Transfer Effects in Antiferromagnet/Ferromagnet Spin Valves Yong-Tao Cui, A. R. Mellnik, K. V. Thadani, R. A. Buhrman, D. C. Ralph Spin transfer effects due to current passing between magnetic layers are of interest as a way to control the orientation of nano-scale ferromagnets. Recent theoretical studies [1] suggest that current-induced torques can also occur when electrons pass between antiferromagnets and ferromagnets. Here we report measurements on nanopillar spin valve devices containing the layer structure: antiferromagnet / normal metal / free ferromagnet / normal metal / pinned ferromagnet. By using spin-transfer-induced ferromagnetic resonance (ST-FMR) [2,3] to detect current-driven oscillations of the free ferromagnetic layer, and comparing to simple free ferromagnet / normal metal / pinned ferromagnet devices, we determine the spin torque applied on the free ferromagnet by current passing through the antiferromagnet. We will discuss the magnitude and direction of the spin torque, its angular dependence relative to the antiferromagnetic director (direction of the antiferromagnet order parameter), and its bias dependence. [1] P. M. Haney and A. H. MacDonald, Phys. Rev. Lett. 100, 196801 (2008). [2] A. A. Tulapurkar et al., Nature 438, 339 (2005). [3] J. C. Sankey et al., Phys. Rev. Lett. 96, 227601 (2006). [Preview Abstract] |
Tuesday, March 16, 2010 1:51PM - 2:03PM |
J37.00010: Electrical Pulse Modification and Reversal of the Exchange-Bias in Magnetic Tunnel Junction Structures Yun Li, Hsin-wei Tseng, Dan Ralph, Robert Buhrman The use of antiferromagnetic layers to exchange-bias (EB) the reference layer is common in spin-torque (ST) experiments. Previous work has shown that the EB in both MTJs and spin valves can be degraded or reversed by electrical pulses, with the effect being attributed to heating or possibly to ST effects in the spin valve case. We have studied EB modification due to individual electrical pulses in the presence of a small external field ($<$50Oe) in FeCoB/MgO/FeCoB/IrMn MTJs as a function of MgO thickness. For MgO thickness = 1.7 nm, \textit{RA} = 5 x 10$^{3}\Omega \mu $m$^{2}$, pulses with $J_{c}$ = 4 x 10$^{4}$A/cm$^{2}$ and $V = $1.8 V, can repeatedly and reliably reverse the EB. For 1.3 nm barriers, \textit{RA =}150$\Omega \mu $m$^{2}$\textbf{,} much higher power pulses, $J_{c}$ = 6 x 10$^{5}$A/cm$^{2}$ and $V = $0.9 V, are required for reversal. Such results indicate that a combination of heating and ST, with the latter possibly involving the field-like spin torque component at high bias, is responsible for EB reversal in our MTJs. We will discuss the details of the EB reversal behavior and report the phase diagram for reversal as function of electrical and field bias. [Preview Abstract] |
Tuesday, March 16, 2010 2:03PM - 2:15PM |
J37.00011: Experimental evidence for thermal spin transfer torque Simon Granville, Haiming Yu, Dapeng Yu, Jean-Philippe Ansermet It has been predicted that a heat current can exert a spin torque on the magnetization in a nanostructure, analogous to the well-known spin-transfer torque induced by an electrical current. A number of recent theoretical studies have aroused interest in this subject, but no experimental evidence of such a thermal spin transfer torque has yet emerged. We present measurements of the second harmonic voltage response of Co-Cu- Co pseudo-spin-valves deposited within Cu nanowires. We exploit the quasi-1D nature of the nanostructures to generate a heat current by way of a large temperature gradient between the Co layers. Both the magnitude of the second harmonic response and the switching field of the Co layers are found to be dependent on the heat current generated. These effects show that the magnetization dynamics of the pseudo-spin-valves are influenced by the heat current, providing evidence for a thermal spin torque exerted on the magnetization of the Co layers. [Preview Abstract] |
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