Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S14: Focus Session: Spin Transfer Torque |
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Sponsoring Units: GMAG DMP FIAP Chair: Tom Silva, National Institute of Standards and Technology, Boulder Room: Colorado Convention Center Korbel 4D |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S14.00001: Spin Transfer Switching and Magnetization Dynamics in Py/Cu/Py Nanopillar Spin-Valves with Sidewall Oxide Passivation and Nonuniform Current Injection Ozhan Ozatay, Kee Wee Tan, Praveen Gowtham, Patrick M. Braganca, Eric Michael Ryan, Gregory D. Fuchs, John C. Read, Andre K. Mkhoyan, Malcolm G. Thomas, Kiran V. Thadani, Jack C. Sankey, John Silcox, Daniel C. Ralph, Robert A. Buhrman The manipulation of magnetization, both to drive precessional dynamics and trigger magnetization reversal in nanomagnets by transferring spin angular momentum from a spin-polarized current, presents opportunities for better scalability in nanoscale magnetic memory devices and microwave oscillators. Some of the major practical concerns include reducing the current level needed to write magnetic bits in an error-free fashion at high operating speeds in memory devices as well as exciting highly coherent dynamic modes for nanoscale microwave oscillator applications. In this work we report on the detrimental effects of the adventitious antiferromagnetic oxides at the perimeter of Py/Cu/Py nanomagnets such as an anomalous increase in magnetic damping at low temperatures and stochastic fluctuations in switching fields. We find that in addition to sidewall oxide passivation, the concentrated spin torque from nonuniform injection also reduces the sidewall effects leading to a more efficient spin transfer switching mechanism as well as microwave dynamics. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S14.00002: Current-Induced Magnetization Switching (CIMS) for `Ballistic' and `Diffusive' Transport Through the Non-Magnetic (N) metal in Permalloy/N/Permalloy Nanopillars Nikoleta Theodoropoulou, Amit Sharma, Mustafa AlHaj-Darwish, William Pratt Jr., Jack Bass . Adding 5{\%} Ge to Cu decreases the mean-free-path, $\lambda $, at 4.2K from $\sim $ 130 nm to $\sim $ 3.8 nm, while still leaving the spin-diffusion length $\ge $ 40 nm. Thus, comparing the CIMS switching currents at 4.2K for sputtered Py/N/Py with layer thickness t$_{N}$ = 10nm for N = Cu or Cu(5{\%}Ge), allows testing of the importance of `quasi-ballistic'-- (t$_{Cu}$/$\lambda _{Cu}) \quad \sim $ 0.08, versus `quasi-diffusive'---(t$_{CuGe}$/$\lambda _{CuGe}) \quad \sim $ 2.6, transport, with at most minor correction for spin-flipping in the N-metals. At 4.2K we find a ratio of switching currents, $\Delta $I$_{s}$(CuGe)/$\Delta $I$_{s}$(Cu) = 1.3 $\pm $ 0.2, where $\Delta $I$_{s}$ is the sum of the magnitudes of the critical current for switching from parallel to anti-parallel magnetic order and vice-versa. We will compare this ratio with values calculated using different models. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S14.00003: Temperature dependence of current induced magnetization switching in spin-valves with a ferrimagnetic CoGd free layer Li Gao, Xin Jiang, Jonathan Sun, Stuart Parkin Current induced magnetization switching (CIMS) has stimulated great interest recently due to its potential for applications, such as magnetic random access memories. Here, we report for the first time, a CIMS effect in spin-valves with a \textit{ferrimagnetic} CoGd free layer. The temperature dependence of the CIMS effect in CoGd-Cu-CoFe spin-valves is explored. At temperatures well above and well below the magnetization compensation temperature ($T_{MC})$ of CoGd, a current flowing from the free layer to the CoFe fixed layer aligns the moments of the two layers parallel, and a current flowing in the opposite direction aligns them antiparallel. However, for intermediate temperatures just above $T_{MC}$, the current-induced alignment of the moments is reversed. We attribute this to the different compensation temperatures of the net magnetization and angular momentum of CoGd. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:42PM |
S14.00004: Spin-torque-induced reversal in nanopillars containing perpendicularly magnetized layers. Invited Speaker: Devices where at least one of the magnetic elements has the anisotropy normal to the film surface are theoretically predicted to increase the efficiency and/or the speed of spin-torque switching. Devices where both the layers have the magnetization normal to the surface increase the efficiency of reversal while devices that combine perpendicular and in-plane magnetized layers are predicted to increase the speed of switching. In this talk we describe recent experimental demonstrations of current-induced magnetic reversal of magnetic elements with perpendicular anisotropy and high coercive fields [1]. The best results are observed for Co/Ni multilayers, which exhibit higher giant magnetoresistance values and spin-torque efficiencies than Co/Pt multilayers. The sample structures are nanopillars with a Co/Pt/Co/Ni composite reference magnetic element and a Co/Ni free layer that responds to the current. The reference layers were designed to have significantly higher anisotropy and coercive allowing a complete current-field phase diagram of the free layer reversal to be explored. The results are compared to micromagnetic modelling that, depending on the bias current and applied field, details regions of irreversible magnetic switching, coherent and incoherent spin waves, or static non-uniform magnetization states. Whereas only the two uniform magnetization states are available under the action of a magnetic field, we observed current induced Bloch domain walls in pillars as small as 100x50 nm2 [2]. This domain wall state can be further controlled by current to restore the uniform states. This ability to manipulate high-anisotropy magnetic elements could prove enabling for a range of spintronic applications. This research is done in collaboration with S. Mangin, D. Ravelsona, Y. Lemaho, Y. Henry, J. Katine, M. Carey, and B. Terris. \newline \newline [1] S. Mangin et al., Nature Materials \textbf{5}, 210 (2006). \newline [2] D. Ravelosona et al., Phys. Rev. Lett. \textbf{96}, 186604 (2006). [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S14.00005: Ferromagnetic resonance studies of nanopillars with Co/Ni multilayer free layers Wenyu Chen, J-M. L. Beaujour, G. de Loubens, Andrew D. Kent, M. J. Rooks, N. Ruiz, Jonathan Z. Sun Recently it has become possible to study ferromagnetic resonance (FMR) of magnetic layers in nanopillar junctions using the spin-transfer interaction [1,2]. This enables powerful new quantitative studies of the layer magnetic anisotropy and damping in confined structures. Here we report studies of Co/Ni multilayer free layers with variable easy plane anisotropy. Experiments were conducted on $\vert \vert $[t nm Co 2t nm Ni] x 1.2/t$\vert $ 10 nm Cu$\vert $ 12 nm Co$\vert \vert $ layer structures patterned to $\sim $50 nm lateral dimensions using a nanostencil process, with t=0.1, 0.2, 0.3 and 0.4. Varying the Co thickness (t) enables systematic variation of the Co/Ni easy-plane anisotropy, while the total magnetic moment and thickness of the free layer is kept constant. Field swept FMR measurements were conducted using a microwave signal generator (1 to 20 GHz) with a magnetic field applied perpendicular to the surface of the layers. The resonance field and linewidth were measured as a function of frequency and DC current bias. Magnetic anisotropy constants and damping parameters are determined and compared to those found in FMR studies of extended films of the same layer structure. [1] A. A. Tulapurkar et al., Nature, 438, 339 (2005) [2] J. C. Sankey et al., Phys. Rev. Lett., 96, 227601 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S14.00006: Reducing the critical switching current of magnetic multilayers -- an ab-initio approach. Paul Haney, Derek Waldron, Alvaro Nunez, Rembert Duine, Hong Guo, Allan MacDonald We examine strategies for reducing the critical switching current density of spin valve structures, including the dual spin filter (DSF) design, and the use of depolarizing materials outside of the magnetic layers. We study both ideas from first principles using the non-equilibrium Green's function formalism and direct microscopic evaluation [1] of spatially resolved spin transfer torque contributions. We compare the spin torques present in simple Co-Cu-Co sandwiches with those in the DSF structure. In addition we study the role of Ru layer in enhancing the spin transfer efficiency, exploring the physical origin of Ruthenium's apparent usefulness in microscopic detail. [1] Haney et al.. cond-mat/0611534 [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S14.00007: Spin-Motive Force Studies in Spin-Valves Jun'ichi Ieda, Sadamichi Maekawa, Stewart Barnes A spin-motive force (smf) is the counterpart of an electro-motive force, which couples to spin degrees of freedom of electrons rather than charge ones. Here we discuss how the smf works in the so-called spin-valves. Usually the observed \textit{dV/dI} for spin-valves is analyzed in terms of magneto-resistance. However when the magnetization makes a sudden jump, there often appears a large peak in \textit{dV/dI}, i.e., a voltage jump that is better interpreted in terms of the smf discussed here. In order to see this, we model spin-valves using an equivalent circuit that involves magnetic dissipation represented by the smf as well as electric dissipation through ordinary resisters for both majority and minority currents. There are four possible conduction paths, e.g., the majority electrons tunnel into the majority band, or into the minority band and vice versa. The first path adds an up electron to the free layer and causes a rotation in a certain sense, while the second path adds a down electron and a rotation in the opposite sense. Since the rotations are in opposite senses so is the work done on the free layer and hence the smf. The equivalent circuit with the relevant parameters predicts a stable large angle precession and the voltage signal. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S14.00008: Magneto-transport and spin-torque effects in current perpendicular to the plane spin-valves with Co-Fe-Al magnetic layers Stefan Maat, Matthew Carey, Jeffrey Childress The magneto-transport of current-perpendicular to the plane giant magneto-resistive spin valves utilizing (Co$_{x}$Fe$_{100-x})_{100-y}$Al$_{y}$ alloys in the reference and free layers is investigated. (Co$_{50}$Fe$_{50})_{75}$Al$_{25}$ is determined to be the alloy composition that maximizes magneto-resistance. At this composition the magnetization is around 1000 emu/cm$^{3}$, which is high enough to be used as magnetic material in spin-valves with ultra-thin read gaps for high recording densities. An improvement in magneto-resistance from 1.7{\%} for spin-valves utilizing Co$_{50}$Fe$_{50}$ in reference and free layers to 3.3{\%} for spin-valves utilizing (Co$_{50}$Fe$_{50})_{75}$Al$_{25}$ with the same ``magnetic'' thickness in both parts of the reference and the free layers were observed. The spin-diffusion length for (Co$_{50}$Fe$_{50})_{75}$Al$_{25}$ is determined to be approximately 30 {\AA}. Spin-torque measurements show that the spin-torque current density threshold is approximately 7$\cdot $10$^{7}$ A/cm$^{2}$ in CoFeAl spin-valves in comparison to 13$\cdot $10$^{7}$ A/cm$^{2}$ in CoFe spin-valves. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S14.00009: Magnetization and resistance noise in spin valves J{\o}rn Foros, Arne Brataas, Gerrit E. W. Bauer, Yaroslav Tserkovnyak Electronic noise hinders the application of spin valves as read heads in magnetic hard drives. We report a theoretical analysis of such noise. Electronic or resistance noise in spin valves is caused by fluctuations in the relative orientation of the magnetic layers via the magnetoresistance-effect. Two sources of thermal magnetization fluctuations can be distinguished: Random fields intrinsic to the bulk ferromagnets, and external spin current fluctuations that affect the magnetizations through the spin-transfer torque. The cross talk between fluctuating magnetizations and the corresponding resistance noise strongly depends on the magnetic configuration. In agreement with experiments by Covington et al.\footnote{M. Covington et al., unpublished} we find that the noise level in the antiparallel configuration can exceed that of the parallel one by an order of magnitude. [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S14.00010: Effect of Bias on Spin-Transfer Torque in Magnetic Tunnel Junctions. Ioannis Theodonis, Alan Kalitsov, Nicholas Kioussis, Mairbek Chshiev, W.H. Butler The current-induced magnetic switching in non-collinear magnetic tunnel junctions (MTJ) through the spin-transfer torque(STT) provides the possibility of manipulating nonvolatile MRAM, without applying cumbersome magnetic fields. Using tight-binding calculations and the non-equilibrium Keldysh formalism, we have studied the effect of applied bias on the components of the STT, parallel $T_{\vert \vert } $ , and perpendicular, $T_\bot $, to the interface. We show that depending on the exchange splitting, $T_{\vert \vert } $ may exhibit a non-monotonic bias dependence: it may change sign without a sign reversal in current, and in some cases it may even have a quadratic bias dependence. Second, we show that $T_{\vert \vert } $ is given by the difference in spin currents between the FM and anti-ferromagnetic (AF) configurations. Third, the bias dependence for the spin current for the FM (AF) alignment is shown to have a linear (quadratic) bias dependence, whose origin lies on the symmetric (asymmetric) nature of the barrier. The interplay of the spin currents for the FM and AF configurations can lead to a rich behavior of the $T_{\vert \vert } $ on bias. Finally, we find that, $T_\bot $(non-equilibrium exchange coupling), is comparable in size with $T_{\vert \vert } $, and has a quadratic bias dependence. [Preview Abstract] |
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