Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P19: Focus Session: Spin Transport & Magnetization Dynamics in Metals VI |
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Sponsoring Units: GMAG Chair: Stephane Mangin, Nancy-Universite Room: D170 |
Wednesday, March 23, 2011 8:00AM - 8:12AM |
P19.00001: Quantum Dynamics of Spin-torqued Nanomagnet Yong Wang, Yaojun Zhang, L.J. Sham Nanomagnet is the key ingredient of many spintronics devices, such as hard disk read head, magnetoresistive random access memory(MRAM),etc. The underlying mechanisms in these structures are due to the interplay between the nanomagnet and spin-polarized electrons. Usually, the nanomagnet is characterized by the classical magnetization vector with its intrinsic quantum fluctuation neglected. By treating the nanomagnet as a huge atom with millions of energy levels, we are able to take account into the magnetization fluctuation, and study the interactions between nanomagnet and spin-polarized electrons on the same footing as quantum objects. We will show that the well-known giant magnetoresistance(GMR) effect and spin transfer torque(STT) effect are the consequences of continuous quantum measurements on the nanomagnet by spin-polarized electrons. We found that the quantum dynamics of nanomagnet is governed by a Fokker-Planck(FP) type equation in the atomic coherent state representation(P- representation). We will also discuss the correlation between the magnetization fluctuation and the electric current fluctuation. [Preview Abstract] |
Wednesday, March 23, 2011 8:12AM - 8:24AM |
P19.00002: Minimum action paths for single domain ferromagnetic nanostructures under the influence of spin transfer torque Gabriel Chaves-O'Flynn, Daniel Stein, Andrew Kent, Eric Vanden-Eijnden Thermally induced magnetization reversal is an important issue for the design of magnetic storage devices. The problem is usually studied using Kramers' theory of reaction rates, which is applicable when the dynamics can be described as gradient forces. Spin Transfer Torque (STT) is an effect of technological importance which does not fall in this category. For Spin Transfer Torque an action minimization is required to find the most probable paths and transition states between metastable states. We calculate these most probable paths for ferromagnetic nanostructures under the influence of STT in the low noise limit for a variety of current strengths and magnetic fields. Previous action minimization were done in the absence of STT and provide a good basis for comparison [1]. We study thin films with an in-plane easy magnetization axis using the geometrical Minimum Action Method (gMAM) [2]. The action obtained using gMAM is in qualitative agreement with activation energy barriers on previous work by Li-Zhang [3].\\[4pt] [1] R.V. Kohn, M.G. Reznikoff, E. Vanden-Eijnden, J. Nonlinear Sci. 15, 223 (2005)\\[0pt] [2] M. Heymann, E. Vanden-Eijnden, Comm. Pure Appl. Math. LXI, 1052(2008)\\[0pt] [3] Z. Li, S. Zhang, Phys. Rev. B 69, 134416 (2004) [Preview Abstract] |
Wednesday, March 23, 2011 8:24AM - 8:36AM |
P19.00003: Giant enhancement of microwave emission in magnetic tunnel junction oscillators by orientating in-plane field Z.M. Zeng, K.H. Cheung, H.W. Jiang, P. Upadhyaya, P. Khalili Amiri, K.L. Wang, J.A. Katine Recently spin-transfer nano-oscillators (STNOs) have attracted considerable attention because they are tunable over a wide frequency rang by varying the applied DC current or magnetic field. One main challenge for practical applications is to boost the relative low emitted power. MgO-based STNOs have exhibited a capability to deliver much larger power. However, they often show multiple emission peaks or broad linewidths. It is necessary to suppress the additional peaks and to reduce the central peak linewidth. In this talk, we present our microwave measurements in MgO-based STNOs as a function of in-plane field orientation. At an optimal orientation, emitted power of a single peak is largely enhanced, together with a significantly narrowed linewidth. The experiment shows that the understanding of intrinsic features of the oscillators as a function of in-plane orientation is important for optimizing the performances of MgO-based nano-oscillators.\\[0pt] [1] see for example, S. I. Kiselev, et.al., Nature 425, 308 (2003). [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P19.00004: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P19.00005: Spin-transfer oscillators in the effective planar approximation Ya. B. Bazaliy Spintronic devices with dominating easy plane anisotropy can be described in an effective planar approximation of the LLG equation. In particular, the effective equation can be used to study the spin-transfer oscillators. We use this approach to study the transitions of the oscillator excited by a combination of an AC and a DC electric currents between the small and large amplitude regimes. [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P19.00006: High Speed Spin Torque Memory with Combined Perpendicular and In-Plane Polarizers Graham Rowlands, Tofizur Rahman, Jordan Katine, Juan Alzate, Alexey Kovalev, Yaroslav Tserkovnyak, Kosmas Galatsis, Pedram Khalili Amiri, Kang Wang, Jian-Ping Wang, Ilya Krivorotov The use of a perpendicular polarizing layer in combination with an in-plane free layer has been proposed [1] as a means of reducing the switching time and write energy of spin-torque MRAM cells. While these structures have been realized in nanopillars with metallic spacers [2-4], memory applications demand the use of magnetic tunnel junctions (MTJs) due to their higher magnetoresistance and better impedance matching to a write transistor. We augment standard in-plane CoFeB/ MgO/ CoFeB MTJs to include an additional fixed layer pulled out-of-plane by coupling to the adjacent [Co/Pd] multilayer designed to possess a strong perpendicular magnetic anisotropy. This additional polarizer generates spin torque with an out-of-plane component, resulting in a fast precessional switching with no incubation time or pre-switching oscillations. For a variety of sample sizes we observe switching times approaching 100 ps. References: [1] A. D. Kent et al. Appl. Phys. Lett. 84, 3897 (2004). [2] C. Papusoi et al. Appl. Phys. Lett. 95, 072506 (2009) [3] O. J. Lee et al. 95, 012506 (2009) [4] R. Sbiaa et al. J. Appl. Phys. 105, 013910 (2009) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:48AM |
P19.00007: Noise in Spin Torque Oscillators Invited Speaker: In a spin torque oscillator {\_}(STO){\_}, a direct current passing through a reference magnetic layer becomes spin polarized and transfers angular momentum to a second magnetic layer that is excited into steady-state oscillation. The oscillating magnetization causes an oscillating device resistance, through either the giant magnetoresistance effect or the tunneling magnetoresistance effect, which in combination with the bias current generates an oscillating voltage as the output signal. Interest in potential applications of STOs in integrated microwave circuits is driven by their rapid frequency tunability, small size {\_}{\_}($<$100 nm){\_}, and compatibility with standard semiconductor processing techniques. For any oscillator, noise is both an important figure of merit for applications and a useful probe of internal physical processes. I will summarize the theoretical and experimental state of our understanding of frequency and phase noise in a variety of oscillators, considering both time domain and frequency domain measurements. Some aspects can be explained by the effects of thermal fluctuations. Others, such as frequency noise that varies as 1/f at low frequencies, are not yet understood. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P19.00008: Optimal spin current pattern for fast domain wall propagation in nanowires Peng Yan, Zhouzhou Sun, John Schliemann, Xiangrong Wang One of the important issues in nanomagnetism is to lower the current needed for a technologically useful domain wall (DW) propagation speed. Based on the modified Landau-Lifshitz-Gilbert (LLG) equation with both Slonczewski spin-transfer torque and the field-like torque, we derive an optimal temporally and spatially varying spin current pattern for fast DW propagation along nanowires. Under such conditions, the DW velocity in biaxial wires can be enhanced as much as tens of times higher than that achieved in experiments so far. Moreover, the fast variation of spin polarization can efficiently help DW depinning. Possible experimental realizations are discussed. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P19.00009: Effects of Spin transfer torques on domain wall nucleation and propagation in perpendicular spinvalve nanopillars Stephane Mangin, Julien Cucchiara, Thomas Hauet, David P. Bernstein, Eric E. Fullerton, Andrew D. Kent, Jordan Katine, Jonathan Z. Sun Controlled manipulation of magnetic domain wall (DW) propagation has spurred intensive research in recent years because of its fundamental interest and the potential impact in spintronic device technology such as racetrack memories. Both magnetic fields and electric currents may be used to control domain walls. Most of the studies have been performed on magnetic nanowires with in plane anisotropy. Here we study domain wall creation and propagation in spinvalve nanopillar composed of magnetic materials with perpendicular anisotropy such Co/Ni multilayers [1]. It is shown that DWs can nucleate and propagate in perpendicularly magnetized nanopillar spin valves as small as 50 $\times $ 100 nm$^{2}$ [2]. The study of the dynamics of DW nucleation and propagation driven by applied fields and injected currents is presented [3]. High domain wall velocities of about 100m/s are found. \\[4pt] [1] S. Mangin, et al , \textit{Nat. Mater.} \textbf{5}, 210 (2006), S. Mangin, et al, \textit{Appl. Phys. Lett.} \textbf{94}, 012502 (2009) \\[0pt] [2] D. Ravelosona, et al (2006) \\[0pt] [3] J. Cucchiara, et al \textit{Appl. Phys. Lett. 94 102503 (2009)} [Preview Abstract] |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P19.00010: Domain Wall Pumping with Spin-Transfer Torque Carl Boone, Ilya Krivorotov We numerically investigate the effects of current-perpendicular-to-the-plane (CPP), angularly asymmetric Slonczewski spin-transfer torque (ST) on transverse domain walls (DW) in nanowires. The CPP ST excites long-range domain wall motion in a direction independent of the current polarity and proportional to the square of the current amplitude. This symmetry with respect to current polarity creates the possibility of DW pumping -- long range DW motion driven by an alternating current. The DW velocity becomes resonantly enhanced near a frequency that depends on the nanowire dimensions, corresponding to the eigenfrequency of a localized, spatially antisymmetric spin-wave mode that exists within the DW. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P19.00011: Current-Perpendicular-to-Plane (CPP) Magnetoresistance at 4.2K of spin-valves with the half-metal Co(2)Fe(Al(0.5)Si(0.5)) and Permalloy Rakhi Acharyya, Reza Loloee, William Pratt, Jack Bass There is interest in current-perpendicular-to-plane (CPP) magnetoresistance (MR) in spin-valves containing half-metallic Heusler alloys such as Co(2)FeAl((0.5)Si(0.5)) (CFAS) [1]. Onto [001] oriented MgO substrates, we sputter epitaxially [001] oriented layers of 150-nm-thick Nb, 10-nm-thick Cu, and chosen thickness CFAS. We complete a spin-valve with 25 nm of Cu, 24 nm of Py = Ni(84)Fe(16), 10 nm of Cu, 25 nm of Nb, and 15 nm of a Au capping layer. With optical lithography and Ar-ion milling, we make 25 micron radius pillars insulated with in-situ deposited SiO. Finally, we lightly ion mill the Au surface and deposit a 150 nm thick Nb cross-strip, then covered by 5 nm of Au. The Nb strips superconduct at our measuring temperature of 4.2K, giving uniform current flow. We will describe how the CPP-MR varies with thicknesses of CFAS ranging from 2 nm to 20 nm. We hope to describe additional studies with Ag instead of Cu and with pinned Py layers.\\[4pt] [1] T.M. Nakatani et al., Appl. Phys. Lett. \textbf{96}, 212501 (2010). [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P19.00012: Modulation of spin torque from spin transport through two nearby domain walls Elizabeth Golovatski, Michael Flatt\'e The motion of domain walls due to the spin torque generated by coherent coherent carrier transport [1] is of considerable interest for the development of spintronic devices [2]. We model two $\pi$ N\'eel walls [3] separated by a variable distance, and calculate transport characteristics and spin torque through the system [4]. We find that for large separations, the domain walls show the resonant transmission behavior of a spin-dependent double barrier; for small separations, the transmission spectrum resembles that of a $2\pi$ wall. We also find that the spin torque across the system initially increases as the separation between the walls increases from zero, then decreases slightly before reaching a saturation value that is larger than both the spin torque of a $2\pi$ wall and that of two individual $\pi$ walls. This work is supported by an ARO MURI.\\[4pt] [1] M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature 428, 539 (2004).\\[0pt] [2] S. Parkin,M . Hayashi, L. Thomas, Science 320, 190 (2008)\\[0pt] [3] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89 (2002).\\[0pt] [4] D. Ralph and M. Stiles, J.M.M.M. 320, 1190 (2008). [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P19.00013: Current-Induced Torques in the Presence of Spin-Orbit Coupling M.D. Stiles, Paul M. Haney In systems without spin-orbit coupling, the simple connection between spin transfer torque and the divergence of the spin current has provided a simple description of current induced torques. Here, we generalize this relationship for systems with strong spin-orbit coupling to a relationship between spin transfer torques, total angular momentum current, and mechanical torques. In such systems, the spin-orbit coupling modifies the behavior of the spin transfer torques. For example, the it can give rise to a persistent spin transfer torque in a spin valve: the spin transfer torque density approaches a constant value rather than decaying away from the interface. This approach also provides a formal expression for the mechanical torque at a single ferromagnetic-nonmagnetic interface. [Preview Abstract] |
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