Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y32: Focus Session: Current-Induced Magnetic Switching |
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Sponsoring Units: GMAG DMP FIAP Chair: Yi Ji, University of Delaware Room: 336 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y32.00001: Effect of spin diffusion in the polarizer on current-induced magnetic switching Scott Button, Sergei Urazhdin Current-induced magnetic switching of a nanomagnet occurs due to the spin transfer torque exerted by current spin-polarized by another ferromagnet. Efficient switching is generally achieved by enhancing the polarizing properties of the latter. However, calculations show that switching is affected not only by the polarizing properties of the polarizer, but also by the electron diffusion in this layer [1,2]. To test the effects of spin diffusion in the polarizer on current-induced switching, we performed measurements of magnetic multilayer nanopillars with three different structures of the polarizing magnetic layer: a thick Co layer, a thin Co layer, and a bilayer consisting of a thin Co layer and a strongly spin-flipping FeMn alloy. In the pillars with a thick Co polarizer, the switching currents dramatically increase below 130 K, while the magnetoresistance exhibits a nonmonotonic dependence on temperature with a peak at 130 K. In contrast, the samples with a thin Co polarizer exhibit weak monotonic dependencies of switching and magnetoresistance on temperature. We discuss the implications of our results for our understanding of spin-dependent diffusion in magnetic multilayers. [1] A.A. Kovalev, A. Brataas, and G.E.W. Bauer, Phys. Rev. B 66, 224424 (2002). [2] Zhang, P.M. Levy, and A. Fert, Phys. Rev. Lett. 88, 236601 (2002). [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y32.00002: Spin transfer torque switching of Co nanoparticles Han Zou, Xiaojun Wang, Yi Ji Spin transfer torque effect has potential application in Magnetic Random Access Memory (MRAM) devices as a way to address the memory elements. Most spin transfer studies are based on patterned multilayer thin films with 100 nm lateral dimension. In this work, we demonstrate the feasibility of the spin transfer switching of a few cobalt nanoparticles with a diameter of $<$ 5 nm at 4.2 K. The motivation arises from the prospect of device miniaturization and the capability to manipulate an individual magnetic nanoparticle. We use a multilayer thin film Cu(100nm)/Co(10nm)/Cu(3nm)/Co(0.5nm)/Au(2nm). The 0.5 nm Co layer is not continuous, and it consists of isolated Co particles formed due to surface tension. A mechanical point contact is formed on the multilayer film at 4.2K. By varying the size of the contact, the number of nanoparticles underneath a point contact can be controlled between $\sim $5 and $\sim $50. Hysteretic loops in \textit{dV/dI }-- $I$ measurements clearly indicates spin-transfer switching. The \textit{dV/dI-I} curves are qualitatively different between point contacts involving only few particles (5-10) and those involving many particles (40-50). [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y32.00003: Stochastic Resonance Driven by Spin Torque Xiao Cheng, Carl Boone, Jian Zhu, Ilya Krivorotov Application of a microwave ac current to a spin valve gives rise to a rectified voltage due to magnetization dynamics driven by ac spin torque. We study the effect of dc current bias on these dynamics in spin valves with superparamagnetic free layers. We observe large enhancement of the rectified voltage (up to two orders of magnitude) along a line in the dc current - magnetic field phase diagram of the system. This enhancement arises from large-amplitude nonlinear dynamics of magnetization of the free layer induced by the combined action of ac and dc spin torques. For small out-of-plane external magnetic field, the enhanced rectified signal is observed at low frequencies ($<$1GHz) of the ac drive. This signal enhancement arises from adiabatic stochastic resonance of magnetization of the free layer driven by ac spin torque. For large out-of-plane magnetic field, the rectified signal enhancement is found at the ac drive frequencies of several GHz. We interpret this new type of large-amplitude high-frequency dynamics as non-adiabatic stochastic resonance of magnetization. Temperature-dependent measurements of the rectified signal confirm the stochastic resonance nature of the observed phenomena. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y32.00004: Reduction of spin-torque switching currents by partially canceling the free layer demagnetization field Luqiao Liu, Takahiro Moriyama, Dan Ralph, Robert Buhrman A small switching or excitation current is crucial for the successful application of spin torque (ST) in magnetic memory and on-chip oscillator devices. The required ST current for an in-plane-polarized nanomagnet is proportional to its effective field, within which the out-of-plane component (H$_{z})$ dominates. This large H$_{z}$, however, does not contribute to the thermal stability of the free layer. So it will be of great advantage if we can reduce H$_{z}$. Co/Ni multilayer structures have been shown to exhibit perpendicular anisotropy and we have precisely controlled the thickness of these multilayer components so that this crystalline anisotropy can be used to cancel the demagnetization field, reducing H$_{z}$ to a value comparable to the in-plane geometry-dependent coercive field. In comparison to a control sample with a relatively higher H$_{z}$ and the same magnetic volume, we find that the low H$_{z}$ sample has much smaller ST reversal currents in both the quasi-state thermally activated and short pulse reversal regimes. The fact that the free layer magnetization lies in plane and the fixed layer(s) can be conventional magnetic material(s) makes it more tractable to deal with the dipole coupling between the free and reference layers, and should also facilitate the incorporation of this approach in high performance ST devices that utilize magnetic tunnel junctions. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y32.00005: Effects of rf current on critical field for magnetization reversal in spin torque devices Wenyu Chen, Sylvia Florez, Jordan Katine, Matthew Carey, Liesl Folks, Bruce Terris Current induced switching assisted by rf current has recently been observed in spin torque devices at low temperature [1, 2]. This effect allows control of spin transfer induced magnetization reversal through the frequency of an injected rf current. In this study, the effects of the rf current injection on critical field for magnetization reversal in spin valve junctions have been investigated. Measurements were conducted at room temperature, and the magnetic field was applied along the easy axis of the junction. An rf current was injected into the nanojunction at various frequencies ranging between 1 and 20 GHz. The dynamic resistance, dV/dI, was measured as a function of the rf frequency, power and the dc bias current while ramping the magnetic field. The rf current injection was observed to change the critical field for free layer magnetization reversal when the intrinsic spin-transfer-induced dynamics is frequency-locked with the injected rf. The results will be discussed in the context of macrospin models of spin transfer in metallic spin valve structures. [1] S. H. Florez et al. Phys. Rev. B 78, 184403 (2008) [2] Y.-T. Cui et al. Phys. Rev. B 77, 214440 (2008) [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:36AM |
Y32.00006: A Three Terminal Approach to Spin-Torque Written MRAM Cells Invited Speaker: Magnetic random access memory (MRAM) is a potentially superior alternative to silicon-based memories due to a combination of properties including non-volatility, fast read/write times, and low power consumption. Future MRAM technologies have been considered which use the spin transfer effect as a mechanism for bit element writing. Here, a spin polarized current passing through a ferromagnetic element is used to reverse its moment via an exchange of angular momentum, as opposed to the magnetic fields from remote write lines used in more conventional toggle MRAM [1]. However, the large current densities required for spin transfer reversal create significant barrier wearout issues in the magnetic tunnel junctions (MTJs) used as bit elements. One possible solution is to develop a nanopillar structure where a third electrode can be made to any point within a thin-film multilayer stack, substantially enhancing the versatility of the device by providing the means of applying independent electrical biases to two separate parts of the device. Using experimental results and micromagnetic simulations, I will discuss a joint magnetic spin valve/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode [2]. A spatially nonuniform spin-polarized current flowing into the free layer via the low-resistance spin valve path can reverse the magnetic orientation of the free layer as a consequence of the spin torque effect, by nucleating a reversal domain at the spin injection site that propagates across the free layer. The free layer magnetic state can then be read out separately via the higher-resistance magnetic tunnel junction. This three-terminal structure provides a strategy for developing high performance spin-torque MRAM cells which avoids the need to apply a large voltage across a MTJ during the writing step, thereby enhancing device reliability, while retaining the benefits of a high-impedance MTJ for read-out. \\[4pt] [1] Slaughter J.M., Dave R.W., DeHerrera M., Durlam M., Engel B.N., Janesky J., Rizzo N.D., Tehrani S., Fundamentals of MRAM technology, \textit{Journal of} \textit{Superconductivity: Incorporating Novel Magnetism }15, 19 (2002). \\[0pt] [2] Braganca P.M., Katine J.A., Emley N.C., Mauri D., Childress J.R., Rice P.M., Delenia E., Ralph D.C., Buhrman R.A., \textit{IEEE Trans. Nanotechnol.}, In press (2008). [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y32.00007: Invariant Form of the Spin-Transfer Switching Condition Inti Sodemann, Yaroslaw Bazaliy Conventional spin-transfer (ST) device with one fixed and one free layer is considered in the macrospin approximation for the case of constant driving current. The expression for the critical current capable of pushing the free layer magnetization out of the local energy minimum is obtained in an invariant form. It is found that the relevant quantity is the divergence of the spin-transfer torque, and not the strength of the torque itself. This shows that there is no essential difference between current induced switching in collinear and non-collinear geometries. The result further provides a qualitative picture of the influence of ST torque angular dependence on the switching current and allows to understand when the Slonczewski spin polarization coefficient $g(\theta)$ can, or cannot be approximated by a constant. We discuss the implications of the derived formulas for the engineering of low current devices. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y32.00008: Exchange assisted spin transfer torque switching Xi Chen, Randall Victora The main challenge in the application of spin transfer torque switching is the high current required to reverse the magnetization. We propose a composite structure containing soft and hard magnetic layers that significantly lowered the switching current. The dynamic phase diagram of the structure is studied using a macrospin model, with Landau-Lifshitz-Gilbert equation including a spin torque term. It is shown that an optimal exchange coupling strength exists with a value around half the anisotropy of the hard layer. By using multiple soft layers with graded anisotropy, a further reduction can be achieved. We also show that the switching current grows linearly with the damping constant in the soft layer. This means that a low damping, soft material can facilitate the reversal of the hard layer and reduce the switching current by over an order of magnitude. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y32.00009: Structural Characterization of Spin-Torque Oscillators Sarah C. Parks, K. Li, A. Hauser, J. E. Thompson, J. Ciraldo, J. Emerick, J. Lucy, F. Y. Yang, E. Johnston-Halperin The discovery of current-induced magnetodynamics in giant magnetoresistive (GMR) trilayers promises a novel platform for microwave electronics. One of the keys to developing this potential has been the development of nanoscale fabrication techniques, typically resulting in either nanopillar or point-contact geometries. As a result, a considerable technical barrier to further progress is the fidelity of current nanoscale patterning techniques. In an effort to address this challenge, we present the results of development efforts aimed at fabricating prototype point-contact spin torque oscillator (PC-STO) structures with a focused ion beam (FIB). The flexibility of FIB-based nanofabrication allows \textit{in situ} cross sectional imaging of contact structure, and these results are correlated with DC magnetotransport. This fabrication approach enables the rapid generation of structures in arbitrary geometries, and in conjunction with cross-sectional imaging promises increased control of device to device variation and the development of novel PC-STO structures. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y32.00010: AC and DC voltage driven magnetization dynamics in magnetic nanostructures Oleg Tretiakov, Aditi Mitra We study a geometry involving a thin ferromagnetic (F) layer sandwiched between two normal metal (N) leads. The system is driven out of equilibrium by the simultaneous application of an external dc and ac voltage across the N-F-N structure. The Keldysh diagrammatic approach is used to study the system which reveals that one of the effects of the external drive is to produce noise with a non-trivial frequency dependence. We determine the effect of the noise on the magnetization dynamics, and also present results for how the current-voltage characteristics of the structure is affected by the nonequilibrium dynamics of the ferromagnetic layer. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y32.00011: All Spin Digital Circuits Behtash Behin-Aein, Deepanjan Datta, Sayeef Salahuddin, Supriyo Datta Switching of a magnetic free layer using spin polarized current has been demonstrated in Magnetic Tunnel Junction (MTJ) devices. Currently MTJ's are being studied for memory and microwave oscillator applications. The purpose of this talk is to explore a modified MTJ where a clock pulse via the fixed layer facilities the switching of the free layer in accordance with a weak bias provided by an input magnet in the form of a spin current. Based on the Landau-Lifshitz-Gilbert equation (LLG) augmented with spin torque functions, we show the switching energy and the switching time of the free layer which indicates the possibility of very low power digital logic applications. Ordinary digital circuits store information in the form of capacitor charges that communicate through electrical interconnects. The purpose of this paper is to show that modified MTJ's can be the basis for all spin digital circuits. Our primary objective is to stimulate proof of concept experiments that could usher in a whole new set of devices suitable for spintronic circuits. [Preview Abstract] |
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