Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A43: Focus Session: Spin Transfer Effect I |
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Sponsoring Units: GMAG DMP Chair: Robert Buhrman, Cornell University Room: LACC 150C |
Monday, March 21, 2005 8:00AM - 8:36AM |
A43.00001: Time-Domain Measurements of Nanomagnet Dynamics Driven by Spin-Polarized Current Invited Speaker: The transfer of spin angular momentum from a spin-polarized current to a nanomagnet exerts torque, and can cause the magnet's moment either to reverse its direction or to enter a state of steady precession. Exploiting a new nanoscale spin valve design, we make first time-resolved measurements of these dynamics. These measurements are made in Py(4 nm)/Cu(8 nm)/Py(4 nm)/IrMn(8 nm) nanopillar spin valves in which exchange bias is used to create a non-zero equilibrium angle between the magnetic moments of the free and fixed permalloy (Py) nanomagnets. In the regime of steady-state precession, the current-driven dynamics exhibit a high degree of coherence, as evidenced by long dephasing times ($\sim $10$^{2}$ ns). Measurements of the onset of the persistent precession in response to a current step demonstrate a fast ($\sim $ 1 ns) response of the nanomagnet to variations of the current. In the switching regime, time-resolved measurements demonstrate that spin-transfer-driven magnetization reversal in our samples is accomplished via a process of coherent precession. We also make time-resolved measurements of magnetic relaxation of the free Py nanomagnet excited by a short current pulse. These measurements, made as a function of spin-polarized current bias, demonstrate that the effective Gilbert damping parameter can be tuned by the spin-transfer torque. The value of the damping parameter in the limit of no current bias significantly exceeds the damping of an extended 4 nm thick Py film -- which is attributable to substantial spin pumping in the nanopillar structure. Our results demonstrate that coherent nanomagnet dynamics can be generated by spin-transfer torques in properly designed magnetic nanostructure devices and directly measured in both the time and frequency domains. This opens a wide range of opportunities for new types of fundamental studies of nanomagnet dynamics and for novel technological applications in the areas of high frequency communications and signal processing. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A43.00002: Current-driven excitations in symmetric magnetic nanopillars M. Tsoi, J. Z. Sun, S. S. P. Parkin An electrical current was shown to induce spin waves and reversal of magnetization in a ferromagnet. A typical experiment on current-driven excitation of a ferromagnet involves two single-domain thin film magnets separated by a nonmagnetic spacer. One magnet is ‘hard’ and used to polarize the current while the spacer is thin enough for the polarized current to get through and excite the second ‘free’ magnet. The free layer is generally thin compared to the hard one thus marking an intrinsic asymmetry of the phenomenon, i.e., for initially parallel magnetizations of the two magnets the current-driven excitation occurs only when electrons flow from the free magnet to the fixed one. In the present work we study experimentally the current-driven excitations in symmetric Co/Cu/Co nanopillars. In contrast to all the previous observations where current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer [Phys. Rev. Lett. 93, 36602 (2004)]. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers. [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A43.00003: Spin-transfer torque driven de-pinning of a domain-wall in a magnetic nano-wire Luc Thomas, Masamitsu Hayashi, Xin Jiang, Rai Moriya, Charles Rettner, Stuart Parkin We present a theoretical study of the dynamics of a magnetic domain wall trapped in a potential well, driven by current-induced spin-transfer torque, in the presence of an external magnetic field. We show the existence of two regimes and two different mechanisms for the de-pinning of the domain wall using a one dimensional model. This model reveals that in small magnetic fields the critical current for de-pinning of the domain wall is completely independent of the pinning potential and weakly dependent on the magnetic field. Whereas, above some critical field, the critical current density is sensitive to the pinning potential and, moreover, strongly decreases with increasing field. Analytical expressions of the field dependence of the critical de-pinning current are derived for each regime, in the zero damping limit. The influence of adiabatic and non-adiabatic spin-transfer torques is also discussed. These theoretical predictions are compared to experimental results and to micromagnetic simulations. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A43.00004: Domain wall motion driven by an electric current Jiexuan He, Zhanjie Li, Shufeng Zhang We have recently proposed [1] that an electric current in a ferromagnetic film generates two mutually orthogonal spin torques, $\mbox{\boldmath $\tau$}_1 = b_J {\bf M}\times ({\bf M} \times \frac{\partial \bf M}{\partial x})$ and $\mbox{\boldmath $\tau$}_2 = c_J {\bf M}\times \frac{\partial \bf M}{\partial x} $ where ${\bf M}$ is the magnetization vector and the constants $b_J$ and $c_J$ are proportional to the current density. By including these two spin torques in the Landau-Lifshitz-Gilbert equation; we have simulated the domain motion in a number of experimentally accessible geometries. We have found that the current-driven domain wall motion displays many unique features compared to that driven by an external field. One particular example is to predict the critical current as a function of the applied magnetic field in a ``constriction'' geometry where the domain wall is originally trapped before applying an electric current. The calculated critical current densities are compared to the existing experimental data. [1] S. Zhang and Z. Li, Phys. Rev. Lett. {\bf 93}, 127204 (2004). [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A43.00005: Current-Driven Domain Wall Motion in Permalloy Nanowires Masamitsu Hayashi, Luc Thomas, Charles Rettner, Xin Jiang, Rai Moriya, Stuart Parkin The current-driven motion of magnetic domain walls (DW) in permalloy (Ni$_{81}$Fe$_{19})$ nanowires is discussed. The nanowires were fabricated by electron-beam lithography from permalloy films 10 to 40 nm thick. The wire lengths and widths were varied from 2 to 10 $\mu $m, and from 70 to 300 nm, respectively. DWs are injected into the nanowires using magnetic fields, generated either by a large electromagnet or locally by using micron-wide gold wires fabricated above and transverse to the permalloy nanowires. The injected DWs are trapped at triangularly-shaped notches fabricated along one or both edges of the nanowires. Current-driven DW motion is probed using anisotropic magnetoresistance measurements and magnetic force microscopy (MFM). DWs can be moved in the absence of any external magnetic field by current pulses, varying in length from nano- to micro- seconds. Current densities of the order of 10$^{8}$ A/cm$^{2 }$are needed. MFM images show unambiguously that DWs can be moved from one notch to another, in either direction along the nanowire, depending on the current pulse polarity, intensity and duration. The dependence of the critical current density required to move the DWs between notches on the nanowire width and notch shape and size will be discussed. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A43.00006: Mobility of field and current-driven domain walls in magnetic nanowires G. S. D. Beach, C. Nistor, C. Knutson, M. Tsoi, J. L. Erskine Recent experiments [1] have demonstrated domain wall displacements in magnetic nanowires resulting from the injection of a spin-polarized current across the wall. These observations have been attributed to spin-momentum transfer. We present direct measurements of domain wall velocities in focused ion beam etched Permalloy nanowires using high spatial resolution scanning Kerr polarimetry with high-bandwidth detection ($<$2 ns risetime). The present experiments provide instantaneous measures of domain wall velocity during the entire course of wall propagation, in contrast to the average velocities determined in previous displacement studies [2]. We will report on field-driven domain wall velocity profiles and mobilities in magnetic nanowires, and the influence of dc spin currents on these dynamic quantities. The relation to various models will be discussed. Supported by NSF-DMR-0404252 and the R. A. Welch Foundation.\newline \newline[1] M. Tsoi, et. al, Appl. Phys. Lett. 83, 2617 (2003)\newline [2] A. Yamaguchi, et. al, Phys. Rev. Lett. 92, 077205 (2004) [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A43.00007: SEMPA measurements of trapped domain walls in thin film nanoconstrictions. W. Casey Uhlig, John Unguris We used scanning electron microscopy with polarization analysis (SEMPA) to image the magnetic nanostructure of domain walls trapped in patterned NiFe thin film nanoconstrictions. Currents were applied to the structures to induce spin torque driven motion of the domain wall in the nanoconstriction. Various film geometries were investigated in order to understand how the size and shape of the constriction affects the magnetic nanostructure of the domain wall. The structures were fabricated using electron beam lithography. Constriction widths varied from 40 nm to 200 nm. The non-invasive nature of SEMPA allowed successful imaging of the unperturbed, remanent state of the trapped domain walls. In 10 nm thick NiFe films, all of the observed trapped walls (within the constriction) were of the transverse type, and the domain wall widths were strongly dependent on both the width of the constriction (approximately equal to the width) as well as the shape of the constriction. Because SEMPA directly measures the magnetization direction, the image data allows meaningful quantitative comparisons to micromagnetic calculations. Simulations with inserted domain walls show good agreement with the behavior of the domain walls observed by SEMPA. Detailed comparisons will be presented. *Work supported in part by the Office of Naval Research [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A43.00008: Nanometer Scale Observation of Current-Induced Narrow Domain Wall Depinning in Perpendicular Spin Valves Dafine Ravelosona, Daniel Lacour, Jordan Katine, Bruce Terris Until now, current driven domain wall (DW) motion in magnetic wires has been experimentally studied for in-plane magnetized films. Since the DW width is large ($\sim $100 nm), only the adiabatic limit in which the current polarization follows the magnetization direction has been studied. Also, this wide DW masks any local variation in the pinning potential, thus making it difficult to probe the depinning process on a nanometer scale. Here, we report the first quantitative study of the depinning of a 1D narrow DW under a current. We use a 12nm wide Bloch DW in wires based on spin valves with perpendicular magnetic anisotropy. High sensitive electrical measurements allow us to observe current-induced DW motion between pinned sites separated by 10 nm. In spite of the strong pinning potential and narrow DW, a low critical current density of the order of 1x 10$^{7}$ A/cm$^{2}$ is found. The study of the depinning process emphasizes the crucial role thermal fluctuations and the pinning potential play in current induced DW motion process. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A43.00009: Interaction of Spin-Transfer Oscillators With AC Currents and Fields M.R. Pufall, W.H. Rippard, S. Kaka, T.J. Silva, S.E. Russek We have shown previously that a DC current flowing through a 40 nm point contact made to a spin valve structure induces high frequency, large-angle coherent magnetic precession.~ The precession frequency ranges from 5-40+ GHz, and is a strong function of the field magnitude and direction, and the current.~ Injection of an additional AC current into the device produces frequency modulation for low injected frequencies, and "injection locking" for injection frequencies near the resonant frequency.~ We will present a detailed analysis of the injection locking process, highlighting the similarities and differences with conventional oscillators, and using the injection locking as a means of determining the precession amplitude at the device.~ In addition, we will show results on the effect of AC magnetic fields on the spin-transfer resonance.~ Comparison with models to describe the effects on the magnetization trajectory will be made. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A43.00010: Current-Induced High Frequency Excitations in Py-based Nanopillars Mustafa AlHajDarwish, Irinel Chiorescu, William Pratt Jr., Jack Bass To study how high frequency excitations induced by high current densities in ferromagnetic/non-magnetic/ferromagnetic (F/N/F) nanopillars vary with applied magnetic field H and current I, we have assembled a system containing 40 GHz picoprobes, a 12 GHz spectrum analyzer, and a 40 GHz Microwave-generator-based system that can be used as a spectrum analyzer up to 40 GHz. Our first measurements, on Permalloy (Py = Ni(84)Fe(16))-based nanopillars of the form Cu(80nm)/Py(30nm)/Cu(10nm)/Py(6nm)/Cu(5nm)/Au(200nm), yielded peaks at frequencies in the range 1 to 2 GHz. We will describe how the frequencies and heights of these peaks vary with H and I. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A43.00011: Bipolar High Field Excitations in Co/Cu/Co Nanopillar Junctions B. \"{O}zyilmaz, W. Chen, A. D. Kent, M. J. Rooks, J. Z. Sun Spin transfer has been studied in Co/Cu/Co pillar devices (PD) in large fields applied perpendicular to the layers and as a function of magnetic layer thickness. Sub-100 nm size junctions have been fabricated by means of a nano-stencil mask process in combination with an in-situ wedge growth mechanism. The junctions consist of a thick`fixed' Co layer and thin (0.5 to 3 nm) `free' Co layer. At high current densities excitations, which lead to a decrease in junction resistance, are observed for both polarities of the current [1]. Our results suggest that current-induced excitation of the magnetization can lead to a lower resistance state than that of a state of static parallel alignment of the layers. Intrinsic asymmetries of bilayer junctions in conjunction with lead asymmetries cause a strong asymmetry in the longitudinal spin accumulation (LSA). Recently it has been found that at high current densities such asymmetries in the LSA can cause non-uniform spin- wave excitations even in PDs with only a single ferromagnetic layer [2]. Here we compare the thickness dependence of these additional excitations in single layer junction with that of the free layer thickness dependence of the bilayer junctions. \\[4pt] [1] B. \"{O}zyilmaz et al. cond-mat/0407210.\\[0pt] [2] PRL,93, 176604 (2004). [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A43.00012: High Speed Spin-Transfer Switching Behavior of Low Critical Current Spin Valve Nanopillars P.M. Braganca, I.N. Krivorotov, O. Ozatay, A.G.F. Garcia, J.C. Sankey, N.C. Emley, D.C. Ralph, R.A. Buhrman For spin transfer writing to be effective for MRAM, the integration of a magnetic device with a scaled CMOS transistor in a memory cell requires that $I_{c}$ for switching a thin, thermally stable element on ns time scales be $<<$ 1 mA. Since $I_{c}$ scales with the volume of the magnetic element and the square of its saturation magnetization $M_{S}$, the use of very small free layers with low $M_{S}$ can result in low $I_{c}$'s. The challenge is obtaining a large enough magnetic anisotropy to ensure thermal stability at $\sim $100 C. We have fabricated 40x120 nm elliptical Py/Cu/Py nanopillar spin valves exhibiting free layer coercive fields in accord with 3-D micromagnetic modeling. For a 4.5 nm thick free layer device, currents necessary for 100{\%} switching go from 0.6 mA for a 10 ns pulse, where thermal activation aids switching, to 2 mA for a 1 ns pulse, where there is insufficient time for thermal fluctuations and $I_{c}$ is set by the current required to transfer enough spin into the free layer to force its reversal. We will discuss the switching mechanisms of these devices in the ns regime, and our progress towards achieving fully stable devices with low $I_{c}$'s. [Preview Abstract] |
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