Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session B22: Focus Session: Current Driven Magnetization Dynamics II |
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Sponsoring Units: GMAG FIAP DMP Chair: Mark Stiles, National Institute of Standards and Technology Room: Baltimore Convention Center 319 |
Monday, March 13, 2006 11:15AM - 11:27AM |
B22.00001: Reduction of the FMR damping in the nonlinear regime. Olivier Klein, Gregoire de Loubens, Vladimir V. Naletov It is well known in magnetic resonance, that the Gilbert damping coefficient cannot be extracted directly from the linewidth measurements. The later quantity only yields an upper bound value of the coupling to the lattice. In contrast simultaneous measurement of the microwave susceptibility and the longitudinal component of the magnetization gives directly the ratio between energy stored over the energy absorbed in the sample. This quantity is proportional to the spin-lattice relaxation rate of the excited system. We have recently measured the power dependence of this ratio in the nonlinear regime of YIG samples. Surprisingly, we find that the energy damping coefficient decreases dramatically above the Suhl's thresold while the linewidth increases. [Preview Abstract] |
Monday, March 13, 2006 11:27AM - 11:39AM |
B22.00002: The Periodic Table of Damping in Doped Permalloy Thin Films James Rantschler, Deepthi Pulugurtha, Lawrence Matthew Connors, Andrew Chen, Audi Castillo, Alexander Shapiro, William Egelhoff, Jr., Robert McMichael, Brian Maranville In a survey of thin film Permalloy doped with transition metals, we have constructed a periodic table of damping. We have co-sputtered 25 nm Permalloy films with twenty-one different dopants (Ti, V, Cr, Mn, Co, Cu, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, Pt, and Au) with at least ten cocnetrations. We measured damping with ferromagnetic resonance spectroscopy (FMR) and concentration with energy dispersive x-ray spectroscopy. Out-of-plane rotation FMR experiments were used to separate the extrinsic linewidth contributions produced by inhomogeneities in the sample from the intrinsic damping of the material. In all samples, doping increased damping, but the effect varied considerably. Trends in the periodic table include larger damping for the heavier elements and larger damping when the d shells are half-full. When using the dimensionless damping parameter of the Landau-Lifshitz-Gilbert equation to describe magnetization dynamics, undoped Permalloy is generally found to have a damping parameter of 0.008. In this study doping increased the damping parameter in all cases; the weakest effects were as little as 0.0001 per atomic percent of copper or silver, while the largest effect was measured to be 0.006 per atomic percent of osmium. [Preview Abstract] |
Monday, March 13, 2006 11:39AM - 11:51AM |
B22.00003: Enhanced Magnetic Damping in Spin-Transfer Excitation E. M. Ryan, P. M. Braganca, J. C. Read, N. C. Emley, G. D. Fuchs, J. C. Sankey, D. C. Ralph, R. A. Buhrman While magnetic damping is understood to play a fundamental role in spin-torque phenomena, little experimental work has been done to study the effect of varying the damping parameter $\alpha $. Recently, light terbium (Tb) doping in thin films of permalloy (Py) has been shown to increase $\alpha $ by several orders of magnitude [1]. To directly study the effect of increased $\alpha $ on spin-transfer systems, we have fabricated 0.008 um$^{2}$ Py/Cu/Py nanopillar spin valves with Tb-doping between 0 and 2{\%} in the free layer. We find that the critical currents for reversibly switching the free layer (proportional to $\alpha )$ are several times larger on average in the 2{\%} Tb samples than in pure Py samples. This substantial increase is still considerably less than the increase in $\alpha $ observed in the bulk film measurements of similar composition samples, suggesting that processes other than intrinsic spin-orbital coupling can dominate $\alpha $ in spin-transfer nanopillars, in agreement with simulation and pulsed measurements [2]. We compare this effect across a range of temperatures, and also discuss how the Tb doping affects the processional dynamics. These results suggest one approach for controllably reducing the negative impact of spin torque effects on nanoscale spin valve and tunnel junction read head sensors. [1] W. Bailey, P. Kabos, F. Mancoff, and S. E. Russek, IEEE Trans. Magn. 37, 1749 (2001). [2] P. M. Braganca, et al. Appl. Phys. Lett. 87, 112507 (2005). [Preview Abstract] |
Monday, March 13, 2006 11:51AM - 12:27PM |
B22.00004: Field and Current-Driven Domain Wall Motion in Permalloy Nanowires Invited Speaker: Ferromagnetic nanowires provide a well-defined conduit for domain walls, which may be manipulated and used in a variety of information storage and processing schemes. A domain wall may be displaced by a magnetic field or by an electric current traversing the wall via spin-momentum transfer. Many experiments have focused on the depinning of a domain wall by a current, but few have addressed the subsequent propagation of a depinned wall, whether driven by current [1], field [2,3], or both. Using high-bandwidth scanning Kerr polarimetry, we have studied time-resolved motion of field and current-driven domain walls in Permalloy nanowires. Domain dynamics models predict that above a threshold field, uniform wall translation gives way to turbulent wall motion, a dynamic internal wall structure, and a substantial drop in wall mobility. We have observed this transition at an unexpectedly low field [3], suggesting that in many experiments, wall motion is far more complex than the smooth translation typically assumed. The interaction of a dc current with a propagating domain wall is likewise more complex than existing spin-torque models predict. We find a nonlinear response of the wall velocity to a dc current, with velocity enhancements approaching 40 m/s at a current density of $\sim $6$\times $10$^{11}$ A/m$^{2}$. The response may be represented by a field-independent linear term, consistent with theory, and a field-dependent nonlinear term that overtakes the linear term at moderate currents. This latter term may arise from the interaction of the current with a vortex in the wall. [1] A. Yamaguchi, \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{92}, 077205 (2004) [2] T. Ono, \textit{et al}., \textit{Science} \textbf{284}, 468 (1999); D. Atkinson, \textit{et al}., \textit{Nature Mater.} \textbf{2}, 85 (2003). [3] G. S. D. Beach, \textit{et al.}, \textit{Nature Mater.} \textbf{4}, 741 (2005) [Preview Abstract] |
Monday, March 13, 2006 12:27PM - 12:39PM |
B22.00005: Direct observation of current driven domain wall distortions, transitions, and propagation in ferromagnetic wires. W. Casey Uhlig, John Unguris Spin transfer torque induced domain wall distortions, transitions between vortex and transverse wall states, and defect-to-defect hopping of domain walls were all observed while flowing current through narrow ferromagnetic wires. Domain walls in 100 nm, 300 nm, and 1 um wide stripes fabricated from 12 nm and 24 nm thick NiFe films were directly imaged using scanning electron microscopy with polarization analysis (SEMPA). SEMPA images revealed that small current densities initially distort the domain walls which are fixed at random pinning defects along the wires. If the current density is increased above 5x10$^{11}$ A/m$^{2}$, the wall is usually swept from the wire, but at smaller currents, the domain wall simply jumps a short distance to the next pinning site before stopping. For vortex type domain walls, the vortex core moves towards the stripe edge, perpendicular to the wall propagation direction. If the core reaches the edge, it is annihilated, thereby converting the vortex into a transverse wall. The direction of the vortex core motion and thus the chirality of the resulting transverse wall are dependent on the applied current direction. The observed domain wall distortions, transitions, and interactions with defects are all expected to play a significant role in spin torque driven magneto-electronics. Work supported in part by the Office of Naval Research. [Preview Abstract] |
Monday, March 13, 2006 12:39PM - 12:51PM |
B22.00006: Spin Transfer Torque for Continuously Variable Magnetization Jiang Xiao, Andrew Zangwill, Mark Stiles We report quantum and semi-classical calculations of spin current and spin-transfer torque in a free-electron Stoner model for systems where the magnetization varies continuously in one dimension. Analytic results are obtained for an infinite spin spiral and numerical results are obtained for realistic domain wall profiles. The adiabatic limit describes conduction electron spins that follow the sum of the exchange field and an effective field produced by the gradient of the magnetization in the wall. Non-adiabatic effects arise for short domain walls but their magnitude decreases exponentially as the wall width increases. Our results cast doubt on the existence of a non-adiabatic contribution to the spin-transfer torque. [Preview Abstract] |
Monday, March 13, 2006 12:51PM - 1:03PM |
B22.00007: Nonlinear regimes of current-induced domain wall motion Ya. B. Bazaliy, M. Hayashi, B. A. Jones, S. S. P. Parkin, A. Joura We study theoretically the current-induced motion of a magnetic domain wall in a nanowire in the presence of external magnetic field and pinning potential. The wall is assumed to be rigid and is described by the one-dimensional model equations [1,2]. Both ``adiabatic" and ``non-adiabatic" spin-transfer terms are taken into account. The current-induced motion differs significantly below and above the Walker breakdown, i.e., in the regimes when the deflection angle is close to a particular value vs. when it makes the full circles. We find two novel phenomena. The first one is the existence of a stable autogeneration regime for a wall trapped in a shallow pinning center. The second is the ``reversed motion window" regime in the situation when current and magnetic field push the domain wall in opposite directions. Both regimes are qualitatively different from the domain wall resonance regime studied in Ref. [1,3], or zero field current induced motion [1,2]. Reversal of the domain wall velocity by a relatively small current above the Walker breakdown was recently achieved in Ref. [4]. \\ \\ 1. Gen Tatara and H. Kohno, Phys. Rev. Lett. {\bf 92}, 086601 (2004).\\ 2. A. Thiaville {\it et al.}, Europhys. Lett., {\bf 69}, 990 (2005).\\ 3. E. Saitoh {\it et al.}, Nature, {\bf 432}, 203 (2004). \\ 4. M. Hayashi {\it et al.}, submitted (2005). [Preview Abstract] |
Monday, March 13, 2006 1:03PM - 1:15PM |
B22.00008: Interactions between domain walls and spin currents M. Klaui, M. Laufenberg, D. Backes, W. Buhrer, U. Rudiger, L. Vila, C. Vouille, G. Faini A promising novel approach for switching magnetic nanostructures is current-induced domain wall propagation (CIDP), where due to a spin torque effect, electrons transfer angular momentum to a head-to-head domain wall and thereby push it in the direction of the electron flow without any externally applied fields. This effect has been observed with a variety of techniques including MFM [1] and spin polarized scanning electron microscopy [2] to directly observe current-induced domain wall propagation in ferromagnetic nanostructures and magnetoresistance measurements to systematically probe the critical current densities as a function of the geometry [3]. The observed wall velocities and critical current densities, where wall motion sets in at room temperature, do not agree well with theoretical 0K calculations [4]. We have therefore measured the critical current densities as a function of the sample temperature. We find that the spin torque effect becomes more efficient at low temperatures, which could account for some of the observed discrepancies between the 300K experiment and the 0K simulation. [1] A. Yamaguchi et al., Phys. Rev. Lett. 92, 77205 (2004); [2] M. Klaui et al., PRL 95, 26601 (2005); [3] M. Klaui et al., PRL 94, 106601 (2005); [4] A. Thiaville et al., EPL 69, 990 (2005); G. Tatara et al., APL 86, 252509 (2005); [Preview Abstract] |
Monday, March 13, 2006 1:15PM - 1:27PM |
B22.00009: Current-Driven Resonance of Single Domain Wall in Permalloy Nanowires Seok-Hwan Chung, Frank Y. Fradin, Axel Hoffmann The manipulation of magnetic domain structures with electrical transport current is of fundamental interest for understanding the magneto-transport properties of spintronic devices. We measured current driven domain wall resonance by using a well-defined single domain wall in half-circle nanowires with 35 $\mu $m radius. Nanowires of 40 nm-thick and 250 nm-wide Permalloy with contact pads were fabricated by multi-step electron beam lithography and subsequent lift-off processes. A single \textit{transverse} magnetic domain wall can be effectively introduced and pinned in the center of the nanowire, due to the strong shape anisotropy, by applying external field perpendicular to the center of the wire. This was confirmed with magneto-transport measurement, magnetic force microscopy and micromagnetic simulation. We observed single domain wall resonance by measuring the reflected power as a function of driving frequency and applied pinning field. From these measurements, we can obtain the domain wall inertia (D\"{o}ring mass), which is 4$\times $10$^{-8}$g/cm$^{2}$ in our sample. The critical current density for the wall motion is $\sim $1$\times $10$^{7}$A/cm$^{2}$. We attribute the mechanism of the current driven wall motion in our sample to the spin and momentum transfer from $s$ electrons in the conduction band to the local $d$ electrons in the domain wall. * Supported by DOE, BES under contract W-31-109-ENG-38. [Preview Abstract] |
Monday, March 13, 2006 1:27PM - 1:39PM |
B22.00010: Spin accumulation and resistance due to a moving domain wall J. Ieda, S. Takahashi, S. E. Barnes, S. Maekawa The resistance of magnetic nanowires containing a moving domain wall (DW) is studied. The excess resistance due to the DW has been shown in a number of transport experiments [1]. Theoretically, the DW resistance is investigated along with the concepts of the spin dependent scattering with spin mistracking and the spin accumulation, where the DW is assumed static just producing the exchange field to conduction electrons. For a moving DW, however, the angular momentum transfer effect [2] should be incorporated to those mechanisms. In this presentation, we discuss this effect along with the spin accumulation mechanism. The frequency dependence of the spin accumulation is also presented. [1] J. F. Gregg et al., PRL77, 1580 (1996); U. Ebels et al., PRL84, 983 (2000); E. Saitoh et al., Nature432, 203 (2004). [2] S. E. Barnes and S. Maekawa, PRL95, 107204 (2005); cond-mat/0410021. [Preview Abstract] |
Monday, March 13, 2006 1:39PM - 1:51PM |
B22.00011: Theory of Antiferromagnetic Metal Spintronics Rembert Duine, Alvaro Nunez, Paul Haney, Allan MacDonald Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. In this talk I will argue that nanostructure circuits containing antiferromagnetic elements have large potential for applications, partly because the critical current for inducing magnetization dynamics may be smaller than for ferromagnets. This occurs in part because spin torques that lead to current-induced switching act through the entire volume of an antiferromagnet. I will explain how this follows from the special symmetry properties of antiferromagnets. [Preview Abstract] |
Monday, March 13, 2006 1:51PM - 2:03PM |
B22.00012: Towards ab-initio calculations for antiferromagnetic metal spintronic devices. Paul Haney, Derek Waldron, Alvaro Nunez, Rembert Duine, Hong Guo, Derek Waldron In conventional spintronics, the electron's spin degree of freedom is exploited to construct systems which exhibit behavior of practical importance and fundamental scientific interest. The prototypical system is a heterostructure which consists of alternating layers of ferromagnetic and paramagnetic materials. Such a system is used to study GMR and spin transfer torque, the two most important phenomena in spintronics today. In GMR, the relative orientation of magnetic layers affects the current flowing through a heterostructure, while in spin transfer the current flowing through the structure effects the orientation of the magnetic layers. It is of interest to consider what type of effects occur in structures containing antiferromagnetic materials. To this end, realistic calculations of GMR and spin transfer in antiferromagnetic heterostructures are presented. The calculations are carried out using ab-initio NEGF methods, using the LSDA extended to noncollinear magnetic configurations. Preliminary results of a Cr-Au-Cr structure are presented as an example of an experimentally realizable antiferromagnetic system. [Preview Abstract] |
Monday, March 13, 2006 2:03PM - 2:15PM |
B22.00013: Nonequilibrium Green's function calculations for antiferromagnetic metal spintronic nanocircuits Alvaro Nunez, Rembert Duine, Paul Haney, Allan MacDonald In this talk I will present results of nonequilibrium Green's function calculations on toy-model heterostructures containing antiferromagnetic elements that are intended to illustrate some generic aspects of their spintronic properties. Using this formalism we calculate the nonequilibrium spin density of the electrons which, in a microscopic picture of spin transfer, gives rise to spin transfer torques. The main result is that, unlike the case of ferromagnets to ferromagnets, the spin transfer acts throughout the entire antiferromagnet, making current a very effective way to induce collective magnetization dynamics. Preliminary results on the influence of disorder and surface roughness will be presented. [Preview Abstract] |
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