Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session H36: Focus Session: Nanomagnetism  Spin Torque 
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Sponsoring Units: DMP GMAG Chair: Liam O'Brien, Imperial College London Room: E146 

H36.00001: ABSTRACT WITHDRAWN 
Tuesday, March 16, 2010 8:12AM  8:24AM 
H36.00002: Observation of fractional synchronization of spin torque nanooscillators to a symmetrybreaking microwave signal Vasyl Tyberkevych, Andrei Slavin, Phil Tabor, Sergei Urazhdin Spin torque nanooscillators (STNO) are novel microwave oscillators. Strong nonlinearity and nonisochronity facilitate synchronization of STNOs to external driving signals. Using novel experimental setup, in which external signal is supplied in form of both microwave magnetic field and microwave current, we studied STNO synchronization in a wide range of driving signal frequencies. We observed multiple synchronization regimes corresponding to different relations $r=f_{e}$/$f$ between external frequency $f_{e}$ and the locked oscillation frequency $f$. Both integer ($r$ = 1, 2, 3, 4) and fractional ($r$ = 3/2, 7/3, 5/2, 7/2) synchronization regimes were clearly resolved. Theoretical analysis and numerical simulations show that pronounced fractional synchronization becomes possible only when the driving signal destroys symmetry of STNO magnetization trajectory with respect to halfperiod rotation, which is possible only when both microwave components (magnetic field and current) act simultaneously on STNO. [Preview Abstract] 
Tuesday, March 16, 2010 8:24AM  8:36AM 
H36.00003: Magnetoresistance in magnetic nanostructures: the role of nonuniform current Tatiana Rappoport, Tiago Machado, Marcio de Menezes, Luiz Sampaio We developed a method to calculate the magnetoresistance of metallic magnetic nanostructures that takes into account the nonuniform magnetization and an additional ingredient: a nonuniform electric current. We discretize the nanostructure and use the LandauLifshitzGilbert equation to numerically determine its magnetic structure. Each cell contributes with a local anisotropic magnetoresistance that depends on the relative orientation between the local magnetization and the current. We then obtain a resistance network where the current distribution is calculated using a relaxation method. As a first example, we considered a disk of Permalloy with dimensions that lead to the appearance of a magnetic vortex. We calculated the resistance and current distribution as a function of an inplane magnetic field and during the dynamical vortex core reversal. We show that the current is concentrated in the vortex core and the resistance curves can be used to observe the vortex core inversion. [Preview Abstract] 
Tuesday, March 16, 2010 8:36AM  8:48AM 
H36.00004: Huge enhancement of the magnetoresistance in nanoparticle arrays Elena Bascones, Virginia Estevez The interplay between ferromagnetism and discrete charging control the controls the electronic transport through nanoparticle placed between two ferromagnetic electrodes. If the spin relaxation time in the nanoparticle is long, spin accumulation in the nanoparticle appears when the magnetic moment in the electrodes have antiparallel orientation, but not for parallel orientation. Charging effects are strongly enhanced in nanoparticle arrays. Here we show that spin accumulation has a very strong effect in the transport properties of metallic nanoparticle arrays placed in between ferromagnetic electrodes. The observed behavior is qualitatively different to the case of a single island. Spin accumulation appears not also for antiparallel but also for parallel orientation of the electrodes's magnetic moments. In the last case, the threshold voltage at which current starts and the magnetoresistance are modified. Within the array the sign of the spin accumulation reverses leading to oscillations in the current and negative differential resistance and a huge enhancement of the differential magnetoresistance for a given range of voltages. [Preview Abstract] 
Tuesday, March 16, 2010 8:48AM  9:00AM 
H36.00005: Measurements of GMR and Spin Valve Effects in Single Co/Cu/Co Nanowires Mingliang Tian, Jian Wang, Joe Kulik, Jeremy Cardellino, David Rench, Thomas Mallouk, Moses Chan, Nitin Samarth The fabrication of metallic multilayer nanowires (NWs) by electrodeposition has allowed studies of spin transport in a variety of NW systems in recent years. Here, we study the transition from giant magnetoresistance (GMR) to spin valve behavior in single crystal Co/Cu/Co multilayer NWs by systematically varying the NW architecture. The NWs are fabricated by electrodeposition in polycarbonate membranes and characterized using transmission electron microscopy and magnetic force microscopy. We carry out fourprobe measurements of the magnetoresistance of single NW devices over the temperature range 4.2 K $<$ T $<$ 300 K. When the Cu spacer thickness t$_{Cu}$ is smaller than 100 nm, we observe GMR with room temperature values as high as $\sim $10 {\%}. In this regime, the reduction of the GMR with increasing spacer thickness is qualitatively similar to that observed in past studies of thin film multilayers. When t$_{Cu}$ approaches 170 nm, we observe a transition to spin valve behavior, suggesting a reduction of the interlayer coupling. Finally, for t$_{Cu} \quad >$ 200 nm, the NWs display only anisotropic magnetoresistance. [Preview Abstract] 
Tuesday, March 16, 2010 9:00AM  9:12AM 
H36.00006: High temperature annealing of MgO based perpendicular MTJ with Co/Pd multilayers M. Tofizur Rahman, Andrew Lyle, Hui Zhao, JianPing Wang MgO based perpendicular MTJ (pMTJ) are becoming increasing demanded due to their thermal stability and unlimited cell aspect ratio [1]. Promising results are reported for pMTJs with TbFeCo or Co/Pt (Pd,Ni) MLs but these materials could not withstand high temperature annealing above 250$^{\circ}$C. However, annealing at or above 300$^{\circ}$C is required to fully crystallize the MgO. We fabricated Sub/Pd/(Co/Pd)n/CoFeB/MgO/CoFeB/Pd/(Co/Pd)n/Pd pMTJ structure by engineering the exchange coupling between CoFeB and (Co/Pd)n and the interface morphology of Co and Pd in (Co/Pd)n layers and annealed at different temperatures up to 350$^{\circ}$C. To clarify the thermal effects on free layer completely, we also studied the annealing of free layer only. The perpendicular squareness of the free layer and the full stack after annealing at 350$^{\circ}$C are 0.90 and 1, respectively. The existence of good perpendicular anisotropy at high temperature is ascribed to the sharp and defect free interfaces. Field and spin torque switching results will also be presented.\\[4pt] [1] H.Meng, APL, \textbf{88}, 172506 (2006),]M. Nakayama, JAP, 103, 07A710. [Preview Abstract] 
Tuesday, March 16, 2010 9:12AM  9:48AM 
H36.00007: Enhanced spintorque efficiency in ferromagnetic metal systems characterized by Rashbatype structural inversion asymmetry Invited Speaker: Ioan Mihai Miron We investigate enhanced spin torque mechanisms based on spinorbit effects in structurally asymmetric ferromagnetic metal layers. It is well known that spinorbit coupling is ultimately responsible for magnetocrystalline anisotropy and damping. Under certain conditions, however, spinorbit effects might additionally enhance or induce specific spin torque mechanisms. We analyze these effects by using two trilayer structures (Pt/Co/Pt and Pt/Co/AlO) with similar magnetic properties but opposite structural inversion parity, evidencing a 50fold increase of the nonadiabatic component of currentinduced spintorque in domain walls (DW) [1]. After characterizing the spintorque by quasistatic measurements, further confirmation of the effect is obtained from the study of DW displacements under ultrashort current pulses. We observe ultrafast DW motion with velocities approaching 400 m/s. Despite the strong pinning characterizing these samples, the DW displacements show high reproducibility demonstrating the potential for applications. Future directions of research in this field as well as the possibility of combining different spintorque mechanisms in Rashbatype magnetic layers will be outlined. \\[4pt] [1] Miron et al. Phys. Rev\textit{. Lett. }\textbf{102}, 137202 (2009). [Preview Abstract] 
Tuesday, March 16, 2010 9:48AM  10:00AM 
H36.00008: Rapid Domain Wall Motion in Permalloy Nanowires Excited by SpinPolarized Current Applied Perpendicular to the Nanowire C. Boone, X. Cheng, J. Katine, J. Childress, M. Carey, I. Krivorotov We make measurements of domain wall (DW) dynamics in permalloy (Py) nanowires with spinpolarized current applied perpendicular to the nanowire. DW motion is excited in the free layer of a 90 nm wide Py(3 nm)/ Cu(5 nm)/ CoFe(7 nm) nanowire spin valve. The DW is trapped under the top lead of the spin valve, and oscillatory motion is excited by AC applied between the top and bottom leads. Using spintorque ferromagnetic resonance to characterize the dynamics [1], we observe DW velocities as high as 800 m/s at current densities below 10$^{7}$ A/cm$^{2}$. This exceeds the maximum DW velocities ($\sim $100 m/s) observed at higher current densities ($>$10$^{8}$ A/cm$^{2})$ when current is applied along the nanowire [2]. This shows that spin torque from current applied perpendicular to a ferromagnetic nanowire excites DW dynamics more efficiently than current flowing along the nanowire. \\[4pt] [1] J.C. Sankey, et al., Phys. Rev. Lett. 96, 227601 (2006)\\[0pt] [2] M. Hayashi, et al., Phys. Rev. Lett. 98, 037204~(2007) [Preview Abstract] 
Tuesday, March 16, 2010 10:00AM  10:12AM 
H36.00009: Spin waves, domain wall, and spin transfer torque interactions Mahdi Jamali, Jaehyun Kwon, Hyunsoo Yang Magnetic nanostructures have shown many interesting aspects. Recent studies of the interaction of spin waves and domain wall have proposed that the domain wall speed can be controlled by the spin wave amplitude and frequency. One recent report showed that the spin waves can be amplified when the nonadiabatic spin torque is sufficiently large. We studied the interaction effect between spin waves and spin transfer torque on the domain wall motion in magnetic nanowires by using micromagnetic simulation. We found that the amplification gain of spin waves is not constant along the nanowires. The direction of motion of domainwall depends on the frequency of spin waves. In addition we show that the spin waves can enhance the current induced domain wall motion and the domain wall motion speed is controllable by changing spin wave properties. [Preview Abstract] 
Tuesday, March 16, 2010 10:12AM  10:24AM 
H36.00010: Current driven magnetization dynamics in ferromagnetic nanowires with DzyaloshinskiiMoriya interaction Oleg Tretiakov, Artem Abanov We study a onedimensional classical spin chain which models a long ferromagnetic wire with DzyaloshinskiiMoriya interaction under the influence of an electric current. The static and dynamic properties of this system are investigated. We find a spiral state configuration of the magnetization in the wire for the static case as well as how it evolves for nonzero current. We also study propagation of a domain wall in such a wire. We obtain an analytical expression for the width of this domain wall as a function of the DzyaloshinskiiMoriya interaction constant, uniaxial anisotropy along the wire, and exchange interaction constant. Our findings show that above a certain value of DzyaloshinskiiMoriya constant a domain wall configuration cannot exist in the wire. Below this value we determine the domain wall dynamics for small currents, and as one of the results we calculate the drift velocity of the domain wall along the wire. Furthermore, we show that DzyaloshinskiiMoriya interaction decreases the minimum value of current at which a domain wall starts to move. [Preview Abstract] 
Tuesday, March 16, 2010 10:24AM  10:36AM 
H36.00011: Spin torque and domain wall velocity in ferromagnetic semiconductor $\pi$ and $2\pi$ N\'eel walls E.A. Golovatski, M. E. Flatt\'e The motion of a domain wall under an applied spinpolarized current[1] has interesting device applications for the development of spintronic devices. We model $2\pi$ N\'eel walls (energetically favorable in thin films) in ferromagnetic semiconductors, and compare the results to those for the morefrequently studied $\pi$ walls. Under coherent transport conditions, analytic solutions for spindependent reflection and transmission coefficients are possible[2,3]. We calculate charge resistance, spin torque, and domain wall velocity. We find the peak spin torque is more than twice as large for a $2\pi$ wall than for a $\pi$ wall. We also find that the peak velocity of a $2\pi$ wall is larger than that of a $\pi$ wall, but the peak velocities of $3\pi$ and $4\pi$ walls are smaller than those of both $\pi$ and $2\pi$ walls. This work was supported by an ARO MURI.\\[4pt] [1] M. Yamanouchi, D. Chiba, F. Matsukura, and H. Ohno, Nature 428, 539 (2004).\\[0pt] [2] G. Vignale and M. Flatt\'e, Phys. Rev. Lett. 89 (2002).\\[0pt] [3] D. Ralph and M. Stiles, Journal of Magnetism and Magnetic Materials 320, 1190 (2008). [Preview Abstract] 
Tuesday, March 16, 2010 10:36AM  10:48AM 
H36.00012: Joule Heating in Ballistic NanoContacts T. Y. Chen, C. L. Chien, M. Manno, L. Wang, C. Leighton Joule heat generation and thermal management are important for any electronic devices. While the origin of Joule heat is nonadiabatic inelastic scattering of electrons, the characteristics of Joule heat are very different depending on the whether the transport is diffusive or ballistic, i.e. whether the structural length scale is respectively larger or smaller than the carrier mean free path. Joule heating in the diffusive limit has been well known but not in the ballistic limit. In this work, we have determined the Joule heating relation of $V^{2}$ = C($T_{m}T)$ in ballistic nanocontacts involving the bias voltage V, the ambient temperature T, and the maximum temperature Tm inside the contact by exploiting the ordering temperature of a magnetic solid as a natural thermometer. The relation has been further corroborated using a single contact at one temperature but different magnetic fields. A simple energy transfer model incorporating ballistic transport can account for this relation, which is essential for transport in nanostructures. Ballistic heat transport also provides a novel method for determining the ordering temperatures of magnets, either ferromagnets or antiferromagnets. [Preview Abstract] 
Tuesday, March 16, 2010 10:48AM  11:00AM 
H36.00013: Robust isothermal electric switching of interface magnetization: A route to voltagecontrolled spin electronics Xi He, Yi Wang, Ning Wu, Siqi Shi, A. Caruso, E. Vescovo, Kirill D. Belashchenko, Peter Dowben, Christian Binek Promising spintronic device concepts utilize the electric control of magnetic interfaces. We present compelling evidence of a roughnessinsensitive and electrically controllable ferromagnetic state at the (0001) surface of antiferromagnetic chromia. If this ferromagnetic surface is placed in close proximity with a ferromagnetic Co/Pd multilayer film, exchange coupling across a Pd interlayer induces an electrically controllable unidirectional anisotropy in the Co/Pd film. This electrically controlled exchange bias effect allows for reversible isothermal shifting of the global hysteresis loop of the Co/Pd film along the magnetic field axis from negative to positive values. Supported by NSF through Career DMR0547887, by NRI, by NSF MRSEC, and by the NRC/NRI supplement. K.D.B. is a Cottrell Scholar of Research Corporation. [Preview Abstract] 
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