Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session X31: Focus Session: Magnetic Nanostructures: Domain Walls, Reversal, Oscillators |
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Sponsoring Units: DMP GMAG Chair: Valnetyn Novosad, Argonne National Laboratory Room: 335 |
Thursday, March 19, 2009 2:30PM - 3:06PM |
X31.00001: Reversal mechanisms and defects in perpendicularly magnetized nanostructures Invited Speaker: The problem of switching field distributions (SFDs) is currently plaguing developing technologies which rely on uniform arrays of magnetic nanostructures such as bit patterned media, magnetic random access memory (MRAM) and spin-torque oscillators. Most of these technologies are shifting towards the use of perpendicularly magnetized materials due to the increased device performance and thermal stability that can be achieved. SFDs in such perpendicularly magnetized nanostructures can result from dot-to-dot interactions and size distribution, but is largely dominated by material defects [1- 4]. Such defects can arise from both the material deposition process, and post-deposition processing that occurs during nanofabrication. By comparing nanostructures fabricated by deposition on pre-patterned wafers to those fabricated by ion milling of continuous films, we show that the anisotropy of the edge region can be greatly different in each case. The size, temperature, and angular dependences of the reversal field indicate that the reversal mechanism also differs. In contrast to fabrication induced defects, microstructural variations manifest themselves as a random distribution of local anisotropies. We studied the anisotropy distribution in patterned elements by imaging the localized reversal of low anisotropy regions and mapping these sites as a function of applied field using MFM imaging and TEM. In addition, we used simulations to show the effect a small localized region of lower anisotropy material (such as a grain) has on the reversal field of the entire nanostructure. We find that the reversal field depends on both the relative anisotropy of the defect to the film, as well as, the location of the defect within the structure. \\[4pt] [1] T. Thomson, PRL 96,257204 (2006).\\[0pt] [2] J.M. Shaw, JAP 101, 023909 (2007).\\[0pt] [3] J. Lau, APL 92, 012506 (2008).\\[0pt] [4] J.M. Shaw, PRB 78, 024414 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:06PM - 3:18PM |
X31.00002: Controlling Double Vortex States in Low-Dimensional Dipolar Systems Sergey Prosandeev, Laurent Bellaiche The reversal process of the chirality of each opposite vortex belonging to a double vortex state in ferromagnetic hysterons, via the application of in-plane magnetic fields, is reported [1]. Simulations reveal that such a process involves the formation of four intermediate states, including original ones. Hysteresis loops can occur only in a counterclockwise fashion because of one of these intermediate states. Double vortex states can also be controlled by electric fields in ferroelectric nanostructures of different shapes, but with some key differences with respect to the ferromagnetic case. This work is supported by ONR grants~ N00014-04-1-0413 and N00014-08-1-0915,~ DOE grant DE-FG02-05ER46188~ and NSF grants DMR-0701558, DMR-0404335 and DMR-0080054 (C-SPIN). Some computations were made possible thanks to the MRI Grants~0421099 and 0722625 from NSF. [1] S. Prosandeev and L. Bellaiche, Physical Review Letters \textbf{101}, 097203 (2008). [Preview Abstract] |
Thursday, March 19, 2009 3:18PM - 3:30PM |
X31.00003: Magnetic domain wall shift registers for data storage applications Dan Read, L. O'Brien, H.T. Zeng, E.R. Lewis, D. Petit, J. Sampaio, L. Thevenard, R.P. Cowburn Data storage devices based on magnetic domain walls (DWs) propagating through permalloy (Py) nanowires have been proposed [Allwood et al. Science 309, 1688 (2005), S. S. Parkin, US Patent 6,834,005 (2004)] and have attracted a great deal of attention. We experimentally demonstrate such a device using shift registers constructed from magnetic NOT gates used in combination with a globally applied rotating magnetic field. We have demonstrated data writing, propagation, and readout in individually addressable Py nanowires 90 nm wide and 10 nm thick. Electrical data writing is achieved using the Oersted field due to current pulses in gold stripes (4 $\mu $m wide, 150 nm thick), patterned on top of and perpendicular to the nanowires. The conduit-like properties of the nanowires allow the propagation of data sequences over distances greater than 100 $\mu $m. Using spatially resolved magneto-optical Kerr effect (MOKE) measurements we can directly detect the propagation of single DWs in individual nanostructures without requiring data averaging. Electrical readout was demonstrated by detecting the presence of DWs at deliberately introduced pinning sites in the wire. [Preview Abstract] |
Thursday, March 19, 2009 3:30PM - 3:42PM |
X31.00004: Ratchet Effects and Domain Wall Energy Landscapes in Amorphous Magnetic Films with 2D Arrays of Asymmetric Holes J.I. Martin, A. Alija, I. Sobrado, A. Perez-Junquera, G. Rodriguez-Rodriguez, M. Velez, J.M. Alameda, V.I. Marconi, A.B. Kolton, J.M.R. Parrondo The driven motion of domain walls in extended magnetic films patterned with 2D arrays of asymmetric holes has been found to be subject to two different crossed ratchet effects [1] which results in an inversion of the sign of domain wall motion rectification as a function of the applied magnetic field. This effect can be understood in terms of the competition between drive, elasticity and asymmetric pinning as revealed by a simple $\phi^4-$model. In order to optimize the asymmetric hole design, the relevant energy landscapes for domain wall motion across the array of asymmetric holes have been calculated by micromagnetic simulations as a function of array geometrical characteristics. The effects of a transverse magnetic field on these two crossed ratchet effects will also be discussed in terms of the decrease in domain wall energy per unit area and of the modifications in the magnetostatic barriers for domain wall pinning at the asymmetric inclusions. Work supported by Spanish MICINN.[1] A. Perez-Junquera et al, Phys. Rev. Lett. 100 (2008) 037203 [Preview Abstract] |
Thursday, March 19, 2009 3:42PM - 3:54PM |
X31.00005: Magnetic Frustration in Nanowires: Domino Effect Samir Lounis, Peter H. Dederichs, Stefan Bl\"ugel The parity of the number of atoms in finite antiferromagnetic nanowires deposited on ferromagnetic substrates is shown to be crucial in predicting whether the magnetic ground state is non-collinear or collinear [1]. Using the full-potential Korringa-Kohn- Rostoker method for non-collinear magnetism [2] and a Heisenberg model we show that the magnetic structure of the whole nanowires dramatically changes if a {\em single} adatom is added. Infinite and finite nanochains with even number of adatoms are always magnetically non-collinear while odd numbers of atoms in the wire lead under given conditions to a collinear ferrimagnetic ground state. This unexpected nano-effect, which resembles a domino-effect, occurs only for wires at finite lengths. [1] S. Lounis, P. H. Dederichs, S. Blügel, Phys. Rev. Lett. 101, 107204 (2008). [2] S. Lounis, Ph. Mavropoulos, P. H. Dederichs, S. Blügel, Phys. Rev. B 72 224437 (2005). [Preview Abstract] |
Thursday, March 19, 2009 3:54PM - 4:06PM |
X31.00006: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 4:06PM - 4:18PM |
X31.00007: Magnetic and mechanical characterizations of ultra-high frequency nanoelectromechanical systems (NEMS) Joe Losby, N. Liu, C. Holt, D. Mitlin, A.E. Fraser, V. Sauer, W.K. Hiebert, M.R. Freeman Recent efforts in our group involve time-domain studies of the motion of silicon NEMS$^{1}$ and spin dynamics in nanometer-scale permalloy elements$^{2}$. Transduction of microwave frequency ($>$ 1 GHz) cantilevers, and time domain coherent control (``unringing'') of nanoscale resonators have been demonstrated. For the next stage of this work, we have fabricated permalloy NEMS cantilevers and doubly clamped beams in order to begin exploration of magnetomechanical dynamics in ferromagnetic nanostructures. The magnetization of these resonators is probed using time-resolved magneto-optical Kerr effect microscopy, while stroboscopic optical interferometry is used for the detection of vibrational modes. \\[0pt] 1. N. Liu, F. Giesen, M. Belov, J. Losby, J. Moroz, A. E. Fraser, G. McKinnon, T. J. Clement, V. Sauer, W. K. Hiebert {\&} M. R. Freeman, Nature Nanotechnology, In Press (2008).2. Z. Liu, R.D. Sydora, and M.R. Freeman, Phys. Rev. B. \textbf{77}. 174410 (2008). [Preview Abstract] |
Thursday, March 19, 2009 4:18PM - 4:54PM |
X31.00008: Synchronization of spin-torque oscillators via phase-shift control Invited Speaker: The Spin Torque Oscillator (STO) shows great promise as a frequency generating device at microwave frequencies. However its very limited output power has to be significantly improved for any realistic application. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase Q. We have recently demonstrated an intrinsic preferred phase shift between an STO and an injected RF current [1, 2]. This phase shift has direct implications for current-mediated synchronization of serially connected STOs [3]. It is exactly at this phase shift where the multi-STO synchronized state develops the highest robustness and by tuning the total circuit I-V phase shift, synchronization can be enhanced by close to 2 orders of magnitude. Since our initial work, we have now determined both the phase shift and the enhancement factor in all types of STOs (standard, perpendicular [4], wavy torque [5], tilted polarizer [6]). More recently we have also found that the perpendicular torque component present in magnetic tunnel junctions enhances synchronization through a decrease of the intrinsic phase shift. These findings are expected to be critical for future applications and will hopefully accelerate the realization of useful STO-based microwave devices. \\[4pt] References \\[0pt] [1] Yan Zhou, J. Persson, and Johan {\AA}kerman, J. Appl. Phys. 101, 09A510 (2007). \\[0pt] [2] Yan Zhou, J. Persson, S. Bonetti, and Johan {\AA}kerman, Appl. Phys. Lett. 92, 092505 (2008). \\[0pt] [3] J. Persson, Yan Zhou, and Johan {\AA}kerman, J. Appl. Phys. 101, 09A503 (2007). \\[0pt] [4] D. Houssameddine, et al, Nat. Mater. 6, 447 (2007). \\[0pt] [5] O. Boulle, et al., Nat. Phys. 3, 492 (2007). \\[0pt] [6] V. S. Pribiag, et al., Nat. Phys. 3, 498 (2007). [Preview Abstract] |
Thursday, March 19, 2009 4:54PM - 5:06PM |
X31.00009: Microwave Emitting Nanomagnet Oscillator: Strongly coupled spin-photon modes \"O.O. Soykal, M.E. Flatt\'e We describe a microwave emitting nanomagnet oscillator confined in a high Q-cavity. The precession of the magnetization of a typical Fe nanomagnet of 100 nm in radius, possessing roughly $10^9$ spins and described as a macrospin, is tuned to be in resonance with a microwave cavity of 1 mm$^3$ in volume with an applied magnetic field. The Hamiltonian of the spin-cavity system is quantized in a fully quantum treatment. Due to the very large number of coherently-oriented spins, the interaction Hamiltonian of the spin-cavity system contains magnet-microwave mode coupling terms that exceed several GHz. Coherent coupling of the microwave field with nanomagnet spins is analyzed in terms of the coherent states of the spin-cavity system, which are characterized by large oscillations in the nanomagnet spin and cavity photon number, as well as by exceptionally long dephasing times. Therefore, the nanomagnet-cavity system is predicted to have distinguishable large total spin, long coherence times, and high power output. This may serve as an efficient means of transferring information between a magnetic and a photonic system. [Preview Abstract] |
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