Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session W32: Focus Session: Domain Wall Motion and Itinerant Magnetism |
Hide Abstracts |
Sponsoring Units: GMAG DMP FIAP Chair: Rembert Duine, Utrecht University Room: Morial Convention Center 225 |
Thursday, March 13, 2008 2:30PM - 2:42PM |
W32.00001: Domain wall motion under the non-uniform transverse magnetic field with rigid domain structure Chun-Yeol You Another method for the domain wall movement in a nanowire geometry with rigid domain structure will be proposed. We find that a transverse domain wall move to the energy minimum position under a non-uniform transverse magnetic field in order to minimize the Zeeman energy. By the collective coordinate approach, the domain wall dynamics under non-uniform transverse magnetic field in nanowire geometry is investigated. The validity of concept of the present method and the domain wall equation of motion are confirmed by micromagnetic simulations. It is found that the domain wall velocity of a few 100 m/s can be obtained for the moderate conditions based on the analytic and numerical studies. The direction of the domain wall movement depends only on the magnetization direction inside of the domain wall itself, not on the one of the domain. Therefore, it is possible to achieve field driven domain wall motion with rigid domain structures. The non-uniform transverse magnetic field driven domain wall motion has a superior nature of the rigidity of the domain structure during the domain walls movement in addition to the all advantages of conventional field driven domain wall movement. [Preview Abstract] |
Thursday, March 13, 2008 2:42PM - 2:54PM |
W32.00002: Crossed Ratchet Effects for Magnetic Domain Wall Motion Jose I. Martin, A. Perez- Junquera, V.I. Marconi, A.B. Kolton, L.M. Alvarez- Prado, Y. Souche, A. Alija, M. Velez, J.V. Anguita, J.M. Alameda, J.M.R. Parrondo The driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes has been studied experimentally and theoretically. We find two crossed ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. These crossed effects have an interesting consequence with potential applications: the system keeps memory of the sign of the last saturating state even in a zero magnetization configuration. By solving numerically a simple $\phi^4$-model we show that the essential physical ingredients for this effect, the competition between drive, elasticiy and asymmetric pinning, are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials. [Preview Abstract] |
Thursday, March 13, 2008 2:54PM - 3:06PM |
W32.00003: In Situ TEM Observation of Current-Induced Domain Wall Motion in Patterned Permalloy Wires Todd Brintlinger, John Cumings Using a transmission electron microscope (TEM) operating in Lorentz mode, we observe the movement of a domain-wall due to the flow of current in a permalloy wire. The wire is formed on electron-transparent silicon nitride membranes using standard electron beam lithography and thermal vacuum deposition. The resulting wire geometry is 30 nm thick, $\sim $100nm wide, and microns long. A custom-built electrical measurement stage and palladium leads deposited on top of the permalloy wires allow in situ measurements on the wire in the TEM. Lorentz mode imaging (Fresnel contrast) allows the determination of the domain wall location. We observe the domain wall to move in the direction of electron flow, with a current density of around 1 x 10$^{11}$ A/m$^{2}$ being required to move the wall. We will present the nanofabrication process, results, and interpretation of these experiments. [Preview Abstract] |
Thursday, March 13, 2008 3:06PM - 3:42PM |
W32.00004: Stochastic Current-Driven Domain-Wall Motion Observed by X-Ray Microscopy Invited Speaker: Transmission x-ray microscopy can directly visualize the influence of a spin-polarized current on the magnetization of micro- and nanostructures. We investigate the stochastic motion of domain walls in curved wires [1] and the motion of vortices in squares [2]. To observe domain-wall motion pulses of nanosecond duration and high current density are send through permalloy wires and either move or deform the domain wall. The current pulses have nanosecond duration and a high current density of up to 1.0 10$^{12}$ A/m$^{2}$ and drive the wall either undisturbed, i.e. as a composite particle through the wire or causes structural changes of the magnetization. Repetitive pulse measurements reveal the stochastic nature of current induced domain-wall motion. From the experiments we estimate the ratio between the degree of nonadiabaticity and the Gilbert damping parameter indicating the importance of the nonadiabatic contribution to current driven domain-wall motion. To compare experimental results with theory the spin-torque transfer model of Zhang and Li [3] is implemented in the micromagnetic framework OOMMF [4]. The code is applied to determine the current-induced domain wall velocity using the material parameters of permalloy. The simulations support the interpretation of the experimental results. Sinusoidal high-density currents are applied to micrometer-sized permalloy squares containing ferromagnetic vortices. Spin-torque induced vortex gyration on the nanosecond timescale is observed. The phase of the gyration in structures with different chirality are compared to an analytical model and micromagnetic simulations, considering both alternating spin-polarized currents and the current's Oersted fields. This analysis reveals that spin-torque is the main source of motion. Supported by the DFG via SFB 668 and GK 1286 as well as by the U.S. DOE Contract No. DE-AC02-05-CH11231. References: \newline [1] G. Meier, M. Bolte, R. Eiselt, U. Merkt, B. Kr\"{u}ger, D. Pfannkuche, D.-H. Kim, and P. Fischer, Phys. Rev. Lett. 98, 187202 (2007) \newline [2] B. Kr\"{u}ger, A. Drews, M. Bolte, U. Merkt, D. Pfannkuche, and G. Meier, Phys. Rev. B 76, in press (2007). \newline [3] S. Zhang and Z. Li, Phys. Rev. Lett. 93, 127204 (2004). \newline [4] M. Donahue and D. Porter, Interagency Report NISTIR 6376, National Institute of Standards and Technology, Gaithersburg, MD (Sept. 1999). [Preview Abstract] |
Thursday, March 13, 2008 3:42PM - 3:54PM |
W32.00005: The dynamics of field and current-driven magnetic domain wall depinning Geoffrey Beach, Carl Knutson, Maxim Tsoi, James Erskine The depinning of a magnetic domain wall from a well-defined potential well was studied experimentally on timescales ranging from minutes down to tens of nanoseconds. At longer timescales, the behavior follows the classical Neel-Brown model of thermal activation, one of the few observations of this process for the ideal case of a single energy barrier. Below one microsecond, however, the depinning rate becomes independent of the activation volume and assumes a more universal behavior. This transition is due to a vanishing of the energy barrier at a critical field, beyond which the rate of depinning depends primarily on the torque supplied by the field and spin current. A dc spin-polarized current flowing across the domain wall has the effect of lowering the energy barrier by an amount that is predominantly quadratic in current, independent of its direction. This is seen to arise from a shift of the wall in the energy potential due to the adiabatic component of spin-transfer torque. [Preview Abstract] |
Thursday, March 13, 2008 3:54PM - 4:06PM |
W32.00006: Dynamics of domain walls in nanostrips via collective coordinates D. Clarke, O. Tretiakov, G.-W. Chern, Ya. B. Bazaliy, O. Tchernyshyov The rich internal structure of domain walls in nanostrips [1-2] greatly affects the motion when an external magnetic field or electric current is applied, leading to reduced mobility when the driving force is strong. We generalize Thiele's equations [3] to describe arbitrary wall motion with any number of collective coordinates [4]. The formalism is sufficiently general as to allow the inclusion of spin current, and can be applied to films with in- or out-of-plane magnetic anisotropy. We examine a model wall [5] with two soft modes corresponding to the coordinates of a vortex core. As in a one-dimensional domain wall [6], the system has a steady-state regime below a critical field and an oscillatory regime above it. We calculate the drift velocity in both regions. The results are compared to numerical simulations and to available experimental data [7]. This work was supported in part by the NSF Grant DMR-0520491. [1] R. D. McMichael and M. J. Donahue, IEEE Trans. Magn. 33, 4167 (1997). [2] O. Tchernyshyov and G.-W. Chern, Phys. Rev. Lett. 95, 197204 (2005). [3] A. A. Thiele, Phys. Rev. Lett. 30, 230 (1973). [4] O. Tretiakov et. al, arXiv:0705.4463 [5] H. Youk et al., J. Appl. Phys. 99, 08B101 (2006). [6] N. L. Schryer and L. R. Walker, J. Appl. Phys. 45, 5406 (1974). [7] G. S. D. Beach et al., Nature Mater. 4, 741 (2005) [Preview Abstract] |
Thursday, March 13, 2008 4:06PM - 4:18PM |
W32.00007: Domain wall motion by subcritical harmonic current Yury Adamov, Artem Abanov, Jairo Sinova We consider the behavior of the domain wall in the bianisotropic magnetic wire. We show that while the domain wall cannot be moved by the constant current below the certain threshold value (critical current), if we add an alternating component to the current the domain wall starts to move with nonzero velocity. We obtain the analytic expression for this velocity and make suggestions of possible experiments. [Preview Abstract] |
Thursday, March 13, 2008 4:18PM - 4:30PM |
W32.00008: Theory of Electromotive Force Induced by Domain Wall Motion Shengyuan Yang, Di Xiao, Qian Niu We formulate a theory on the dynamics of conduction electrons in the presence of moving magnetic textures in ferromagnetic materials. We show that the variation of local magnetization in both space and time gives rise to topological fields, which induce electromotive forces on the electrons. Universal results are obtained for the emf induced by both transverse and vortex domain walls traveling in a magnetic film strip, and their measurement may provide clear characterization on the motion of such walls. [Preview Abstract] |
Thursday, March 13, 2008 4:30PM - 4:42PM |
W32.00009: Oscillatory domain wall motion in a single-crystal ultrathin Au/Co/Au system Keoki A. Seu, Sujoy Roy, Sungkyun Park, Charles M. Falco, Stephen D. Kevan We have used x-ray photon correlation spectroscopy together with resonance soft x-ray scattering to measure domain dynamics in a Au/Co/Au system that exhibits a spin reorientation phase transition (SRT) in the temperature range of 200-300 K. The incoming photon energy was tuned at the Co L$_{3}$ edge and the coherence is established with a $\sim$10 um pinhole. The resultant speckle pattern is measured with a CCD camera in time as a function of temperature 200 K to 300 K. The correlation coefficient, which is an indicator of domain wall dynamics, shows damped oscillatory behavior in time. The period of the oscillations is approximately 120 sec. The frequency and damping constant were found to depend on the length-scale and temperature changes through the phase transition. Our results show that the SRT dynamics on a mesoscopic length scale and slow time scale can be surprisingly complex. [Preview Abstract] |
Thursday, March 13, 2008 4:42PM - 4:54PM |
W32.00010: Thermodynamics of Itinerant Magnets: A Simple Classical Model with Longitudinal Spin Fluctuations James Glasbrenner, Aleksander Wysocki, Kirill Belashchenko The effects of longitudinal spin fluctuations (LSF) on the thermodynamics of magnetic metals are studied using a model Hamiltonian with only one ``itinerancy parameter.'' We performed Monte Carlo simulations and compared the results with the mean-field theory. A non-trivial complication is the choice of phase space measure. We explored two options: the ``classical'' measure and the ``flat'' measure. Our central result is that magnetic short-range order is always weak, and the mean-field theory is in a very good agreement with Monte Carlo results. Additionally, the results are very sensitive to the choice of the phase space measure, which is a limitation of our model. Nevertheless LSF are essential for the correct description of magnetic thermodynamics and their absence in the adiabatic approximation leads to unphysical results for itinerant systems. Deviations from the Curie-Weiss law due to LSF are also observed and discussed. [Preview Abstract] |
Thursday, March 13, 2008 4:54PM - 5:06PM |
W32.00011: High Pressure Study of Magnetic Order in an Itinerant Electron System: Investigating Weak vs. Strong Coupling Rafael Jaramillo, Y. Feng, J. C. Lang, Z. Islam, T. F. Rosenbaum We measure directly the spin- and charge-density-wave order parameters of the itinerant antiferromagnet Cr via x-ray diffraction as the system is driven towards its quantum critical point with pressure using diamond anvil cell techniques. The exponential decrease of the spin and charge diffraction intensities with pressure confirms the harmonic scaling of spin and charge in this incommensurate system, while the evolution of the incommensurate ordering vector provides important insight into the difference between tuning with pressure and chemical doping. Measurement of the charge density wave over several orders of magnitude of diffraction intensity provides the clearest demonstration to date of a weakly coupled BCS-like ground state. Evidence for coexistence of this weak coupling ground state with incipient magnetic fluctuations at high temperatures in chromium and other more strongly coupled systems raises intriguing questions about the meaning of weak vs. strong coupling and suggests a new category of quantum phase transitions. [Preview Abstract] |
Thursday, March 13, 2008 5:06PM - 5:18PM |
W32.00012: Dynamic susceptibility of itinerant ferromagnets in the ordered state Matthew Vannette, Sergey Bud'ko, Paul Canfield, Ruslan Prozorov Measurements of radio-frequency dynamic susceptibility of ferromagnets exhibit striking differences between local moment and itinerant systems. Whereas local moment systems show a sharp peak at the Curie temperature ($T_c$) which evolves to higher temperatures and lower amplitudes with applied dc magnetic field, itinerant systems show a broad maximum at temperatures well below $T_c$. The itinerant system's maximum is suppressed in amplitude and shifts to lower temperatures with applied dc magnetic field. Existing Stoner or spin fluctuations theories derive strictly zero-field susceptibility and we propose a generalization of these models to incorporate the effect of applied dc field. A good agreement between our semi-phenomenological approach and experimental results obtained on several generally accepted itinerant materials with various $T_c$'s is presented. [Preview Abstract] |
Thursday, March 13, 2008 5:18PM - 5:30PM |
W32.00013: Elementary excitations in antiferromagnetic Heisenberg spin segments. Marco Affronte, Alberto Ghirri, Marco Evangelisti, Andrea Candini, Stefano Carretta, Paolo Santini, Giuseppe Amoretti, Rachel Davies, Grigore Timco, Richard Winpenny We report on \textit{ac}-susceptibility, low temperature magnetization and specific heat measurements on molecular compounds, shortly named (Cr$_6$)$_2$, (Cr$_7$)$_2$, Cr$_8$Cd, (NiCr$_6$)$_2$Zn, that comprise different variants of spin arrays. These systems constitute real examples of collections of identical antiferromagnetic Heisenberg spin segments. We indeed show that this picture, with dominant exchange term in the spin Hamiltonian ($J/k_B$ ranging from 13 to 16 K in all compounds) and weak anisotropy term, fits well the measured physical properties. The character of energy spectra and the low lying magnetic excitations are discussed accordingly. The direct comparison of experimental results and of the energy spectra of these variants with those of similar cyclic spin systems evidences effects associated to: i) the breaking the cyclic boundary conditions and ii) odd and even nuclearity of spin segments. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700