Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A9: Magnetic Domains and Dynamics |
Hide Abstracts |
Sponsoring Units: GMAG Chair: Mark Stiles, NIST Room: LACC 153A |
Monday, March 21, 2005 8:00AM - 8:12AM |
A9.00001: Magnetic Reversal Energy Loss and Dynamics in Permalloy Thin Films and Microstructures Corneliu Nistor, Eshel Faraggi, James L. Erskine Magnetic hysteresis energy loss scaling and switching dynamics was studied in thin (20 nm) permalloy films and patterned microstructures using the magneto-optic Kerr effect. Sinusoidal, triangular and square magnetic waveforms of peak amplitudes up to 100 Oe and frequencies covering 10 decades (1mHz to 10 MHz) were applied to the samples while monitoring the magnetic response at 1 ns temporal resolution and 1 $\mu $m spatial resolution. All films and microstructures exhibited similar loss scaling behavior characterized by the dynamic coercivity H$_{c}$*($\omega )$: an ``adiabatic'' region described by the averaged static coercivity H$_{0 }$, followed by a region of monotonically increasing loss described by a power law: H$_{c}$*($\omega )$= H$_{0}$ + A(dH/dt)$^{\alpha }$. The scaling function is derived from a domain wall dynamics model based on a linear ramp field. Exponents, $\alpha $, obtained from fits to scaling measurements are independent of H$_{0 }$ for microstructures obtained from the same parent film, suggesting universal behavior. The basic loss mechanism (thin film limit) at both low and high frequencies appear to result from large-angle local spin damping. Supported by NSF-DMR-0404252. [Preview Abstract] |
Monday, March 21, 2005 8:12AM - 8:24AM |
A9.00002: Ferromagnetic resonance modes of the vortex state in Permalloy dot arrays Chengtao Yu, Mark Zimmerman, Michael Pechan, Jordan Katine, Leisl Folks, Matthew Carey Permalloy dot arrays (circular dot 1000nm in diameter and 40nm thick, arranged in square lattice at 1100 nm period) have been fabricated with e-beam lithography. Spin dynamics of the magnetic dots were measured via ferromagnetic resonance on a microstrip in the frequency range 5-36 GHz with the field applied in the film plane. A single mode is present at high microwave frequency, where resonance occurs well above the saturation field of the dots. With decreasing frequency, however, an additional mode appears on the high field side of the main mode, which may result from edge domains as indicated by micromagnetic simulation. At frequencies below 15 GHz, additional modes are observed at fields lower than the two above modes when sweeping up in field, but not when sweeping down in field. These additional modes are attributed to the influence of a vortex structure (the equilibrium state of the dot at zero field). This is inferred from simulation, wherein the vortex state persists up to 750 Oe when increasing field from zero and does not reappear with decreasing field until 100 Oe. [Preview Abstract] |
Monday, March 21, 2005 8:24AM - 8:36AM |
A9.00003: Magnetic Vortex Interactions in Double Vortex Stadium Structures R.L. Compton, J.P. Park, P. Eames, P.A. Crowell We have used time-resolved Kerr microscopy (TRKM) to study the spin dynamics of individual Permalloy stadium structures having thickness 50~nm, width 600~nm, and lengths ranging from 800~nm to 1200~nm. The stadium geometry relaxes from saturation into either a single vortex or double vortex state, depending on the orientation of the applied field during relaxation. From the zero field double vortex state, the separation distance between the two vortices decreases with applied field until annihilation near 270 Oe. TRKM measurements on a 1200~nm long stadium reveal a zero field gyrotropic mode frequency of $\sim0.4$~GHz that shifts downward in frequency to $\sim0.2 $~GHz near 270 Oe. This behavior is consistent with theoretical predictions for a coupled vortex system with decreasing vortex separation [1]. Above 270 Oe, a single vortex remains in the system, with a gyrotropic mode frequency $\sim0.5$~GHz and non-monotonic field dependence. Finally, above 500 Oe, the dynamic behavior is characteristic of the saturated state. Similar TRKM measurements on increasingly shorter stadia, at zero field in the double vortex state, show the gyrotropic mode frequency shifting from $\sim0.4$~GHz for the 1200 nm long stadium to $\sim0.2$~GHz for a 900 nm stadium. [1] J. Shibata, K. Shigeto, Y. Otani, Phys. Rev. B \textbf{67}, 224404 (2003). This work was supported by NSF DMR 04-06029 and the University of Minnesota MRSEC (NSF DMR-02-12032). [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 8:48AM |
A9.00004: Atomic Spin Dynamics during reversal in composite media Sonali Mukherjee, Luc Berger We present spin dynamics during reversal for composite material (exchange coupled hard and soft phases). Up until now, consensus has been that reversal is by coherent rotation with the field determined by the average of the intrinsic reversal fields of the two pure phases. Atomic scale simulations show non-coherent reversal. Reversal is initiated in the soft phase and a domain wall is formed at the interface between the hard and soft phase which propagates through the hard phase under the action of the field. The two important fields associated with the reversal process are Hk1 (reversal field for soft phase) and Hdw (domain wall propagation field from soft to hard phase). The switching field is determined by max(Hk1,Hdw). Hdw is found to be 1) proportional to anisotropy difference of the two phases and 2) inversely proportional to the total moment of the two phases. In the limit of zero anisotropy difference between the phases Hdw becomes negligible as expected. Hk1 on the other hand depends on the geometrical length of the soft phase. The lowest limit of Hk1 is equal to the intrinsic reversal field of the soft phase when its length (L1) is sufficient to support the intrinsic domain wall width (Ldw). When $ L1 < Ldw $, Hk1 increases in proportion to the excess energy required to accomodate the domain wall in the soft phase. Analytical expressions for both Hk1 and Hdw will be given and shown to agree very well with our simulations and experiments in Appl. Phys. Lett. 82, 2859 (2003). [Preview Abstract] |
Monday, March 21, 2005 8:48AM - 9:00AM |
A9.00005: Fast magnetization switching of Stoner particles: A nonlinear dynamics picture Xiang Rong Wang, Zhouzhou Sun We reexamine the problem of the magnetization switching of Stoner particles in the presence of dissipation from the point of view of nonlinear dynamics. Within the Landau-Lifshiz-Gilbert formulation, we illustrate how the fixed points and their basins change under a perpendicular and a parallel field. This change explains well why a non-parallel field gives a small minimal switching field and a short switching time. Furthermore, we clarify that the so-called Stoner-Wohlfarth (SW) limit is exact only when the dissipation is infinitely large. However, for a give magnetic anisotropic energy function, there is a critical dissipation above which the minimal switching field is the same as that of SW-limit. The reason and meaning of such a critical disspistion is also given. [Preview Abstract] |
Monday, March 21, 2005 9:00AM - 9:12AM |
A9.00006: Barkhausen Noise, Domain Structure and Entropy in Magnetic Amorphous Ribbons Under Stress Alberto Guimaraes, Andre Gundel, Luciana Santi, Rubem Sommer The effect of applied stress on the magnetization process of magnetostrictive Fe78B13Si9 and amorphous Fe73.5Cu1Nb3Si13.5B9 metallic ribbons was investigated by estimating the entropy of the Barkhausen noise time series. The Barkhausen series is formed of voltage pulses detected by a coil around a ferromagnetic sample under an applied magnetic field. The stress induces a preferred orientation on the domain walls and reduces their thickness. The entropies calculated from the Random Field Ising Model simulations with different degrees of disorder were also analyzed and the results compared to the experimental data. In both cases, the relative entropy is calculated from the size of the Barkhausen noise time series packed by applying the LZ77 (zipping) algorithm. In the case of the experimental curves, an increase in the relative entropy was observed above a given stress level, possibly as a result of the decrease of the domain wall width. The results are also compared with domain structures observed by Kerr microscopy and to magnetization curves obtained by the inductive method. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A9.00007: Spin-Torque Stimulated Barkhausen Jumps in a Thin-Film Permalloy Microstructure Shuqiang Yang, James Erskine Barkhausen Jumps (BJs) are studied in a 60$\mu $m x 50$\mu $m x 30nm thick permalloy microstructure as a function of the bipolar current pulse amplitude applied during field-driven magnetization reversal. Magnetic force microscopy is used to characterize the quasi-static domain structure and the magneto-optic Kerr effect is used to measure BJs. Above a threshold current density J$_{T} \quad \sim $ 10$^{10}$ A/m$^{2}$, the BJs become correlated with the current pulses. The observed behavior is consistent with models of spin-torque transfer domain wall motion and compatible with recent experiments after accounting for difference in sample static coercivity. The threshold current density for current-stimulated domain wall motion in the (low coercivity) micron-scale structures is about two orders of magnitude lower than the threshold reported for sub-micron wire structure [1] (10$^{12}$ A/m$^{2})$, which is near the damage threshold. The effect can be used to control dynamic coercivity and offers opportunities for studying current-driven domain dynamics near the depinning threshold, and under conditions permitting a wide dynamic range below the damage threshold. [1] A. Yamaguchi et al. Phys. Rev. Lett. 92, 077205-1 (2004). [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A9.00008: Magnetization reversal and anisotropy at the Fe/AlGaAs (001) interface Gunter Luepke, Haibin Zhao, Diyar Talbayev, Aubrey Hanbicki, Connie Li, Olaf van't Erve, George Kioseoglou, Berry Jonker We distinguish the magnetic reversal process of an Fe interface layer from that of the bulk in Fe/AlGaAs heterostuctures using magnetization induced second harmonic generation (MSHG) and the magneto-optical Kerr effect (MOKE). We find that the switching characteristics are distinctly different -- single step switching occurs at the interface layer, while two jump switching occurs in the bulk for the magnetic field orientations employed. This indicates a larger contribution from uniaxial versus cubic anisotropy at the interface layer, causing the absence of an intermediate single domain state during reversal. The general assumption that spins within a ferromagnetic metal films are strictly parallel due to strong exchange coupling is therefore incorrect at the interface. Our results show that MSHG is a powerful technique to probe interface magnetic properties in non-centrosymmetric hybrid structures. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A9.00009: Imaging antiferromagnetic domains of GdNi2Ge2 by x-ray resonant magnetic scattering J.W. Kim, A. Kreyssig, L. Tan, B. Sieve, P.C. Canfield, S.L. Bud'ko, S. Law, D. Wermeille, A.I. Goldman The body-centered tetragonal compound GdNi$_{2}$Ge$_{2}$ orders antiferromagnetically below T$_{N}$ = 27.5 K. By using x-ray resonant magnetic scattering we have determined that the magnetic phase transforms from a collinear structure to a cycloidal structure below T$_{t} $ = 16 K. Both magnetic structures lower the symmetry and result in magnetic domains. The excellent quality of our single crystal and the resulting high intensity allowed us to image these domains using the x-ray resonant magnetic scattering technique. By reducing the illuminated area to 100x100~$\mu $m$^{2}$ we succeeded in investigating a single magnetic domain. The plane in which the magnetic moments lie is determined to have tilt about 10 degrees away from the \textbf{a} direction for both magnetic structures. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A9.00010: X-ray imaging of chiral domains in Dy metal J.C. Lang, A. Cady, D. Haskel, G. Srajer, D. McWhan Domain growth has been measured in Dy metal using a circularly polarized x-ray beam both on cooling through the PM to AF transition, and on warming through the FM to AF transition. The difference in the scattered intensity between right and left handed incident x-rays was measured at the (0,0,4+$\tau$) peak, where $\tau$ is the wave vector of the AF structure. On cooling from the PM to the AF phase, the chiral domains nucleate and grow to several hundred microns and no further change is observed with decreasing temperature. The size of the domains is assumed to be limited by defects. On warming from the FM phase the domain size is resolution limited and little domain growth is observed between T$_c$=90K and about 140K. With further increase in temperature the domains grow to the hundreds of microns observed on cooling. Early neutron scattering measurements revealed a second harmonic with an intensity that decreased linearly to zero at 140K.$^1$ We observe, a weak non- resonant reflection at 2$\tau$ in the charge ($\sigma-\sigma$) channel rather than the magnetic ($\sigma-\pi$) channel. This suggests that there is a distortion of the structure that is hindering domain wall motion at low temperatures. Work at the Advanced Photon Source was supported by the DOE, Office of Basic Sciences, under contract no. W- 31-109-Eng-38. \\ \\$^1$M.K. Wilkinson et al., J. Appl. Phys. 32, 485 (1961). [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A9.00011: Domain Walls and Roughening Transition Possibilities in a Transverse-field Ising Model with Long-range Interactions George Mias, Steven Girvin We have studied domain walls and domain wall roughening in the presence of long-range interactions. The insulating system LiHoF$_4$ is a physical realization of the transverse-field Ising model, which is known to have an order-disorder quantum phase transition between ferromagnetic and paramagnetic states. Furthermore due to long-range dipole interactions, LiHoF$_4$ naturally forms thin needle-like domains which might suggest the possibility of a roughening transition for the domain walls, on the grounds that it is expected for the standard short-range transverse-field Ising model. We will describe how the long-range forces which are responsible for the domain formation also affect the nature of the domain wall structure and account for the absence of a roughening transition.\\ \\ This work is supported by NSF DMR-0342157. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A9.00012: Imaging Antiferromagnetic Domain Walls with the Hall Effect R. Jaramillo, T. F. Rosenbaum, E. Isaacs, G. Aeppli We find that the Hall effect in the spin-density-wave state of elemental chromium is acutely sensitive to the underlying domain structure. A large (20\% effect) hysteresis in the linear Hall coefficient emerges as a function of temperature between the spin-flip (123 K) and Neel (311 K) transitions. The hysteresis is accompanied by a pronounced increase in the noise. Scratching the surface of a clean single crystal can pin the domains, suppressing the hysteresis loop and curtailing the motion of the spin domain walls in the transverse antiferromagnetic phase. Changing the relative orientations of the current flow and the pinning directions alters the preferred state of the system. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A9.00013: Domain Walls and Macroscopic Spin-Flip-Like States in Gd$_{x}$Co$_{1-x}$/Gd$_{y}$Co$_{1-y}$ Bilayers Jose I. Martin, R. Morales, J.M. Alameda Exchange coupled double layers (ECDL) made of rare earth -- transition metal amorphous alloys are of basic and technological interest, as they present different magnetization configurations when the composition is changed or when the temperature is varied crossing the compensation temperatures (T$_{comp})$ of both ferrimagnetic alloys. In this work, amorphous Gd$_{x}$Co$_{1-x}$(100 nm)/Gd$_{y}$Co$_{1-y}$(100 nm) ECDL have been prepared to investigate the magnetization reversal and the stable magnetic configurations when the compositions of both layers are similar: x = 0.22, y = 0.24. The samples have been grown by co-sputtering on corning glass substrates, which has allowed to analyze the behaviour within each layer by transverse Kerr effect measurements. A rich variety of behaviours has been found in the temperature range between the T$_{comp}$ of both layers, including magnetization reversal by annihilation/creation of a Bloch wall across the sample thickness, and a macroscopic spin-flip-like metamagnetic state where the magnetic moments form a double antiferromagnetic state with the presence of a N\'{e}el-like wall when the magnetizations of both layers are similar [1]. The whole observed behavior can be understood in terms of a deduced general magnetic field -- temperature phase diagram. \newpage [1] R. Morales et al. Phys. Rev. B 70, 174440 (2004). \newpage Work supported by Spanish CICYT. [Preview Abstract] |
Monday, March 21, 2005 10:36AM - 10:48AM |
A9.00014: Putting a spin on speckle: the twisted way magnets remember Trieu Mai Major hysteresis loops in magnetic systems have generally been thought to be symmetric under reversal of the two axes. We show why this is incorrect, and how the asymmetry provides an explanation for recent x-ray speckle experiments on thin films that show a nontrivial microscopic difference between the zero field states produced by starting from large positive and large negative fields, and generalizations thereof. This unexpected result is due to the dynamics of vector spins; scalar models such as the Ising model are inadequate for this purpose, even if the anisotropy is high. We compare data from magnetic thin film speckle experiments with our numerical model, and find good agreement for many features. In certain regions of parameter space of the model, we find an unusual `mixed phase' of magnetic domains. We also present results on nanomagnetic pillar arrays, where a similar symmetry violation and multicycles for minor hysteresis loops are seen. [Preview Abstract] |
Monday, March 21, 2005 10:48AM - 11:00AM |
A9.00015: Displacement Profile of Charge Density Waves and Domain Walls at Critical Depinning Thomas Nattermann, Andreas Glatz The influence of a strong surface potential on the critical depinning of an elastic system driven in a random medium is considered. If the surface potential prevents depinning completely the elastic system shows a parabolic displacement profile. Its curvature C exhibits at zero temperature a pronounced rhombic hysteresis curve of width $2f_c$ with the bulk depinning threshold $f_c$. The hysteresis disappears at non-zero temperatures if the driving force is changed adiabatically. If the surface depins by the applied force or thermal creep, C is reduced with increasing velocity. The results apply, e.g., to driven magnetic domain walls, flux-line lattices and charge-density waves. [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