Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X47: Domain Wall MotionFocus
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Sponsoring Units: GMAG DMP FIAP Chair: Andy Balk, Los Alamos National Laboratory Room: 394 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X47.00001: Current driven domain wall motion in ferrimagnetic Heusler thin racetracks Panagiotis Filippou, Jaewoo Jeong, See-Hun Yang, Yari Ferrante, Teya Topuria, Mahesh Samant, Stuart Parkin Heusler compounds are a large family of materials with a wide range of tunable properties. Of particular interest are ferrimagnetic binary Heusler compounds that have low magnetization and high perpendicular magnetic anisotropy in their tetragonally distorted forms. We have investigated a number of binary Mn based Heusler compounds and have prepared ultrathin films with thicknesses ranging from 1 to several unit cells. By forming racetracks from these materials we have demonstrated, for the first time, the current induced motion of domain walls with speeds up to 106 m/sec. We discuss the mechanisms by which the domain walls are moved with current which we find is surprisingly complex. Ferrimagnetic Heuslers are particularly interesting for spintronic applications because their low moment and high anisotropy allow for very narrow domain walls and, therefore, dense applications. We estimate the domain wall widths are of the order of a few unit cells. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X47.00002: Strain induced parametric pumping of a domain wall and its depinning from a notch Rabindra Nepal, Utkan Gungordu, Alexey Kovalev Using Thiele's method and detailed micromagnetic simulations, we study resonant oscillation of a domain wall in a notch of a ferromagnetic nanowire due to the modulation of magnetic anisotropy by external AC strain. Such resonant oscillation results from the parametric pumping of domain wall by AC strain at frequency about double the free domain wall oscillation frequency, which is mainly determined by the perpendicular anisotropy and notch geometry. This effect leads to a substantial reduction in depinning field or current required to depin a domain wall from the notch, and offers a mechanism for efficient domain wall motion in a notched nanowire. Our theoretical model accounts for the pinning potential due to a notch by explicitly calculating ferromagnetic energy as a function of notch geometry parameters. We also find similar resonant domain wall oscillations and reduction in the domain wall depinning field or current due to surface acoustic wave in soft ferromagnetic nanowire without uniaxial anisotropy that energetically favors an in-plane domain wall. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X47.00003: Barkhausen noise in the Ising Ferromagnet LiHo$_{\mathrm{x}}$Y$_{\mathrm{1-x}}$F$_{\mathrm{4}}$ Jian Xu, Daniel Silevitch, Thomas Rosenbaum LiHoF$_{\mathrm{4}}$ is an S$=$1/2 dipole-coupled Ising ferromagnet with a Curie temperature, T$_{\mathrm{c}}=$1.53 K. Partially substituting non-magnetic yttrium for the magnetic holmium suppresses T$_{\mathrm{c}}$ for moderate doping, but crucially also introduces disorder. The combination of the random dilution of the Ho$^{\mathrm{3+}}$ ions, the off-diagonal components of the dipole interaction, and the application of a magnetic field transverse to the Ising axis, produces a site-random-field along the Ising axis. The phase boundary as a function of temperature and transverse field is known and the random-field domain pinning can be tuned reversibly. Here, we use Barkhausen noise techniques to study domain reversal and avalanche dynamics in this model quantum magnet, focusing on the competing effects of random-field pinning and quantum tunneling on the avalanche dynamics. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X47.00004: Spin wave beam splitter Xiansi Wang, Xiangrong Wang Spin waves are promising information carriers in spintronics. A spin wave beam splitter, which can divide a spin wave beam into two or more beams, is a fundamental element of spin wave circuits. Here we demonstrate a low-loss and integrable spin wave beam splitter utilizing a domain wall in a ferromagnetic strip. We show that with a proper lattice structure and spin-spin interaction, the ferromagnetic spin system supports topologically protected chiral edge states, and a spin wave beam propagating along one edge towards a domain wall will be guided along the domain wall and split into two spin wave beams propagating in two opposite directions along the other edge. It is found that there are two branches of bound spin waves in the domain wall and their superpositions can result in different power division ratios depending on the strip width (domain wall length). Various types of devices are designed based on these findings. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X47.00005: Transport properties of exchange biased mesoscale wires James Delles, David Harrison, Dan Endean, Dan Dahlberg We used anisotropic magnetoresistance (AMR) measurements to investigate the magnetization processes and reversal mechanisms in both straight and zigzag ferromagnetic wires with an exchange bias. The wires were fabricated from a 7 nm CoFe layer coupled to a 7 nm IrMn layer with the exchange bias either perpendicular or collinear to the length of the wires. All wires were 80 nm wide with the straight wires 1.5 microns long and the individual segments of the zigzag wires 800 nm long. The four terminal resistance data were taken while sweeping a 10 kG applied field. For the straight wire segments, we show that the magnetization reversal is explained by coherent rotation in all but one of the six measured applied field, exchange bias field directions. In the sixth, the reversal includes domain wall formation and propogation. Using the AMR data from the straight wires segments, we modeled the AMR data for the zigzag wires as a superposition of two perpendicular wire segments. This superposition describes the general behavior but not in detail. Simulations that solve the Landau-Lifschitz-Gilbert equation show that the differences can be explained by domain wall nucleation in the corners of the zigzag wires for all measured applied field directions. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X47.00006: The Analog Information Limit of Magnetic Domain Wall Positions in Nanowires Sumit Dutta, Saima Siddiqui, Joseph Finley, Caroline Ross, Marc Baldo Nonvolatile memory and logic devices are often made of magnetic nanowires with mobile domain walls (DWs). When the wire width is scaled to below 100 nm, the line edge roughness (LER) plays a larger role in domain wall dynamics. We explain how discrete domain wall pinning sites are distributed in sub-100-nm-wide wires with LER. Based on measurements of 60-nm-wide Co wires and micromagnetic modeling of DW motion and pinning in such wires, we understand LER-influenced trap distributions and their effects on the information density and control in DW memory and logic applications such as racetrack memory. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X47.00007: Periodic Magnetic Domains on Single-Crystalline Cobalt Filaments. Fei Chen, Fan Wang, Fei Jia, Jingning Li, Ru-Wen Peng, Mu Wang Magnetic structures with controlled domain wall pattern may be applied as potential building blocks for three-dimensional magnetic memory and logic devices. Using a unique electrochemical self-assembly method, we are able to achieve regular single-crystalline cobalt filament arrays with specific geometric profile and crystallographic orientation, and the magnetic domain configuration can be conveniently tailored [1]. We observe the transition of periodic anti-parallel magnetic domains to a compressed vortex magnetic domains, which depends on the ratio of height vs. width of the wires. A "phase diagram" has been obtained to describe the dependence of the type of magnetic domains and the geometrical profiles of the wires. Magnetoresistance of the filaments demonstrates that the contribution of series of 180\textdegree domain walls is about 0.15{\%} of the zero-field resistance $\rho $(H$=$0). These self-assembled magnetic nano-filaments, with controlled periodic domain patterns, offer an interesting platform to explore domain-wall-based memory and logic devices. [1] F. Chen et al., Phys. Rev. B 93, 054405 (2016). [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X47.00008: Temperature Dependence Characterization of Layered Materials via the Magneto-Optical Kerr Effect Haoxiang Zhang, Christopher Stevens, Jagannath Paul, Denis Karaiskaj, Casey Miller The Curie temperature of PyCu alloy films can be controlled by Cu content. The additional thickness in layered materials changes the Cure temperature and hence the magnetic coupling between permalloy and Cu layers. The decoupling is investigated by the Magneto-Optical Kerr Effect (MOKE) as a function of temperature around the Curie temperature. The measurements reveal the coupling dynamics between permalloy and Co in novel magnetic heterostructures. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X47.00009: Parity-time symmetry breaking in magnetic systems Alexey Galda, Valerii Vinokur The understanding of out-of-equilibrium physics, especially dynamic instabilities and dynamic phase transitions, is one of the major challenges of contemporary science, spanning the broadest wealth of research areas that range from quantum optics to living organisms. Focusing on nonequilibrium dynamics of an open dissipative spin system, we introduce a non-Hermitian Hamiltonian approach, in which non-Hermiticity reflects dissipation and deviation from equilibrium. The imaginary part of the proposed spin Hamiltonian describes the effects of Gilbert damping and applied Slonczewski spin-transfer torque. In the classical limit, our approach reproduces Landau-Lifshitz-Gilbert-Slonczewski dynamics of a large macrospin. We reveal the spin-transfer torque-driven parity-time symmetry-breaking phase transition corresponding to a transition from precessional to exponentially damped spin dynamics. Micromagnetic simulations for nanoscale ferromagnetic disks demonstrate the predicted effect. Our findings can pave the way to a general quantitative description of out-of-equilibrium phase transitions driven by spontaneous parity-time symmetry breaking. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X47.00010: Directional heat dissipation generated by surface acoustic waves in Co/Pt/LiNbO$_{\mathrm{3}}$ structures. Ferran Macia, Blai Casals, Nahuel Statuto, Rafael Cichelero, Florencio Sanchez, Alberto Hernandez-Minguez, Joan Manel Hernandez, Gervasi Herranz Surface acoustic waves (SAWs) are used as filters and oscillators in mobile telephones. SAWs are also considered as an efficient route towards low-energy control of nanomagnetic devices. For instance, SAWs can be used in spintronics to generate spin-pumping in ferromagnetic structures as well as to modulate dynamically spin states in spintronic devices. Interestingly, the shear strains associated to SAW propagation can induce local heating that eventually is used to control magnetic. Here we report an experimental study on scanning thermal microscopy that analyzes propagation and spatial distribution of the heat generated by SAWs on the surface of LiNbO$_{\mathrm{3}}$ crystal. Our study reveals that heat generated by SAWs is directional and causes local temperature increases up to 10 K. We tested the effects of the SAW-induced heat dissipation in Co/Pt nanostructures having small magnetostriction and with a Curie temperature that can be engineered by film's thickness. We show that the effect of the fast and local heating can be used to variate the magnetic properties of the nanoelements. Our work provides interesting perspectives towards control magnetic nanodevices. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X47.00011: Impact of lattice dynamics on the phase stability of metamagnetic FeRh: Bulk and thin films Michael Wolloch, Markus E. Gruner, Werner Keune, Peter Mohn, Josef Redinger, Florian Hofer, Dieter Suess, Raimund Podloucky, Joachim Landers, Soma Salamon, Franziska Scheibel, Detlef Spoddig, Ralf Witte, Beatrize Roldan Cuenya, Oliver Gutfleisch, Michael Y. Hu, Jiyong Zhao, Thomas Toellner, Ercan E. Alp, Mario Siewert, Peter Entel, Rossitza Pentcheva, Heiko Wende We present phonon dispersions, element-resolved vibrational density of states (VDOS) and corresponding thermodynamic properties obtained by a combination of density functional theory (DFT) and nuclear resonant inelastic x-ray scattering (NRIXS) across the metamagnetic transition of B2 FeRh in the bulk material and thin epitaxial films. We see distinct differences in the VDOS of the antiferromagnetic (AF) and ferromagnetic (FM) phases, which provide a microscopic proof of strong spin-phonon coupling in FeRh. In the bulk phase, lattice vibrations contribute with the same sign and in similar magnitude to the isostructural AF-FM phase transition as excitations of the electronic and magnetic subsystems demonstrating that lattice degrees of freedom need to be included in thermodynamic modeling. We also propose a new monoclinic groundstate. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X47.00012: Combined Molecular and Spin Dynamics Simulation of Lattice Vacancies in BCC Iron Mark Mudrick, Dilina Perera, Markus Eisenbach, David P. Landau Using an atomistic model that treats translational and spin degrees of freedom equally, combined molecular and spin dynamics simulations have been performed to study dynamic properties of BCC iron at varying levels of defect impurity. Atomic interactions are described by an empirical many-body potential\footnote{Dudarev S L, Derlet P M 2005 \textit{J. Phys.: Cond. Mat.} \textbf{17} 7097}, and spin interactions with a Heisenberg-like Hamiltonian with a coordinate dependent exchange interaction\footnote{Ma P W, Woo C H, Dudarev S L 2008 \textit{Phys. Rev. B} \textbf{78} 024434}. Equations of motion are solved numerically using the second-order Suzuki-Trotter decomposition for the time evolution operator\footnote{Perera D, et al. 2014 \textit{J. Phys.: Conf. Ser.} \textbf{487} 012007}. We analyze the spatial and temporal correlation functions for atomic displacements and magnetic order to obtain the effect of vacancy defects on the phonon and magnon excitations. We show that vacancy clusters in the material cause splitting of the characteristic transverse spin-wave excitations, indicating the production of additional excitation modes. Additionally, we investigate the coupling of the atomic and magnetic modes. These modes become more distinct with increasing vacancy cluster size. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X47.00013: First-principles studiesy of the order-disorder phase transition in FeCo using Wang-Landau Monte-Carlo method Zongrui Pei, Markus Eisenbach, G. Malcolm Stocks Simulating order-disorder phase transitions in magnetic materials requires the accurate treatment of both the atomic and magnetic interactions, which span a vast configuration space. Using FeCo as a prototype system, we demonstrate that this can be addressed by combining the Locally Self-consistent Multiple Scattering (LSMS) method with the Wang-Landau (WL) Monte-Carlo algorithm. Fe-Co based materials are interesting magnetic materials but a reliable phase diagram of the binary Fe-Co system is still difficult to obtain. Using the combined WL-LSMS method we clarify the existence of the disordered A2 phase and predict the Curie temperature between it and the ordered B2 phase. The WL-LSMS method is readily applicable to the study of second-order phase transitions in other binary and multi-component alloys, thereby providing a means to the direct simulation of order-disorder phase transitions in complex alloys without need of intervening classical model Hamiltonians. We also demonstrate the capability of our method to guide the design of new magnetic materials. [Preview Abstract] |
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