Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session X42: Structured MaterialsFocus
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Sponsoring Units: GMAG Chair: Amal El-Ghazaly, Cornell Room: 709/711 |
Friday, March 6, 2020 11:15AM - 11:27AM |
X42.00001: Vacancies in artificial spin ice Pablo Díaz, Felipe Brevis, Eugenio Vogel We study artificial spin-ice systems (in the form of square arrays of islands of magnetic materials) from the numerical point of view. Typical Permalloy elongated and anisotropic islands [1] occupy the positions completely filling a layer L×L, with variable L; several equivalent layers are then piled up. We focus on the vertices (that are the convergence points of islands) as the internal layers are diluted upon introducing vacancies (lack of islands). This is both a random and controlled process for a single internal layer; the upper-most and lower-most layers remain without vacancies. The introduction of vacancies generated several new types of vertices that can be characterized according to their distribution depending of vacancy concentration. We found that a small amount of vacancies has strong influence on the vertex distributions. Moreover, we considered a code based format for the different vertices and found that is possible to write on each layer complete and robust messages. The propagation of information in a transverse way (from top to lower layer) is also explored. |
Friday, March 6, 2020 11:27AM - 11:39AM |
X42.00002: Field-tunable correlations in perpendicular artificial spin ice arrays Susan Kempinger, Yu-Sheng Huang, Paul Edward Lammert, Michael Vogel, john pearson, Axel Hoffmann, Vincent Crespi, Peter E Schiffer, Nitin Samarth Artificial spin ice (ASI) provides an effective platform for the study of custom designed frustration and its relationship with geometry, interaction, and stochasticity. Perpendicular ASI is particularly useful in this context, as the state of each element in a lattice is readily accessed using Kerr microscopy and the microstate of the entire lattice can be characterized through an applied field protocol. Unfortunately, studies of perpendicular ASI have been limited by weak interactions between elements. We have overcome this limitation by fabricating perpendicular ASI systems from Pt/Co islands on a soft-magnetic Ni80Fe20(Py) underlayer to increase interactions. In the simplest case, the Py is saturated and serves to break the lateral symmetry in the arrays. We show that this configuration leads to a highly tunable system with unusual properties such as directionally-tunable interactions, preferred next-nearest neighbor coupling, and in situ adjustable coordination number. This project was funded by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under Grant No. DE- SC0010778 |
Friday, March 6, 2020 11:39AM - 11:51AM |
X42.00003: Competition between topological order and long range order in Santa Fe spin ice Ayhan Duzgun, Cristiano Nisoli Artificial spin ices are two-dimensional arrays of interacting single-domain ferromagnetic nano-islands [1,2]. Santa Fe Ice is a newly proposed and recently realized artificial spin ice lattice whose vertex-frustrated [3,4] spin ensemble can be described by strings of topologically protected excitations. We have investigated numerically Santa Fe Ice in and out of equilibrium. Depending on structural parameters, its ground state can either be a disordered topological phase [5,6] or an ordered, anti-ferromagnetic state. In the latter case, however, the topological phase is still present and can trap the system, delaying or preventing the relaxation to the ordered state either after quenches or even after a slow annealing when in the neighborhood of the structural transition. |
Friday, March 6, 2020 11:51AM - 12:03PM |
X42.00004: Shape transformations of flexible ferromagnetic ribbons Kostiantyn Yershov, Volodymyr P. Kravchuk, Denis D. Sheka, Jeroen Van den Brink, Yuri Gaididei We propose a minimal extension of the anisotropic 2D Heisenberg model in order to describe flexible magnetic systems with coupled magnetic and mechanical subsystems[1,2]. The coupling between the magnetic and mechanical subsystems is driven by uniaxial anisotropy with the easy-axis normal or tangential to the magnetic film and by the Dzyaloshinskii--Moriya interaction (DMI). We show that the presence of DMI results in a spontaneous deformation of a flexible magnetic ribbon. The final state is characterized by the geometrical chirality whose sign is determined by the sign of the DMI constant. Depending on the mechanical, magnetic, and geometric parameters of the system one can obtain two different states: a twisted-state with zero curvature of the central line is typical for small DMI constants and narrow ribbons, while a DNA-like state with nonzero curvature of its central line is preferable for large DMI constants and wider ribbons. All these states can be efficiently controlled by magnetic fields opening new possibilities in the development of nanorobots in the context of organicelectronics and spintronics. |
Friday, March 6, 2020 12:03PM - 12:15PM |
X42.00005: A New Approach of Constructing Shape-controlled Nanowire Arrays Fei Chen, Ruwen Peng, Mu Wang Magnetic nanowires with controlled morphology and crystallinity are enabling devices with novel functionalities. The fabrication of the nanowires with specific shapes such as standing U-shaped nanowires and helical nanowires is still challenging, especially on a rough and bumpy surface. Here we demonstrate a unique way to fabricate cobalt nanowire arrays on an arbitrarily-shaped surface by electrodeposition. When the pH of the electrolyte is around 2.3, the deposit shows monocrystalline hcp structure with the c-axis perpendicular to the long-axis of the nanowire arrays. On the other hand, when the pH is around 4.5, the deposit shows polycrystalline hcp structure. Counter intuitively, here the growth direction of nanowire is perpendicular to its longitudinal axis, and the specific geometry of nanowires can be achieved by introducing specially designed shaped substrate. The spatial separation and the width of the nanowires can be tuned by voltage, electrolyte concentration and temperature in electrodeposition. By taking cobalt nanowire array as an example, we show that head-to-head and tail-to-tail magnetic domain walls can be easily introduced and modulated in the nanowire arrays, which is enlightening to construct new devices such as domain wall racetrack memory. |
Friday, March 6, 2020 12:15PM - 12:27PM |
X42.00006: S-shaped configurational magnetic states in square permalloy dots Barry Costanzi, Anthony Cho, Spencer Weeden, Gregor Dairaghi We experimentally observe both U- and S-shaped zero-field magnetic configurations in single square permalloy dots, with a dot’s configurational state dependent on field history and temperature. Magnetization of 170 nm x 170 nm x 7 nm dots is probed using 4-terminal anisotropic magnetoresistance (AMR) measurements. Room temperature field sweeps parallel to the dot edge yield AMR curves characteristic of both a U-shaped state [1], and of a previously unreported S-shape, for identical field sweep parameters. The S-shape sweep pattern is not observed at low T, implying the state is thermally activated, a result confirmed by micromagnetic simulations. The S-state also occurs when sweeping the field off-axis by several degrees, consistent with minimizing Zeeman interaction energy. The existence of multiple zero-field configurations precludes use of the Stoner-Wolfarth model typically used at this scale, instead necessitating a more complicated configurational picture as dot size decreases. |
Friday, March 6, 2020 12:27PM - 1:03PM |
X42.00007: Functional ferrogels: from magnetic biosensors to regenerative medicine Invited Speaker: Galina Kurlyandskaya G.V. Kurlyandskaya, A.P. Safronov, F.A. Blyakhman, S.V. Shcherbinin N.A. Buznikov |
Friday, March 6, 2020 1:03PM - 1:15PM |
X42.00008: Tunability of magnetism in Heusler Mn3Ga: Crossover between cubic and tetragonal phase upon Co-substitution Thi Quynh Anh Nguyen, Huynh Thi Ho, Soon Cheol Hong, Sonny H. Rhim Structural and magnetic properties of Co substituted Mn3-xCoxGa (x = 0~1) is investigated using ab initio calculations. Ferrimagnetic Mn3Ga possesses two inequivalent Mn sites, octahedral Mn-I and tetrahedral Mn-II, where Co prefers Mn-II site, which leads to intriguing physics. Notably, structural transition, from tetragonal to cubic phase, occurs around x=0.6. Unlike other Co compounds, local moment of Co nearly vanishes in tetragonal phase, which results in decreasing total moment with Co concentration. In contrast, in cubic phase Co retains usual moment: Magnetic moment increases linearly with x obeying Slater-Pauling rule of cubic Heusler, M = NV - 24, where M is the magnetization, NV is numbers of valance electrons. In particular, the integral M, a signature of half-metallicity, emerges in cubic phase for x=0.5 and 1.0. Furthermore, the vanishing moment of Co for tetragonal phase is analyzed phenomenologically using Heisenberg model, where the exchange coefficients of two Mn sites tends to compensate. |
Friday, March 6, 2020 1:15PM - 1:27PM |
X42.00009: Deconvoluting Magnetic contributions in BiFeO3 NPs using First Order Reversal Curves Alexander Cardona Rodríguez, Edwin ER RAMOS, Nicolás Vergara, Diego Carranza, Andreas Reiber, Juan Ramirez BiFeO3 (BFO) in bulk is ferroelectric (FE) and antiferromagnetic (AF) at room temperature. Additional it also possesses a Dzyaloshinski-Moria (DM) anisotropy that produces a 62 nm wavelength spin spiral. Therefore, in nanostructured BFO systems, a complex ferromagnetism-like behavior appears that depends on the balance between size and magnetic length scales . Furthermore, it is known that the physical properties in perovskites in general, and BFO in particular, are very sensitive to changes in their crystal structure. Hence, one can exploit such effects to control their degrees of freedom. Here we present first-order reversal curves (FORC) as a reliable method to deconvolute the BFO magnetic contributions that appear when nanostructured in the form of nanoparticles with sizes below and above the wavelength spin spiral . We use FORC to obtain coercivity distributions for different nanoparticles sizes. Our results suggest two main contributions to BFO magnetism that can be related to an intrinsic core-shell structure that forms due to strain relaxation. |
Friday, March 6, 2020 1:27PM - 1:39PM |
X42.00010: Multiscale modeling of magnetorheological elastomers with a twist David Marchfield, Tong Dang, Andy T Clark, Jiajia Li, Xuemei Cheng, Kristen S. Buchanan Magnetorheological elastomers (MREs) are polymers with embedded micron or submicron ferromagnets. MREs are a promising platform for studying cell responses to dynamic mechanical changes in their environment, as the shear modulus is tunable via an applied magnetic field [1]. MREs deform under applied magnetic fields, including visible twisting. In this work, we used a multi-scale approach to model the magnetic reversal and field-dependent deformations. Ferromagnetic particles in the polymer interact via dipolar coupling, but are also permitted to move; the balance of dipole, elastic energies, and Zeeman energy dictates the deformations of MREs. A point-dipole based model was used to study the behavior of a collection of particles in an elastomer matrix, and micromagnetic simulation results for a ferromagnetic sphere provided the susceptibilities used as input parameters. Volume-preserving in- and out-of-plane deformations were incorporated in the model, as well as, twisting of the matrix as observed in experiments. Modelling results will be presented and compared to experimental hysteresis measurements. |
Friday, March 6, 2020 1:39PM - 2:15PM |
X42.00011: Ferromagnetic Liquid Droplets Invited Speaker: Xubo Liu Ferrofluids are dispersions of ferromagnetic or superparamagnetic |
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