Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E39: Magnetic Nanostructures and NanoparticlesFocus
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Sponsoring Units: GMAG DMP Chair: Katrin Schultheiss, Helmholtz-Zentrum Dresden-Rossendorf Room: BCEC 207 |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E39.00001: Topological Magnetic Writing: Controlling the magnetic state of nanostructures Invited Speaker: Will Branford Networks of magnetic nanostructures are of broad interest from novel data storage and computation to magnonic crystals. One family of nanomagnetic arrays, characterized by strong and frustrated magnetic interactions are the artificial spin ices (ASI).1 ASI structures have provided vast amounts of physical insight in recent years in part due to their ability to model complex systems and exhibit exotic phenomena such as ‘magnetic monopole’-like states.2 The power of these networks stems from the extraordinary number of unique microstates, even in systems comprising relatively few nanostructures. However, magnetic nanoarrays in general, and ASI structures in particular, have yet to realise their full potential as the majority of microstates remain inaccessible due to the rudimentary state-writing tools currently available. An experimental means to prepare all potential microstates has huge implications, including realising ASI as a tunable-bandgap magnonic crystal3 or reconfigurable neural-network4. We present a novel MFM-tip based state writing technique,5 building on our previously demonstrated domain-wall injection process.6 It requires no global fields and is applicable to all nanostructure architectures, providing control over the spin-configuration and access to every possible microstate. We demonstrate our method via realisation of several exotic and thus-far unobserved states, unachievable via global field-protocols: ‘magnetic monopole-defect’ chains and the spin-crystal ground state of kagome ASI. |
Tuesday, March 5, 2019 8:36AM - 8:48AM |
E39.00002: Enhanced interaction strength in perpendicular artificial spin ice arrays Susan Kempinger, Yu-Sheng Huang, Paul Edward Lammert, MICHAEL VOGEL, JOHN E. PEARSON, Axel F Hoffmann, Vincent Henry Crespi, Peter E Schiffer, Nitin Samarth Perpendicular implementations of artificial spin ice are appealing as a custom designed, frustrated system due to the optical accessibility of the magnetic components. This allows for complete microstate characterization in situ with an applied field using magneto-optical Kerr effect microscopy. We have fabricated arrays of lithographically patterned, single-domain Pt/Co multilayer islands. Studies on this system thus far have been limited by the weak interactions between islands that fit the constraints necessary for optical accessibility. New samples fabricated with a permalloy underlayer show an enhancement in correlation and allow for studies beyond the weakly interacting regime. We study both the quasi-dynamic hysteresis and demagnetized states of these systems to gain an understanding of how the coupling is affected by the soft underlayer. |
Tuesday, March 5, 2019 8:48AM - 9:00AM |
E39.00003: Role of Lattice Defects in Artificial Spin Ice Studied with Coherent X-ray Scattering and XMCD-PEEM Xiaoqian M Chen, Barry W Farmer, Justin Woods, Lance De Long, Sujoy Roy, Jeffrey T Hastings Magnetically frustrated materials are unable to satisfy all competing interactions simultaneously. The introduction of a small defect in such a system can completely alter the ground state. We used coherent x-ray scattering and XMCD-PEEM to investigate the role of lattice defects in artificial spin ice. We observed reduced blocking temperature and the broadening of the phase coexistence region with increased doping. In addition, we observed defect pinnings of superdomain walls in a nontrivial manner as the temperature is varied through the transition. |
Tuesday, March 5, 2019 9:00AM - 9:12AM |
E39.00004: Real-Space Dynamics via Increased Effective Dimension in Artificial Spin Glass Michael Saccone, Alan Farhan From ordering piles of sand [1] to comprehending the human mind [2], spin glass systems continue to inspire theorists and experimentalists alike. Artificial nanomagnet fabrication [3] offers a unique opportunity to directly visualize the dynamics of glassy systems. However, spin glass phases have only been discovered at finite temperature in systems embedded in dimensions greater than or equal to 3 [1,4]. In this study we fabricate Nanomagnet arrays such that the network of their interactions possesses an effective dimension [5] higher than 2. Dubbed the Bethe glass, this is first finite temperature spin glass embedded in two dimensions. Using XMCD [6] the time scale free relaxation of magnetic moments is filmed in real space, directly observing the supermetric qualities of [1] spin glass. Further, the effective dimension is varied to show the tunability of glass properties. |
Tuesday, March 5, 2019 9:12AM - 9:48AM |
E39.00005: Three-Dimensional Magnetic Nanostructures Grown by Focused Electron Beam Induced Deposition (FEBID) Invited Speaker: Javier Pablo-Navarro The fabrication of three-dimensional (3D) magnetic nanostructures is currently a central topic in nanomagnetism [1]. Ferromagnetic nanowires (NWs) and nanotubes (NTs) are potential candidates for magnetic data storage, logic and sensing, and Focused Electron Beam Induced Deposition (FEBID) could play a crucial role in the fabrication of these architectures [2]. |
Tuesday, March 5, 2019 9:48AM - 10:00AM |
E39.00006: Asymmetric Electrical Transport in Symmetric Permalloy Artificial Honeycomb Lattice Ashutosh Dahal, Yiyao Chen, Brock T Summers, George Yumnam, Deepak K Singh We have observed asymmetric electrical transport in macroscopic samples of artificial magnetic honeycomb lattice of Permalloy having ultra-small elements with length scales of sub - 10nm to 30nm. The unidirectional enhancement of conductivity, analogues to a semiconductor diode, is dependent on thickness of the Permalloy as well as the order of honeycomb and can persist up to room temperature. The Permalloy honeycomb lattice exhibit very small rectification voltage and power dissipation compared with semiconductor diodes, which could have strong implication for spintronics. We will present detailed investigation done on the system and shed some light on the possible mechanism behind the asymmetric transport phenomena. |
Tuesday, March 5, 2019 10:00AM - 10:12AM |
E39.00007: Field and current control of the electrical conductivity of an artificial two-dimensional honeycomb lattice Yiyao Chen, Brock T Summers, Ashutosh Dahal, Valeria Lauter, Giovanni Vignale, Deepak K Singh The conductivity of a neodymium-based artificial honeycomb lattice undergoes dramatic changes upon application of magnetic fields and currents. We attribute these changes to a redistribution of magnetic charges that are formed at the vertices of the honeycomb due to the non-vanishing net flux of magnetization from adjacent magnetic elements. We suggest that the application of a large magnetic field or a current causes a transition from a disordered state, in which magnetic charges are distributed at random, to an ordered state, in which they are regularly arranged on the sites of two interpenetrating triangular Wigner crystals. |
Tuesday, March 5, 2019 10:12AM - 10:24AM |
E39.00008: Dynamic hysteresis loops of iron oxide nanoflowers Zoe Boekelheide, Jackson Miller, Cordula Grüttner Iron oxide nanoparticles have many potential applications in biomedicine, including magnetic hyperthermia and magnetic particle imaging. These applications depend sensitively on the magnetization (M) of the particles under a rapidly oscillating applied field H. The internal structure of nanoparticles strongly affects the magnetic behavior. Iron oxide nanoflowers, particles composed of several small crystallites fused together with an irregular surface, are promising for magnetic hyperthermia due to their large loss power [1]. We present dynamic measurements of M(H) at 230 kHz for iron oxide nanoflowers. The nanoflowers were synthesized by a polyol method described previously [2]. The cores are 22 ± 9 nm and are coated in dextran (synomag®-D). The particles are suspended in H2O at 14.2 mg(Fe)/mL. The resulting M(H) curves show linear behavior (open ellipsoidal loops) at low fields, transitioning to nonlinear behavior at a field of 6 kA/m. These results show directly the magnetic reversal behavior of iron oxide nanoflowers in the linear and nonlinear regimes, and will help guide applications of these particles. |
Tuesday, March 5, 2019 10:24AM - 10:36AM |
E39.00009: Room temperature ferromagnetic MnCr2O4 spinel chromite nanoparticles Eduardo Martínez-Terán, Raju Baral, Harikrishnan Nair, Ahmed El-Gendy MnCr2O4 spinel is a non-collinear ferrimagnet with transition temperature around 43 K. By means of reducing the particle size to nanoscale regime, the magnetic behavior changes to long-range ferromagnetic order and the transition temperature shifts to above 400 K. MnCr2O4 nanoparticles were synthesized successfully using supercritical conditions of liquid. Changing the reaction conditions, in particular, increasing the reaction time leads to reducing the particles size as well as increasing the magnetic properties of the formed MnCr2O4 nanoparticles. Crystal structures of the synthesized bulk and their nano-scaled candidates were investigated using XRD revealing the same spinel cubic symmetry Fd-3m. Morphology of all samples has been identified using SEM yielding average size of 535±75 nm for bulk sample and 223±50, 195±40, 172±30, 148±27 nm for nanoparticles prepared at reaction time of 2, 4, 6, and 8 hours respectively. The results yield room temperature ferromagnetic behavior at 300 K with magnetization of 6.5, 8.4, 10.8, 17 emu/g, for 223±50, 195±40, 172±30, 148±27 nm respectively. The obtained results confirm the role of reducing the particles size in enhancing the magnetic properties and open new route to probe the ferroelectricity as well as multiferroicity of this system. |
Tuesday, March 5, 2019 10:36AM - 10:48AM |
E39.00010: Coercivity Distributions in BiFeO3 nanoparticles Edwin RAMOS, Alexander Cardona, Juan Ramirez We study the effect of distribution of coercivities with the calcination temperature, using the first order reversal curves (FORC) treatment in BiFeO3 nanoparticles prepared by sol-gel method. We varied the calcination temperature in order to control the NP size. XRD measurements suggest that at lower temperatures (450°C and 550°C) the BFO NPs are single phase. At higher temperatures an impurity phase at higher appears with magnetic contribution. The FORC treatment was built as a linear combination for each temperature value. The coercivity distributions were obtained extracting the central ridge of the resultant FORC diagrams and contrasted with magnetization as a function of magnetic field measures. These results evidence a decreasing in the coercivity average that we associate to a decrease of magnetic domain size; then, suggesting a transition from single domain to superparamagnetic particles. |
Tuesday, March 5, 2019 10:48AM - 11:00AM |
E39.00011: Magnetically controlled geometry of flexible ferromagnetic rings Kostiantyn Yershov, Denis D. Sheka, Volodymyr P. Kravchuk, Avadh Saxena, Yuri Gaididei We propose a minimal extension of the anisotropic Heisenberg model in order to describe flexible magnetic wires (rings) [1]. Flexible ferromagnetic wires are spin-chain magnets, in which the magnetic and mechanical subsystems are coupled. The coupling between the magnetic and mechanical subsystems is driven by uniaxial anisotropy with the easy-axis oriented along the tangential direction. First, we show that depending on the magnetic and elastic parameters and the size of the system one can obtain two different states: the onion state with the quasi-uniform magnetization is typical for small enough rings, while the vortex state with the magnetization oriented tangential to the wire is preferable for large systems. Second, we demonstrate that according to the system symmetry there can appear normal modes with zero frequency (i. e. zero modes) on the background of equilibrium states. [1] Yu. Gaididei et al, arXiv:1809.10622 (2018). |
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