Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session B29: Focus Session: Artificial Spin Ice |
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Sponsoring Units: GMAG DMP Chair: Cristiano Nisoli, Los Alamos National Laboratory Room: 206A |
Monday, March 2, 2015 11:15AM - 11:51AM |
B29.00001: Thermalization, Charge Ordering, and other Recent Developments in Artificial Spin Ice Invited Speaker: Peter Schiffer Artificial spin ice consists of arrays of lithographically fabricated single-domain ferromagnetic elements, arranged in different geometries such that the magnetostatic interactions between the moments are frustrated. Magnetic force microscopy imaging of these arrays allows us to study the accommodation of frustration through the local correlations between the moments as a function of both the strength of the interactions and the geometry of the frustration. The interactions can be closely mapped onto those of the ``spin ice'' materials, and allow a detailed analysis of the local correlations and monopole-like excitations. We have probed a number of different lattice geometries and find both local ordering and disordered states that match classic models for frustrated spin systems. Our recent work has focused on thermalization of these arrays as well as investigation of lattice geometries that are unavailable in natural systems and are specifically designed to exhibit unusual behavior associated with frustration, e.g., the shakti lattice. Thermalization reveals ordering both of the moments and of the effective magnetic charges that characterize correlated many-body dynamics in these systems. Other recent work has involved studies of return point memory as well as measurements of electrical transport in these systems. [Preview Abstract] |
Monday, March 2, 2015 11:51AM - 12:03PM |
B29.00002: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 12:03PM - 12:15PM |
B29.00003: Dynamics of artificial square spin ice during a non-equilibrium field ramp and quench Juan Carlos Andresen, Shrawan Mishra, James Lee, Xiaowen Shi, Barry Farmer, Lance De Long, Patrik Henelius, Steve Kevan, Sujoy Roy Recent advances in nanotechnology make it possible to create arrays of single-domain ferromagnetic nanoislands that can be fabricated to mimic a variety of Ising-like model systems. This has opened up new ways of studying frustrated systems, such as artificial square spin ice. One of the main advantages of studying these nanomagnet systems is that the Ising-like moments can be directly visualized; but a persistent drawback has been the inaccessibility of the ground state, due to the highly athermal nature of these systems. We present the magnetic autocorrelation function of artificial square spin ice, as measured by XPCS following a non-equilibrium field quench. Our large-scale Monte Carlo simulations agree qualitatively with the experimental relaxation measurements. Furthermore, our simulation results indicate that a simple field ramping demagnetization protocol can be a viable way of reaching a low-energy state. [Preview Abstract] |
Monday, March 2, 2015 12:15PM - 12:27PM |
B29.00004: Exchange Bias and Magnetotransport in Permalloy Connected Kagome Artificial Spin Ice Brian Le, David Rench, Rajiv Misra, Liam O'Brien, Chris Leighton, Nitin Samarth, Peter Schiffer Artificial spin ice consists of nanoscale ferromagnets arranged in a periodic lattice, with the resultant magnetostatic interactions emulating the local magnetic behavior of spin ice. Kagome artificial spin ice consists of elongated ferromagnetic islands or nanowires arranged in a honeycomb lattice. We present magnetotransport results in connected kagome artificial spin ice composed of permalloy (Ni$_{\mathrm{81}}$Fe$_{\mathrm{19}})$ nanowires. Magnetoresistance was measured as a function of applied field strength at different temperatures. At temperatures below 20 K, the field reversal symmetry of the magnetoresistance is broken. This asymmetry appears to be associated with exchange bias due to the surface oxidation of permalloy and is suppressed in aluminum-capped samples. These results signify that exchange bias can play a substantial role in the physics of artificial spin ice that has potential as a new mode of controlling its behavior. Supported by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under grant number DE-SC0010778. Work at the University of Minnesota was supported by the NSF MRSEC under award DMR-0819885 and a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (project no. 299376). [Preview Abstract] |
Monday, March 2, 2015 12:27PM - 12:39PM |
B29.00005: Return Point Memory in Artificial Spin Ice Ian Gilbert, Gia-Wei Chern, Bryce Fore, Yuyang Lao, Cristiano Nisoli, Peter Schiffer Return point memory, in which the spins of a magnet return to their original configuration after the magnet is driven through a hysteresis loop, has been studied extensively with theory, simulations, and bulk experimental probes. However, due to the difficulties associated with directly imaging single spins, microscopic experimental examination of return point memory has proven to be elusive. Here we describe a study of return point memory in arrays of single-domain nanomagnets known as artificial spin ice. In this system, the individual moments can be experimentally resolved by magnetic force microscopy (MFM), so we can both verify the existence of return point memory and explore the mechanism by which it develops. We find that, in artificial spin ice, magnetic monopole excitations drive the development of return point memory through a ratchet-like interaction with the local field produced by the surrounding nanoislands. The number of hysteresis loops required to produce return point memory can be adjusted by tuning the applied magnetic field and array geometry. [Preview Abstract] |
Monday, March 2, 2015 12:39PM - 12:51PM |
B29.00006: Charge-ordering in FePd$_{3}$ artificial kagome ice Jasper Drisko, Stephen Daunheimer, John Cumings Artificial spin ices (ASIs) are arrays of lithographically-patterned, Ising-like nanomagnets built-by-design to be geometrically frustrated. ASI has proven to be a novel and powerful tool for studying the effects of frustration due to its success in modeling real frustrated materials like spin ice, its highly tunable nature, and its amenability to a variety of techniques to directly characterize exact spin configurations. A fundamental question for frustrated systems is how they find long-range ordered states or whether this is even possible at all in the presence of frustration. In this work we investigate theoretical predictions of charge-ordering in the kagome ice-II state [1]. We employ ASI fabricated from FePd$_{\mathrm{3}}$, which has a relatively low Curie temperature and thus easily allows for thermally activated reversal of individual spins. We have fabricated samples with magnets of varying lengths and investigate them using Lorentz Transmission Electron Microscopy. Samples are heated above their Curie temperature and cooled slowly back to room temperature, allowing the macro-spins to interact, flip, and relax during the cooling process. We find that shorter lattice constant samples tend to exhibit better ordering of magnetic charges after cooling. We have also performed simulations of our samples using a kinetic Monte Carlo technique. We find very good agreement between the simulations and experiment when we incorporate a disordered spread of magnet widths into the simulations, representative of our real samples due to lithography artifacts.\\[4pt] [1] G.-W. Chern et al., \textit{Phys. Rev. Lett. }\textbf{106}, 207202 (2011) [Preview Abstract] |
Monday, March 2, 2015 12:51PM - 1:03PM |
B29.00007: Nanoscale SEMPA imaging of an artificial quasicrystal spin ice at remanence Andrew Balk, Vinayak Bhat, Barry Farmer, Lance DeLong, John Unguris Artificial spin ice has emerged in the past decade as a model metamaterial for studying frustrated magnetic ordering at length scales large enough to be experimentally probed in real space. Recently, complex designs have been engineered to explore exotic behavior in non-square lattices. However, direct measurements of the actual moment directions have not been very common, and interpretation from techniques such as magnetic force microscopy and magneto-optical Kerr effect magnetometry can be complicated by the more complex geometries. Here we demonstrate using SEMPA (scanning electron microscopy with polarization analysis) as a method to robustly measure the ordering direction of elements in a connected artificial quasicrystal. We discuss the applicability of SEMPA to this system, details of the imaging and potential artifacts, and conclusions that can be drawn from the nanoscale two dimensional maps of the moment direction. [Preview Abstract] |
Monday, March 2, 2015 1:03PM - 1:15PM |
B29.00008: First observation of ferromagnetic order in an artificial 2D quasicrystal Barry Farmer, Vinayak Bhat, Andrew Balk, John Unguris, Lance De Long Magnetic order in bulk quasicrystals is not well understood and known materials exhibit short-range, spin-glass order. We patterned ferromagnetic (FM) thin films into artificial quasicrystals, a new class of metamaterials that exhibits complex magnetic reversal and dynamics that can be controlled via tiling design.\footnote{V.S. Bhat\textit{, et al.} \textit{Phys. Rev. Lett.} \textbf{111}, 077201 (2013).} We analyzed two-dimensional SEMPA images of magnetization textures of Penrose P2 tilings (P2T) patterned into Permalloy. The diverse, asymmetric vertex coordinations drive novel, \textit{non-stochastic switching} and \textit{complex spin-ice} behaviors that reflect the influence of vertex domain wall energies. Monte Carlo and OOMMF simulation analyses of SEMPA images of slowly grown, never-field-cycled P2T reveal low energy, long-range ordered sublattices that form building blocks of a ground state. A fully ordered ground state is unresolved without long-range dipolar interactions that stabilize a magnetically ordered state with a net moment. Our P2T constitute a set of quasicrystalline metamaterials in which frustration and magnetic order among classical Ising spins can be directly studied. [Preview Abstract] |
Monday, March 2, 2015 1:15PM - 1:27PM |
B29.00009: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 1:27PM - 1:39PM |
B29.00010: Honeycomb artificial spin ice at low temperatures Katharina Zeissler, Megha Chadha, Lesley Cohen, Will Branford Artificial spin ice is a macroscopic playground for magnetically frustrated systems. It consists of a geometrically ordered but magnetically frustrated arrangement of ferromagnetic macros spins, e.g. an arrangement of single domain ferromagnetic nanowires on a honeycomb lattice. Permalloy and cobalt which have critical temperature scales far above 290 K, are commonly used in the construction of such systems. Previous measurements have shown unusual features in the magnetotransport signature of cobalt honeycomb artificial spin ice at temperatures below 50 K which are due to changes in the artificial spin ice's magnetic reversal. In that case, the artificial spin ice bars were 1 micron long, 100 nm wide and 20 nm thick. Here we explore the low temperature magnetic behavior of honeycomb artificial spin ice structures with a variety of bar dimensions, indirectly via electrical transport, as well as, directly using low temperature magnetic imaging techniques. We discuss the extent to which this change in the magnetic reversal at low temperatures is generic to the honeycomb artificial spin ice geometry and whether the bar dimensions have an influence on its onset temperature. [Preview Abstract] |
Monday, March 2, 2015 1:39PM - 1:51PM |
B29.00011: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 1:51PM - 2:03PM |
B29.00012: Magneto-optical and magneto-transport studies of hexagonal artificial spin ice nano-structures Simon Olivari, Kane Esien, Dan Read Artificial spin ice structures have attracted a great deal of attention recently and may prove to be useful analogues for frustrated magnetic systems, such as bulk spin ice materials. We will present the results of studying these structures by utilising magneto optical Kerr effect (MOKE) and magneto-transport measurements. We have fabricated hexagonal (also sometimes known as honeycomb) ASI structures from metallic ferromagnetic islands having dimensions close to 1 $\mu$m long, 100nm wide and 10nm thick. We have made electrical transport measurements of two types of structure both having similar geometry and electrically connected islands, however the first samples have magnetically connected elements forming the honeycomb networks whereas the second set of samples are formed from magnetically isolated islands. Comparing these structures allows an assessment of the relative contributions from magnetic domain wall (DW) motion and from magnetostatic interactions. The magneto-optical measurements have been made as a function of angle between the field direction and the lattice. The properties observed with NiFe and Co fabricated nanostructures are discussed in relation to the geometries described above. [Preview Abstract] |
Monday, March 2, 2015 2:03PM - 2:15PM |
B29.00013: A new class of aperiodic, long-range ordered artificial spin ices based upon Fibonacci distortions of 2D periodic lattices Justin Woods, Vinayak Bhat, Barry Farmer, Joseph Sklenar, Eric Teipel, John Ketterson, J. Todd Hastings, Lance De Long Artificial spin ice (ASI) systems are composed of nanoscale ferromagnetic segments whose shape anisotropy dictates they behave as mesoscopic Ising spins. Most ASI have segments patterned on periodic lattices and a single vertex topology. We have continuously distorted 2D honeycomb and square lattices such that the pattern vertex spacings follow a Fibonacci chain sequence along primitive lattice directions. The Fibonacci distortion is related to the aperiodic translational symmetry of 2D artificial quasicrystals1 that cannot be viewed as continuous distortions of periodic lattices due to their forbidden (e.g., fivefold) rotational symmetries. In contrast, Fibonacci distortions of 2D periodic lattices can be ``turned on'' by control of the ratio of two lattice parameters d1 and d2. Distortions alter film segments such that pattern vertices are no longer equivalent and traditional spin ice rules are no longer strictly valid. We have performed OOMMF simulations of magnetization reversal for samples having different levels of distortion, and found the magnetic reversal to be dramatically slowed by small distortions (d1/d2 $\approx $ 1). [Preview Abstract] |
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