Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S39: Invited Session: Artificial Spin Ice and Artificial Frustrated Systems: Desiging Topology, Controlling Frustration, Engineering Emergence |
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Sponsoring Units: DCMP GMAG Chair: Cristiano Nisoli, Los Alamos National Laboratory Room: Mile High Ballroom 2A-3A |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S39.00001: Magnetic Charge Organization and Screening in Thermalized Artificial Spin Ice Invited Speaker: Ian Gilbert Artificial spin ice is a material-by-design in which interacting single-domain ferromagnetic nanoislands are used to model Ising spins in frustrated spin systems. Artificial spin ice has proved a useful system in which to directly probe the physics of geometrical frustration, allowing us to better understand materials such as spin ice. Recently, several new experimental techniques have been developed that allow effective thermalization of artificial spin ice [1-3]. Given the intense interest in magnetic monopole excitations in spin ice materials and artificial spin ice's success in modeling these materials, it should not come as a surprise that interesting monopole physics emerges here as well. The first experimental investigation of thermalized artificial square spin ice determined that the system's monopole-like excitations obeyed a Boltzmann distribution and also found evidence for monopole-antimonopole interactions [1]. Further experiments have implicated these monopole excitations in the growth of ground state domains [2]. Our recent study of artificial kagome spin ice [3], whose odd-coordinated vertices always possess a net magnetic charge, has revealed a theoretically-predicted magnetic charge ordering transition which has not been previously observed experimentally. We have also investigated the details of magnetic charge interactions in lattices of mixed coordination number. This work was done in collaboration with Sheng Zhang, Cristiano Nisoli, Gia-Wei Chern, Michael Erickson, Liam O'Brien, Chris Leighton, Paul Lammert, Vincent Crespi, and Peter Schiffer. \\[4pt] [1] J.P$.$ Morgan et al., Nature Phys. 7, 75 (2011).\\[0pt] [2] A. Farhan et al., Phys. Rev. Lett. 111, 057204 (2013).\\[0pt] [3] S. Zhang, et al., Nature 500, 553 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S39.00002: Thermally active two dimensional artificial spin-ice systems: experiment and simulation Invited Speaker: Peter Derlet Recently it has been possible to fabricate two dimensional arrays of interacting nano-magnetics which are thermally active within the time-frame of a photoemission electron microscopy (PEEM) experiment. Employing X-ray magnetic circular dichroism, such a local experimental probe can image the changing magnetic state of finite kagome and square lattice systems. Both equilibrium and non-equilibrium conditions have been considered revealing non-trivial dynamics which for the case of the kagome system depends strongly on the underlying magnetic frustration. To give insight into the observed dynamics, monte carlo and kinetic monte carlo methods are performed using a simple Ising-like Hamiltonian. This talk will discuss the origins of such an Ising-like Hamiltonian and its application to specific experiments. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S39.00003: Collective Properties of Nanomagnet Arrays; Electric and Magnetic Currents in Artificial Spin Ice Invited Speaker: Will Branford I will discuss arrays of single domain nanomagnets. The shape of each nanomagnets controls the magnetic anisotropy and the elements are closely spaced so dipolar interactions are important. Lattices are chosen such that the geometry prevents all dipole interactions from being satisfied. The building block of such frustrated lattices is the equilateral triangle because it cannot support simple antiparallel ordering. A two dimensional array of corner sharing triangles is known as the kagome lattice and a three-dimensional array of corner sharing tetrahedral is known as pyrochlore. Magnetic pyrochlore chemical compounds (spin ices) have recently attracted much attention with the observation of emergent magnetic monopoles, but they have limitations as model frustrated systems: tuning the lattice parameter by chemical doping tends to break the symmetry, specific defects cannot be engineered and the spins cannot be directly imaged. The use of frustrated artificial nanostructures overcomes these problems through the tremendous versatility in array fabrication and compatibility with a suite of magnetic imaging techniques. Here I will show direct magnetic imaging studies of monopole defects [1-2] and magnetic charge flow. [3-4] The magnetic charge is carried by transverse domain walls and the chirality of the domain wall is found to control the direction of propagation. In addition to magnetic imaging studies of the magnetization state, I will also present magnetoresistance and Hall effect measurements. These techniques probe the array as a whole and can be very sensitive to the details of the spin structure. A change in symmetry in the Hall response of connected honeycomb nanostructures is observed at low temperatures indicating a collective response of the array of nanomagnets. [5] \\[4pt] [1] S. Ladak, D. E. Read, G. K. Perkins, L. F. Cohen {\&} W. R. Branford. Nature Physics 6, 359, (2010).\\[0pt] [2] S. Ladak, D. Read, T. Tyliszczak, W. R. Branford {\&} L. F. Cohen. New Journal of Physics 13, 023023, (2011).\\[0pt] [3] S. Ladak, S. K. Walton, K. Zeissler, T. Tyliszczak et al. New Journal of Physics 14, 045010, (2012).\\[0pt] [4] K. Zeissler, S. K. Walton, S. Ladak, D. E. Read et al. Sci. Rep. 3, 01252, (2013).\\[0pt] [5] W. R. Branford, S. Ladak, D. E. Read, K. Zeissler {\&} L. F. Cohen. Science 335, 1597, (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S39.00004: Resonant dynamics of topological magnetic structures Invited Speaker: Olle Heinonen A variety of topological magnetic structures have recently been observed and discussed in atomic structures. Examples are spin ices in pyrochlore lattices [1], or skyrmion lattices [2] in helical magnets, such as MnSi. Underlying these structures are competing interactions, which cannot all be simultaneously minimized. Patterned magnetic nanostructures can be engineered to have competing interactions that give rise to frustration, which can enable the formation of topological magnetic structures on the nanoscale and at room temperatures that can rather conveniently be observed [3-5]. In addition to interesting ground states or metastable states, the resonant dynamics of topological structures can be very interesting and different from the dynamics of the non-topological states [6-8]. This leads to the possibility of changing the resonant dynamics in magnetic system rather dramatically both in in frequency and space by small variations in a control parameter. In this introductory talk of the symposium, I will give examples of such states and the ensuing dynamics, and discuss possible future directions and applications. \\[4pt]Argonne National Laboratory is a US DOE Science Laboratory operated under contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC. \\[4pt] [1] M. J. Harris et al, Phys. Rev. Lett., {\bf79}, 2554 (1997).\newline [2] S. M\"uhlbauer et al., Science {\bf323}, 915 (2009). \newline [3] R.F. Wang et al.,, Nature {\bf439}, 303 (2006).\newline [4] L. Sun et al., Phys. Rev. Lett. {\bf110}, 167201 (2013).\newline [5] C. Phatak et al., Phys. Rev. B {\bf83}, 174431 (2011).\newline [6] S. Gliga et al, Phys. Rev. Lett. {\bf110}, 117205,(2013).\newline [7] I. Makhfudz, B. Krueger, and O. Tchernyshyov, Phys. Rev. Lett. {\bf109}, 217201 (2012).\newline [8] Y.Y Dai et al., Phys. Rev. B {\bf88}, 054403 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S39.00005: Two-dimensional artificial skyrmion crystals stabilized by nano-patterning Invited Speaker: Haifeng Ding The skyrmion crystal is a new material of current interest. It carries a topological charge and a Berry phase in real space and is anticipated to produce unconventional spin-electronic phenomena, such as the topological Hall effect and to exhibit spectacular dynamic properties. Technologically, skyrmion crystal may be exploited as a new class of spintronic material due to its unusual response to an electric charge current and spin current. A skyrmion crystal typically arises from helical spin structures induced by the Dzyaloshinskii--Moriya (DM) interaction. Experimentally, what has impeded its property exploration is that it is only to be found in few systems and within a narrow temperature and magnetic field range. In this talk, we present a practical design of a 2D skyrmion crystal, which completely by-passing the need for strong (or, indeed, any) DM interaction. The methodology is demonstrated with micromagnetic simulations and the computed skyrmion number per unit cell. The created skyrmion crystal has a robost working regime including room temperature, much broader than that for DM-driven skyrmion crystals. The method can dramatically widen the scope of the properties exploration and practical applications of the skyrmion crystal. In addition, from a more general point of view, previous experimental and theoretical studies of systems with DM interactions have already shown amply that the DM interaction is not sufficient for the spontaneous formation of a skyrmion crystal all by itself, since many systems with DM interaction do not display skyrmion-crystal self-assembly. Our method demonstrates that the DM interaction is not necessary either. [Preview Abstract] |
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