Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y24: Artificial Frustrated Spin SystemsFocus
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Sponsoring Units: GMAG DMP Chair: Oleg Tchernyshyov, Johns Hopkins Univ Room: LACC 403A |
Friday, March 9, 2018 11:15AM - 11:27AM |
Y24.00001: Fragmentation of magnetism in artificial kagome spin ice Nicolas Rougemaille, Benjamin Canals Artificial spin ices (ASI) are systems designed to explore the intriguing physics observed in magnetically frustrated materials. Generally fabricated by using lithography techniques, they offer almost infinite possibilities to construct a wide variety of spin models which can be accessed experimentally in a controlled manner. |
Friday, March 9, 2018 11:27AM - 11:39AM |
Y24.00002: Optimizing Thermalization Process for Square Artificial Spin Ice Xiaoyu Zhang, Peter Schiffer Artificial spin ice systems are arrays of two-dimensional nanoscale islands, each of which possesses a discrete magnetic moment pointing along easy axis, that are used to study collective behavior of interacting magnetic systems. Square artificial spin ice has a well-defined ground state, and previous studies have reported that high temperature annealing can thermalize these systems into that ground state. We used Magnetic Force Microscope (MFM) imaging of the magnetic moment configuration after thermal treatments to study the effectiveness of different annealing protocols of this system. By varying temperature profiles and island dimensions, we explored a set of parameters to optimize thermal treatment and study the thermalization effect for arrays of islands with varying thickness and lateral dimensions. This work can be extended to better thermalize other artificial spin ice systems, such as the kagome lattice and will help us to understand the underlying mechanism of the thermalization process. |
Friday, March 9, 2018 11:39AM - 12:15PM |
Y24.00003: Emergent dynamic chirality in a thermally driven artificial spin ratchet Invited Speaker: Sebastian Gliga The study of emergent phenomena in two-dimensional artificial spin ices is presently the focus of intense research. Artificial spin ices are composed of geometrically frustrated arrangements of nanomagnets and have so far mainly been used to investigate fundamental aspects of the physics of frustration. Recently, it has become clear that artificial spin ice has the potential to become a class of functional material with technological applications. I will present a spin ice based active material – consisting in a repeating pattern of chiral units – in which energy is converted into unidirectional dynamics, thus functioning like a ratchet [1]. Measurements performed using x-ray photoemission electron microscopy show that following saturation by an external field, thermal relaxation proceeds through the rotation of the average magnetization in a unique sense. Micromagnetic simulations demonstrate that this emergent chiral behavior is driven by the topology of the magnetostatic field at the boundaries of the nanomagnet array, resulting in an asymmetric energy landscape. This opens the possibility of implementing a Brownian ratchet, which may find applications in nanomotors, actuators or memory cells. I will also discuss other routes towards applications based on spin wave manipulation [2] as well as perspectives for functionalizing artificial spin ices in three dimensions in light of recent experimental advances in magnetic nanotomography [3]. |
Friday, March 9, 2018 12:15PM - 12:27PM |
Y24.00004: Spontaneous Domain Wall Fluctuation in Artificial Square Spin Ice Studied with Soft X-Ray Photon Correlation Spectroscopy Xiaoqian Chen, James Lee, Sujoy Roy, Barry Farmer, Justin Woods, Lance De Long, Jeffrey Hastings Artificial spin ice (ASI) systems, initially introduced to model spin ice systems in nature, are 2D array of frustrated dipolar-coupled nanomagnets. We performed soft x-ray photon correlation spectroscopy (XPCS) on the antiferromagnetic Bragg peak of thermally active square ASI and studied its domain wall dynamics as functions of temperature. Our result shows seconds-to-milliseconds timescale nucleation and annihilation of a single domain wall in the region under-study. We calculated the respective autocorrelation functions of domain fluctuation, whose characteristic curvatures indicates the distinct fluctuation time scale of the ASI systems that are coupled through dipole interaction. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y24.00005: Artificial spin ice assisted creation and tuning of frustrated flux quanta Yong-Lei Wang, Xiaoyu Ma, Jing Xu, Zhili Xiao, Boldizsar Janko, Wai-Kwong Kwok We report the controlling of geometric frustration in a novel hybrid system consisting of a reconfigurable artificial spin ice placed on top a superconducting thin film. The magnetic charge ground state of the artificial-spin-ice generates an ice-like “two attractive, two repulsive” pinning potential for the underlying flux-quanta in the superconducting film. This creates a 2D geometrically frustrated flux-quantum system which possesses extensive degeneracy. The geometric frustration in this flux-quantum system can be transformed in-situ into a variety of crystallized states with different symmetries. We achieved this by either changing the flux-quanta density with magnetic field or reconfiguring the magnetic charge ordering of the artificial-spin-ice. A tangible direct result of this study is a tunable superconducting critical current and a flux-quantum rectifier. |
Friday, March 9, 2018 12:39PM - 12:51PM |
Y24.00006: Temperature driven macro-ferromagnetism in 'pinwheel' artificial spin ice Rair Macedo, Gavin Macauley, Fabio Nascimento, Robert Stamps Artificial spin ice are patterned structure of arranged magnetic elements such that their interactions produce correlations and competitions that can be used to generate non-trivial dynamics and complex magnetic orderings. In this work, we investigate thermodynamic properties of geometries which are simple but fundamental variants of the original square lattice. We shown how a simple rotation of the elements induces a drastic change in the dipolar energy. |
Friday, March 9, 2018 12:51PM - 1:03PM |
Y24.00007: Visualization of Spontaneous Domain Wall Fluctuation in Artificial Spin Ice using Bragg Coherent Diffraction Imaging (BCDI) Xiaoqian Chen, Wen Hu, Xiaojing Huang, Justin Woods, Sujoy Roy, Jeffrey Hastings, Ian Robinson Inhomogeneity caused by a defect is known to play an important role in the emergence of long-range order and phase stability in strongly correlated systems. But their role in an artificial mesoscopic system is not very well understood. We studied spontaneous domain wall fluctuations in a dipolar-coupled antiferromagnetic (AF) artificial lattice for a wide temperature range by introducing defects to the magnetic lattice. BCDI reconstruction was performed using speckles under the half-integer AF peaks, revealing seconds-to-milliseconds timescale nucleation and annihilation of single domain wall. Our result shows that BCDI is an effective method of measuring fast fluctuation in the vicinity of a phase transition. |
(Author Not Attending)
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Y24.00008: Magnetic Diode-type Rectification in an Artificial Honeycomb Lattice of Ultra-small Elements Brock Summers, Deepak Singh, Ashutosh Dahal, Yiyao Chen The two-dimensional artificial magnetic honeycomb lattice system is evolving into a new research arena to explore a plethora of novel magnetism. We have created macroscopic samples of artificial magnetic honeycomb lattices of Permalloy having connected ultra-small elements (bonds), with length scales of sub-10 nm to 30 nm, which have never before been possible. The equivalent energy of the resulting systems is 10-100 K and is thus amenable to both temperature- and field-dependent exploration of novel magnetic phenomena. We have also performed detailed electrical measurements that have shown unidirectional electronic properties, analogous to a semiconductor diode. The unidirectional transport behavior, characterized by the asymmetric colossal enhancement in differential conductivity at a modest current application of 10-15 microamps, persists to T = 300 K in honeycomb lattice of thickness 6 nm. The asymmetric behavior arises without the application of magnetic field. A qualitative analysis of experimental data suggests the role of magnetic charge or monopoles in the unusual observations with strong implication for spintronics. |
Friday, March 9, 2018 1:15PM - 1:27PM |
Y24.00009: Abstract Withdrawn
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Friday, March 9, 2018 1:27PM - 1:39PM |
Y24.00010: Discontinuous phases and phase coexistence in an artificial Potts lattice Joseph Sklenar, Yuyang Lao, Justin Watts, Alan Albrecht, Cristiano Nisoli, Gia-Wei Chern, Peter Schiffer The study of artificial spin ice makes strong allusions to Ising-model-like physics. This is intuitive because the state of each ferromagnetic island can be described by a magnetic domain with a binary degree of freedom. To move past the Ising-paradigm one can strongly couple pairs of islands to act as a composite object. We have fabricated a checkerboard array of island pairs with alternating horizontal or vertical orientation. We find that the quadrupole moment at each lattice site can be used to describe an effective spin state. The extra degrees of freedom allow the system to be described by a Potts Hamiltonian similar to the Blume-Emery-Griffiths model. We study this system by thermalizing the lattice in the presence of an in-situ applied magnetic field. We observe, for the first time, a field-induced, discontinuous phase transition. Our experimental method allows us to globally reconfigure an array of nanomagnets to a state which would be difficult to write using even the most advanced on-demand writing protocols. |
Friday, March 9, 2018 1:39PM - 1:51PM |
Y24.00011: Spin Fluctuations and Correlations of XY Macrospins Robert Streubel, Noah Kent, Scott Dhuey, Andreas Scholl, Stephen Kevan, Peter Fischer Uniformly magnetized nano islands resembling macrospins have become prototypical systems for studying spin frustration and fluctuations at the microscale. The capability to mimic magnetic exchange via dipole-dipole interaction has been used to study artificial spin ice, and recently extended to colloidal spin glass. In view of spin liquids, we have investigated XY macrospins utilizing nanodisk systems with absent energy barriers, various thickness and spacing. We have used XPEEM to study fluctuations and correlations in planar square and honeycomb XY macrospin arrays with various geometries at remanence and at temperatures from 120-390K. The lack of an energy barrier in individual disks leads to a high sensitivity of the magnetization configuration on the lattice parameters and temperature, with fluctuation rates orders of magnitude larger than those of biaxial spin systems. We find experimental evidence of a Berezinskii-Thouless-Kosterlitz-like phase transition solely mediated by magnetostatics that favors a trigonal vortex lattice similar to Skyrmion lattices stabilized by the Dzyaloshinskii-Moriya interaction. Our results open a new avenue for artificial spin ices, spin liquids, Kitaev honeycomb lattices, magnonics. |
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