Magnetization reversal and emergent magnetic monopole-like state in square artificial spin ice vertex with defects (original) (raw)
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Physical Review B
In artificial spin ice systems, an interplay of defects and dipolar interactions is expected to play important roles in stabilizing different collective magnetic states. In this work, we investigated the magnetization reversal of individual defective square artificial spin ice vertices where defects break fourfold rotational symmetry of the system. By varying the angle between the applied field and the geometrical axis of individual vertices, we are able to access different energy landscapes of the system. Thus, we find a tuning parameter to stabilize different collective magnetic configurations of a defective vertex. Interesting among them is the collective low-energy state which is otherwise observed in a fully demagnetized state. Micromagnetic simulations are performed for the exact experimental conditions the results of which are consistent with the experimental data.
Journal of Applied Physics
We report here the results of micromagnetic simulations of square artificial spin ice (ASI) systems with defects. The defects are introduced by misaligning of a nanomagnet at the vertex. In these defective systems, we are able to stabilize emergent monopole-like state by applying a small external field. We observe a systematic change of dipolar energies of the systems with varying misalignment angle. The fields at which the emergent monopoles are created vary linearly with the dipolar energies of the systems. Our results clearly show that the magnetization reversal of the ASI systems is intricately related to the interplay of defects and dipolar interactions.
Magnetic interactions and reversal of artificial square spin ices
New Journal of Physics, 2012
Artificial spin ices are nanoscale geometrically engineered systems that mimic the behavior of bulk spin ices at room temperature. We describe the nanoscale magnetic interactions in a square spin ice lattice by an experimentally verified model that accounts for the correct shape of the magnetic islands. Magnetic force microscopy measurements on lithographically fabricated lattices are compared to Monte Carlo simulations of the reversal process of two lattices with different lattice spacings. Lattice node statistics and correlations show significant differences in the reversal mechanism for lattices with different spacings. The effect of structural variations is also compared for the two lattice reversals.
Heavy and light monopoles in magnetic reversion in artificial spin ice
Current Applied Physics, 2013
This work makes a theoretical study of the dynamics of emergent elemental excitations in artificial spin ice systems with hexagonal geometry during the magnetic reversion of the system. The magnetic and physical parameters of the nanoislands that form the array are considered as variables in the study. The parameters considered are: the energy barrier for the inversion of each nanoisland, the magnetic moment of the nanomagnets and the possible disorder in the sample. Our results show that the reversion dynamic presents two distinct mechanisms of magnetic reversion, with different elemental excitations for each mechanism. The first mechanism presents a reversion with the appearance of magnetic monopoles that do not move in the samples (heavy monopoles) and the absence of Dirac chains. In the other mechanism elemental magnetic excitations (light monopoles) appear that move great distances in the sample, giving rise to extensive Dirac chains during the magnetic reversion.
Conditions for free magnetic monopoles in nanoscale square arrays of dipolar spin ice
Physical Review B, 2010
We study a modified frustrated dipolar array recently proposed by Möller and Moessner [Phys. Rev. Lett. 96, 237202 (2006)], which is based on an array manufactured lithographically by Wang et al. [Nature (London) 439, 303 (2006)] and consists of introducing a height offset h between islands (dipoles) pointing along the two different lattice directions. The ground-states and excitations are studied as a function of h. We have found, in qualitative agreement with the results of Möller and Moessner, that the ground-state changes for h > h1, where h1 = 0.444a (a is the lattice parameter or distance between islands). In addition, the excitations above the ground-state behave like magnetic poles but confined by a string, whose tension decreases as h increases, in such a way that for h ≈ h1 its value is around 20 times smaller than that for h = 0. The system exhibits an anisotropy in the sense that the string tension and magnetic charge depends significantly on the directions in which the monopoles are separated. In turn, the intensity of the magnetic charge abruptly changes when the monopoles are separated along the direction of the longest axis of the islands. Such a gap is attributed to the transition from the anti to the ferromagnetic ground-state when h = h1.
Scientific Reports
Magnetic analogue of an isolated free electric charge, i.e., a magnet with a single north or south pole, is a long sought-after particle which remains elusive so far. In magnetically frustrated pyrochlore solids, a classical analogue of monopole was observed as a result of excitation of spin ice vertices. Direct visualization of such excitations were proposed and later confirmed in analogous artificial spin ice (ASI) systems of square as well as Kagome geometries. However, such magnetically charged vertices are randomly created as they are thermally driven and are always associated with corresponding equal and opposite emergent charges, often termed as monopole–antimonopole pairs, connected by observable strings. Here, we demonstrate a controlled stabilisation of a robust isolated emergent monopole-like magnetically charged vertices in individual square ASI systems by application of an external magnetic field. The excitation conserves the magnetic charge without the involvement of a...
Topology by Design in Magnetic Nano-materials: Artificial Spin Ice
Springer Series in Solid-State Sciences, 2018
Artificial Spin Ices are two dimensional arrays of magnetic, interacting nano-structures whose geometry can be chosen at will, and whose elementary degrees of freedom can be characterized directly. They were introduced at first to study frustration in a controllable setting, to mimic the behavior of spin ice rare earth pyrochlores, but at more useful temperature and field ranges and with direct characterization, and to provide practical implementation to celebrated, exactly solvable models of statistical mechanics previously devised to gain an understanding of degenerate ensembles with residual entropy. With the evolution of nano-fabrication and of experimental protocols it is now possible to characterize the material in real-time, real-space, and to realize virtually any geometry, for direct control over the collective dynamics. This has recently opened a path toward the deliberate design of novel, exotic states, not found in natural materials, and often characterized by topological properties. Without any pretense of exhaustiveness, we will provide an introduction to the material, the early works, and then, by reporting on more recent results, we will proceed to describe the new direction, which includes the design of desired topological states and their implications to kinetics.
Spin wave spectral probing of degenerate microstates in building-block of square artificial spin ice
AIP Advances
We have investigated the spin wave modes of dipolar coupled, highly shape anisotropic magnetic nanoislands forming square artificial spin ice system by performing micromagnetic simulations using MUMAX3 in combination with Matlab coding. Here, artificial spin ice is considered to be formed by the four identical square ring-type structure of elliptical shaped nanoislands of permalloy. Our results demonstrate a direct relation between the spin wave modes generated and the micro-states formed in the system. Thus, we show that single ring type structure may alone be adequately used to understand the spin wave modes of square artificial spin ice.
Dynamics of Magnetic Charges in Artificial Spin Ice
Physical Review Letters, 2010
Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic magnetic wires. We propose a theoretical model of magnetization dynamics in artificial spin ice under the action of an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge. They are emitted by lattice junctions when the the local field exceeds a critical value Hc required to pull apart magnetic charges of opposite sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches in magnetization reversal.