Accessing low-energy magnetic microstates in square artificial spin ice vertices of broken symmetry in static magnetic field (original) (raw)
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AIP Advances
We have investigated the micro-magnetic behavior as well as magnetization reversal of dipolar coupled magnetic nanoislands with strong shape anisotropy arranged in a square artificial spin ice geometry. Our 0 K-temperature simulation results show that metastable two-in two-out state is stabilized at remanence of regular configurations. A complex interplay of defects and dipolar interaction leads to a predictable three-in one-out or three-out one-in higher energy state. Switching of the magnetic states is defined by the reduction of the no. of high-energy head-to-head or tail-to tail magnetic state.
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.
Efficient demagnetization protocol for the artificial triangular spin ice
Applied Physics Letters, 2013
In this work we study demagnetization protocols for an artificial spin ice in a triangular geometry. Our results show that a simple hysteresis-like process is very efficient in driving the system to its ground state, even for a relatively strong disorder in the system, confirming previous expectations. In addition, transitions between the magnetized state and the ground state were observed to be mediated by the creation and propagation of vertices that behave like magnetic monopoles pseudo-particles. This is an important step towards a more detailed experimental study of monopole-like excitations in artificial spin ice systems.
Sculpting the spin-wave response of artificial spin ice via microstate selection
Physical Review B, 2019
Artificial spin ice (ASI) systems have emerged as promising hosts for magnonic applications due to a correspondence between their magnetic configuration and spin dynamics. Though it has been demonstrated that spin-wave spectra are influenced by the ASI microstate the precise nature of this relationship has remained unclear. Recent advances in controlling the magnetic configuration of ASI make harnessing the interplay between spin dynamics and the microstate achievable. This could allow diverse applications including reconfigurable magnonic crystals and programmable microwave filters. However, extracting any novel functionality requires a full understanding of the underlying spin wave/microstate interaction. Here, we present a systematic analysis of how the microstate of a honeycomb ASI system affects its spin-wave spectrum through micromagnetic simulations. We find the spectrum to be highly tunable via the magnetic microstate, allowing the (de)activation of spinwave modes and bandgap tuning via magnetic reversal of individual nano-islands. Symmetries of ASI systems and the chirality of "monopole" defects are found to play important roles in determining the high-frequency magnetic response.
Emergent ice rule and magnetic charge screening from vertex frustration in artificial spin ice
2014
Artificial spin ice comprises a class of frustrated arrays of interacting single-domain ferromagnetic nanostructures. Previous studies of artificial spin ice have focused on simple lattices based on natural frustrated materials. Here we experimentally examine artificial spin ice created on the shakti lattice, a structure that does not directly correspond to any known natural magnetic material. On the shakti lattice, none of the near-neighbour interactions is locally frustrated, but instead the lattice topology frustrates the interactions leading to a high degree of degeneracy. We demonstrate that the shakti system achieves a physical realization of the classic six-vertex model ground state. Furthermore, we observe that the mixed coordination of the shakti lattice leads to crystallization of e ective magnetic charges and the screening of magnetic excitations, underscoring the importance of magnetic charge as the relevant degree of freedom in artificial spin ice and opening new possibilities for studies of its dynamics.
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.
Observation of magnetic fragmentation in spin ice
Nature Physics, 2016
Fractionalized excitations that emerge from a many-body system have revealed rich physics and concepts, from composite fermions in two-dimensional electron systems, revealed through the fractional quantum Hall e ect 1 , to spinons in antiferromagnetic chains 2 and, more recently, fractionalization of Dirac electrons in graphene 3 and magnetic monopoles in spin ice 4. Even more surprising is the fragmentation of the degrees of freedom themselves, leading to coexisting and a priori independent ground states. This puzzling phenomenon was recently put forward in the context of spin ice, in which the magnetic moment field can fragment, resulting in a dual ground state consisting of a fluctuating spin liquid, a so-called Coulomb phase 5 , on top of a magnetic monopole crystal 6. Here we show, by means of neutron scattering measurements, that such fragmentation occurs in the spin ice candidate Nd 2 Zr 2 O 7. We observe the spectacular coexistence of an antiferromagnetic order induced by the monopole crystallization and a fluctuating state with ferromagnetic correlations. Experimentally, this fragmentation manifests itself through the superposition of magnetic Bragg peaks, characteristic of the ordered phase, and a pinch point pattern, characteristic of the Coulomb phase. These results highlight the relevance of the fragmentation concept to describe the physics of systems that are simultaneously ordered and fluctuating. The physics of spin ice materials is intimately connected with the pyrochlore lattice, composed of corner-sharing tetrahedra. On the corners of these tetrahedra reside rare-earth magnetic moments J i , which, as a consequence of the strong crystal electric field, are constrained to point along their local trigonal axes z i , and behave like Ising spins. The magnetic interactions are composed of nearestneighbour exchange J and dipolar interactions between spins i and j separated by a distance r ij (ref.
Experimental and theoretical evidences for the ice regime in planar artificial spin ices
Journal of Physics: Condensed Matter, 2018
Geometry induced dynamics yields remarkable physical phenomena. In the macrocosms, the curvature of spacetime (geometrodynamics) tells matter how to move (gravity). In the microcosms, an example is the geometrical frustration in magnetic materials, whereas under certain conditions, can lead to the formation of spin liquids, in which the constituent spins still fluctuate strongly down to a temperature of absolute zero. In this work, we would like to explore a geometrical effect in artificial spin ices (ASI). It is well known that, in general, such artificial materials are athermal because they are constructed with elongated nanomagnets containing a large number of atomic spins, generating a big net magnetic moment that need a great amount of energy to flip. Therefore, recently, thermally driven dynamics in ASI materials became an important subject of investigation. We then expand this picture by showing that geometrically driven dynamics in ASI can open up the panorama of exploring distinct ground states and thermally magnetic monopole excitations. Here, it is shown that a particular ASI lattice, whereas four spins meet at every vertex, will provide a richer thermodynamics only due to its geometry. Indeed, for all kinds of planar ASI geometries, with ground states obeying the familiar 'two-in, two-out' ice rule in each vertex, the nanomagnets spin will experience less restriction to flip precisely in a kind of rhombic lattice. This can be observed by analysing only three types of rectangular artificial spin ices (RASI). Denoting the horizontal and vertical lattice spacings by a and b, respectively, then, a RASI material can be described by its aspect ratio γ ≡ a/b. The rhombic lattice emerges when γ = √ 3. So, by comparing the impact of thermal effects on the spin flips in these three appropriate different RASI arrays, it is possible to find the phenomenon we call ASI geometrothermodynamic. The comparison is done among RASI with γ = √ 2, γ = γR = √ 3 and γ = √ 4. The experimental data and the direct imaging of individual nanomagnets and their magnetization are obtained, as a function of temperature, by using Photoemission Electron Microscopy (P EEM) combined with X-ray Magnetic Circular Dichroism (XM CD) and Magneto-optic Kerr effect (M OKE) measurements. Our experimental data corroborates the unusual behavior of the critical temperatures in the RASI materials investigated here, as predicted by our Monte Carlo simulations.
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.