Artificial Spin Ice Lattice Research Articles

Stochastic hexagonal injectors in artificial spin ice

Artificial spin ice (ASI) systems have emerged as powerful platforms for exploring the fundamental aspects of magnetic frustration and topological phenomena in condensed matter physics. In this study, we investigate the bountiful effects that result from introducing hexagonal magnetic defects into an ASI lattice. The stochastic stabilisation of a plethora of metastable states in the hexagonal defects are explored, as well as harnessing the defect magnetisation state for the selective injection of emergent monopoles of different polarities and proximities within the lattice. We demonstrate a mechanism for tailoring the ASI behaviour using the magnetic state of the defect, which is of interest to applications including magnetic memory devices and spin-based logic.

Coupled magnetic nanostructures: Engineering lattice configurations

We present a systematic investigation of tunable magnetization dynamics of coupled magnetic nanostructures, arranged in one-dimensional arrays of horizontally and vertically coupled linear chains and in two-dimensional arrays of square artificial spin ice lattice. The spatial distribution of the demagnetization field is markedly sensitive to the lattice arrangement, leading to a significant modification of the collective behavior of static and dynamic properties of the arrays. Using ferromagnetic resonance spectroscopy, the engineering of demagnetizing factors with various lattice arrangements has been established quantitatively. The signature of distinct spin wave modes, spatially localized in the constituent nanomagnets, was observed and tuned by the lattice arrangements and applied field orientation. The experimental results are well complemented with micromagnetic simulations.

Observation of mode splitting in artificial spin ice: A comparative ferromagnetic resonance and Brillouin light scattering study

We report the dependence of the magnetization dynamics in a square artificial spin-ice lattice on the in-plane magnetic field angle. Using two complementary measurement techniques—broadband ferromagnetic resonance and micro-focused Brillouin light scattering spectroscopy—we systematically study the evolution of the lattice dynamics, for both a coherent radio frequency excitation and an incoherent thermal excitation of spin dynamics. We observe a splitting of modes facilitated by inter-element interactions that can be controlled by the external field angle and magnitude. Detailed time-dependent micromagnetic simulations reveal that the split modes are localized in different regions of the square network. This observation suggests that it is possible to disentangle modes with different spatial profiles by tuning the external field configuration.

Fabrication of hybrid artificial spin ice arrays with periodic site-specific local magnetic fields

We describe the microfabrication and magnetic behavior of a composite/hybrid, two-dimensional, magnetostatically interacting array of nanomagnets of Fe and exchange-biased bilayer Fe/IrMn heterostructures. Such an interacting array of nanomagnets, forming an artificial spin ice lattice but with a hybrid structure, has not been demonstrated before. These devices are fabricated of epitaxially grown Fe/IrMn thin films by a two-stage electron beam lithography process involving metal mask transfer and controlled ion milling. Following the epitaxial deposition of Fe/IrMn bilayer films, the first step involves electron beam lithography fabrication of nanomagnet arrays, followed by selective removal of exchange-bias by etching away IrMn layer at specific nanomagnet elements by ion milling. The technique described provides a way to apply a site-specific magnetic field at the nanometer length scale, utilizing the phenomenon of exchange-bias, as demonstrated here for an array with local fields applied at twice the period of the artificial spin ice lattice. This technology can also be readily extended to different spintronic devices requiring spatial distribution of exchange-bias fields.

Emergent Dynamics of Artificial Spin-Ice Lattice Based on an Ultrathin Ferromagnet.

We present high-frequency dynamics of magnetic nanostructure lattices, fabricated in the form of "artificial spin-ice", that possess magnetically frustrated states. Dynamics of such structures feature multiple resonance excitation that reveals rich and intriguing microwave characteristics, which are highly dependent on field-cycle history. Geometrical parameters such as dimensions and ferromagnetic layer thickness, which control the interplay of different demagnetizing factors, are found to play a pivotal role in governing the dynamics. Our findings are highlighted by the evolution of unique excitations pertaining to magnetic frustration, which are well supported by static magnetometry studies and micromagnetic simulations.

Coupled macrospins: Mode dynamics in symmetric and asymmetric vertices

We report the microwave response of symmetric and asymmetric threefold clusters with nearly contacting segments that can serve as the node in a Kagome artificial spin ice lattice. The structures are patterned on a coplanar waveguide and consist of elongated and nearly-contacting ellipses with uniform thickness. Branches of the ferromagnetic resonance spectra display mode softening that correlates well with the calculations, whereas agreement between the measured and simulated static magnetization is more qualitative.

Monte Carlo simulation on a new artificial spin ice lattice consisting of hexagons and three-moment vertices

A new artificial spin ice lattice called vortex lattice is proposed based on the Kagome lattice. Monte Carlo simulations were performed to investigate the magnetization reversal process of the new artificial spin ice lattice at external magnetic field and different lattice parameters. The results demonstrate some interesting phenomena which are different from Kagome lattice. There are four typical sub-structures emerged in the vortex lattice, which are clockwise and counter-clockwise hexagons, and frustrated +3q and -3q vertices. The occurrence frequency of the four sub-structures change dramatically at different lattice parameter. The new lattice can be partially frustrated at different lattice parameter.

Observation of ice-rule violation and monopole dynamics via edge nucleation of domain walls in artificial spin ice lattice

In a patterned Co honeycomb spin ice structure, we show that violation in the ice-rule or magnetic monopoles, can be observed during a magnetization reversal process in 430Oe≤H≤760Oe magnetic field (H) range. The monopoles are shown to originate from the nucleation of domain walls at the edges, and they hop towards the other edge via the propagation of magnetic domain walls. The paths that the domain walls traveled or the Dirac strings, are shown to increase in length with magnetic fields increment and no random flipping of the bars are observed in the structure.

Direct observation of deterministic domain wall trajectory in magnetic network structures.

Controlling the domain wall (DW) trajectory in magnetic network structures is crucial for spin-based device related applications. The understanding of DW dynamics in network structures is also important for study of fundamental properties like observation of magnetic monopoles at room temperature in artificial spin ice lattice. The trajectory of DW in magnetic network structures has been shown to be chirality dependent. However, the DW chirality periodically oscillates as it propagates a distance longer than its fidelity length due to Walker breakdown phenomenon. This leads to a stochastic behavior in the DW propagation through the network structure. In this study, we show that the DW trajectory can be deterministically controlled in the magnetic network structures irrespective of its chirality by introducing a potential barrier. The DW propagation in the network structure is governed by the geometrically induced potential barrier and pinning strength against the propagation. This technique can be extended for controlling the trajectory of magnetic charge carriers in an artificial spin ice lattice.

Spectral analysis of topological defects in an artificial spin-ice lattice.

Arrays of suitably patterned and arranged magnetic elements may display artificial spin-ice structures with topological defects in the magnetization, such as Dirac monopoles and Dirac strings. It is known that these defects strongly influence the quasistatic and equilibrium behavior of the spin-ice lattice. Here, we study the eigenmode dynamics of such defects in a square lattice consisting of stadiumlike thin film elements using micromagnetic simulations. We find that the topological defects display distinct signatures in the mode spectrum, providing a means to qualitatively and quantitatively analyze monopoles and strings that can be measured experimentally.

Propagation of magnetic charge monopoles and Dirac flux strings in an artificial spin-ice lattice

We systematically investigate magnetic reversal of permalloy islands in a square spin-ice geometry with in situ Lorentz microscopy. Differential phase imaging reveals the presence of a flux channel similar to a Dirac string between the magnetic charge monopoles during the reversal. Analysis of the phase images shows that positively and negatively charged monopoles always move together with the flux channel. Statistical analysis of monopole populations and system correlations shows the emergence of a highly frustrated state for low magnetizations. This state is explained by a strong influence of charge ordering, which limits monopole densities.

Direct observation and control of magnetic monopole defects in an artificial spin-ice material

Magnetic monopoles have stimulated a great amount of theoretical and experimental interest since their prediction by Dirac in 1931. To date, their presence has evaded detection in high energy experiments despite intensive efforts. Recently, entities that mimic magnetic monopoles have been observed in bulk and planar frustrated materials known as spin-ice materials, and artificial spin-ice materials, respectively. In this paper we discuss the formation of these so-called monopole defects within a cobalt honeycomb artificial spin-ice lattice. Experimental results and micromagnetic simulations show that monopole defects of opposite sign are created at the boundaries of the lattice, and move in opposing directions. Discrepancies between simulations and experimental results demonstrate the importance of quenched disorder. Furthermore, we show that controlled edge nucleated monopole defect formation can be realized with the use of soft magnetic injection pads, which is a very promising development for technological applications based upon magnetic charge.