Emerging Chirality in Artificial Spin Ice

Artificial spin ice, made up of planar nanostructured arrays of simple ferromagnetic bars, is a playground for rich physics associated with the spin alignment of the bars and spin texture associated with the magnetic frustration at the bar vertices. The phase diagram is exotic, showing magnetic monopole-like defects and liquid and solid phases of spins arranged in loop states with predicted chiral order. We show that magnetotransport measurements in connected honeycomb structures yield the onset of an anomalous Hall signal at 50 kelvin. The temperature scale can be attributed to the long-range dipolar ice phase. The topological Hall signal arises because chiral loops form at the sample edges, indicating a generic route to exotic states via nanoarray edge structure

Dynamic dependence to domain wall propagation through artificial spin ice

Domain wall propagation dynamics has been studied in nanostructured artificial kagome spin-ice structures. A stripline circuit has been used to provide localized pulsed magnetic fields within the artificial spin-ice (ASI) structure. This provides control of the system through electrically assisted domain wall nucleation events. Synchronization of the pulsed fields with additional global magnetic fields and the use of a focused magneto-optical Kerr effect magnetometer allows our experiments to probe the domain wall transit through an extended ASI structure. We find that the propagation distance depends on the driving field revealing field-driven properties of domain walls below their intrinsic nucleation field

Spectral-fingerprinting: microstate readout via remanence ferromagnetic resonance in artificial spin systems

Artificial spin ices (ASIs) are magnetic metamaterials comprising geometrically tiled strongly-interacting nanomagnets. There is significant interest in these systems spanning the fundamental physics of many-body systems to potential applications in neuromorphic computation, logic, and recently reconfigurable magnonics. Magnonics focused studies on ASI have to date have focused on the in-field GHz spin-wave response, convoluting effects from applied field, nanofabrication imperfections ('quenched disorder') and microstate-dependent dipolar field landscapes. Here, we investigate zero-field measurements of the spin-wave response and demonstrate its ability to provide a 'spectral fingerprint' of the system microstate. Removing applied field allows deconvolution of distinct contributions to reversal dynamics from the spin-wave spectra, directly measuring dipolar field strength and quenched disorder as well as net magnetisation. We demonstrate the efficacy and sensitivity of this approach by measuring ASI in three microstates with identical (zero) magnetisation, indistinguishable via magnetometry. The zero-field spin-wave response provides distinct spectral fingerprints of each state, allowing rapid, scaleable microstate readout. As artificial spin systems progress toward device implementation, zero-field functionality is crucial to minimize the power consumption associated with electromagnets. Several proposed hardware neuromorphic computation schemes hinge on leveraging dynamic measurement of ASI microstates to perform computation for which spectral fingerprinting provides a potential solution

The non-random walk of chiral magnetic charge carriers in artificial spin ice

The flow of magnetic charge carriers (dubbed magnetic monopoles) through frustrated spin ice lattices, governed simply by Coulombic forces, represents a new direction in electromagnetism. Artificial spin ice nanoarrays realise this effect at room temperature, where the magnetic charge is carried by domain walls. Control of domain wall path is one important element of utilizing this new medium. By imaging the transit of domain walls across different connected 2D honeycomb structures we contribute an important aspect which will enable that control to be realized. Although apparently equivalent paths are presented to a domain wall as it approaches a Y-shaped vertex from a bar parallel to the field, we observe a stark non-random path distribution, which we attribute to the chirality of the magnetic charges. These observations are supported by detailed statistical modelling and micromagnetic simulations. The identification of chiral control to magnetic charge path selectivity invites analogy with spintronics

Limitations in artificial spin ice path selectivity: the challenges beyond topological control

Magnetic charge is carried through nanowire networks by domain walls, and the micromagnetic structure of a domain wall provides an opportunity to manipulate its movement. We have shown previously that magnetic monopole defects exist in artificial spin ice (ASI) and result from two bar switching at a vertex. To create and manipulate monopole defects and indeed magnetic charge in general, path selectivity of the domain wall at a vertex is required. We have recently shown that in connected ASI structures, transverse wall chirality (or topology) determines wall path direction, but a mechanism known as Walker breakdown, where a wall mutates into a wall of opposite chirality partially destroys selectivity. Recently it has been claimed that in isolated Y-shaped junctions that support vortex walls, selectivity is entirely determined by chirality (or topology), the suggestion being that vortex wall chirality is robust in the Walker breakdown process. Here we demonstrate that in Y-shaped junctions, magnetic switching in the important topologically protected regime exists only for a narrow window of field and bar geometry, and that it will be challenging to access this regime in field-driven ASI. This work has implications for the wider field of magnetic charge manipulation for high density memory storage

Current-controlled nanomagnetic writing for reconfigurable magnonic crystals

Strongly-interacting nanomagnetic arrays are crucial across an ever-growing suite of technologies. Spanning neuromorphic computing, control over superconducting vortices and reconfigurable magnonics, the utility and appeal of these arrays lies in their vast range of distinct, stable magnetization states. Different states exhibit different functional behaviours, making precise, reconfigurable state control an essential cornerstone of such systems. However, few existing methodologies may reverse an arbitrary array element, and even fewer may do so under electrical control, vital for device integration. We demonstrate selective, reconfigurable magnetic reversal of ferromagnetic nanoislands via current-driven motion of a transverse domain wall in an adjacent nanowire. The reversal technique operates under all-electrical control with no reliance on external magnetic fields, rendering it highly suitable for device integration across a host of magnonic, spintronic and neuromorphic logic architectures. Here, the reversal technique is leveraged to realize two fully solid-state reconfigurable magnonic crystals, offering magnonic gating, filtering, transistor-like switching and peak-shifting without reliance on global magnetic fields

Low-power continuous-wave all-optical magnetic switching in ferromagnetic nanoarrays

All-optical magnetic switching promises ultrafast magnetization control without a magnetic field. Existing schemes typically require power-hungry femtosecond-pulsed lasers and complex magnetic materials. Here, we demonstrate deterministic, all-optical magnetic switching in simple ferromagnetic nanomagnets (Ni81Fe19, Ni50Fe50) with sub-diffraction limit dimensions using a focused low-power, linearly polarized continuous-wave laser. Isolated nanomagnets are switched across a range of dimensions, laser wavelengths, and powers. All square-geometry artificial spin ice vertex configurations are written at low powers (2.74 mW). Usually, switching with linearly polarized light is symmetry forbidden; here, the laser spot has a similar size to the nanomagnets, producing an absorption distribution that depends on the nanoisland-spot displacement. We attribute the deterministic switching to the transient dynamics of this asymmetric absorption. No switching is observed in Co or Ni nanostructures, suggesting the multi-species nature of NiFe plays a role. These results usher in inexpensive, low-power, optically controlled devices with impact across data storage, neuromorphic computation, and reconfigurable magnonics

Signatures of filamentary superconductivity in antiferromagnetic BaFe2As2 single crystals

In this paper, we present ac susceptibility and magnetotransport measurements on aged single crystals of the ferropnictide parent compound, BaFe2As2 with a paramagnetic-to-antiferromagnetic transition temperature of 134 K. The ac susceptibility shows the clear onset of a partial diamagnetic response with an onset temperature, commensurate with a subtle downturn in resistivity at approximately 20 K. Below 20 K the magnetotransport shows in-plane anisotropy, magnetic-field history dependence and a hysteretic signature. Above 20 K the crystals show the widely reported high-field linear magnetoresistance. An enhanced noise signature in ac susceptibility is observed above 20 K, which varies in character with amplitude and frequency of the ac signal. The hysteresis in magnetoresistance and the observed sensitivity of the superconducting phase to the amplitude of the ac signal are indicative characteristics of granular or weakly linked filamentary superconductivity. These features taken together with the observed noise signature above $T_{\mathrm{c}}$ suggests a link between the formation of the superconducting filamentary phase and the freezing of antiphase domain walls, known to exist in these materials

Realization of ground state in artificial kagome spin ice via topological defect-driven magnetic writing

Arrays of non-interacting nanomagnets are widespread in data storage and processing. As current technologies approach fundamental limits on size and thermal stability, enhancing functionality through embracing the strong interactions present at high array densities becomes attractive. In this respect, artificial spin ices are geometrically frustrated magnetic metamaterials that offer vast untapped potential due to their unique microstate landscapes, with intriguing prospects in applications from reconfigurable logic to magnonic devices or hardware neural networks. However, progress in such systems is impeded by the inability to access more than a fraction of the total microstate space. Here, we demonstrate that topological defect-driven magnetic writing—a scanning probe technique—provides access to all of the possible microstates in artificial spin ices and related arrays of nanomagnets. We create previously elusive configurations such as the spin-crystal ground state of artificial kagome dipolar spin ices and high-energy, low-entropy ‘monopole-chain’ states that exhibit negative effective temperatures