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Register a new userObservation of Coherent Spin Waves in a Three-Dimensional Artificial Spin Ice Structure
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Harnessing high-frequency spin dynamics in three-dimensional (3D) nanostructures may lead to paradigm-shifting, next generation devices including high density spintronics and neuromorphic systems. Despite remarkable progress in fabrication, the measurement and interpretation of spin dynamics in complex 3D structures remain exceptionally challenging. Here we take a first step and measure coherent spin waves within a 3D artificial spin ice (ASI) structure using Brillouin light scattering. The 3D-ASI was fabricated by using a combination of two-photon lithography and thermal evaporation. Two spin-wave modes were observed in the experiment whose frequencies showed a monotonic variation with the applied field strength. Numerical simulations qualitatively reproduced the observed modes. The simulated mode profiles revealed the collective nature of the modes extending throughout the complex network of nanowires while showing spatial quantization with varying mode quantization numbers. The study shows a well-defined means to explore high-frequency spin dynamics in complex 3D spintronic and magnonic structures.
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Artificial spin ice systems have seen burgeoning interest due to their intriguing physics and potential applications in reprogrammable memory, logic and magnonics. In-depth comparisons of distinct artificial spin systems are crucial to advancing the field and vital work has been done on characteristic behaviours of artificial spin ices arranged on different geometric lattices. Integration of artificial spin ice with functional magnonics is a relatively recent research direction, with a host of promising early results. As the field progresses, studies examining the effects of lattice geometry on the magnonic response are increasingly significant. While studies have investigated the effects of different lattice tilings such as square and kagome (honeycomb), little comparison exists between systems comprising continuously-connected nanostructures, where spin-waves propagate through the system via exchange interaction, and systems with nanobars disconnected at vertices where spin-waves are transferred via stray dipolar-field. Here, we perform a Brillouin light scattering study of the magnonic response in two kagome artificial spin ices, a continuously-connected system and a disconnected system with vertex gaps. We observe distinctly different high-frequency dynamics and characteristic magnetization reversal regimes between the systems, with key distinctions in system microstate during reversal, internal field profiles and spin-wave mode quantization numbers. These observations are pertinent for the fundamental understanding of artificial spin systems and the design and engineering of such systems for functional magnonic applications.
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, both for a coherent radiofrequency 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.
Artificial square spin ices are structures composed of magnetic elements arranged on a geometrically frustrated lattice and located on the sites of a two-dimensional square lattice, such that there are four interacting magnetic elements at each vertex. Using a semi-analytical approach, we show that square spin ices exhibit a rich spin wave band structure that is tunable both by external magnetic fields and the configuration of individual elements. Internal degrees of freedom can give rise to equilibrium states with bent magnetization at the edges leading to characteristic excitations; in the presence of magnetostatic interactions these form separate bands analogous to impurity bands in semiconductors. Full-scale micromagnetic simulations corroborate our semi-analytical approach. Our results show that artificial square spin ices can be viewed as reconfigurable and tunable magnonic crystals that can be used as metamaterials for spin-wave-based applications at the nanoscale.
Band topology, or global wave-function structure that enforces novel properties in the bulk and on the surface of crystalline materials, is currently under intense investigations for both fundamental interest and its technological promises. While band crossing of non-trivial topological nature was first studied in three dimensions for electrons, the underlying physical idea is not restricted to fermionic excitations. In fact, experiments have confirmed the possibility to have topological band crossing of electromagnetic waves in artificial structures. Fundamental bosonic excitations in real crystals, however, have not been observed to exhibit the counterpart under ambient pressure and magnetic field, where the difficulty is in part because natural materials cannot be precisely engineered like artificial structures. Here, we use inelastic neutron scattering to reveal the presence of topological spin excitations (magnons) in a three-dimensional antiferromagnet, Cu3TeO6, which features a unique lattice of magnetic spin-1/2 Cu2+ ions. Beyond previous understanding, we find that the materials spin lattice possesses a variety of exchange interactions, with the interaction between the ninth-nearest neighbours being as strong as that between the nearest neighbours. Although theoretical analysis indicates that the presence of topological magnon band crossing is independent of model details, Cu3TeO6 turns out to be highly favourable for the experimental observation, as its optical magnons are spectrally sharp and intense due to the highly interconnected spin network and the large magnetic cell. The observed magnon band crossing generally has the form of a special type of Z2-topological nodal lines that are yet to be found in fermion systems, rendering magnon systems a fertile ground for exploring novel band topology.
Artificial spin ices are ensembles of geometrically-arranged, interacting nanomagnets which have shown promising potential for the realization of reconfigurable magnonic crystals. Such systems allow for the manipulation of spin waves on the nanoscale and their potential use as information carriers. However, there are presently two general obstacles to the realization of artificial spin ice-based magnonic crystals: the magnetic state of artificial spin ices is difficult to reconfigure and the magnetostatic interactions between the nanoislands are often weak, preventing mode coupling. We demonstrate, using micromagnetic modeling, that coupling a reconfigurable artificial spin ice geometry made of weakly interacting nanomagnets to a soft magnetic underlayer creates a complex system exhibiting dynamically coupled modes. These give rise to spin wave channels in the underlayer at well-defined frequencies, based on the artificial spin ice magnetic state, which can be reconfigured. These findings open the door to the realization of reconfigurable magnonic crystals with potential applications for data transport and processing in magnonic-based logic architectures.
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