Session Y45: Frontiers in Artificial Spin Ice

Artificial Spin Ices: Magnetization Dynamics and X-ray Orbital Angular Momentum Generation

Artificial spin ices (ASI) are patterned arrays of nanoscale, ferromagnetic elements that can serve as model systems and are also being considered for applications in energy-efficient computing.  Our group has studied permalloy, square ASIs with and without programmed topological defects using resonant soft X-ray techniques.  These include X-ray photon correlation spectroscopy, Bragg coherent diffraction imaging, and X-ray photoemission electron microscopy.  These studies revealed the importance of superdomain-wall nucleation, annihilation, and motion for understanding equilibrium fluctuations of ASIs near room temperature.  They also showed that square ASIs with even-order topological defects can equilibrate into single-domain antiferromagnetic ground states that impart orbital angular momentum (OAM) to scattered X-rays.  The X-ray OAM beams can be switched on and off with changes in temperature and applied magnetic field and may prove to be a first step toward reconfigurable X-ray optics.  In addition to affecting X-ray OAM generation, the charge of the topological defect also influences both frustration and fluctuations of the ASIs. 

This work is the product of a collaboration among researchers including J. Woods^1,2, X. Chen^1,3,5 B. Farmer¹, S. Dhuey³, R. Chopdekar³, R. Koch³, A. Scholl³, S. Kevan³, A. Tremsin⁴, C. Mazzoli⁵, S. Wilkins⁵, W. Hu⁵, A. Barbour⁵, I. Robinson⁵, W. Kwok², L. De Long¹, and S. Roy³.

¹ University of Kentucky, ² Argonne National Laboratory, ³ Lawrence Berkeley National Laboratory, ⁴ University of California, Berkeley, and ⁵ Brookhaven National Laboratory   

Qubit spin ice

Artificial spin ices are frustrated spin systems that can be engineered, in which fine tuning of geometry and topology has allowed the design and characterization of exotic emergent phenomena at the constituent level. Here, we report a realization of spin ice in a programmable lattice of superconducting qubits in a quantum annealing processor. Unlike conventional artificial spin ice, our system is disordered by both quantum and thermal fluctuations. The ground state is classically described by the ice rule, and we achieved control over a fragile degeneracy point, leading to a Coulomb phase. The ability to pin individual spins allows us to demonstrate Gauss's law for emergent effective monopoles in two dimensions. Finally, we identify a purely entropic screening between emergent monopoles, in the absence of energetic interaction. The demonstrated qubit control lays the groundwork for potential future study of artificial quantum spin liquids and a broad range of spin ice phenomena in various programmable geometries.   

Artificial Spin Ice in Exchange-biased Systems

Exchange bias is used to fabricate a hybrid artificial spin ice composed of athermal Fe nanomagnets that are subject to site-specific unidirectional anisotropy, or a local magnetic field, applied in integer multiples of the lattice period along one sublattice of a classic square artificial spin ice (ASI). By varying this period and applying external fields, we demonstrate that the ground state is tunable in this hybrid ASI, and identify three distinct magnetic textures –– a striped ferromagnetic phase, an antiferromagnetic phase, and a state with magnetically charged pairs embedded in an antiferromagnetic matrix.  Monte Carlo simulations broadly support the ground state tunability of this hybrid ASI, and demonstrate that the pinning tunes relaxation timescales and their critical behavior.   

Magnetization Noise and Demonstration of a Field-Induced Magnetic Monopole Plasma in Artificial Spin Ice

Not all noise in experiments is unwelcome. Certain types of fundamental noise contain extremely valuable information about the system itself. In magnetic systems, noise can exist in the form of thermal magnetization fluctuations, especially associated with changes in ferromagnetic domain structure.  Here we develop and apply a broadband magneto-optical noise spectroscopy to artificial spin ice (ASI) materials, which are interacting arrays of lithographically-defined single-domain nanomagnets in which novel frustrated magnetic phases can be intentionally designed. A key emergent description of fundamental excitations in ASIs is that of magnetic monopoles -- mobile quasiparticles that carry an effective magnetic charge. We demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, plasma-like regimes containing a high density of mobile magnetic monopoles [1]. By passively "listening" to spontaneous monopole noise under conditions of strict thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are most diffusive (that is, minimally correlated) in the plasma regime. These regimes result from the magnetic field-tunable tension on the Dirac strings that connect mobile monopoles. These results open the door to on-demand monopole regimes having continuously field-tunable densities and dynamic properties, thereby providing a new paradigm for probing the physics of effective magnetic charges in synthetic matter. [1] M. Goryca et al., Physical Review X 11, 011042 (2021).