Abstract
Artificial spin ices (ASIs) are interacting arrays of lithographically-defined 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. Here we demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, high-density plasma-like regimes of mobile magnetic monopoles. By passively "listening" to spontaneous monopole noise in thermal equilibrium, we reveal their intrinsic dynamics and show that monopole kinetics are minimally correlated (that is, most diffusive) in the plasma phase. These results open the door to on-demand monopole regimes having field-tunable densities and dynamic properties, thereby providing a new paradigm for probing the physics of effective magnetic charges in synthetic matter.
Highlights
Artificial spin ices (ASIs) are interacting arrays of lithographically defined nanomagnets in which novel, frustrated magnetic phases can be intentionally designed
We demonstrate that the archetypal square ASI lattice can host, in specific regions of its magnetic phase diagram, plasmalike regimes containing a high density of mobile magnetic monopoles
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 in the plasma regime
Summary
The field-tunable tension on the Dirac strings connecting mobile monopoles. detailed noise spectra demonstrate that monopole kinetics are minimally correlated (i.e., most diffusive) in this plasmalike regime. Our study focuses on a previously unexplored characteristic of thermal square ASI: namely, that its fielddependent magnetic phase diagram and ground-state moment configuration must include regions where monopole excitations play a dominant and active role, even under conditions of strict thermal equilibrium. This requirement can be understood by considering the relative energies of the four possible vertex types (I–IV), shown and described in Fig. 1(b) in order of increasing energy at zero applied magnetic field. To search for dynamic monopole regimes in square ASI and to quantify their timescales and correlations—all under conditions of strict thermal equilibrium—we developed a broadband magnetization noise spectrometer to measure (a) PBS
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