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).