In particle physics, a magnetic monopole remains a hypothetical elementary particle that has never been observed. However, certain condensed matter systems can contain effective magnetic monopoles. One of such systems are Artificial Spin Ices (ASIs) - arrays of interacting nanomagnets that have allowed the design of geometrically frustrated exotic collective states not found in natural magnets. A key emergent description of fundamental excitations in ASIs is that of mobile quasiparticles that carry an effective magnetic charge - that is magnetic monopoles. These charge excitations can interact with each other and with applied magnetic fields via the magnetic analog of the electronic Coulomb interaction, representing the emergence of a range of novel phenomena, including the possibility of "magnetricity" – magnetic analog of electricity. While the presence of monopoles in ASI has been observed in pioneering imaging measurements, dynamical studies of monopole kinetics, and (especially) the ability to tune continuously through monopole-rich regimes in thermal equilibrium, remain at an early stage.

In the seminar I will present a noise-based experimental approach that we have developed  to passively "listen" to spontaneous magnetization fluctuations in archetypal, thermally active square ASI. The noise reveals specific regions in the magnetic field-dependent phase diagram  where the density of mobile monopoles increases well over an order of magnitude compared with neighboring regimes. Moreover, detailed noise spectra demonstrate that monopole kinetics are minimally correlated (i.e., most diffusive) in this plasma-like regime. Experiments and Monte-Carlo simulations of more complex ASIs (including quadrupolar and vertex-frustrated Shakti and Tetris lattices) show similarly fascinating behavior, revealing surprisingly rich field-dependent phase diagrams of these systems.

The discovery of on-demand monopole regimes with tunable kinetic properties opens the door to new probes of magnetic charge dynamics and provides a new paradigm for the studies of magnetricity in artificial magnetic materials.