Deconstructing Magnetization Noise: Degeneracies, Phases, and Mobile Fractionalized Excitations in Tetris Artificial Spin Ice

Direct detection of spontaneous spin fluctuations, or magnetization noise, is emerging as a powerful means of studying magnetic excitations in both natural and artificial frustrated magnets. These excitations can often be described as fractionalized quasiparticles possessing an effective magnetic charge. They are topologically protected, can diffuse through the crystal lattice in thermal equilibrium, and can move in response to applied magnetic fields, motivating studies of magnetricity. In archetypal square ASI lattices, magnetization noise was detected via optical magnetometry and the appearance of excess noise at certain applied magnetic fields revealed the presence of phases rich in mobile magnetic charges.

Motivated by these studies, here we use optical magnetometry of spontaneous noise to reveal new families of fractionalized excitations in the low-symmetry frustrated ASI known as tetris ice. By applying small magnetic fields, tetris ice can be tuned through a variety of complex spin configurations, whose degeneracies and boundaries are directly revealed by noise. In particular, we find particularly intense and narrow bands of noise for certain directions and ranges of the applied field. Using Monte Carlo simulations to deconstruct these noise signatures, these bands are shown to herald novel regimes wherein magnetic quasiparticles delocalize and proliferate, demonstrating the power of noise-based studies to probe microscopic details of complex magnetic phenomena.