Broadband Spectroscopy of Magnetic Monopole Noise in Artificial Spin Ice

Arrays of lithographically-patterned interacting nanomagnets known as Artificial Spin Ice (ASI) have allowed the design of geometrically-frustrated collective states not found in natural magnets, as well as the ability to characterize the magnetic state at the level of the individual elements. Particularly for thermally-active ASIs, real-time studies of magnetization dynamics over a wide frequency range are highly desirable to reveal the underlying dynamics and to test theoretical models of the kinetics. Here we present an all-optical method to study the spontaneous magnetization fluctuations in ASIs over a broad frequency range (1 Hz - 1 MHz), which allows us to investigate magnetization dynamics through temperature- and field-dependent phase transitions. We focus first on the simple case of thermally-active square ASIs (using ~3 nm thick permalloy), to develop and benchmark tools for future investigations of more complex topologies. The field- and temperature-dependent total activity of the material follows theoretical models describing the creation, annihilation, and motion of ‘magnetic monopoles’ in the square ASI lattice. However, more subtle quantities such as correlation/relaxation rates deviate from widely-accepted theoretical paradigms.