DoE EPSCoR: Magnetization Dynamics and Soft X-Ray Vortex Beam Formation in Nanoscale Magnetic Metamaterials

Frustration in physical systems arises when geometrical constraints or competing interactions block their reaching a global energy minimum, and induces non-equilibrium or metastable phases in a variety of amorphous, magnetic, and polymeric systems. The unique behavior of microscopic spin ices derives from frustrated magnetic interactions caused by crystal symmetry, resulting in a large number of lowenergy, nearly-degenerate magnetic configurations that generate a large entropy without magnetic order at low temperatures. These concepts have been generalized to include mesoscale arrays of magnetic dots with frustrated dipole-dipole interactions, such as square artificial spin ice (ASI), which consists of a square array of ferromagnetic wire segments sufficiently small and elongated to be singledomain with bi-stable (Ising) polarization. The segment shape anisotropy and mesoscopic size of ASI generate large thermal barriers to segment reversal and depress thermal fluctuations, but also ensure their magnetic textures can be directly imaged at variable temperature and magnetic field. Understanding athermal dynamics is extremely challenging, as non-equilibrium systems can be so far from equilibrium that perturbative tools of statistical mechanics fail to predict their dynamics. ASI therefore constitute a novel class of metamaterials for systematic studies of athermal, nonequilibrium dynamics, and their relationship with the non-attainment of a true ground state.