Dynamics of a fractal set of first-order magnetic phase transitions in frustrated Lu2 CoMnO6

The axial next-nearest-neighbor Ising model predicts a fractal (infinite) set of phases with incommensurate wave vectors that are separated by first-order phase boundaries. This complexity results from a simple frustration condition between nearest- and next-nearest-neighbor interactions along a chain of Ising spins. Using x-ray photon correlation spectroscopy (XPCS), we investigate the surprising antiferromagnetic dynamics that emerge from such a complex phase diagram over a wide range of temperatures. We present XPCS measurements of the frustrated magnetic chain compound Lu2CoMnO6 and Monte Carlo simulations. Incommensurate magnetic Bragg peaks slide towards commensurate "up-up-down-down"spin order with decreasing temperature and increasing time. Both simulation and experiment support a counterintuitive "upside-down"temperature dependence of the magnetic dynamics: at higher temperatures in the region of first-order phase boundaries, slower dynamics are observed where the speckle maintains its coherence. At the lowest temperatures, where part of the sample adopts commensurate order, the dynamics speed up and result in fast decoherence.