Dynamics of frustrated antiferromagnets studied using pulsed magnetic fields and X-ray photon correlation spectroscopy

Frustration in antiferromagnets can lead to surprising dynamic behavior as well as spatial inhomogeneities. We study these using a combination of pulsed magnetic fields with different timescales, resonant coherent X-ray measurements, and Monte Carlo simulations. Here I will discuss two compounds, Lu₂CoMnO₆ and Ca₃Co₂O₆, in which nearest-neighbor frustration gives rise to a version of the axial next-nearest neighbor Ising model (ANNNI). This 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. The incommensurate wave vector slides with temperature, finally giving rise to a commensurate ground state at the lowest temperatures. Strong dynamics and inhomogeneities of the antiferromagnetic order have been observed in both compounds. I will talk about X-ray photon correlation spectroscopy (XPCS) results in Lu₂CoMnO₆ that show unusual dynamics of antiferromagnetic fluctuations as a function of temperature that match the predictions of the ANNNI model. I will also talk about pulsed magnetic field results in Ca₃Co₂O₆ and finally DC annealing experiments designed to elucidate the metastable magnetization steps and reach the ground state of this compound.