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Layered antiferromagnets are under investigation as possible axion insulators. Recent neutron scattering measurements have revealed that the layered magnet EuIn2As2 exhibits an unprecedented non-colinear magnetic structure in which conventional antiferromagnetic order coexists with an incommensurate spin helix. We are using ultrafast measurements of transient Faraday rotation and birefringence to identify the elementary excitations of this unusual magnetic structure. So far, we have detected six distinct spin wave modes of this structure and measured their dependence on magnetic field. The six modes include three nearly degenerate quasi-Goldstone modes that are very beautifully seen in the time domain. We are currently working towards a theoretical understanding of magnetic ground state and its excitations. The next step will be to measure the propagation dynamics of spin waves in this structure.
