Incommensurate spin-density wave and magnetic lock-in transition in CaFe₄As₃

The magnetic structure for the recently synthesized iron-arsenide compound CaFe₄As₃ has been studied by neutron-powder diffraction. Long-range magnetic order is detected below 85 K, with an incommensurate modulation described by the propagation vector k= (0,δ,0), δ∼0.39. Below ∼25 K, our measurements detect a first-order phase transition where δ locks into the commensurate value 3/8. A model of the magnetic structure is proposed for both temperature regimes, based on Rietveld refinements of the powder data and symmetry considerations. The structures correspond to longitudinal spin-density waves with magnetic moments directed along the b axis. A Landau analysis captures the change in thermodynamic quantities observed at the two magnetic transitions, in particular, the drop in resistivity at the lock-in transition.