Incommensurate spin-density wave and magnetic lock-in transition in CaFe4As3
Abstract
The magnetic structure for the newly discovered iron-arsenide compound has been studied by neutron powder diffraction. Long-range magnetic order is detected below 85K, with an incommensurate modulation described by the propagation vector k=(0,δ,0), δ 0.39. Below 25K, 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.