Incommensurate spin-density wave and magnetic lock-in transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>CaFe</mml:mtext></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mtext>As</mml:mtext></mml:mrow><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>

Pascal Manuel, L. C. Chapon, Iliya Todorov, Duck Young Chung, J. P. Castellan, Stephan Rosenkranz, R. Osborn, P. Tolédano, M. G. Kanatzidis

Abstract

The magnetic structure for the recently synthesized iron-arsenide compound ${\text{CaFe}}_{4}{\text{As}}_{3}$ 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 $k=(0,\ensuremath{\delta},0)$, $\ensuremath{\delta}\ensuremath{\sim}0.39$. Below $\ensuremath{\sim}25\text{ }\text{K}$, our measurements detect a first-order phase transition where $\ensuremath{\delta}$ locks into the commensurate value $\frac{3}{8}$. 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.

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