Magnetic structure of the spin-density wave antiferromagnet CaFe4As3from magneto-elastic coupling

Abstract

We report an ultrasonic study of the magneto-elastic coupling of the spin-density wave antiferromagnet CaFe4As3. Longitudinal waves propagating along the a axis reveal anomalies on the acoustic velocity at both the incommensurate (ICM) (TN1 = 89.3 K) and commensurate (CM) (TN2 = 26.3 K) spin-density phases, which are consistent with the magnetic structure established from neutron diffraction experiments. Moreover, at higher temperatures, magnetic fluctuations are likely responsible for a reduced stiffening of the velocity below 150 K. Although the ICM phase appears elastically inhomogeneous below 50 K, a precise magnetic field dependence of the ICM-CM transition at TN2 specifies a preferential orientation of the in-plane easy and hard axes respectively parallel and perpendicular to the vectord (â ·d = cos 30• ). Within the CM phase, a magnetic field aligned along the ribbon b axis reveals a new magnetic transition of the spin-flop type near 16 teslas. For this particular field direction a phase diagram is proposed.