Field-induced spin reorientation and giant spin-lattice coupling in EuFe₂As₂

We have studied a EuFe₂As₂ single crystal by neutron diffraction under magnetic fields up to 3.5 T and temperatures down to 2 K. A field-induced spin reorientation is observed in the presence of a magnetic field along both the a and c axes, respectively. Above critical field, the ground-state antiferromagnetic configuration of Eu²⁺ moments transforms into a ferromagnetic structure with moments along the applied field direction. The magnetic phase diagram for Eu magnetic sublattice in EuFe₂As₂ is presented. A considerable strain (∼0.9%) is induced by the magnetic field caused by the realignment of the twinning structure. Furthermore, the realignment of the twinning structure is found to be reversible with the rebound of magnetic field, which suggested the existence of magnetic shape-memory effect. The Eu moment ordering exhibits close relationship with the twinning structure. We argue that the Zeeman energy in combined with magnetic anisotropy energy is responsible for the observed spin-lattice coupling.