  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 Eu$^{2+}$ 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 ($\sim$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.
