Etude magnétique du composé SrTb2Fe2O7, détermination des structures magnétiques par diffraction neutronique

Magnetic properties of the compound SrTb2Fe2O7 (space group P4 2 mnm ) have been investigated by magnetic measurements of susceptibility and magnetization and by neutron diffraction. The Neel temperature of the iron sublattice is 628 K. At 295 K the configuration of the Fe3+ spins is Gx− ± Ay− (antiferromagnetic). At 4.2 K the configuration of Tb3+ spins is represented by the modes Fx+, Cy+, Az− coupled in the representation Eg with the mode Az− of the Fe3+ spins. The values of the magnetic moments corresponding to the different modes are obtained from the magnetic structure refinement. The value of the Fx+ ferromagnetic mode of Tb3+ is 2.6 μB/ion at 4.2 K. At room temperature the orientation of Fe3+ spins perpendicularly to the c axis seems to be due to a site anisotropy, the easy direction coinciding with the axis of the FeO5 bipyramids. The reorientation of Fe3+ spins along the c axis observed at 4.2 K can be explained by a strong Fe3+Tb3+ coupling which overcomes the Fe3+ one ion anistropy. The low site symmetry (m) of Tb lifts the degeneracy completely and gives rise to a pseudodoublet (formed by two singlets of small separation) associated with an Ising-like behavior. In fact the Tb3+ spins are in the m planes, containing the c axis.