Etude magnétique du composé SrTb 2Fe 2O 7, détermination des structures magnétiques par diffraction neutronique

Magnetic properties of the compound SrTb ₂Fe ₂O ₇ (space group {P4 ₂}/{mnm}) have been investigated by magnetic measurements of susceptibility and magnetization and by neutron diffraction. The Néel temperature of the iron sublattice is 628 K. At 295 K the configuration of the Fe ³⁺ spins is G_x^- ± A_y^- (antiferromagnetic). At 4.2 K the configuration of Tb ³⁺ spins is represented by the modes F_x⁺, C_y⁺, A_z^- coupled in the representation E_g with the mode A_z^- of the Fe ³⁺ spins. The values of the magnetic moments corresponding to the different modes are obtained from the magnetic structure refinement. The value of the F_x⁺ ferromagnetic mode of Tb ³⁺ is 2.6 μ _B/ion at 4.2 K. At room temperature the orientation of Fe ³⁺ spins perpendicularly to the c axis seems to be due to a site anisotropy, the easy direction coinciding with the axis of the FeO ₅ bipyramids. The reorientation of Fe ³⁺ spins along the c axis observed at 4.2 K can be explained by a strong Fe ³⁺Tb ³⁺ coupling which overcomes the Fe ³⁺ 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 Tb ³⁺ spins are in the m planes, containing the c axis.