Symmetry analysis of the magnetic structures in TbIG and Tb:YIG at low temperature

Magnetic structures in TbIG and in terbium-yttrium ferrite garnet Tb0.37Y2.63IG are revisited using Bertaut' representation analysis for the highest crystallographic subgroups of the space group Ia d with the wave vector k 0. Powder neutron diffraction experiments performed only on TbIG are combined with single crystal magnetization measurements. Three field-induced phase transitions are observed at 4.2 K in high magnetic fields up to 150 kOe along the direction while remain the easy axis of magnetization. New parameters of the previous Tb3+ double umbrella magnetic structure are found at 5 K in a modified model of the rhombohedral subgroup R c . From magnetizations on Tb0.37Y2.63IG single crystal, non collinear magnetic structure is evidenced at 4.2 K along the easy axis . The parameters of a 'square pyramid' umbrella are predicted using both the basis vectors of the tetragonal subgroup I41/acd and a model based on the effective spin Hamiltonian with anisotropic exchange and magnetic tensors with the requirement gyGy < gzGz ≪ gxGx. Anisotropy of magnetization is observed by comparison with the hard axis below Ta 80 K where the earlier spin reorientation phase transitions ↔↔ occur between T1 40 K and T2 140 K. The basis vectors of the monoclinic subgroup C2/c are tentatively proposed for the low symmetry angular phases . All results are compared to previous works.