Double umbrella structure in terbium iron garnet

The umbrella magnetic structure of the terbium iron garnet (TbIG) has been studied using neutron diffraction experiments performed in the 4.2 to 650 K temperature range which covers both the Néel temperature (TN= 550 ± 10 K) and the compensation temperature (Tcomp= 244 ± 2 K). When T> TN, the observed peak intensities originate from nuclear contribution only. When Tcomp< T <TN, the results have been interpreted in the Néel theory of ferrimagnetism. Below Tcomp, three salient features have been observed : there appear superstructure lines forbiden by the cubic space group Ia3d, such as