Anomalous magnetic reordering in magnetodielectric terbium iron garnet at low temperatures

• We examine the changes of the “double umbrella” in TbIG using neutron diffraction.
• Symmetry arguments of Bertaut clarify suitable rhombohedral space group at 13 K.
• Its opening leads to an increasing of anisotropy of the Tb components below 160 K.
• The “low-temperature point” of Belov explains partly its intricate behavior near 54 K.
• A relationship with the magnetodielectric effects revealed in this garnet is discussed.

The paper deals with five topics: i) the single three-dimensional irreductible representation (Г4g=T1g) of the paramagnetic space group Ia3¯d No. 230 is chosen according to the representation analysis of Bertaut for the interpretation of the neutron powder diffraction experiments performed on terbium iron garnet (Tb3Fe5O12); ii) the use of the method of the “symmetry lowering device” of Bertaut in order to select the appropriate rhombohedral subgroup of Ia3¯d which allows to deal with the case where the cubic description provides an incomplete answer to the changes observed below 160 K in the ferrimagnetic structure around the [1 1 1] axis from the Néel model toward the “double umbrella” observed at 13 K; iii) the magnetic modes belonging to the one-dimensional irreductible representation A2g of the highest rhombohedral subgroup R3¯c No. 167 are able to describe the occurrence of its anisotropic character which steeply increases below 160 K due to the concomitant anisotropic effects; iv) the broad anomaly observed near 54 K in the temperature dependences of the components of both sublattices of the Tb3+ ions in the Wyckoff positions (6e) and (6e′) is explained partially on the basis of the concept of Belov of the strong paraprocess which has been termed “exchange-enhanced paramagnetism” at the so-called “low-temperature point” (TB); v) the results are related to the magnetodielectric effect in low magnetic field and to the significant coupling between exchange magnons and ligand-field excitations reported recently in this compound.