Anomalous magnetic reordering in magnetodielectric terbium iron garnet at low temperatures

Abstract

The paper deals with five topics: i) the single three-dimensional irreductible representation (Г 4 g =T 1 g ) of the paramagnetic space group Ia 3 ¯ 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 (Tb 3 Fe 5 O 12 ); ii) the use of the method of the “symmetry lowering device” of Bertaut in order to select the appropriate rhombohedral subgroup of Ia 3 ¯ 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 A 2 g of the highest rhombohedral subgroup R 3 ¯ 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 Tb 3+ 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” ( T B ); 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. • 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.