Magnetic Properties of the TbCr3(BO3)4 Single Crystals Synthesized under Different Conditions: Successive Ordering of the Cr3+ and Tb3+ Magnetic Subsystems

Abstract

The magnetic properties of rare-earth borates evoke keen interest, since the coexistence of two interconnected magnetic subsystems, 3d and 4 f ions, in the crystals of this family causes a diversity of magnetic structures and phase transitions between them. A comparative study of the effect of synthesis conditions on the structure and magnetic properties of the terbium chromoborate TbCr₃(BO₃)₄ single crystals grown from bismuth molybdate and lithium tungstate solvents has been carried out. In the single crystals with the monoclinic symmetry synthesized from the bismuth molybdate solvent, partial substitution of Bi³⁺ions for Tb³⁺ ions occurs, which entails a change in the magnetic anisotropy of the crystal and the formation of an angular magnetic structure, in which, unlike the crystals grown from another solvent, the Ising axis of Tb³⁺ ions deviates from the С₃ pseudo-axis direction by an angle of ~40°.In the case of the lithium tungstate solvent, single crystals with the trigonal and monoclinic symmetry are formed and their temperature and field dependences of the magnetization coincide. At a Néel temperature of T_N1 = 9.2 K, the antiferromagnetic ordering with the magnetic moments lying in the basal plane is established in the Cr³⁺ ion subsystem. Due to the weakness of the exchange interaction between Cr³⁺ and Tb³⁺ ions, the antiferromagnetic order in the terbium subsystem of all the investigated TbCr₃(BO₃)₄ crystals is formed at a lower temperature: T_N2 = 5.5 K. The existence of two temperatures of the successive ordering of the magnetic subsystems of 3d and rare-earth ions has been found for the first time in the crystals of the huntite family and confirmed by the results of the magnetization and specific heat investigations. The lower ordering temperature of the terbium subsystem, which has a strong easy-axis magnetic anisotropy, also explains the nature of the easy plane→easy axis orientational transition in the chromium subsystem discovered previously at a temperature of ~5 K.