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D LIQUIDS

LowTemperature Thermal Conductivity of Terbium–Gallium Garnet A. V. Inyushkin and A. N. Taldenkov Russian Research Centre, Kurchatov Institute, Moscow, 123182 Russia email: [email protected] Received March 26, 2010

Abstract—Thermal conductivity of paramagnetic Tb3Ga5O12 (TbGG) terbium–gallium garnet single crys tals is investigated at temperatures from 0.4 to 300 K in magnetic fields up to 3.25 T. A minimum is observed in the temperature dependence κ(T) of thermal conductivity at Tmin = 0.52 K. This and other singularities on the κ(T) dependence are associated with scattering of phonons from terbium ions. The thermal conductivity at T = 5.1 K strongly depends on the magnetic field direction relative to the crystallographic axes of the crys tal. Experimental data are considered using the Debye theory of thermal conductivity taking into account res onance scattering of phonons from Tb3+ ions. Analysis of the temperature and field dependences of the ther mal conductivity indicates the existence of a strong spin–phonon interaction in TbGG. The lowtempera ture behavior of the thermal conductivity (field and angular dependences) is mainly determined by resonance scattering of phonons at the first quasidoublet of the electron spectrum of Tb3+ ion. DOI: 10.1134/S1063776110110075

1. INTRODUCTION Several years ago, experimental observation of a new magnetically transverse thermal effect, viz., the phonon Hall effect, in a paramagnetic dielectric was reported in [1]. In a Tb3Ga5O12 (TbGG) monocrystal line plate, in which a thermal flux q was established in a transverse magnetic field H, a temperature gradient was observed in the direction perpendicular to q and H (i.e., in a geometry analogous to that of the Hall effect). The existence of the phonon Hall effect was confirmed in [2]. A distinguishing feature of the new effect is that it takes place in a dielectric that contains no charged particles moving over the crystal lattice and, accordingly, does not exhibit the Righi–Leduc effect. It was shown in theoretical works [3–6] that the phonon Hall effect is possible in the case of strong electron spin–orbit and spin–lattice interactions for a paramagnetic ion. The latter interaction is responsible for phonon scattering from a paramagnetic ion and may lead to considerable suppression of the diagonal component of the thermal conductivity. Despite the observation of the nondiagonal com ponent of the thermal conductivity tensor in TbGG, information on its diagonal component and, in partic ular, on the role of the spin–phonon interaction in thermal conductivity of this paramagnet is scarce. The TbGG crystal possesses cubic symmetry, and 10 its structure is described by space group Ia3d ( O h ). The unit cell consists of eight formula units (160 atoms) of {Tb3}[TbxGa2 – x](Ga3)O12. The brack ets denote three different cation sites with different

oxygen surroundings: {…} denote 24 dodecahedral c sites with an orthorhombic point symmetry group D2, which are occupied by Tb3+ ions; […] denote 16 octa hedral