Magnetic order and single-ion anisotropy in Tb₃Ga₅O₁₂ rare-earth garnet.

Terbium gallium garnet(TGG), Tb₃Ga₅O₁₂ exhibits complex low-temperature magnetism, that involves frustration, single-ion anisotropy and induced-moment type ordering. Tb³⁺ magnetic ions form two interpenetrating half-garnet lattices – three-dimensional arrangements of corner-sharing triangles. The low-symmetry local environment of Tb³⁺ sites provides a complete lifting of the degeneracy within the lowest crystal electric field (CEF) manifold. The resulting ground-state observed with inelastic neutron scattering is a quasidoublet comprised of two closely-lying singlets. The spectroscopic data allowed for refinement of the CEF Hamiltonian parameters and retrieval of its eigenstates. Despite the singlet ground-state TGG orders magnetically at T_N=0.25 K, much lower than |θ_CW|=8 K given by the bulk susceptibility. Neutron powder diffraction has shown the ordered structure to be of a multiaxial antiferromagnet type with strong single ion anisotropy. The results of powder magnetization measurements are in good agreement with values based on the CEF model down to 20 K, where the onset of magnetic correlations is seen. Also, a strong temperature-dependence of the quasidoublet ground-state is found. These observations suggest existence of a correlated paramagnet regime of unknown nature.