Magnetic order and single-ion anisotropy in Tb\(_3\)Ga\(_5\)O\(_{12}\)

Terbium gallium garnet (TGG), Tb\(_3\)Ga\(_5\)O\(_{12}\), is well known for its applications in laser optics but also exhibits complex low-temperature magnetism that is not yet fully understood. Its low-temperature magnetic order is determined by means of time-of-flight neutron powder diffraction. It is found to be a multiaxial antiferromagnet with magnetic Tb\(^{3+}\) ions forming six sublattices of magnetic moments aligned parallel and antiparallel to the ⟨100⟩ crystallographic directions of the cubic unit cell. The structure displays strong easy-axis anisotropy with respect to a twofold axis of symmetry in the local orthorhombic environment of the Tb\(^{3+}\) sites. The crystal-field splitting within the single-ion ground-state manifold is investigated by inelastic neutron scattering on powder samples. A strong temperature dependence of the quasidoublet ground state is observed and revised parameters of the crystal-field Hamiltonian are given. The results of bulk magnetic susceptibility and magnetization measurements are in good agreement with values based on the crystal-field model down to 20 K, where the onset of magnetic correlations is observed.