Analysis of Magnetic Ground State in Thulium Iron Garnets

Abstract

Tlulium iron garnet (TmΙG) and also its derivatives are known to display unusual magnetic behaviour. A large number of questions still remain open. For instance, it has been recently reported that single-crystal TmIG thin films doped with bismuth and gallium undergo a continuous re-orientation of their easy axis of magnetization from the in-plane to the perpendicular to the plane [1, 2]. In TmIG the octahedral [α] and tetraledral (d) sites are occupied by Fe3+ whose ground state is the orbital S singlet, 6S5/2. Thus, the ferrous magnetism is purely spin-like in its nature. Thulium ions Tm 3+ occupy the dodecahedral sublattice c. The ground state of Tm 3+ is the multiplet, 3H6 with a large orbital contribution to the total angular momentum operator, J. The overall action of the crystal field which at the c sites is considered to be cubic with a trigonal distortion, on the thulium magnetic moments produces a considerable reduction of their large values (J = 6). The total angular momentum of Τm3+ projected on the lowest-lying crystal-fleld energy level, is reduced to that of one [3]. In order to understand the magnetic properties of both pure and doped TmIG, it is necessary to determine a magnetic ground state of the system in a wide range of its superexchange parameters. A three-sublattice Heisenberg Hamiltonian is assumed with the superexchange nearest-neighbour interactions: a-a, a-d' d-d and d—c. The α—c and c-c are considered negligibly small.