Exchange and dipolar interactions in TbF3

Measurements of neutron-diffraction scattering performed earlier have shown that TbF3 undergoes a cooperative transition at low temperature. The observed magnetic structures in two different samples can be described either as canted ferromagnetic F/sub x/C/sub z/ structure or as noncommensurate structure with a propagation vector k = (0, k/sub y/, 0) where k/sub y/ = 0.082. To analyse for the interaction, a model is proposed in which the dominant interaction contributions are assumed to be of the form μ(R) A/sub RR'/ μ(R') where A/sub RR'/ accounts for both, exchange and dipolar coupling. These assumptions are tested using Bertaut's matrix formalism. It is shown, that in presence of strong crystalline anisotropy, a delicate balance between exchange and dipolar forces may be easily perturbed. As a result either commensurate F/sub x/C/sub z/ or noncommensurate modulated structure is stable. The present studies reveal that the field induced transition from noncommensurate phase to F/sub x/C/sub z/ structure can be easily achieved. The possibility of field induced ordering at 4.2 K confirms additionally early findings that TbF3 is a two level system, strongly anisotropic with two easy axes of magnetization. No indications for change of crystal symmetry are found at 4.2 K. (author)