MAGNETIC INCOMMENSURABILITY IN HEXAGONAL HOGA2

Specific heat, powder neutron diffraction experiments and magnetizatio n measurements on a single crystal of the hexagonal HoGa2 compound are presented. Below T-N = 7.6 K two collinear antiferromagnetic phases ( M parallel to a of the orthohexagonal cell) are observed: (i) a simple collinear antiferromagnetic structure with Q(1) =(0, 1, 0) (in the or thohexagonal cell) below T-t = 6.5 K; (ii) a mixing of this phase (abo ut 36%) and of an incommensurate amplitude modulated structure (about 64%) with Q(2) =(0.123, 1, 0) for T-t < T < T-N. Below T-N smooth meta magnetic processes are observed along both directions of the easy basa l plane. The second order crystal field parameter A(2)(0), deduced fro m the shift of the reciprocal susceptibilities along and perpendicular to c, is significantly smaller than in the other RGa(2) compounds.