Magnetic incommensurability in hexagonal HoGa2

Specific heat, powder neutron diffraction experiments and magnetization 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 orthohexagonal cell) below T-t = 6.5 K; (ii) a mixing of this phase (about 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 metamagnetic processes are observed along both directions of the easy basal plane. The second order crystal field parameter A(2)(0), deduced from the shift of the reciprocal susceptibilities along and perpendicular to c, is significantly smaller than in the other RGa(2) compounds.