Long period magnetic commensurability in PrGa2

We present neutron diffraction, specific heat and magnetization measurements on the hexagonal PrGa2 compound. Two successive magnetic phases appear below TN=7.2 K. Between TN and Tt=3.5 K, the structure is amplitude modulated with Q=(0.148, 0.148, 0.023). Below Tt the propagation vector locks into a value Q=(p/q, p/q, 0) commensurate with the crystal lattice but corresponding to a long period (p/q≅0.149), including a small ferromagnetic component (≅0.10μB at 1.5 K). The magnetic moments are parallel to the propagation vectors. In the low temperature phase, the magnetization processes along the [100] and [120] axes of the hexagonal plane exhibit a multi-step behaviour associated with the reversal of a few magnetic moments antiparallel to the field, leading to a complex magnetic phase diagram. (orig.)