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 T(N) = 7.2 K. Between T(N) and T(t) = 3.5 K, the structure is amplitude modulated with Q = (0.148, 0.148, 0.023). Below T(t) 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 almost-equal-to 0.149), inducing a small ferromagnetic component (almost-equal-to 0.10mu(B) 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.