[en] The pyrochlore slab (kagome bilayer) compounds SrCr_9xGa_12-9xO₁₉ (SCGO; x<1) and Ba₂Sn₂ZnCr_7xGa_10-7xO₂₂ (BSZCGO; x<1), are frustrated systems with quite similar magnetic properties of the spin S = 3/2 Cr³⁺ ions. Neutron scattering studies have shown that the two compounds have a completely dynamic magnetic response in a broad temperature range. In both systems the development of short-ranged dynamic correlations leads to a low-T state that can be understood as local clusters with antiferromagnetic character. At liquid He temperatures a partial freezing of the magnetic fluctuations is observed as an increase of the elastic resolved response. A large majority of the magnetic moments remain fluctuating and one also observes a low-energy (long-time) relaxation in the vicinity of the macroscopic freezing. Time and temperature dependence of this relaxation appear system dependent without critical behavior, and we conclude that the freezing is a consequence of the establishment of a coherent quantum state