  A system defined by two coupled Ising models, with a bimodal random field
acting in one of them, is investigated. The interactions among variables of
each Ising system are infinite-ranged, a limit where mean field becomes exact.
This model is studied at zero temperature, as well as for finite temperatures,
representing physical situations which are appropriate for describing real
systems, such as plastic crystals. A very rich critical behavior is found,
depending directly on the particular choices of the temperature, couplings, and
random-field strengths. Phase diagrams exhibiting ordered, partially-ordered,
and disordered phases are analyzed, showing the sequence of transitions through
all these phases, similarly to what occurs in plastic crystals. Due to the wide
variety of critical phenomena presented by the model, its usefulness for
describing critical behavior in other substances is also expected.
