  We report the achievement of stimulated Raman adiabatic passage (STIRAP) in
the microwave frequency range between internal states of a Bose-Einstein
condensate (BEC) magnetically trapped in the vicinity of an atom chip. The
STIRAP protocol used in this experiment is robust to external perturbations as
it is an adiabatic transfer, and power-efficient as it involves only resonant
(or quasi-resonant) processes. Taking into account the effect of losses and
collisions in a non-linear Bloch equations model, we show that the maximum
transfer efficiency is obtained for non-zero values of the one- and two-photon
detunings, which is confirmed quantitatively by our experimental measurements.
