We search for evidence of a scalar top quark partner (stop) predicted by supersymmetry, using the 2012 dataset recorded with the ATLAS detector. The search extends earlier work by studying two classes of simplified models with a reduced sparticle content and specific stop decay scenarios: stop to top+neutralino decays with an off-shell top quark, and mixed models where two different stop decay modes are simultaneously allowed at different branching ratios. Using datasets of simulated events, selection criteria for proton-proton collision events are established, defining regions of phase-space enriched in hypothetical supersymmetric events. Suitable kinematic variables are defined and the background from Standard Model events is estimated. As no significant excess of recorded events over the background expectation is observed, the signal hypothesis can be ruled out at 95% confidence level for certain mass assumptions of the supersymmetric particles.