We present magnetization studies in a series of phase separated La_0.5Ca_0.5MnO₃ samples, with different low temperature fractions of the ferromagnetic (FM) and charge ordered-antiferromagnetic (CO-AFM) phases. A particular experimental procedure probes the effect of the magnetic field applied while cooling the samples, which promotes FM fraction enlargement and enhances the melting of the CO phase. The response of the system depending on its magnetic field history indicates the existence of three different regimes in the phase separated state which develops below TC. Our data allows us to identify the onset temperature below which the system becomes magnetic and structurally phase separated and an onset field above which FM fraction enlargement occurs.