Results of thermal annealing on Fe66Co18B15Si1 amorphous alloy by means of linear heating and isothermal process are presented. The analysis of the DSC curve and X-ray diffraction of the annealed samples indicate the synthesis of magnetic nanocrystals in the Fe7Co3 phase, and the co-existence of two coupled magnetic phases; one soft magnetic phase due to the remaining amorphous matrix and a hard magnetic phase associated to the crystalline nanoparticles. The presence of these different magnetic phases and their coupling are unhidden by analysis of FORC diagrams of the samples.