The development of rare-earth free permanent magnets continues to be a challenging unsolved problem. The growing demand for strong permanent magnetic materials, which maintain performance at high temperatures but contain as little of very expensive rare-earth and transition metal elements as possible, requires a quantitatively accurate ab initio modeling of such alloys their properties. In all hard magnets, a highly coercive state occurs as a result of specific structural and phase changes, which depend on the alloy composition and heat treatment. The decomposition in Fe-Co-based alloys will be systematically investigated using first-principles calculations based on density functional theory and employing both Green’s function technique and the full potential projector augmented wave method. This approach, which includes an accurate first principles description of a disordered magnetic state as well as a vibrational free energy, allows the atomic-scale modeling of the decomposition in these alloys. This project is a part of my postdoctoral project at Luleå University of Technology.