Ferromagnetic Mn-Al-C L10 Formation by Electric Current Assisted Annealing

The ferromagnetic Mn-Al-C tau-phase (L10 tetragonal structure) shows intrinsic potential to be developed as a permanent magnet; however, this phase is metastable and is easily decomposed to nonmagnetic stable phases, affecting negatively the magnetic properties. Giving the necessity to careful control of its synthesis, the use of a novel approach is investigated using electric current-assisted annealing to obtain pure tau-phase samples. The temperature and electrical resistance of the samples are monitored during annealing and it is shown that the change in resistance can be used to probe the phase transformation. Upon increase of electric current density, the required temperature for the ferromagnetic phase formation is reduced, reaching a maximum shift of 140 degrees C at 45 A mm(-2). Even though this noticeable shift is achieved, the magnetic properties are not affected showing coercivity of 0.13 T and magnetization of 90 Am-2 kg(-1), independently from the electric current density used during annealing. Microstructural investigation reveals the nucleation of the tau-phase at the grain boundaries of the parent epsilon-phase. In addition, the existence of twin boundaries upon nucleation and growth of the metastable phase for all evaluated annealing conditions is observed, resulting in similar extrinsic magnetic properties.