Mn-Al permanent magnets and their native defects: A first principle investigation

Abstract

We have studied the ferromagnetic Mn-Al-C alloys (Mn50Al50, Mn50+xAl50-xCy with x = 2, y = 0, 1) and their native defects by employing density functional theory (DFT) based simulations. These calculations elucidate the experimentally observed trends for magnetic properties (e.g., saturation magnetization, anisotropy constant) with the variation of Mn and C concentrations. With the aid of Monte Carlo simulations, it is further shown that the magnetic properties of tau-MnAlC alloys diminish noticeably with temperature which may help to explain the large difference found between previous theoretical and experimental estimates of highest achievable maximum energy product. As for point defects, antisite defects (Mn antisite or Al antisite) were found to be the most stable defect type for pure tau-MnAl (i.e., without C addition), but their range of stabilities and formation energies depend upon the chemical environment during the growth. However, the electronic structure of tau-MnAl remains largely unchanged due to point defects. For tau-MnAlC alloys, C vacancy is the most dominant defect with very low formation energy (similar to 0.097 eV). It is shown that the presence of antisite defects contribute to the erosion of magnetic performance of tau-MnAl permanent magnets: a 1 % Al antisite disorder reduces the magnetic anisotropy by similar to 4 % and maximum energy product by over 3 %.