Paramagnetic susceptibility of ferrite and cementite obtained from ab initio calculations

Paramagnetic susceptibility of BCC ferrite (αα-Fe) and of cementite (Fe3C), was calculated in a temperature range above the Curie point, using a density functional-based method in conjunction with statistical approximations. The electronic structure and the magnetic energy of both systems were calculated using the full potential linear augmented plane wave (FPLAPW) method as implemented in the Wien2k Code. The temperature effect was captured by introducing the Boltzmann statistical distribution to describe the orientation perturbation of the magnetic moments caused by thermal agitation. The magnetic moment was calculated in the temperature range from 1045 to 1175 K for alpha-iron and from 540 to 640 K for cementite. The modeling was performed for applied magnetic fields ranging from 1–2 T. The main assumption was that the effect of temperature and that of the applied magnetic field can be decoupled and, consequently, be treated separately. The calculated moments were used to estimate the paramagnetic susceptibility according to the linear relation χ=M/Bχ=M/B well established for paramagnetic systems. The calculated values were compared with published experimental measurements for iron and with values obtained from the Curie–Weiss law for cementite.