Thermodynamic prediction on the formation of oxides in MgO-based magnetic tunnel junctions

The equilibrium oxygen partial pressures for various oxides, which can be formed in CoFeB/MgO/CoFeB junctions, are calculated over a wide temperature range of 298 K ∼ 1000 K by using available data for the temperature dependences of the heat capacities of the constituent elements. The equilibrium oxygen partial pressure is lowest in MgO over the entire temperature range, indicating that MgO is the most stable oxide among the various oxides. The next most stable oxide is B₂O₃, followed by Fe oxides (FeO, Fe₃O₄ and Fe₂O₃) and Co oxides (CoO and Co₃O₄). The same order of stability is predicted from the results for the standard free energy of formation per mol of oxygen and from those for the standard enthalpy of formation per mol of oxygen, except for the Fe oxides. The relative stability among the Fe oxides is incorrectly predicted from the standard enthalpy of formation per mol of oxygen, indicating the need for great care in using this thermodynamic parameter for the prediction of phase stability. The standard free energy of formation and the standard enthalpy of formation per mol of oxides, which were used frequently in the past, are shown not to be suitable for the prediction of oxide formation.