The interfacial properties of the Co ₂ FeAl _0.5 Si _0.5/ MgO based magnetic tunnel junction have been investigated using x ray absorption spectroscopy (XAS), angle resolved x ray photoelectron spectroscopy (ARXPS), x ray magnetic circular dichroism (XMCD), and element-specific hysteresis loops. The XAS demonstrates a multiplet structure at the Co L₃ edge which could be attributed to the formation of CoO at the interface due to the high annealing temperature. The XMCD sum-rule analysis and the element-specific hysteresis loops show a higher magnetic moment, a change in the loop shape, and an increase in the Co coercive field when probing more close to the interface layer. The chemical and structural disorder at the interface has been further revealed by the ARXPS measurements.