The spin tunnel junction with perfectly spin-polarized ferromagnet La1-_xSr_xMnO₃ (LSMO) can be one of the best candidates for magnetic random access memory. However, in the actual junctions, e.g. LSMO/SrTiO₃ (STO)/LSMO, only an inferior tunnel magnetoresistance has been observed. We have succeeded in probing the local magnetic properties at the magnetic heterointerface by Magnetization-induced Second Harmonic Generation (MSHG), and we confirmed that the interface ferromagnetism is deteriorated at the LSMO/STO interface. We have shown that by grading the doping profile on an atomic scale at the interface, robust ferromagntism can be realized even around room temperature, which leads to the improvement of the performance of spin tunnel junctions.