Anomalous second ferromagnetic phase transition as a signature of spinodal decomposition in Fe-doped GeTe diluted magnetic semiconductor

Structural and magnetic properties of diluted magnetic semiconductor Ge1-xFexTe thin films are investigated. The conventional structure analysis shows c-axis orientation with columnar growth of the films and no indication of Fe clusters or second phase. Magnetic measurements combined with theory models reveal that two ferromagnetic phase transitions occur. We consider that the second ferromagnetic phase transition in high Fe content thin film is from a ferromagnetic phase with long range exchange interaction to a superparamagnetic phase with dipole interaction between Fe clusters, which can be viewed as a signature of spinodal decomposition in Ge1-xFexTe material.