Oscillation of coercivity between positive and negative in Mn_xGe_1-x:H ferromagnetic semiconductor films

Amorphous Mn_xGe_1-x:H ferromagnetic semiconductor films prepared in mixed Ar with 20% H₂ by magnetron co-sputtering show global ferromagnetism with positive coercivity at low temperatures. With increasing temperature, the coercivity of Mn_xGe_1-x:H films first changes from positive to negative, and then back to positive again, which was not found in the corresponding Mn_xGe_1-x and other ferromagnetic semiconductors before. For Mn_0.4Ge_0.6:H film, the inverted Hall loop is also observed at 30 K, which is in consistence with the negative coercivity. The negative coercivity is explained by the antiferromagnetic exchange coupling between the H-rich ferromagnetic regions separated by the H-poor non-ferromagnetic spacers. Hydrogenation is a useful method to tune the magnetic properties of Mn_xGe_1-x films for the application in spintronics.