Magnetic and electrical transport properties of Ge_1-x Mn _x thin films

We report on preparation and systematic study of the structural, magnetic, and electrical transport properties of Ge1-x Mnx (x=25%-42%) alloy thin films grown on GaAs (001) substrates by molecular beam epitaxy. Amorphous Ge1-x Mnx, and coexistence of amorphous Ge1-x Mnx, Ge crystallites, and high TC secondary phases (Mn11 Ge8 and Mn5 Ge3) are obtained at substrate temperatures of 160 and 200-300 °C, respectively. The amorphous samples are found to consist of a low-temperature highly ordered spin-glass-like phase with an ordering temperature of TC ∼20 K (x∼0.39), increasing with the Mn composition, and a high-temperature "clustered dopant" phase with an ordering temperature of TC* ∼104 K (x∼0.39), increasing with both the Mn composition and applied external field. The magnetization of the low-temperature phase is found to be coupled antiferromagnetically with that of the high-temperature phase, leading to the appearance of a negative thermal remanent magnetization. Detailed magnetic and electrical measurement revealed that the low-temperature highly ordered spin-glass-like phase consists of both spin-glass-like phase and ferromagnetically ordered region. The amorphous samples also exhibit a negative magnetoresistance and an anomalous Hall effect at low temperature. The good agreement between the values of TC and TC* for amorphous samples and those of the two characteristic temperatures reported in literature for epitaxially grown samples suggests that the ferromagnetic phase of the latter observed in the temperature range of 110-120 K by some reports may not be of intrinsic origin. On the other hand, the samples grown at 300 °C are ferromagnetic up to room temperature and exhibit a positive magnetoresistance.