Synthesis and magnetic characterization of coaxial Ge_1-xMn _x/a-Si Heterostructures

A method for synthesizing Ge_1-xMn_x/a-Si core-shell nanowires (x = 0.3(1)-1.0(2)) using a supercritical fluid deposition technique, with a homogeneous distribution of manganese along the entire lengths of the crystalline Ge cores but not in the a-Si shells, is reported. Investigations into the magnetic properties of the heterostructured nanowires revealed a significant influence of the amorphous Si shell covering the surface of the core Ge_0.997Mn_0.003 nanowires compared to pristine Ge _0.997Mn_0.003 nanowires with no a-Si coating. The magnetic data revealed diminished values of both the remanence and the saturation magnetization for pristine Mn-doped Ge nanowires at higher temperatures when compared to the Ge_1-xMn_x/a-Si core-shell nanowires, whereas both parameters increased as the temperature dropped down to 5 K. Differences in the temperature-dependent evolution of the coercivity were observed in the magnetically harder core-shell nanowires compared to the pristine Ge_0.997Mn_0.003 nanowires, showing a drop of 26% at 5 K vs room temperature compared to a drop of 66% for the pristine nanowires. The low dopant concentration (0.3(1)%) of Mn in the core-shell nanowires, combined with the observed ferromagnetic properties, suggests a combination of hole-mediated exchange and confinement processes are responsible for the observed properties. Our observations show the importance of a protective layer in covering the oxidation-sensitive dilute magnetic semiconductor nanowires.