Magnetotransport Studies of Mn Ion-Implanted Nanowires

Sammanfattning: This thesis focuses on the magnetotransport properties of highly Mn-doped crystalline GaAs nanowires. The GaAs nanowires were first grown by metal-organic vapor phase epitaxy from gold seed particles, and subsequently implanted with Mn ions under varying conditions, e.g., ion fluence and acceleration voltage. The implantation process was carefully analyzed and optimized within the research project. The resulting Mn-concentration in the nanowires ranges from 0.0001% to 5%. The implantation was carried out at elevated temperatures to facilitate dynamic annealing conditions at which most of the implantation-related defects are removed. After implantation, the nanowires were mechanically removed from the substrate to specially designed insulating SiO2/Si substrates optimized for magnetotransport measurements. The single nanowires were supplied with four contacts, defined by electron-beam lithography, for accurate transport measurements. The resistance of GaAs and GaAs: Zn nanowires was meticulously measured and analyzed in the temperature range from 300K to 1.6K, and with magnetic fields ranging from 0T to 8T. The magnetic field was applied both parallel and perpendicular to the nanowires. In addition, the magnetic properties of nanowires were probed using a superconductivity quantum interference device (SQUID) setup. The typical resistance for a highly Mn-doped (5%) nanowire increases from a few MOhm at 300K to several GOhm at 1.6K. More specifically, the temperature-dependence of the resistance shows transport regimes described by different models. The current-voltage characteristics become strongly non-linear as the temperature decreases and shows apparent power-law behavior at low temperatures. The transport data, from 50K to 180K, are interpreted in terms of the variable range hopping (VRH) mechanism and from 180K to 300K in terms of a nearest neighbor hopping (NNH) mechanism; both occur due to the disorder in the nanowires resulting from the implantation of Mn. Below 50K, the magnetotransport data exhibit a large 40% negative magnetoresistance with the magnetic field applied either in parallel or perpendicular to the nanowire. Complementary SQUID measurements under zero-field-cooled and field-cooled conditions, recorded at low magnetic fields, exhibit clear signs of the onset of a spin-glass phase with a spin-freezing temperature of about 16K. The high magnetoresistance is explained in terms of spin-dependent hopping in a complex magnetic nanowire landscape of magnetic polarons, separated by intermediate regions of Mn-impurity spins, forming a paramagnetic/spin-glass phase. Finally, magnetotransport experiments were carried out in a series of in-situ Zn-doped (p-type) GaAs nanowires implanted with different Mn-concentrations. The nanowires with the lowest Mn-concentration exhibit a low resistance of a few kOhms at 300K and a 4% positive magnetoresistance at 1.6K, which is well described by invoking a spin-split sub-band model, unlike nanowires with the highest Mn-concentration which show a high resistance of several MOhms at 300K and a large negative magnetoresistance of 85% at 1.6K. Sweeping the magnetic field back and forth for the samples with highest Mn-concentration reveals a small hysteresis, which signals the presence of a weak ferromagnetic state. Thus, co-doping with Zn appears promising for the goal of realizing ferromagnetic GaMnAs nanowires for future nanospintronics. In summary, this thesis shows that Mn-implanted GaAs nanowires indeed represent an interesting novel type of nanometer-scale building block for miniaturized spintronic devices compatible with mainstream silicon technology.