Type of Document	Dissertation
Author	Li, Yongqing
URN	etd-08192004-105801
Title	Optimization of Hall Magnetometry and Single Magnetic Nanoparticle Measurements
Degree	Doctor of Philosophy
Department	Physics, Department of
Advisory Committee	Advisor Name Title Peng Xiong Committee Co-Chair Stephan von Molnar Committee Co-Chair Joe Schlenoff Committee Member Pedro Schlottmann Committee Member Susan Blessing Committee Member Zachary Fisk Committee Member
Keywords	Magnetometry Nanoparticle
Date of Defense	2003-12-01
Availability	unrestricted
Abstract This dissertation presents work on improving the sensitivity of Hall magnetometry for single magnetic nanoparticle measurement by miniaturizing the devices down to submicron range. Limiting factors for Hall device performance, including noise and mesoscopic eŽects, will be explored. The first systematic low-frequency Hall noise measurements on submicron GaAs/AlGaAs 2DEG devices have been carried out at temperatures between 1.5K and 75K in order to understand the microscopic origin of 1/f noise in the Hall signals and to improve device performance. A surprisingly large gating eŽect was found, which suppresses the 1/f noise level up to several orders of magnitude with application of a modest gate voltage. Detailed temperature and gate voltage dependences of the noise spectra have been analyzed, and the data suggest that the noise originates predominantly from impurity switching processes and their dynamics in the selectively doped AlGaAs layer. This remote origin of the noise is further supported by the fact that the noise is almost independent of electron temperature but varies strongly with lattice temperature. A crossover from thermal activation to quantum tunneling was observed at T < 10K. A moment sensitivity of 104 ľB/Hz1/2 at 1 Hz has been achieved in large applied background fields. With this improved sensitivity we have performed magnetic measurements on individual iron nanoparticles fabricated with a scanning tunneling microscopy assisted chemical vapor deposition technique. Magnetization reversal of such cylinder-shaped Fe nanoparticles with high aspect ratios (d ˇ 11 nm and h ź 100-120 nm) have been studied in tilted applied fields and at diŽerent temperatures. The results show that the magnetization reversal in these particles is an incoherent process despite the small diameters of the particles. These results are discussed in the context of recent work on individual electrodeposited nanowires, as well as various micromagnetic models.
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