Capacitance effect on spin-torque oscillators with thermal fluctuation

xviii, 131 p. : ill. ; 30 cm.PolyU Library Call No.: [THS] LG51 .H577M AP 2009 GuanThis thesis describes the theoretical study and numerical simulation of spin-torque oscillators (STOs) based on a typical giant magneto-resistance (GMR) trilayer spin valve system and magnetic tunneling junction (MTJ) structure. Theory predicted that a spin-polarized current can exert a torque on a nano-scale magnet. This torque can excite magnetization precessional motion with a steady microwave frequency ranging from 5 to 40 GHz that is tunable by adjusting current and applied magnetic ffeld. The quality factor Q can be as high as 18,000. The high Q value and the current tunability of these STOs suggest their potential application in microwave signal processing. However, the very limited power output (typically less than 1 nW) has to be improved dramatically up to about 1 mW for any realistic application. In our STO model, typical trilayer spin valve device structures have been adopted, with one 'fixed' and one 'free' ferromagnetic layer separated by a nonmagnetic spacer. The dynamics of the 'free' layer magnetization is determined by the Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. To study the temperature effects on spin torque valve behavior, we model the thermal fhctuations by adding a Langevin random feld into the effective ffeld in the LLGS equation. The added thermal field is a fhctuating random field whose statistical properties obey the Gaussian distribution. The capacitance effect on the microwave power spectra of an STO was initially investigated with thermal fluctuation. Here, an ideal capacitance is connected in parallel with the STO to represent the intrinsic capacitance of the structure or that of connecting leads, and a dc current source is applied. The emitted power at a given frequency is derived by Fast Fourier Transform, and the data are fitted to a Lorentzian profle, from which the quality factors Q are calculated. In the presence of thermal fhctuations, the microwave power spectrum gets broadened with increasing temperature. We have found that the capacitance effect does inflience the stability of the system, and it can improve the Q factor value of the STO element even in the presence of thermal fluctuation. It is hence possible that one of the underlying reasons for the high quality factors observed in experiments may be ascribed to either intrinsic or extrinsic sources of capacitance in parallel with the STO. Apart from the thermal stability, we studied the capacitive tuning effect on oscillation frequencies. The LLGS equation has been developed to simulate the dynamics of precessional modes in the case of MTJ by adding a perpendicular term in the Slonczewski 'in-plane' spin-torque term. It is found that the oscillation frequency only varies with the values of capacitance, and will not change much with the temperature in both GMR and MTJ based situations. The precessional mode is determined by the applied dc current and the oscillation frequencies can be tuned by varying the capacitance. For the 'in-plane' (IP) precessional mode, an increase in capacitance would bring about a decrease in oscillation frequency for the GMR configuration; for the 'out-of-plane' (OOP) mode, the frequency shifts in the opposite way. For MTJ, the frequency does not vary monotonically with capacitance, which is quite an unexpected result. When the dc current is just below the IP/OOP critical region, where only IP precessional mode is expected, it is found that the mode can be switched to OOP with increasing capacitance. An analytical theory has been borrowed to explain the mechanism.Department of Applied PhysicsM.Phil., Dept. of Applied Physics, The Hong Kong Polytechnic University, 200