SNR Enhancement Technique for FMR

• Main Authors: Han-Zhang Chen, 陳含章
• Other Authors: Chi-Kuen Lo
• Format: Others
• Language: zh-TW
• Online Access: http://ndltd.ncl.edu.tw/handle/72380452959582090353

碩士 === 國立臺灣師範大學 === 物理學系 === 103 === Abstract The measurement of Ferromagnetic resonance is a powerful tool to study the characteristics of spin dynamics. Though having developed a Ferromagnetic Resonance Spectrometer (FMR) based on the Vector-Network-Analyzer (VNA) in our laboratory, the sensitivity and acquisition rate of the spectrometer has more room for improvement. Thus, we studied the spectrometer with different methods and technique to improve the signal-to noise ratio (SNR), and found the best parameters for measurements. It is because the internal signal processing of VNA slower the acquisition rate. If the period of measurements is too long, it will overheat the electromagnet coil and suspend the experiment. In order to accelerate the acquisition rate, we skipped the internal signal processing of VNA and let the microwave detector to accomplish the measurements. Thus, VNA was just the source of X-band microwave which emits the wave into cavity via the circulator. The reflection of the wave by the sample in the cavity will be transmitted to the detector through another port of circulator, so we can combine the signal of reflection power with the measurements of applied field to get the FMR spectrum. Though enhancing the acquisition rate, it sacrifices the signal to noise ratio (SNR). In order to improve the SNR, we employed lock-in technique with a small modulation field which superimposes on the DC magnetic field. Due to the effect of the Eddy current on the metallic surface of the cavity, the modulation field decreases as modulation frequency increases, thus , the best performance of signal was about 3kHz with modulation field 5.5 gauss. In the situation that time constant of Lock-in amplifier was set to 100ms and thickness of Permalloy film is about 10nm, the SNR was higher than 300 which was better than the performance of VNA-FMR.To further enhance the SNR, it should improve the modulation field between 10 and 20 Gauss at high frequency, so the problem of the eddy current must be solved. The phenomenon of ferromagnetic resonance can be described by the Landau-Lifshitz-Gilbert equation(LLG eq.).With the solution of LLG eq. and the experiment data of FMR, we can obtain magnetic parameters such as anisotropy constant, exchange coupling factor, damping constant, etc. In the thesis, we use the Matlab program code to obtain the magnetic parameters of the Permalloy thin film.