The magnetic behavior of cubic B20 crystal structure FeSi thin film has been previously probed at macroscopic level using a magnetometer. The results revealed ferromagnetic state with significant hysteresis. This is contrary to the bulk with the same cubic B20 crystal structure that is paramagnetic. The origin of ferromagnetism in thin films in contrast to paramagnetism in the bulk is unclear and unexplained. Electron spin resonance technique (ESR) was used as a tool to characterize the magnetic behavior of FeSi thin films at microscopic level. With ESR technique, B20 crystalline FeSi show microwave power absorption centred at zero field (HDC = 0) termed low-field microwave absorption (LFA) in addition to usual ferromagnetic resonance (FMR) typical of magnetic materials. LFA was observed as a distinct signal in these films. Magnetic thin films are promising candidates in SPINTRONICS and low-field magnetic sensors. This book is a centre of interest to researchers in SPINTRONICS and electronics who are keen to construct advanced memory devices. This work is also ideal to researchers working with magnetic sensors.