Low-field microwave absorption in pulsed lased deposited FeSi thin films

Abstract

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. This signal has been observed in several other materials other than FeSi thin films. However in FeSi thin films it was for the first time that LFA signal was observed. The LFA is closely connected to the magnetization process that occurs at low applied field. LFA is a new technique that has recently been used to detect the magnetic transition in materials, sensitive detection of magnetic order and more importantly to distinguish between different dynamics of microwave absorption centres. The LFA measurements were made at 9.4 GHz (X-band) on pulse laser deposited (PLD) polycrystalline B20 cubic structure FeSi thin film grown on Si (111) substrate. PLD is regarded as a powerful tool for thin film growth. The LFA properties of the films were investigated as a function of DC field, temperature, microwave power and orientation of DC field with respect to the film surface. The LFA signal is very strong when the DC field is parallel to the film surface and diminishes at higher angles. This is attributed to induced anisotropy field (IAF) and surface anisotropy field (SAF) contributing to total anisotropy field (TAF). The LFA signal strength increases as the microwave power is increased, such increase is due to impedance and thus showing that LFA and magnetoimpedance (MI) has common origin. The LFA signal disappears around 340 K which can be attributed to the disappearance of long range order giving us a positive signature of surviving magnetic state well above room temperature in these films. We believe that domain structure evolution in low-fields, which in turn modifies the low field permeability as well as the anisotropy, could be the origin of LFA observed in these films. MI and LFA can be understood as the absorption of electromagnetic radiation by spin systems that are modified by domain configuration and strongly depend on anisotropy field. The observation of LFA opens the possibility of FeSi films to be used as potential candidates of low magnetic field sensors in the microwave and radio frequency regions. Copyright