Abstract:
This thesis comprehensively investigates the AC magnetic susceptibility of α-Fe2O3, which is a well-known spin-canted antiferromagnet (AFM) with Neel temperature TN around 948 K. Interestingly, it also turns to a pure AFM below 260 K, the Morin transition. This system was already explored in our lab through linear and non-linear susceptibility along the basal plane in a single crystal. The purpose was to explore the Morin transition, especially through the 3rd order susceptibility. However, some unusual features were observed not only in the 3rd order ac-χ but also in the 2nd order ac-χ. These measurements needed to be repeated along the basal plane and the c-axis. Additionally, field and frequency-dependent ac-χ data were also required. The present thesis reports both linear and nonlinear ac-χ, including 1st, 2nd and 3rd order susceptibility along both the axes of the crystal. These measurements are repeated in a new ac-χ setup with better sensitivity at low frequencies. The data are obtained as a function of temperature in various magnetic fields (H) and frequencies (f). Earlier measurements of DC magnetometry on the same crystal revealed signatures of double transition. Similar features are clearly observed in the 1st order ac-χ when data are recorded in close temperature intervals in the vicinity of the Morin transition. We also find unusual features in the 3rd order susceptibility as a function of temperature above the Morin transition. These oscillatory features are observed for linear and non-linear susceptibilities when the frequency is fixed at 133 Hz, but the field varies. Such oscillations are also observed at slightly higher frequencies. In addition, the 2nd harmonic is also measured as a function of time at various temperatures above the Morin temperature. Overall, some unusual features are observed, especially in the non-linear susceptibility. These features are reproducible, especially at lower frequencies. Further experiments and data analysis are needed to confirm the reproducibility and origin of unusual features as observed here.
