Magnetisation of the Aurivillius phase with m =<sans-serif> 4</sans-serif> revisited

We report the successful synthesis of the Aurivillius phase Bi(m+1)Fe(m-3)Ti3O3(m+3) with m = 4 via the solid-state reaction method. X-ray diffraction confirms the structure as a four-layer Aurivillius phase, and elemental analysis (EDA) shows no impurities. The magnetic properties, including temperature-dependent FC and ZFC magnetisation, were measured from helium to room temperature, aligning well with literature data. These dependences, when plotted on double logarithmic scales, show linear behaviour. However, a significant difference between simulated and experimental magnetisation profiles was observed. The simulation predicted a logistic S-shaped temperature dependence, which was not reflected in the experimental results. To address this discrepancy, we analyzed the noisy experimental data by applying the least squares local linear filter before differentiation. This analysis revealed a hidden pattern that matches the theoretical predictions, suggesting that the observed magnetisation profile results from the simultaneous contributions of two magnetic phases in the m = 4 Aurivillius multiferroic: the Aurivillius phase and an ensemble of oxygen vacancies associated with F+ defects.