Perturbed angular correlation and x-ray diffraction studies on the alpha-beta phase transition in multiferroic bismuth ferrite

Multiferroic materials draw attention owing to the exhibition of more than one ferroic form (ferroelectricity, ferromagnetism or ferroelasticity) in the same phase. The main focus of this work lies on materials combining ferroelectricity with ferromagnetism as this combination theoretically allows direct electrically controlled magnetism and magnetically controlled electric polarisation. The advantages of such combinations are obvious by opening a whole new degree of freedom in material and device design, potentially expanding possibilities in storage management and computing power. One problem with most multiferroics is the fact that the magnetic Neél temperature and the electric Curie temperature often are low, usually below room temperature, leading to a loss of multiferroic behaviour in technically desired temperature ranges. However, Bismuth ferrite, also referred to as $BiFeO_3$ or simply BFO, is one promising candidate for industrial applications as its Neél and Curie temperature points are well above room temperature. Bismuth ferrite appears in three different phases, the $\alpha$-phase at room temperature and two high temperature phases namely the $\beta$-phase and the $\gamma$-phase before decomposing and melting, with all off them showing different crystal structures. As the outcome of numerous research groups showed different results regarding the $\beta$-phase, the nature of the $\beta$-phase and the $\alpha$-$\beta$ phase transition were supervised in this work. Perturbed Angular Correlation (PAC) measurements done at the ISOLDE Solid State Physics Group at ISOLDE, CERN in Geneva, Switzerland were combined with X-Ray diffraction (XRD) measurements done at the X-Ray center of the Vienna University of Technology in Vienna, Austria. The results confirm a proposed $\textit{Pbnm}$ crystal symmetry of the $\beta$-phase with a phase transition temperature of $T_{\alpha - \beta} = 820^\circ C$. The combination of the two measurement methods and especially the usage of $^{111m}$Cd as the PAC probe to characterize BFO was performed the first time by our group.