Magnetism in four-layered Aurivillius Bi5_5FeTi3_3O15_{15} at high pressures

We report the structural and magnetic properties of four-layer Aurivillius compound Bi$_5$FeTi$_3$O$_{15}$ (BFTO) at high hydrostatic pressure conditions. The high-pressure x-ray diffraction (XRD) data does not explicitly show structural phase transitions with pressure, however the observed changes in lattice parameters indicate structural modifications at different pressure values. In the initial pressure region values (up to 2.2 GPa), the lattice parameters a- and b- are nearly equal implying a quasi-tetragonal structure, however as the pressure increases a- and b- diverges apart and exhibits complete orthorhombic phase at pressure values of about 8 GPa. Principal component analysis (PCA) of high pressure Raman measurements point out an evident change in the local structure at about 5.5 GPa indicating that the evolution of the local structure under applied pressure seems to not follow crystallographic changes (long range order). Nuclear forward scattering (NFS) measurements reveal the development of magnetic ordering in BFTO at 5K with high pressures. A progressive increase in magnetic order is observed with increase in pressure at 5 K. Further, NFS measurements carried out at constant pressure (6.4 GPa) and different temperatures indicate that the developed magnetism disappears at higher temperatures (20 K). It is attempted to explain these observations in terms of the observed structural parameter variation with pressure