An evidence of local structural disorder across spin-reorientation transition in DyFeO3\mathrm{DyFeO_{3}} : an extended x-ray absorption fine structure (EXAFS) study

The present work is aimed at exploring the local atomic structure modifications related to the spin reorientation transition (SRT) in DyFeO$_3$ orthoferrite exploiting x-ray absorption fine structure (XAFS) spectroscopy. For this purpose we studied by XAFS the evolution of the local atomic structure around Fe and Dy as function of temperature (10–300 K) in a DyFeO$_3$ sample having the SRT around 50–100 K. For sake of comparison we studied a YFeO$_3$ sample having no SRT. The analysis of the extended region has revealed an anomalous trend of Fe–O nearest neighbour distribution in DFO revealing (i) a weak but significant compression with increasing temperature above the SRT and (ii) a peculiar behavior of mean square relative displacement (MSRD) [$\sigma^2$] of Fe–O bonds showing an additional static contribution in the low temperature region, below the SRT. These effects are absent in the YFO sample supporting these anomalies related to the SRT. Interestingly the analysis of Dy L$_3$-edge data also reveal anomalies in the Dy–O neighbour distribution associated to the SRT, pointing out a role of magnetic Re ions across $T_{SRT,Fe}$. These results point out micro-structural modification at both Fe and Dy sites associated to the magnetic transitions in DFO, it can be stated in general terms that such local distortions across $Fe^{3+}$ and magnetic Re$^{3+}$ may be present in other orthoferrites exhibiting multiferroic nature

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