Abstract
This work addresses the study of the high-temperature phase sequence of by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that exhibits an incommensurate modulated orthorhombic structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above , the low-temperature modulated evolves monotonically into a fractionally growing Pnma structure up to . At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The space group is obtained from the Pnma one through the polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below , revealed by the relation between the weak ferromagnetism of the canted iron spins and the octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (β) value of 0.38. The lower limit of the phase coexistence temperature , marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.
1 More- Received 23 September 2021
- Accepted 2 November 2021
DOI:https://doi.org/10.1103/PhysRevB.104.174109
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