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At ambient pressure, the heavy rare earth manganites TmMnO3 and YbMnO3 form with the hexagonal YMnO3-type structure. However, they can also be synthesised under high pressure to form with the orthorhombic GdFeO3-type structure. These manganites are currently under intense scrutiny because each structural form exhibits a different multiferroic behaviour. In this project, a suite of Mössbauer resonances (57Fe, 170Yb and 169Tm) is used to probe the temperature dependence of the local magnetic and electrostatic interactions at the atomic level for both phases of these two manganites. A dilute concentration of 57Fe is demonstrated to provide a sensitive probe of the Mn sub-lattice. The Mn magnetisation is monitored as a function of temperature for both structural phases of YbMnO3 and the Néel temperatures are determined. In combination with point charge model calculations, the fitted orientation of the magnetic hyperfine field gives the alignment of the local Mn moments with respect to the crystallographic axes. For the hexagonal phases, the 170Yb resonance is used to determine the temperature of the onset of long range magnetic order for the Yb sub-lattice and the 169Tm-resonance yields an extensive set of hyperfine field data that are interpreted in terms of a simple 2-singlet ground state model based on crystal field parameters determined elsewhere using optical spectroscopy. In the investigation of o-TmMnO3, the magnetic Néel and IC E transitions reveal themselves as subtle increases in the absorption line widths. Jumps in the quadrupole interaction strength at these same transitions indicate an unexpected sensitivity to multiferroic influences. In addition to the Mössbauer investigation, the relationship between the average size of the ions at the rare earth site and the formation of an orthorhombic structure is explored. It is confirmed that the critical tolerance factor depends on the extent of the ionic size mismatch. |
