Mössbauer spectroscopy measurements were performed for the temperature range between 4.2 K and 300 K in a transmission geometry applying 14.41-keV resonant line in 57 Fe for PrFeAsO the latter being a parent compound of the iron-based superconductors belonging to the '1111' family. It was found that an itinerant 3d magnetic order develops at about 165 K and it is accompanied by an orthorhombic distortion of the chemical unit cell. A complete longitudinal 3d incommensurate spin density wave (SDW) order develops at about 140 K. Transferred hyperfine magnetic field generated by the praseodymium magnetic order on iron nuclei is seen at 12.8 K and below, i.e., below magnetic order of praseodymium magnetic moments. It is oriented perpendicular to the field of SDW on iron nuclei. The shape of SDW is almost rectangular at low temperatures and it transforms into roughly triangular form around "nematic" transition at about 140 K. Praseodymium magnetic order leads to the substantial enhancement of SDW due to the large orbital contribution to the magnetic moment of praseodymium. A transferred field indicates presence of strong magnetic susceptibility anisotropy in the [b-c] plane while following rotation of praseodymium magnetic moments in this plane with lowering temperature. It was found that "nematic" phase region is a region of incoherent spin density wavelets typical for a critical region.2
The paper deals with the hyperfine interactions observed on the 57 Fe nucleus in multiferroic BiFeO 3 by means of the 14.41-keV resonant transition in 57 Fe, and for transmission geometry applied to the random powder sample. Spectra were obtained at 80 K, 190 K and at room temperature. It was found that iron occurs in the high spin trivalent state. Hyperfine magnetic field follows distribution due to the elliptic-like distortion of the magnetic cycloid. The long axis of the ellipse is oriented along 111 direction of the rhombohedral unit cell. The hyperfine magnetic field in this direction is about 1.013 of the field in the perpendicular direction at room temperature. This ratio diminishes to 1.010 at 80 K. Axially symmetric electric field gradient (EFG) on the iron atoms has the principal axis oriented in the same direction and the main component of the EFG is positive. Our results are consistent with the finding that iron magnetic moments are confined to the ] 1 2 1 [ crystal plane.
This contribution is a review concerned with the microscopic characterization of complex materials by using transmission Mössbauer spectroscopy -mainly 14.4-keV resonant transition in 57 Fe. Attention is focused on the novel superconductors, i.e. iron-based superconductors, which are extensively investigated in our Mössbauer laboratory, primarily versus sample temperature. Iron-based superconductors make four major families based on the corrugated nearly-two-dimensional sheets of either strongly bound iron-pnictogen or iron-chalcogen atoms. Usually, superconductivity is induced by doping or applying pressure to the parent compound, except the simplest compounds of the '11' family. One can dope any kind of atom within the compound in isovalent, hole-doping or electrondoping fashion. Parent compounds exhibit itinerant magnetic order of the 3d (iron) character. It appears as spin density wave (SDW) of the antiferromagnetic type incommensurates with the respective lattice period and of the complex shape. For a majority of cases, it is a longitudinal SDW propagating along the a-axis of the orthorhombic unit cell being created at the magnetic order from the tetragonal celldue to the magneto-elastic forces. On the other hand, the 3d magnetism and orthorhombic distortion are gone for superconductors as shown by the Mössbauer spectra obtained versus temperature, and by spectra obtained in the strong external magnetic field at low temperatures -stronger than the first critical field for these second kind superconductors. However, superconductivity is intimately related to these layered structures with the electronic charge modulation, leading to the charge density wave (CDW) on iron nuclei -observed as variation of the isomer shift. What is more, one observes closely related modulation of the electric field gradient on iron nuclei called electric field gradient wave (EFGW). The shape of these modulations changes rapidly at the superconducting gap: opening and relaxing back once the bosonic system of Cooper pairs is well separated from the rest of the electronic system. It was found that localized 4f magnetic moments order within the superconducting phase in a similar fashion as in the normal phase.
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