Mn substitution effect on the local structure of La(Fe_1−x Mn _x )AsO studied by temperature dependent x-ray absorption measurements

Abstract

The local structure of La(Fe1−x Mn x )AsO has been investigated using temperature dependent Fe K-edge extended x-ray absorption fine structure (EXAFS) measurements. The EXAFS data reveal distinct behavior of Fe–As and Fe–Fe atomic displacements with a clear boundary between x ⩽ 0.02 and x > 0.02. The Fe–As bondlength shows a gradual thermal expansion while the Fe–Fe bond manifests a temperature dependent anomaly at ∼180 K for x > 0.02. It is interesting to find characteristically different nature of Fe–As and Fe–Fe bondlengths shown by the temperature dependent mean square relative displacements. Indeed, the Fe–As bond, stiffer than that of the Fe–Fe, gets softer for x ⩽ 0.02 and hardly shows any change for x > 0.02. On the other hand, Fe–Fe bond tends to be stiffer for x ⩽ 0.02 followed by a substantial softening for x > 0.02. Such a distinction has been seen also in the As K-edge x-ray absorption near edge structure, probing local geometry around As atom together with the valence electronic structure. The results suggest that local atomic displacements by Mn substitution inducing increased iron local magnetic moment that should be the main reason for its dramatic effect in iron-based superconductors.

Temperature Dependence of the Local Structure and Iron Magnetic Moment in the Self-Doped CaKFe4As4 Iron-Based Superconductor

“…Figure shows the σ i 2 ( T ) of the two bond lengths in CaKFe 4 As 4 together with the corresponding Einstein model fits. The Fe–As bond is well described by the Einstein model with the θ E = (314 ± 12) K, very similar to the case of the other pnictides. − Similarly, temperature dependence of the σ i 2 for the Fe–Fe bond can be described by the Einstein model with θ E ∼ 228 ± 5 K, consistent with the earlier reports on the 122 system . The Einstein temperature of Fe–As bond ∼314 K (∼220 cm –1 ) should be associated with the infrared active Fe–As stretching mode (E u -symmetry) that has been observed ∼250 cm –1 in the optical studies of iron-based superconductors. ,, On the other hand, Fe–Fe Einstein temperature ∼228 K (∼158 cm –1 ) is possibly contributed by a hybrid Raman mode (A 1g + B 1g , the earlier is due to As axial motion while the latter is due to axial Fe vibrations).…”

Structural strain and competition between charge density wave and superconductivity in La(Fe,Mn)As(O0.89FMartinelli1, Carretta2, Moroni3 et al. 2021Phys. Rev. BSelf Cite3120View full textAdd to dashboardCite

“…The sample preparation and characterization by means of electrical resistivity, superconducting quantum interference devices (SQUID) magnetometry, muon spin rotation (μSR), nuclear quadrupole, 75 As nuclear magnetic resonance, and extended x-ray absorption fine structure (EXAFS) are accurately described in Refs. [13][14][15]22,23]. In particular, μSR analysis evidenced that for samples with x = 0.0020, 0.0010, and 0.0075 a fully magnetic state with 100% of magnetic volume is gained just below the magnetic transition temperature, similarly to what observed in the La(Fe 1-x Ru x )AsO system [24].…”

“…These studies disclose an alternative view for the phenomenology of Fe-based superconductors, indicating that CDW and superconductive (SC) states are adjacent and competing in the phase diagrams. Local structural analyses of the same samples evidenced an emerging disorder in the structure as well as a sharp increase in the Fe moment even with a very small amount of Mn substitution [22,23].…”