Mechanically-induced disorder in CaFe2As2: a 57Fe Mossbauer study

TL;DR

This study uses 57Fe Mossbauer spectroscopy to investigate how pressure and strain affect the magnetic and electronic properties of CaFe2As2, revealing strain-induced inhomogeneity and reduced lattice asymmetry.

Contribution

It provides the first microscopic insight into how mechanical strain influences magnetic phase transition and electronic environment in CaFe2As2 using Mossbauer spectroscopy.

Findings

Strain broadens the magnetic sextet and introduces a doublet in Mossbauer spectra.

Strain suppresses magnetic order in parts of the sample down to 4.6 K.

Strain reduces lattice asymmetry at Fe sites, approaching zero quadrupole shift.

Abstract

57Fe Mossbauer spectroscopy was used to perform a microscopic study on the extremely pressure and strain sensitive compound, CaFe2As2, with different degrees of strain introduced by grinding and annealing. At the base temperature, in the antiferromagnetic/orthorhombic phase, compared to a sharp sextet Mossbauer spectrum of single crystal CaFe2As2, which is taken as an un-strained sample, an obviously broadened sextet and an extra doublet were observed for ground CaFe2As2 powders with different degrees of strain. The Mossbauer results suggest that the magnetic phase transition of CaFe2As2 can be inhomogeneously suppressed by the grinding induced strain to such an extent that the antiferromagnetic order in parts of the grains forming the powdered sample remain absent all the way down to 4.6 K. However, strain has almost no effect on the temperature dependent hyperfine magnetic field in the grains with magnetic order. Additional electronic and asymmetry information was obtained from the isomer shift and quadrupole splitting. Similar isomer shift values in the magnetic phase for samples with different degrees of strain, indicate that the stain does not bring any significant variation of the electronic density at 57Fe nucleus position. The absolute values of quadrupole shift in the magnetic phase decrease and approach zero with increasing degrees of strain, indicating that the strain reduces the average lattice asymmetry at Fe atom position.