Mechanically - induced disorder in CaFe$_2$As$_2$: a $^{57}$Fe M\"ossbauer study

$^{57}$Fe M\"ossbauer spectroscopy was used to study an extremely pressure and strain sensitive compound, CaFe$_2$As$_2$, with different degrees of strain introduced by grinding and annealing. At the base temperature, in the antiferromagnetic/orthorhombic phase, compared to a sharp sextet M\"ossbauer spectrum of single crystal CaFe$_2$As$_2$, which is taken as an un-strained sample, an obviously broadened sextet and an extra doublet were observed for ground CaFe$_2$As$_2$ powders with different degrees of strain. The M\"ossbauer results suggest that the magnetic phase transition of CaFe$_2$As$_2$ 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. The quadrupole shift in the magnetic phase approachs zero with increasing degrees of strain, indicating that the strain reduces the average lattice asymmetry at Fe atom position.