Magnetic anisotropy and lattice dynamics in FeAs studied by M\"ossbauer spectroscopy

TL;DR

This study uses Mössbauer spectroscopy to analyze magnetic anisotropy and lattice dynamics in FeAs across a wide temperature range, revealing complex magnetic behavior and lattice softening without structural transition.

Contribution

It provides detailed insights into the magnetic and lattice properties of FeAs, including the effects of spin spiral propagation and anisotropic lattice softening, with temperature-dependent vibrational anisotropy analysis.

Findings

Complex variation of hyperfine magnetic field with spin orientation.

Lattice softening occurs at 69.2 K without structural transition.

Distinct iron sites show different electron densities up to 300 K.

Abstract

Iron mono-arsenide in the powder form has been investigated by transmission 57Fe Moessbauer spectroscopy in the temperature range 4.2 - 1000 K. Additional spectra have been obtained at 20 K and 100 K applying external magnetic field of 7 T. It was found that the spin spiral propagating along the c-axis leads to the complex variation of the hyperfine magnetic field amplitude with the spin orientation varying in the a-b plane. The magnitude of the hyperfine field pointing in the direction of the local magnetic moment depends on the orientation of this moment in the a-b plane. Patterns are vastly different for iron located in the [0 k 0] positions and for iron in the [0 k+1/2 0] positions within the orthorhombic cell set to the Pnma symmetry. Lattice softens upon transition to the paramagnetic state at 69.2 K primarily in the a-c plane as seen by iron atoms. This effect is quite large considering lack of the structural transition. Two previously mentioned iron sites are discernible in the paramagnetic region till 300 K by different electron densities on the iron nuclei. The anisotropy of the iron vibrations developed at the transition to the paramagnetic state increases with the temperature in accordance with the harmonic approximation, albeit tends to saturation at high temperatures indicating gradual onset of the quasi-harmonic conditions. It seems that neither hyperfine fields nor magnetic moments are correct order parameters in light of the determined static critical exponents. Sample starts to loose arsenic at about 1000 K and under vacuum.