1. Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors
A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, and Z. Bukowski3
1 Mössbauer Spectroscopy Division, Institute of Physics,
Pedagogical University, Cracow, Poland
2 Department of Solid State Physics, Faculty of Physics and Applied Computer Science,
AGH University of Science and Technology, Cracow, Poland
3 Institute of Low Temperature and Structure Research, Polish Academy of Sciences,
Wrocław, Poland
This work was supported by the National Science Center of Poland, Grant DEC-2011/03/B/ST3/00446
XVI KKN - XVI National Conference on Superconductivity
October 7-12, 2013: Zakopane, Poland
Seminarium Instytutu Fizyki UP, Kraków, 25 października 2013 r. (piątek): sala 513: 9.35

2. Mössbauer Spectroscopy Laboratory at MSD Institute of Physics, Pedagogical University Cracow, Poland

4. Mössbauer Spectroscopy
-ray energy is modulated by the Doppler effect due to the source motion vs. absorber
Mössbauer spectrum
1 mm/s  48 neV

5. Hyperfine Interactions
Isomer Shift
Quadrupole Splitting
Magnetic Splitting
57Fe Mössbauer spectra
B = 10 T
Electron Density
Electric Field Gradient
Magnetic Hyperfine Field

6. Electric Field Gradient + Magnetic Hyperfine Field
B = 10 T
 = 0°
 = 90°

7. A bit of formalism Relevant hyperfine Hamiltonian:
Choice of the “convenient” reference frame:
Transition and parameter dependence of the Hamiltonians:

8. For such axial ellipsoid aligned with the Cartesian
Lattice dynamics and transition intensity corrections:
Thermal ellipsoid for FeAs:
For such axial ellipsoid aligned with the Cartesian
quantization axes one has single anisotropy parameter.
For the present case ellipsoid is flattened along y-axis.

9. Spiral structure of the magnetic hyperfine field
Parameterization of the spiral field:

10. Iron-arsenic phase diagram
Landolt-Börnstein New Series IV/5

11. Structure of FeAs Orthorhombic structure The Pnma symmetry group
Arrows show Pna21 distortion
Quantization axes: abc - xyz
All Fe atoms are equivalent within Pnma
Thermal ellipsoid is flattened along b-axis
[0 k+1/2 0] iron and [0 k 0] iron
Orientation of magnetic spirals

12. p-T phase diagram of FeAs
J. R. Jeffries et al., Phys. Rev. B 83, (2011)

13. Magnetic structure of FeAs
Polarized neutron scattering results
E. E. Rodriguez et al., Phys. Rev. B 83, (2011)

14. Low temperature spectra of FeAs

15. Anisotropy of the hyperfine magnetic fields (spiral projections onto a-b plane) in FeAs
Left column shows [0 k+1/2 0] iron, right column shows [0 k 0] iron.
Ba and Bb - iron hyperfine field components along the a-axis and b-axis, respectively.
Orientation of the EFG
and
hyperfine magnetic field
in the main crystal axes
Average hyperfine fields <B>
for
[0 k+1/2 0] and [0 k 0] irons.
Tc - transition temperature
 - static critical exponent

16. FeAs Spectral shift S and quadrupole coupling constant AQ
versus temperature
for
[0 k+1/2 0] iron and [0 k 0] iron.
Line at 72 K separate
magnetically ordered region
from paramagnetic region.
Relative recoilless fraction <f>/<f0>
versus temperature
Green points correspond to magnetically ordered region.
Red point is the normalization point.
Inset shows
relative spectral area RSA plotted versus temperature.

17. Anisotropy of the recoilless fraction - FeAs
Anisotropy disappears in the magnetic region

18. Spectra in the external field anti-parallel to the beam - FeAs
Model 1 (different electron densities) is preferred, as for Model 2 one obtains unphysical diamagnetic „susceptibility”. There is significant anisotropy of the „susceptibility” even high above transition temperature.

19. High temperature spectra of FeAs
Model 1
Saturation of the recoilless fraction anisotropy above RT is an indication of the onset of the quasi-harmonic behavior.
Arsenic starts to evaporate at 1000 K and under vacuum leading to the Fe2As phase – irreversible process.

20. Conclusions
The iron hyperfine field along the electronic spin spiral varies enormously in amplitude
in the magnetically ordered region. The pattern resembles symmetry of 3d electrons in
the a-b plane with the significant distortion caused by the arsenic bonding p electrons.
Another unusual feature is strong coupling between magnetism and lattice dynamics i.e. very strong phonon-magnon interaction.
Static critical exponents suggest some underlying transition leading to the magnetic order. Due to the lack of the structural changes one can envisage some subtle order-disorder transition with very small latent heat and hysteresis driven by the itinerant charge/spin ordering.
The sample starts to loose arsenic at about 1000 K under vacuum, what might be explanation for the specific heat anomaly observed at high temperature.