Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors

1. Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2,3, and Z. Bukowski4 1 Mössbauer Spectroscopy Division, Institute of Physics Pedagogical University, Cracow, Poland 2 Academic Centre for Materials and Nanotechnology 3Department of Solid State Physics, Faculty of Physics and Applied Computer Science AGH University of Science and Technology, Cracow, Poland 4 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 X Ogólnopolskie Seminarium Spektroskopii Mössbauerowskiej Wrocław, 15 – 18 czerwca 2014 Prezentacja ustna

2. Mössbauer Spectroscopy Laboratory at MSDInstitute of Physics, Pedagogical UniversityCracow, Poland

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

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. 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: www.elektron.up.krakow.pl/mosgraf-2009

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 Featoms 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, 134520 (2011)

13. Magnetic structure of FeAs Polarized neutron scattering results E.E. Rodriguez et al., Phys. Rev. B83, 134438 (2011)

14. Preparation of the FeAs single crystals by the Sn-flux methoddr Zbigniew Bukowski

15. Low temperature spectra of FeAs

16. 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. Baand 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

17. 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 tomagnetically ordered region. Red point is the normalization point. Inset shows relative spectral area RSA plotted versus temperature.

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

19. 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” evenhigh above transition temperature.

20. High temperature spectra of FeAs Model 1 Saturation of the recoilless fractionanisotropy 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.

21. 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. Previous important Mössbauer work: L. Häggström et al., Europhys. Lett. 9, 87 (1989)

22. Thank you very much for your attention !