Presentation on theme: "Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, and Z. Bukowski 3 1."— Presentation transcript:
Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, and Z. Bukowski 3 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
Mössbauer Spectroscopy Laboratory at MSD Institute of Physics, Pedagogical University Cracow, Poland
Mössbauer Spectroscopy 1 mm/s 48 neV -ray energy is modulated by the Doppler effect due to the source motion vs. absorber Mössbauer spectrum
Hyperfine Interactions Isomer Shift Quadrupole Splitting Magnetic Splitting Electron Density Electric Field Gradient Magnetic Hyperfine Field B = 10 T 57 Fe Mössbauer spectra
Electric Field Gradient + Magnetic Hyperfine Field = 0° = 90° B = 10 T
A bit of formalism Relevant hyperfine Hamiltonian: Choice of the “convenient” reference frame: Transition and parameter dependence of the Hamiltonians:
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.
Spiral structure of the magnetic hyperfine field Parameterization of the spiral field:
Iron-arsenic phase diagram Landolt-Börnstein New Series IV/5
Structure of FeAs 1.Orthorhombic structure 2.The Pnma symmetry group 3.Arrows show Pna2 1 distortion 4.Quantization axes: abc - xyz Fe 5.All Fe atoms are equivalent within Pnma 6.Thermal ellipsoid is flattened along b-axis Orientation of magnetic spirals [0 k+1/2 0] iron and [0 k 0] iron
p-T phase diagram of FeAs J. R. Jeffries et al., Phys. Rev. B 83, (2011)
Magnetic structure of FeAs Polarized neutron scattering results E. E. Rodriguez et al., Phys. Rev. B 83, (2011)
Low temperature spectra of FeAs
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. B a and B b - 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 for [0 k+1/2 0] and [0 k 0] irons. T c - transition temperature - static critical exponent
FeAs Spectral shift S and quadrupole coupling constant A Q 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 / versus temperature Green points correspond to magnetically ordered region. Red point is the normalization point. Inset shows relative spectral area RSA plotted versus temperature.
Anisotropy of the recoilless fraction - FeAs Anisotropy disappears in the magnetic region
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.
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 Fe 2 As phase – irreversible process.
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.