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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

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

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

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

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

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

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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.

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

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

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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

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

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

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Low temperature spectra of FeAs

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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

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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.

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

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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.

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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.

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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.

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