Presentation on theme: "A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, and Z. Bukowski3"— Presentation transcript:
1Peculiar magnetism of the FeAs – grand parent of the iron-based superconductors A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, and Z. Bukowski31 Mössbauer Spectroscopy Division, Institute of Physics,Pedagogical University, Cracow, Poland2 Department of Solid State Physics, Faculty of Physics and Applied Computer Science,AGH University of Science and Technology, Cracow, Poland3 Institute of Low Temperature and Structure Research, Polish Academy of Sciences,Wrocław, PolandThis work was supported by the National Science Center of Poland, Grant DEC-2011/03/B/ST3/00446XVI KKN - XVI National Conference on SuperconductivityOctober 7-12, 2013: Zakopane, PolandSeminarium Instytutu Fizyki UP, Kraków, 25 października 2013 r. (piątek): sala 513: 9.35
2Mössbauer Spectroscopy Laboratory at MSD Institute of Physics, Pedagogical University Cracow, Poland
4Mössbauer Spectroscopy -ray energy is modulated by the Doppler effect due to the source motion vs. absorberMössbauer spectrum1 mm/s 48 neV
5Hyperfine Interactions Isomer ShiftQuadrupole SplittingMagnetic Splitting57Fe Mössbauer spectraB = 10 TElectron DensityElectric Field GradientMagnetic Hyperfine Field
6Electric Field Gradient + Magnetic Hyperfine Field B = 10 T = 0° = 90°
7A bit of formalism Relevant hyperfine Hamiltonian: Choice of the “convenient” reference frame:Transition and parameter dependence of the Hamiltonians:
8For such axial ellipsoid aligned with the Cartesian Lattice dynamics and transition intensity corrections:Thermal ellipsoid for FeAs:For such axial ellipsoid aligned with the Cartesianquantization axes one has single anisotropy parameter.For the present case ellipsoid is flattened along y-axis.
9Spiral structure of the magnetic hyperfine field Parameterization of the spiral field:
10Iron-arsenic phase diagram Landolt-Börnstein New Series IV/5
11Structure of FeAs Orthorhombic structure The Pnma symmetry group Arrows show Pna21 distortionQuantization axes: abc - xyzAll Fe atoms are equivalent within PnmaThermal ellipsoid is flattened along b-axis[0 k+1/2 0] iron and [0 k 0] ironOrientation of magnetic spirals
12p-T phase diagram of FeAs J. R. Jeffries et al., Phys. Rev. B 83, (2011)
13Magnetic structure of FeAs Polarized neutron scattering resultsE. E. Rodriguez et al., Phys. Rev. B 83, (2011)
15Anisotropy 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 EFGandhyperfine magnetic fieldin the main crystal axesAverage hyperfine fields <B>for[0 k+1/2 0] and [0 k 0] irons.Tc - transition temperature - static critical exponent
16FeAs Spectral shift S and quadrupole coupling constant AQ versus temperaturefor[0 k+1/2 0] iron and [0 k 0] iron.Line at 72 K separatemagnetically ordered regionfrom paramagnetic region.Relative recoilless fraction <f>/<f0>versus temperatureGreen points correspond to magnetically ordered region.Red point is the normalization point.Inset showsrelative spectral area RSA plotted versus temperature.
17Anisotropy of the recoilless fraction - FeAs Anisotropy disappears in the magnetic region
18Spectra 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.
19High temperature spectra of FeAs Model 1Saturation 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.
20ConclusionsThe iron hyperfine field along the electronic spin spiral varies enormously in amplitudein the magnetically ordered region. The pattern resembles symmetry of 3d electrons inthe 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.