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**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

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

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**Mössbauer Spectroscopy**

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

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**Hyperfine Interactions**

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

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**Electric Field Gradient + Magnetic Hyperfine Field**

B = 10 T = 0° = 90°

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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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**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.

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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 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

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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. 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

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**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.

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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 Fe2As 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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