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Magnetism in CaFe2As2 and Phase Separation in Superconducting Ba0.5K0.5Fe2As2 and Sr0.5Na0.5Fe2As2 Single Crystals: A Mössbauer Study

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500/1-001 - Main Auditorium (CERN)

500/1-001 - Main Auditorium

CERN

CH - 1211 Geneva 23 Switzerland
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POSTER Magnetism and Magnetic materials - Bulk and thin layers

Speaker

Dalber Sanchez (Universidade Federal Fluminense)

Description

abstract

Summary

The ternary A1-xMxFe2As2 (A=Ca, Sr, Ba and Eu; M=K and Na) were found to have similar structural, magnetic and superconducting properties with the related RFeAsO1-xFx [1]. The Ca2Fe2As undergoes a first-order high-temperature h-T tetragonal to low-temperature l-T orthorhombic phase transition at TS~170K [2]. Concomitant with the structural transition the Fe moments order in a commensurate AFM structure [3]. This compound becomes superconducting either under moderate applied pressure and or Na-doping [4,5]. The Ba0.5K0.5Fe2As2 and Sr0.5Na0.5Fe2As2 are superconductors with Tc ~37 K and ~35 K, respectively. SR measurements have shown a coexistence of superconductivity and phase separated static magnetic order in these compounds [6].
Mössbauer spectroscopy have been used to investigate the magnetic and structural phase transition of CaFe2As2 as well as the occurrence of phase separation in superconducting Ba0.5K0.5Fe2As2 and Sr0.5Na0.5Fe2As2 single crystals. A mosaic of single crystal plates, with the c axes parallel to -ray direction, were built to perform the Mössbauer transmission measurements. Room temperature measurements revealed that the main component of electric field gradient Vzz is along c axis for these ternary compounds. For the non superconducting CaFe2As2 an abrupt increase of the magnetic hyperfine field Bhf below TN~170K was observed indicating a first-order magnetic transition. Low temperature spectra fits lead to Vzz >0 with Fe moments lying in the (a,b) plane. The quadrupole splitting EQ values have a discontinuity at ~170K confirming that structural and magnetic transition occurs concomitantly. The Mössbauer spectra of Ba0.5K0.5Fe2As2 and Sr0.5Na0.5Fe2As2 have a unique crystal site for Fe at room temperature, however at 4.2K the presence of two phases is clearly seen. For Ba0.5K0.5Fe2As2 ~ 51% of Fe is in a paramagnetic state while the remaining is in a magnetic phase with small magnetic moments (~0.15B). For Sr0.5Na0.5Fe2As2 only ~12% of Fe are paramagnetic, the remaining Fe are in a magnetic state with magnetic moments of the order of ~0.57B.

[1]C. Krellner, et al., Phys. Rev B 78 (2008)100504.
[2] N. Ni, et al., Phys. Rev. B 78 (2008) 014523.
[3] A. I. Goldman, et al., Phys. Rev B 78 (2008) 100506.
[4] M. S. Torikachvili, et al., Phys. Rev Lett. 101 (2008) 057006.
[5] P. M. Shirage, et al., Appl. Phys. Express 1 (2008) 081702.
[6] T. Goko, et al., Phys. Rev B 80 (2009) 024508.

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Primary author

Dalber Sanchez (Universidade Federal Fluminense)

Co-authors

Elisa Baggio-Saitovitch (Centro Brasileiro de Pesquisas Fisicas. Rua Xavier Sigaud 150. Urca. CEP 22290-180. RJ, Brazil) G. F. Cheng (Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, Peoples Republic of China) Julian Munevar (Centro Brasileiro de Pesquisas Fisicas. Rua Xavier Sigaud 150. Urca. CEP 22290-180. RJ, Brazil) Mariella Alzamora (Centro Brasileiro de Pesquisas Fisicas. Rua Xavier Sigaud 150. Urca. CEP 22290-180. RJ, Brazil) P. C. Canfield (3Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA) Sergey Budko (Ames Laboratory, U.S. DOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA)

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