57 Fe and 151 Eu Mssbauer studies of

57 Fe and 151 Eu Mössbauer studies of 3 d-4 f spin interplay in Eu. Fe 2 -x. Nix. As 2 K. Komędera 1, J. Gatlik 1, T. T. H. Nguyen 1, A. Błachowski 1, J. Żukrowski 2, D. Rybicki 3, M. Babij 4, and Z. Bukowski 4 1 Mössbauer Spectroscopy Laboratory, Institute of Physics, Pedagogical University, Kraków, Poland 2 AGH University of Science and Technology, Academic Centre for Materials and Nanotechnology, Kraków, Poland 3 AGH University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland 4 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland ----------------------------------------------------------------------------------------------------- The International Conference on Solid Compounds of Transition Elements April 12 -15, 2021 1 / 14

Outline 1. 122 family of iron-based supeconductors 2. Eu. Fe 2 As 2 3. 57 Fe 4. Eu. Fe 2 -x. Nix. As 2 and 151 Eu Mössbauer spectroscopy ----------------------------------------------------------------------------------------------------- The International Conference on Solid Compounds of Transition Elements April 12 -15, 2021 2 / 14

Fe-based Superconducting Families pnictogens: P, As chalcogens: S, Se, Te nonsuperconducting parent compounds Ba. Fe 2 As 2 Sr. Fe 2 As 2 Ca. Fe 2 As 2 Eu. Fe 2 As 2 KFe 2 As 2 Rb. Fe 2 As 2 Cs. Fe 2 As 2 low Tc superconductors 3 / 14

Eu. Fe 2 As 2 Two magnetic sublattices • Fe 2+ 3 d itinerant electrons Spin Density Wave Fe saturation moment 0. 99 µB along a axis TSDW = 190 K • Eu 2+ 4 f localised electrons A-type Antiferromagnet , µeff = 7. 94 µB Y. Xiao et al. PRB 80, 174424 (2009) TN = 19 K A-type AFM of Eu 2+ moments means that they are coupled FM within the plane and AFM between the planes. Adjacent Eu planes are about 6 Å apart, hence direct overlap of interplanar 4 f orbitals can be neglected. Eu. Fe 2 As 2 -based compounds seem to be a unique laboratory for investigations of interplay between magnetism and superconductivity, as well as they are a playground for peculiar competition between itinerant 3 d order and localized 4 f moments. 4 / 14

_______Parent compound_______ Eu. Fe 2 As 2 TSDW = 190 K , TN = 19 K _____Superconductors_____ Eu 1 -x. Kx. Fe 2 As 2 Eu(Fe 1 -x. Cox)2 As 2 Eu. Fe 2(As 1 -x. Px)2 hole-doping electron-doping isovalent-substitution x = 0. 50 Tsc = 32 K x = 0. 14 Tsc = 11 K x = 0. 30 Tsc = 27 K J. Maiwald et al. PRB 85, 024511 (2012) W. T. Jin et al. PRB 94, 184513 (2016) G. Cao et al. J. Phys: Cond. Matt. 23, 464204 (2011) 5 / 14

57 Fe Mӧssbauer spectroscopy Hyperfine interactions Electric Quadrupole Interaction D – Quadrupole Splitting Electric Field Gradient EFG Magnetic Dipole Interaction Magnetic Splitting Magnetic hyperfine field B B 6 / 14

Spin density wave (SDW) seen by 57 Fe Mössbauer spectroscopy h 2 n-1 – amplitudes of SDW subsequent harmonics q – wave number x – relative position of the resonant nucleus along SDW hyperfine field distribution 57 Fe Mössbauer spectrum 7 / 14

Spin reorientation seen by 151 Eu Mössbauer spectroscopy Electric Field Gradient EFG + Magnetic hyperfine field B 8 / 14

57 Fe Mössbauer spectra Eu. Fe 2 As 2 151 Eu Mössbauer spectra Shape of SDW B = 27. 4 T = 90º TSDW = 192 K 0 = 0. 124 Mean squared amplitude of SDW vs. temperature 9 / 14

Eu. Fe 2 -x. Nix. As 2 The superconductivity is not observed down to 1. 8 K Relative resistivity vs. temperature Z. Ren et al. PRB 79, 094426 (2009) Our samples: Resistivity anomaly associated with SDW transition is clearly visible for x 0. 12 10 / 14

Eu. Fe 2 -x. Nix. As 2 - 57 Fe Mössbauer spectra <B> - Fe-SDW Bt / Bt - transferred from Eu 11 / 14

Eu. Fe 2 -x. Nix. As 2 - 57 Fe Mössbauer spectra Hyperfine field of SDW <B> and corresponding Fe itinerant moment vs. Ni-content x 1. The spin-density-wave (SDW) order of the Fe itinerant moments is monotonically suppressed by Ni-substitution. However, the 3 d magnetic order survives at the lowest temperature up to at least x = 0. 12 and it is certainly completely suppressed for x = 0. 2. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu 2+ ordering for Ni-substituted samples with x > 0. 04, while the transferred field is undetectable in Eu. Fe 2 As 2 and for compound with a low Ni-substitution level. It seems that the 4 f ferromagnetic component arising from a tilt of the Eu 2+ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms. 12 / 14

Eu. Fe 2 -x. Nix. As 2 - 151 Eu Mössbauer spectra Eu moments rotate from a-axis to c-axis 3. The Eu localized moments order regardless of the Ni concentration, but undergo a spin reorientation with increasing x from the alignment parallel to the a-axis in the parent compound, toward c-axis alignment for x > 0. 07. 2+ 13 / 14

Eu. Fe 2 -x. Nix. As 2 3 d-4 f spin interplay The change of the 4 f spins order from AFM to FM takes place simultaneously with a disappearance of the SDW 3 d spins order. It is the evidence of a strong coupling between magnetism of Eu 2+ ions and the conduction electrons of [Fe 2 -x. Nix. As 2]2 - layers. 14 / 14