Abstract
The EuFe2−xNixAs2 (with 0 ≤ x ≤ 0.4) compounds exhibiting 3d and/or 4f magnetic order were investigated by means of 57Fe and 151Eu Mössbauer spectroscopy. Additionally, results for EuNi2As2 are reported for comparison. It was found that spin-density-wave order of the Fe itinerant moments is monotonically suppressed by Ni-substitution. However, the 3d magnetic order survives at the lowest temperature up to at least x = 0.12 and it is certainly completely suppressed for x = 0.20. The Eu localized moments order regardless of the Ni concentration, but undergo a spin reorientation with increasing x from alignment parallel to the a-axis in the parent compound, toward c-axis alignment for x > 0.07. Change of the 4f spins ordering from antiferromagnetic to ferromagnetic takes place simultaneously with a disappearance of the 3d spins order what is the evidence of a strong coupling between magnetism of Eu2+ ions and the conduction electrons of [Fe2−xNixAs2]2- layers. The Fe nuclei experience the transferred hyperfine magnetic field due to the Eu2+ ordering for Ni-substituted samples with x > 0.04, while the transferred field is undetectable in EuFe2As2 and for compound with a low Ni-substitution level. It seems that the 4f ferromagnetic component arising from a tilt of the Eu2+ moments to the crystallographic c-axis leads to the transferred magnetic field at the Fe atoms. Superconductivity is not observed down to 1.8 K, although a comparison with 57Fe and 151Eu Mössbauer data for EuFe2As2-based superconductors indicates a similar magnetic structure.
Highlights
The EuFe2−xNixAs2 compounds exhibiting 3d and/or 4f magnetic order were investigated by means of 57Fe and 151Eu Mössbauer spectroscopy
The EuFe2As2-based compounds are unique laboratory for investigations of interplay between magnetism and superconductivity (SC), as well as they are a playground for peculiar competition between itinerant 3d magnetic order of the spin-density-wave (SDW) type and the localized 4f magnetic moments
The E u2+ AFM order and the Néel temperature TN reveal no changes up to 3 GPa, the applied pressure above 6 GPa causes a 4f spin reorientation and the Eu2+ FM order, while further pressure increase above 8 GPa results in suppression of the FM state, which is connected with the Eu valence change from a divalent state to a nearly trivalent s tate6,7
Summary
The EuFe2−xNixAs2 (with 0 ≤ x ≤ 0.4) compounds exhibiting 3d and/or 4f magnetic order were investigated by means of 57Fe and 151Eu Mössbauer spectroscopy. The present contribution reports Mössbauer results obtained by means of 57Fe and 151Eu spectroscopy applied to systematic investigations of the magnetic properties of the Ni-substituted EuFe2As2 compounds with an emphasis on study of the 3d-4f spin interaction.
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