(57)Fe and (151)Eu Mössbauer studies of 3d-4f spin interplay in EuFe(2−x)Ni(x)As(2)

The EuFe(2−x)Ni(x)As(2) (with 0 ≤ x ≤ 0.4) compounds exhibiting 3d and/or 4f magnetic order were investigated by means of (57)Fe and (151)Eu Mössbauer spectroscopy. Additionally, results for EuNi(2)As(2) 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 Eu(2+) ions and the conduction electrons of [Fe(2−x)Ni(x)As(2)](2-) layers. 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 EuFe(2)As(2) and for compound with a low Ni-substitution level. It seems that the 4f 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. Superconductivity is not observed down to 1.8 K, although a comparison with (57)Fe and (151)Eu Mössbauer data for EuFe(2)As(2)-based superconductors indicates a similar magnetic structure.