Abstract. Superconductivity and ferromagnetism are two antagonistic cooperative phenomena, which makes it difficult for them to coexist. Here we demonstrate experimentally that they do coexist in EuFe 2 (As 1−x P x ) 2 with 0.2 ≤ x ≤ 0.4, in which superconductivity is associated with Fe-3d electrons and ferromagnetism comes from the long-range ordering of Eu-4f moments via Ruderman-Kittel-KasuyaYosida (RKKY) interactions.The coexistence is featured by large saturated ferromagnetic moments, high and comparable superconducting and magnetic transition temperatures, and broad coexistence ranges in temperature and field. We ascribe this unusual phenomenon to the robustness of superconductivity as well as the multi-orbital characters of iron pnictides.(Some figures in this article are in colour only in the electronic version)

Ternary iron phosphide EuFe2P2 with ThCr2Si2-type structure has been systematically studied by the measurements of crystal structure, magnetization, Mössbauer effect, transport properties and specific heat. The structural refinement result confirms no direct P-P covalent bonding. The Mössbauer spectra indicate no magnetic moment for the Fe atoms and, that the Eu ions are divalent in the whole temperatures. The Eu 2+ spins order ferromagnetically at TC =29 K, followed by a possible helimagnetic ordering below THM =26 K, where the Eu 2+ moments tilt a little from the c-axis. External magnetic field increases the TC gradually, but suppresses the THM rapidly. (Magneto)resistivity data indicate characteristic dense Kondo behavior above the Curie temperature. The result is discussed in terms of the interplay between intersite RKKY and intrasite Kondo interactions.

A neutron powder diffraction study of EuFe 2 P 2 at a wavelength of 2.3672(1)Å shows that the 6.6(3) μ B Eu 2+ moments order ferromagnetically at T C = 30(1) K and are canted at an angle of 17(3) • from the c axis. No evidence for the previously proposed antiferromagnetic or helimagnetic structures was found. The almost axial ferromagnetic structure of EuFe 2 P 2 contrasts sharply with the planar antiferromagnetism seen in EuFe 2 As 2 and many other europium-transition metal pnictides, suggesting a delicate interplay between the Eu 4f and transition metal 3d electrons.

We have measured the spin-polarized electron momentum density distributions of EuFe2(As0.73P0.27)2 by magnetic Compton scattering (MCS) measurements. For the first time, we show direct evidence of competing ferromagnetism and superconductivity (SC) on FeAs layers in this iron pnictide system. The MCS orbitalwise decomposition of the density distributions reveals that between 16 and 19 K, the spin-polarized Fe-3d character is enhanced (as the ferromagnetic character supersedes superconducting character), where the resistivity shows a maximum, reentrant SC-like peak, at 18 K. The spin polarization of the Fe-3d orbital, enhanced by ferromagnetic Eu ions, suppresses the SC around 18 K, while at other temperatures the system indeed exhibits SC where the Fe-3d spin polarization is suppressed or collapses.

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