The magnetic structure of the Eu${}^{2+}$ moments in the superconducting EuFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ sample with $x=0.15$ has been determined using element-specific x-ray resonant magnetic scattering. Combining magnetic, thermodynamic, and scattering measurements, we conclude that the long-range ferromagnetic order of the Eu${}^{2+}$ moments aligned primarily along the c axis coexists with the bulk superconductivity at zero field. At an applied magnetic field $\ge$0.6 T, superconductivity still coexists with the ferromagnetic Eu${}^{2+}$ moments, which are polarized along the field direction. We propose a spontaneous vortex state for the coexistence of superconductivity and ferromagnetism in EuFe${}_2$(As${}_{0.85}$P${}_{0.15}$)${}_2$.

The magnetic-field-induced assembly of magnetic nanoparticles (NPs) provides a unique and flexible strategy in the design and fabrication of functional nanostructures and devices. We have investigated the field-induced self-assembly of core–shell iron oxide NPs dispersed in toluene by means of small-angle neutron scattering (SANS). The form factor of the core–shell NPs was characterized and analyzed using SANS with polarized neutrons. Large-scale aggregates of iron oxide NPs formed above 0.02 T as indicated by very-small-angle neutron scattering measurements. A three-dimensional long-range ordered superlattice of iron oxide NPs was revealed under the application of a moderate magnetic field. The crystal structure of the superlattice has been identified to be face-centred cubic.

The magnetic structure of the ${\mathrm{Eu}}^{2+}$ moments in the superconducting ${\mathrm{EuFe}}_2{\left({\mathrm{As}}_{1-x}{\mathrm{P}}_x\right)}_2$ sample with $x=0.19$ has been determined using neutron scattering. We conclude that the ${\mathrm{Eu}}^{2+}$ moments are aligned along the c direction below $T_{\mathrm{C}}=19.0\left(1\right)$ K with an ordered moment of 6.6(2) ${\mu }_B$ in the superconducting state. An impurity phase similar to the underdoped phase exists within the bulk sample which orders antiferromagnetically below $T_{\mathrm{N}}=17.0\left(2\right)$ K. We found no indication of iron magnetic order, nor any incommensurate magnetic order of the ${\mathrm{Eu}}^{2+}$ moments in the sample.

The magnetic ground state of the ${\mathrm{Eu}}^{2+}$ moments in a series of $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals grown from the Sn flux has been investigated in detail by neutron diffraction measurements. Combined with the results from the macroscopic properties (resistivity, magnetic susceptibility and specific heat) measurements, a phase diagram describing how the Eu magnetic order evolves with Co doping in $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ is established. The ground-state magnetic structure of the ${\mathrm{Eu}}^{2+}$ spins is found to develop from the A-type antiferromagnetic (AFM) order in the parent compound, via the A-type canted AFM structure with some net ferromagnetic (FM) moment component along the crystallographic $\mathit{c}$ direction at intermediate Co doping levels, finally to the pure FM order at relatively high Co doping levels. The ordering temperature of Eu declines linearly at first, reaches the minimum value of 16.5(2) K around $\mathit{x}=0.100\left(4\right)$, and then reverses upwards with further Co doping. The doping-induced modification of the indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between the ${\mathrm{Eu}}^{2+}$ moments, which is mediated by the conduction $\mathit{d}$ electrons on the (Fe,Co)As layers, as well as the change of the strength of the direct interaction between the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Fe}}^{2+}$ moments, might be responsible for the change of the magnetic ground state and the ordering temperature of the Eu sublattice. In addition, for $\mathrm{Eu}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$ single crystals with 0.$10\le \mathit{x}\le 0.18$, strong ferromagnetism from the Eu sublattice is well developed in the superconducting state, where a spontaneous vortex state is expected to account for the compromise between the two competing phenomena.