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.

We present results of ac-magnetic susceptibility, dc-magnetization, and magnetotransport measurements on a single crystal of a magnetic Eu(Fe0.81Co0.19)2As2 superconductor. The compound undergoes two phase transitions: into a canted antiferromagnetic state at TN=16.5 K and a superconducting state at Tc$\sim${}5.15 K in a zero field. Based on our experimental results, we conclude that Tc can further be increased up to 6.5 K with magnetic fields applied parallel to the ab plane of crystals. We argue that the tuning superconductivity may be actuated by suppressing the ferromagnetic component along the c axis of the Eu2+ moments, diminishing the orbital pair-breaking effect.

A thorough study of ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ single crystals, which were grown from Sn flux, was performed using both bulk (heat capacity, ac susceptibility, dc magnetization, electrical resistivitivity, magnetoresistance) and microscopic (Mössbauer spectroscopy) techniques. Electrical resistance and magnetic susceptibility were measured also under high pressure conditions (up to 19 and 9.5 GPa, respectively). Further insight into electronic properties and phonons is provided by ab initio calculations. The results indicate that ${\mathrm{EuZn}}_2{\mathrm{P}}_2$ is an antiferromagnet with strong Eu-Eu exchange coupling of ferromagnetic type within the basal plane and weaker antiferromagnetic interaction along the $c$ axis. The Eu magnetic moments are tilted from the basal plane. Hydrostatic pressure strongly affects both magnetic (increase of the Néel temperature) and electronic (suppression of the band gap and semimetallic behavior) properties, indicating a strong interplay of structure with magnetic and electronic degrees of freedom.

Low-temperature dc-magnetization, ac electrical resistivity and specific heat measurements were performed on single crystals of the intermetallic compound β-IrSn4. The compound crystallizes in the tetragonal MoSn4-type structure (space group I41/acd) and exhibits superconductivity below Tc = 0.9 ± 0.05 K. Further, the magnitude of the ratios ΔCp/(γnkBTc) = 1.29, 2Δ/(kBTc) = 3.55 and of the electron-phonon coupling λ[overline](e-ph) = 0.5 imply that superconductivity in β-IrSn4 can be ascribed to a s-wave weak coupling regime. We determined crucial thermodynamic characteristics of the superconducting state. It turned out that depending on the assumption of either a spherical or non-spherical Fermi surface, the superconductivity can be ascribed to either a type-I and type-II/1 or type-II in clean limit, respectively. However, the behavior of the upper critical field and the anisotropic crystalline structure of the studied compound provide strong support to the type-II superconductivity. In the normal state the resistivity exhibits a prominent quadratic temperature dependence, which together with a large Kadowaki-Woods ratio and with the enhanced effective mass indicate that the electrons in β-IrSn4 are strongly correlated.

The simultaneous presence of an Fe-related spin-density wave and the antiferromagnetic order of ${\mathrm{Eu}}^{2+}$ moments ranks ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ among the most interesting parent compounds of iron-based pnictide superconductors. Here we explore the consequences of the dilution of ${\mathrm{Eu}}^{2+}$ magnetic lattice through on-site Ca substitution. By employing macro- and microscopic techniques, including electrical transport and magnetometry, as well as muon-spin spectroscopy, we study the evolution of Eu magnetic order in both the weak and strong dilution regimes, achieved for Ca concentration $x\left(\mathrm{Ca}\right)=0.12$ and 0.43, respectively. We demonstrate the localized character of the Eu antiferromagnetism mediated via RKKY interactions, in contrast with the largely itinerant nature of Fe magnetic interactions. Our results suggest a weak coupling between the Fe and Eu magnetic sublattices and a rapid decrease of the Eu magnetic interaction strength upon Ca substitution. The latter is confirmed both by the depression of the ordering temperature of the ${\mathrm{Eu}}^{2+}$ moments $T_{\mathrm{N}}$, and the decrease of magnetic volume fraction with increasing $x\left(\mathrm{Ca}\right)$. We establish that, similarly to the ${\mathrm{EuFe}}_2{\mathrm{As}}_2$ parent compound, the investigated Ca-doped compounds have a twinned structure and undergo a permanent detwinning upon applying an external magnetic field.

We report transport, electromechanical, and structural properties of single core MgB₂/Fe wire produced using a new fabrication method, called designed internal Mg diffusion (IMD) process, which relies on the use of non-stoichiometric Mg + B pellets with excess Mg in place of a central Mg rod used in the standard IMD method. Structural analysis revealed the successful formation of a porous MgB₂ structure in the center and a dense circular MgB₂ layer surrounding this structure in the designed-IMD wire. Fast transport I–V measurements showed that the designed IMD method increased engineering critical current density (J _e) up to twice that of the IMD wires in self-field. The central porous MgB₂ structure shared the applied current and indirectly behaved as an internal stabilizer against quench damage at high applied currents.