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The trilayer nickelate ${\mathrm{Nd}}_{4}{\mathrm{Ni}}_{3}{\mathrm{O}}_{10\ensuremath{-}\ensuremath{\delta}}$ was investigated by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The crystal structure data suggest a higher Ni valence in the inner perovskitelike layer. At ambient pressure the resistivity shows a jump at 162 K, indicating a metal-to-metal transition (MMT). The MMT is also characterized by a magnetic susceptibility drop, a sharp specific-heat peak, and an isotropic lattice contraction. Below $\ensuremath{\sim}50\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, a resistivity upturn with $logT$ dependence shows up, accompanied with negative thermal expansion. External hydrostatic pressure suppresses the resistivity jump progressively, coincident with the weakening of the $logT$ behavior. The low-temperature electronic specific-heat coefficient is extracted to be $\ensuremath{\sim}150\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{mol}\text{\ensuremath{-}}{\mathrm{f}.\mathrm{u}.}^{\ensuremath{-}1}$, equivalent to $\ensuremath{\sim}50\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}\phantom{\rule{0.28em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{mol}\text{\ensuremath{-}}{\mathrm{Ni}}^{\ensuremath{-}1}$, indicating an unusual heavy-electron correlated state. The novel heavy-electron state as well as the logarithmic temperature dependence of resistivity is explained in terms of the ${\mathrm{Ni}}^{3+}$-centered Kondo effect in the inner layer of the ${({\mathrm{NdNiO}}_{3})}_{3}$ trilayers.

We have investigated the superconducting ground state of the newly discovered superconductor ThFeAsN with a tetragonal layered crystal structure using resistivity, magnetization, heat capacity, and transverse-field muon-spin rotation ($\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$) measurements. Our magnetization and heat-capacity measurements reveal an onset of bulk superconductivity with ${T}_{c}\ensuremath{\sim}30\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A nonlinear magnetic-field dependence of the specific heat coefficient $\ensuremath{\gamma}(H)$ has been found in the low-temperature limit, which indicates that there is a nodal energy gap. Our analysis of the $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ results shows that the temperature dependence of the superfluid density is better described by a two-gap model either isotropic $s+s$ wave or $s+d$ wave than a single-gap isotropic $s$-wave model for the superconducting gap, consistent with other Fe-based superconductors. The combination of $\ensuremath{\gamma}(H)$ and $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ results suggest that the ($s+d$)-wave model is the most consistent candidate for the gap structure of ThFeAsN. The observation of two gaps in ThFeAsN suggests a multiband nature of the superconductivity possibly arising from the $d$ bands of Fe ions. Furthermore, from our $\mathrm{TF}\ensuremath{-}\ensuremath{\mu}\mathrm{SR}$ study we have estimated the magnetic penetration depth in the polycrystalline sample of ${\ensuremath{\lambda}}_{L}(0)=375\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, superconducting carrier density ${n}_{s}=4.97\ifmmode\times\else\texttimes\fi{}{10}^{27}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{\ensuremath{-}3}$, and carrier's effective-mass ${m}^{*}=2.48{m}_{e}$. We compare the results of our present paper with those reported for the Fe-pnictide families of superconductors.

We have studied a quinary Fe-based superconductor ${\text{Sr}}_{2}{\text{VFeAsO}}_{3}$ by the measurements of x-ray diffraction, x-ray absorption, M\"ossbauer spectrum, resistivity, magnetization, and specific heat. This apparently undoped oxyarsenide is shown to be self-doped via electron transfer from the ${\text{V}}^{3+}$ ions. We observed successive magnetic transitions within the ${\text{VO}}_{2}$ layers: an antiferromagnetic transition at 150 K followed by a weak ferromagnetic transition at 55 K. The spin orderings within the ${\text{VO}}_{2}$ planes are discussed based on mixed valence of ${\text{V}}^{3+}$ and ${\text{V}}^{4+}$.

We report the measurements of resistivity and magnetization under magnetic fields parallel and perpendicular to the basal plane, respectively, on a cobalt-doped $\text{Eu}{({\text{Fe}}_{0.89}{\text{Co}}_{0.11})}_{2}{\text{As}}_{2}$ single crystal. We observed a resistivity drop at ${T}_{c}\ensuremath{\sim}21\text{ }\text{K}$, which shifts toward lower temperatures under external fields, suggesting a superconducting transition. The upper critical fields near ${T}_{c}$ show large anisotropy, in contrast with those of other ``122'' FeAs-based superconductors. Low-field magnetic susceptibility data also show evidence of superconductivity below 21 K. Instead of the expected zero resistance below ${T}_{c}$, however, a resistivity re-entrance appears at 17 K under zero field, coincident with the magnetic ordering of ${\text{Eu}}^{2+}$ moments. Based on the temperature and field dependences of anisotropic magnetization, a helical magnetic structure for the ${\text{Eu}}^{2+}$ spins is proposed. External magnetic fields easily change the helimagnetism into ferromagnetism with fully polarized ${\text{Eu}}^{2+}$ spins, accompanying by disappearance of the resistivity re-entrance. Therefore, superconductivity coexists with ferromagnetic state of ${\text{Eu}}^{2+}$ spins under relatively low magnetic field. The magnetic and superconducting phase diagrams are finally summarized for both $H\ensuremath{\parallel}ab$ and $H\ensuremath{\parallel}c$.

We have studied Ni-substitution effect in ${\text{LaFe}}_{1\ensuremath{-}x}{\text{Ni}}_{x}\text{AsO}$ $(0\ensuremath{\le}x\ensuremath{\le}0.1)$ by the measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. The nickel doping drastically suppresses the resistivity anomaly associated with spin-density-wave ordering in the parent compound. Superconductivity emerges in a narrow region of $0.03\ensuremath{\le}x\ensuremath{\le}0.06$ with the maximum ${T}_{c}$ of 6.5 K at $x=0.04$, where enhanced magnetic susceptibility shows up. The upper critical field at zero temperature is estimated to exceed the Pauli paramagnetic limit. The much lowered ${T}_{c}$ in comparison with ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ system is discussed.

We present a systematic study on the physical properties of EuFe$_{2-x}$Ni$_{x}$As$_{2}$ (0$\leq$\emph{x}$\leq$0.2) by electrical resistivity, magnetic susceptibility and thermopower measurements. The undoped compound EuFe$_{2}$As$_{2}$ undergoes a spin-density-wave (SDW) transition associated with Fe moments at 195 K, followed by antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at 20 K. Ni doping at the Fe site simultaneously suppresses the SDW transition and AFM ordering of Eu$^{2+}$ moments. For $x\geq$0.06, the magnetic ordering of Eu$^{2+}$ moments evolves from antiferromagnetic to ferromagnetic (FM). The SDW transition is completely suppressed for $x\geq$0.16, however, no superconducting transition was observed down to 2 K. The possible origins of the AFM-to-FM transition and the absence of superconductivity in EuFe$_{2-x}$Ni$_{x}$As$_{2}$ system are discussed.

Ternary iron arsenide ${\text{EuFe}}_{2}{\text{As}}_{2}$ with ${\text{ThCr}}_{2}{\text{Si}}_{2}$-type structure has been studied by magnetic susceptibility, resistivity, thermopower, Hall, and specific-heat measurements. The compound undergoes two magnetic phase transitions at about 200 and 20 K, respectively. The former was found to be accompanied with a slight drop in magnetic susceptibility (after subtracting the Curie-Weiss paramagnetic contribution), a rapid decrease in resistivity, a large jump in thermopower, and a sharp peak in specific heat with decreasing temperature, all of which point to a spin-density-wave-like antiferromagnetic transition. The latter was proposed to be associated with an $A$-type antiferromagnetic ordering of ${\text{Eu}}^{2+}$ moments. Comparing with the physical properties of the isostructural compounds ${\text{BaFe}}_{2}{\text{As}}_{2}$ and ${\text{SrFe}}_{2}{\text{As}}_{2}$, we expect that superconductivity could be induced in ${\text{EuFe}}_{2}{\text{As}}_{2}$ through appropriate doping.