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Polycrystalline samples of two complex intermetallic borides Zr 2 Fe 1− δ Ru 5+ δ B 2 and Zr 2 Fe 1− δ (Ru 1− x Rh x ) 5+ δ B 2 ( δ =ca. 0.10; x =0.20) were synthesized by high-temperature methods and characterized by single-crystal X-ray diffraction, energy dispersive spectroscopy, and magnetization measurements. Both structures are variants of Sc 2 Fe(Ru 1− x Rh x ) 5 B 2 and crystallize in the space group P 4/ mbm (no. 127) with the Ti 3 Co 5 B 2 -type structure. These structures contain single-atom, Fe-rich Fe/Ru or Fe/Ru/Rh chains along the c -axis with an interatomic metal-metal distance of 3.078(1) A, a feature which makes them viable for possible low-dimensional temperature-dependent magnetic behavior. Magnetization measurements indicated weak ferrimagnetic ordering with ordering temperatures ca. 230 K for both specimens. Tight-binding electronic structure calculations on a model “Zr 2 FeRu 5 B 2 ” using LDA yielded a narrow peak at the Fermi level assigned to Fe–Fe antibonding interactions along the c -axis, a result that indicates an electronic instability toward ferromagnetic coupling along these chains. Spin-polarized calculations of various magnetic models were examined to identify possible magnetic ordering within and between the single-atom, Fe-rich chains.

The upper critical field ${H}_{c2}$ of optimally doped, iron-based superconductor SrFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ ($x$ $=$ 0.35, ${T}_{c}$ $=$ 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion-irradiated single crystals, with columnar defect density corresponding to a matching field ${B}_{\ensuremath{\phi}}$ $=$ 25 T. The ${H}_{c2,c}(T)$ is close to $T$ linear, while clear saturation is observed for ${H}_{c2,a}(T)$, leading to a strongly temperature-dependent anisotropy parameter $\ensuremath{\gamma}$. The linear shape of ${H}_{c2,c}(T)$ is very similar to that observed in nodal KFe${}_{2}$As${}_{2}$, but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on the ${H}_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both ${T}_{c}$ and ${H}_{c2}$, keeping their ratio constant.