Crossover from Kondo semiconductor to metallic antiferromagnet with 5d -electron doping in CeFe2Al10

We report a systematic study of the $5d$-electron-doped system $\mathrm{Ce}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x})}_{2}{\mathrm{Al}}_{10}$ $(0\ensuremath{\le}x\ensuremath{\le}0.15)$. With increasing $x$, the orthorhombic $b$ axis decreases slightly while accompanying changes in $a$ and $c$ leave the unit cell volume almost unchanged. Inelastic neutron scattering, along with thermal and transport measurements, reveal that for the Kondo semiconductor ${\mathrm{CeFe}}_{2}{\mathrm{Al}}_{10}$, the low-temperature energy gap, which is proposed to be a consequence of strong $c\text{\ensuremath{-}}f$ hybridization, is suppressed by a small amount of Ir substitution for Fe and that the system adopts a metallic ground state with an increase in the density of states at the Fermi level. The charge or transport gap collapses (at $x=0.04$) faster than the spin gap with Ir substitution. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements demonstrate that the system undergoes long-range antiferromagnetic order below a N\'eel temperature ${T}_{\mathrm{N}}$ of 3.1(2) K for $x=0.15$. The ordered moment is estimated to be smaller than 0.07(1) ${\ensuremath{\mu}}_{\mathrm{B}}$/Ce, although the trivalent state of Ce is confirmed by Ce ${L}_{3}$-edge x-ray absorption near edge spectroscopy. It is suggested that the $c\text{\ensuremath{-}}f$ hybridization gap, which plays an important role in the unusually high ordering temperatures observed in $\mathrm{Ce}{T}_{2}{\mathrm{Al}}_{10}$ ($T$ = Ru and Os), may not be necessary for the onset of magnetic order with a low ${T}_{\mathrm{N}}$ seen here in $\mathrm{Ce}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x})}_{2}{\mathrm{Al}}_{10}$.