Evolution of complex magnetic phases and metal-insulator transition through Nb substitution in La0.5Sr0.5Co1−xNbxO3

We report the evolution of structural, magnetic, transport, and electronic properties of bulk polycrystalline ${\mathrm{La}}_0.5{\mathrm{Sr}}_0.5{\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Nb}}_x{\mathrm{O}}₃$ ($x=$ 0.025--0.25) samples. The Rietveld refinement of the x-ray diffraction patterns with $R\overline{3}c$ space group reveals that the lattice parameters and rhombohedral distortion monotonously increase with the ${\mathrm{Nb}}⁵⁺(4{d}⁰$) substitution ($x$). The magnetic susceptibility exhibits a decrease in the magnetic ordering temperature and net magnetization with $x$, which manifests that the Nb substitution dilutes the ferromagnetic (FM) double-exchange interaction and enhances the antiferromagnetic (AFM) superexchange interaction. Interestingly, for the $x>$ 0.1 samples the FM order is completely suppressed and the emergence of a glassy state is clearly evident. Moreover, the decrease in the coercivity (${H}_C$) and remanence (${M}_r$) with $x$ in the magnetic isotherms measured at 5 K further confirms the dominance of AFM interactions and reduction of FM volume fraction for the $x>$ 0.1 samples. More interestingly, we observe resistivity minima for the $x=$ 0.025 and 0.05 samples, which are analyzed using the quantum corrections in the conductivity, and found that the weak localization effect dominates over the renormalized electron-electron interactions in the three-dimensional (3D) limit. Further, a semiconducting resistivity behavior is obtained for $x>$ 0.05, which follows the Arrhenius law at high temperatures ($\ensuremath{\sim}160$--320 K), and the 3D variable range hopping prevails in the low-temperature region ($<160$ K). The core-level photoemission spectra confirm the valence state of constituent elements and the absence of ${\mathrm{Co}}²⁺$ is discernible.