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We have discovered spin-state transition (S = 2 to S = 5/2) of Co ions due to Mg substitution in the Ca3Co2O6 apparent in the magnetic susceptibility, XPS, and first-principles study. We also examine the effect of Mg substitution on the magnetic and electronic structure of Ca3Co2O6 by first- principles calculations. It involves generalized gradient approximation with Coulomb interaction (U) in exchange-correlation energy functional. Our study shows a reasonable agreement between effective magnetic moment (μef f ) determined from the Curie–Weiss fit with that from the XPS analysis and first-principles calculations study. We have attributed the decrease in positive intra-chain exchange interaction constant (J1/kB ) to the antiferromagnetically coupled induced Co4+ ions (S = 5/2) arising from the Mg2+ ions substitution. The in-field meta-magnetic transitions in the isothermal M(H) curves below the critical field (Hc ) have been accurately mapped and successfully explained by the change in magnetic entropy (ΔS) calculations and Arrott plots. Electronic structure study reveals hole-type doping of Mg atom, and the Fermi level (EF ) shifts below. Density of state and band structure calculation indicates strong hybridization between partial states of Co-3d and O-2p orbitals for the Mg-doped compound due to which the band crossing at Fermi level is observed, and a hole- type Fermi surface is formed.

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We studied the structural, static, and dynamic properties of Ca3Co2–xBixO6 (x = 0 and 0.2) by means of powder X-ray diffraction and magnetization measurements. We reveal the rejuvenation phenomena in magnetic relaxation under cyclic temperature change and quenching below T c2. The reproduced step in the memory curve confirms the striking feature of rejuvenation effect in these samples. The observed memory and rejuvenation effects are discussed in the framework of hierarchical model of glassy phase.