Unraveling the nature of Fe-doping mediated inter- and intra-chain interactions in Ca₃Co₂O₆

The structural and magnetic properties of quasi-one-dimensional (1D) spin-chain compounds Ca₃Co_2-xFe_xO₆ (x = 0, 0.1, 0.2 and 0.3) synthesized by a sol-gel method have been systematically studied by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), neutron powder diffraction (NPD), dc and ac magnetization measurements. The samples adopt a rhombohedral crystal structure with the space group R-3c in a temperature range of 5–100 K. Fe ions are found to locate at the trigonal prism Co2 crystallographic sites. Fe doping converts some of low-spin Co³⁺ ions at Co1 octahedral sites into Co²⁺ ions. For the high-temperature paramagnetic phase, the Curie paramagnetic temperature θ_p changes its sign from positive to negative at x = 0.2, implying that the dominant magnetic interaction is driven from ferromagnetic (FM) to antiferromagnetic (AFM) upon Fe substitution. A partial low-spin to high-spin state crossover of the Co1 ions is observed at high Fe-doping level x = 0.3. All the samples exhibit a long-range spin-density wave (SDW) AFM ordering below T_N, followed by a spin-glass-like transition at T_f. These low-temperature magnetic phases were significantly suppressed upon Fe doping. The Fe substitution in Ca₃Co_2-xFe_xO₆ weakened both intrachain and interchain magnetic interactions, caused by enhanced magnetic disorder due to the different magnetic characters of Ising Co and Heisenberg Fe spins.