The Effects Of Fe, Cr And Sc Doping On Ca₃Co₂O₆

The physical properties of transition metal oxide have been one of the hot topics inthe study of condensed matter physics. Quasi-one-dimensional spin system Ca₃（Co,T）₂O₆（T=transition metal ions） have been attracting a lot of interests due to its thermoelectricproperties and rich magnetic behavior. In this paper, we performed our studies focus onthe synthesis, crystal structure and magnetic properties under low temperature of Fe-, Cr-and Sc-doped compounds. The main contents can be summarized as follows:（1）We have synthesized polycrystalline samples of Ca₃Co_2-xFe_xO₆and singlecrystal of Ca₃Co_1.6Fe_0.4O₆by sol-gel reaction and flux method respectively. Themagnetization of Ca₃Co_2-xFe_xO₆show that the high-temperature curve complies well withthe Curie-Weiss law, while the low-temperature part does not but exhibit an upturn due tothe presence of short-range ordering. This uptown becomes ambiguous as x increase andalmost disappears at x=0.7. Simultaneously, the value of pdecreases and eventuallyreaches zero because the intrachain FM coupling（J） is reduced. These results all showthat FM interactions decrease as x increase. The ESR spectra of polycrystalline samplex=0.4show that a resonance consisting of eight hyperfine-structure peaks starts to emergeas temperature is lowed to be50K. Such hyperfine peaks are due to the splitting producedby Co nuclei. The nuclear spin quantum number of59Co is I=7/2and a resonancesuperimposed with eight （2I+1） hyperfine splittings is expected. The corresponding gvalue is derived to be4.2, much larger than2.0. This again shows that observedresonance is due to Co ions with strong crystal-field effects of octahedra and trigonalprisms. The magnetization of single-crystal Ca₃Co_1.6Fe_0.4O₆show that the magneticanisotropy is much modified when the FM intrachain interaction is much suppressed byFe-doping.（2） Polycrystalline samples of Ca₃Co_2-xCr_xO₆have been prepared by sol-gel reaction.XRD patterns and lattice constant reveal that solid solubility of Cr in Ca₃Co₂O₆is aboutx=0.4for rhombohedral phase. The magnetization of Ca₃Co_2-xCr_xO₆show that the valueof p, FM coupling（J） and saturation magnetization all decreased as x increased from x=0.0to x=0.4. These results all show that FM interactions decrease as x increase.（3） We studied the magnetic relaxation of Ca₃（Co,T）₂O₆（T=Fe, Cr and Sc）. We haveillustrated the presence of spin-glass-like freezing in these compounds. The magneticrelaxation of Ca₃Co_2-xFe_xO₆are similar with that of Ca₃Co₂O₆while the magnetizationsof Ca₃Co_2-xCr_xO₆and Ca3Co2-xScO6decay more slowly than x=0.0. So the relaxationcharacteristic time is significantly enhanced as compared to x=0.0. Different from thecases in x=0.0with strong spin frustration, the spin freezing in doped compounds isclosely associated with chemical disorder with randomness or imperfects.