Microscopic evidence of destroying the order of magnetic sequence in CuO₂ chain by Zn, Ni, Co Doping in Sr₁₄Cu₂₄O₄₁ compound

Abstract

The Zn²⁺, Ni²⁺ and Co³⁺ doped Sr₁₄Cu₂₄O₄₁ compounds were synthesized by standard solid state method. X-ray diffraction results show that the changes in lattice parameters are very small. Selected area electron diffraction patterns (EDPs) show that the diffraction spots corresponding to the CuO₂ chain substructure are extended to streaks along a* and b* directions for all the samples, while the diffraction spots produced merely by the Cu₂O₃ ladder substructure are still very sharp. This means that the periodicities of chains in a* and b* directions are partially destroyed upon doping of Zn, Ni and Co due to that the initial phase of each chain becomes a random variable. The temperature dependence of magnetic properties was measured for every sample. And the number of dimers in CuO₂ chain per formula unit (f.u.) and dimer coupling constant are obtained by fitting the temperature dependence of the magnetic susceptibility. It is found that the degree of initial phase disorder is related to the order degree of magnetic sequence in CuO₂ chain. For the un-doped sample, the decoupling of dimers is weak, the magnetic sequence is slightly destroyed, and the streaks in EDP are also very weak, which implies the degree of initial phase disorder in CuO₂ chain is very low. When Zn²⁺ and Ni²⁺ ions are doped, the number of dimers per f.u. decreases, and the intensity of diffraction streaks increases in comparison with the corresponding spots. Furthermore, when the high spin magnetic ions Co³⁺ are doped, the number of holes in Sr₁₄Cu₂₄O₄₁ decreases, the magnetic sequence is destroyed very seriously, and the spots in EDP are extended to streaks almost completely. The phenomenon that the diffraction spots of CuO₂ chain extend to streaks in EDP appears as evidence that the magnetic sequence in the CuO₂ chain is destroyed by doping of Zn, Ni, Co.

Keywords

spin ladder and chain / composite compounds / initial phase disorder

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