Magnetism of the “114” orthorhombic charge ordered CaBaCo4O7 doped with Zn or Ga: a spectacular valency effect

Abstract

The substitution of zinc and gallium for cobalt in CaBaCo₄O₇ has allowed the oxide series CaBaCo_4−xM_xO₇ (M = Zn, Ga) with 0.02 ≤ x ≤ 1.20, belonging to the “114” structural family, to be synthesized. From the structural view point two domains can be distinguished: for x ≤ 0.70 (for Zn) and x ≤ 0.90 (for Ga), the orthorhombic symmetry of CaBaCo₄O₇ is retained (albeit with reduced orthorhombic distortion for higher values of x), whereas for x ≥ 1.0 the symmetry is hexagonal. More importantly, we observe a contrasting effect of the valency of Zn²⁺ and Ga³⁺ upon the magnetic properties of the doped CaBaCo_4−xM_xO₇ cobaltites in the low doping regime (x ≤ 0.40). While the doping of Ga³⁺ for Co³⁺ leads to a progressive disappearance of ferrimagnetism with a simultaneous appearance of magnetic frustration, the doping of Zn²⁺ for Co²⁺ induces an abrupt switching from ferrimagnetism to antiferromagnetism. We explain these effects as arising from a perturbation of the ferromagnetic Co²⁺ chains which form the magnetic structure of CaBaCo₄O₇ in the case of substitution by zinc, and a destruction of the magnetic order by substitution of Ga³⁺ on the Co³⁺ sites in the case of gallium doping. For higher substitution levels (x ≥ 0.50), both Zn and Ga substitutions lead to a spin glass behavior, irrespective of the symmetry, orthorhombic or hexagonal.