High sensitivity of magnetism of the “114” ferrimagnet CaBaCo4O7 to stoichiometry deviation

The effect of oxygen/cobalt off-stoichiometry upon magnetism in CaBaCo4O7 has been investigated. It is shown that the oxides CaBaCo4O7+δ and CaBaCo4−xO7−δ (0≤x≤0.20) synthesized below 1100 °C in air exhibit phase separation, where ferrimagnetic regions with TC~56 K to 64 K coexist with regions of magnetic clusters. The latter are detected from ac-susceptibility measurements, which show various frequency dependent peaks at ~14-20 K, 37 K, and 45 K, depending on the stoichiometry. The origin of this phenomenon is attributed to the great sensitivity of the material to oxidation as the synthesis of temperature is lowered, leading to the introduction of additional Co3+ cations, with respect to the ideal formula CaBaCo22+Co23+O7. This excess Co3+ tends to destroy the ferromagnetic zig-zag chains of the ferrimagnetic structure and creates various cobalt spin clusters, leading to the inherent phase separation in the samples.