Oxygen hole-doping effects on magnetic properties of the spin-chain system Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10-\delta}$

The magnetic properties of the low-dimensional spin-chain system Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10-\delta}$ were studied as a function of oxygen content. The temperature dependence of the magnetic moment and specific heat were measured for a series of samples with different oxygen contents, prepared by solid state reaction under various oxygen pressures up to 225 atm and characterized by x-ray diffraction and iodometric titration. At fixed oxygen pressure, oxygen deficiency increases with Ca doping. For example, when annealed at 1 atm O$_2$ pressure, the $x=0$ sample was multiphased due to excess oxygen, whereas the samples with $x>1.2$ were also multiphased due to oxygen deficiency. With decreasing oxygen deficiency, the antiferromagnetic transition temperature decreases for $x=0.50$, $x=0.75$, $x=0.90$, and $x=1.00$ doped samples. In particular, for the fully oxygenated $x=1.00$ sample the transition is completely suppressed, which is contrary to the single crystal result recently published by K. Kudo $et$ $al$.\footnote{K. Kudo, S. Kurogi, and Y.Koike, Physical Review B \textbf{71}, 104413 (2005)}, where long range order disappeared at $x\approx1.4$ for the apparently oxygen-deficient crystals. A new magnetic phase diagram is proposed to include both Ca doping and oxygen deficiency.