Doping effects in the coupled, two-leg spin ladder BiCu(2)PO(6)

We report preparation, x-ray diffraction, magnetic susceptibility χ(T) and heat capacity C(p)(T) measurements on undoped samples as well as samples with Zn-doped (S = 0) at the Cu site BiCu(2(1-x))Zn(2x)PO(6), Ni-doped (S = 1) at the Cu site BiCu(2(1-y))Ni(2y)PO(6), and Ca doped (holes) at the Bi site Bi(1-z)Ca(z)Cu(2)PO(6) in the coupled two-leg spin ladder system BiCu(2)PO(6). While, Zn shows complete solid solubility, Ni could be doped to about 20% and Ca to about 15%. Magnetization and heat capacity data in the undoped compound point towards the existence of frustration effects. In all the samples, the χ(T) at low temperature increases with doping content. The Zn-induced susceptibility is smaller than that due to effective S = 1/2 moments, possibly due to frustrating next-nearest-neighbor interactions along the leg. For Zn content x>0.01, χ(T) deviates from the Curie law at low temperatures. The magnetic specific heat data C(m)(T) for the Zn-doped samples show weak anomalies at low temperature, in agreement with the χ(T) behavior. The anomalies are suggestive of spin freezing at low- T. In contrast, prominent effects are observed in χ(T) and C(m)(T) on Ni-doped samples. The zero-field-cooled (ZFC) and field-cooled (FC) χ(T) data are different from each other at low temperature, unlike that for Zn-doped samples, clearly indicating a transition to a spin-glass-like phase. No anomalies were found in Ca- or Pb-doped samples.