Finite-size effects in the quasi-one-dimensional quantum magnetsSr2CuO3,Sr2Cu0.99

We studied the finite-size effects on the magnetic behavior of the quasi-one-dimensional spin $S=\frac{1}{2}$ Heisenberg antiferromagnets ${\text{Sr}}_{2}{\text{CuO}}_{3}$, ${\mathrm{Sr}}_{2}{\mathrm{Cu}}_{0.99}{M}_{0.01}{\mathrm{O}}_{3}$ ($M=\text{Zn}$ and Ni), and ${\text{SrCuO}}_{2}$. Magnetic susceptibility data were analyzed to estimate the concentration of chain breaks due to extrinsically doped defects and/or due to slight oxygen off-stoichiometry. We show that the susceptibility of ${\text{Sr}}_{2}{\text{Cu}}_{0.99}{\text{Ni}}_{0.01}{\text{O}}_{3}$ can be described by considering ${\text{Ni}}^{2+}$ as a scalar defect (${S}_{\mathrm{eff}}=0$) indicating that the Ni spin is screened. In ${\text{Sr}}_{2}{\text{Cu}}_{0.99}{\text{Zn}}_{0.01}{\text{O}}_{3}$ susceptibility analysis yields a defect concentration smaller than the nominal value which is in good qualitative agreement with crystal growth experiments. Influence of doping on the low-temperature long-range spin ordered state is studied. In the compound ${\text{SrCuO}}_{2}$, consisting of zigzag $S=\frac{1}{2}$ chains, the influence of spin frustration on the magnetic ordering and the defect concentration determined from the susceptibility data is discussed.