Muon-spin-rotation measurements in infinite-layer and infinite-chain cuprate antiferromagnets: Ca_0.86Sr_0.14CuO₂ and Sr₂CuO₃

We have performed zero-field muon-spin-rotation μSr measurements of the ‘‘infinite-layer’’ cuprate compound Ca_0.86Sr_0.14CuO₂ and the ‘‘infinite chain’’ system Sr₂CuO₃. A spontaneous magnetic field from the ordered Cu moments is observed below T_N=540±5 K in Ca_0.86Sr_0.14CuO₂: below T=360 K, we observe muon-spin precession with a frequency ν(T→0=16 MHz (corresponding to a local field of 1.2 kG). The precession signal is replaced by a rapid depolarization above T=360 K due to the onset of muon diffusion. The hopping rate followed an Arrhenius law, with an activation energy of E_a=0.39(1) eV. The sublattice magnetization M_s, proportional to ν(T), showed a slower decay with increasing temperature in Ca_0.86Sr_0.14CuO₂, compared with that observed in La₂CuO₄ and Sr₂CuO₂Cl₂, indicating that a wider CuO₂-layer separation results in a more two-dimensional magnetic behavior. In the infinite-chain system Sr₂CuO₃, the onset of magnetic order was found at T∼5 K with a local field of ∼30 G at the muon site at T→0. The exchange interaction, inferred from susceptibility measurements is on the order of 10³ K, implying a remarkable suppression of the ordering temperature with k_BT_N/J≤0.01 in Sr₂CuO₃. These results demonstarate clear signatures of low-dimensional magnetic behavior in the CuO chains.