Spin-dimer-like magnetic coupling in the infinite-chain compound Ca_0.85CuO₂

The nature of the magnetically ordered state in calcium cuprate Ca_0.85CuO₂ has been studied by electron-spin resonance and magnetic-susceptibility measurements. A gap of 83 K in the spin excitation spectrum has been detected arising from the ordering of Cu²⁺ spins in the CuO₂ chains. The magnetic structure can be considered as one dimensional and consists of short even-number spin segments with an antiferromagnetic coupling. The segments are separated by nonmagnetic Cu³⁺ ions that make the intersegment superexchange coupling constant vanishingly small. The observed magnetic structure can be explained by a simple geometrical model of a distribution of Cu²⁺ and Cu³⁺ ions that is determined by the Ca_0.85CuO₂ specific structure. For a stoichiometric approximant Ca_0.833CuO₂ the magnetic chains consist of regularly spaced noninteracting quartets of spins whereas the incommensurability of Ca and Cu lattices in Ca_0.85CuO₂ implies that in addition to the quartets there exist also segments with larger but still relatively small number of spins. The observed structure closely resembles the spin-Peierls dimerization where the insertion of the Cu³⁺ ions into the Cu²⁺ linear chain plays the role of a lattice distortion.