SPIN-DIMER-LIKE MAGNETIC COUPLING IN THE INFINITE-CHAIN COMPOUND CA0.85CUO2

The nature of the magnetically ordered state in calcium cuprate Ca0.85 CuO2 has been studied by electron-spin resonance and magnetic-suscepti bility measurements. A gap of 83 K in the spin excitation spectrum has been detected arising from the ordering of CU2+ spins in the CuO2 cha ins. The magnetic structure can be considered as one dimensional and c onsists of short even-number spin segments with an antiferromagnetic c oupling. The segments are separated by nonmagnetic CU3+ ions that make the intersegment superexchange coupling constant vanishingly small. T he observed magnetic structure can be explained by a simple geometrica l model of a distribution of CU2+ and CU3+ ions that is determined by the Ca0.85CuO2 specific structure, For a stoichiometric approximant Ca 0.833CuO2 the magnetic chains consist of regularly spaced noninteracti ng quarters of spins whereas the incommensurability of Ca and Cu latti ces in Ca0.85CuO2 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 dimerizatio n when the insertion of the CU3+ ions into the Cu2+ linear chain plays the role of a lattice distortion.