Spin-dimer-like magnetic coupling in the infinite-chain compound Ca 0.85 CuO 2 J. Dolins ˇ ek, D. Arc ˇ on, and P. Cevc Jozef Stefan Institute, University of Ljubljana, Jamova 39, 1000 Ljubljana, Slovenia O. Milat, M. Miljak, and I. Aviani Institute of Physics, University of Zagreb, Bijenicka 46, 41000 Zagreb, Croatia Received 2 September 1997; revised manuscript received 12 November 1997 The nature of the magnetically ordered state in calcium cuprate Ca 0.85 CuO 2 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 Cu 2+ spins in the CuO 2 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 Cu 3+ ions that make the intersegment superexchange coupling constant vanishingly small. The observed magnetic structure can be explained by a simple geometri- cal model of a distribution of Cu 2+ and Cu 3+ ions that is determined by the Ca 0.85 CuO 2 specific structure. For a stoichiometric approximant Ca 0.833 CuO 2 the magnetic chains consist of regularly spaced noninteracting quartets of spins whereas the incommensurability of Ca and Cu lattices in Ca 0.85 CuO 2 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 Cu 3+ ions into the Cu 2+ linear chain plays the role of a lattice distortion. S0163-18299807313-5 I. INTRODUCTION The discovery of high-T c superconductivity has triggered a renewed interest in low-dimensional quantum magnetism. All high-T c superconductors found so far contain two- dimensional 2D CuO 2 planes and the ground state of the insulating parent compound shows antiferromagnetic AFM ordering. The dimensionality of these planes is reduced to become more one dimensional in the spin-ladder compounds Ref. 1 SrCu 2 O 3 a two-leg ladder and Sr 2 Cu 3 O 5 a three- leg ladder. The difference between CuO 2 planes in high-T c superconductors and Cu 2 O 3 or Cu 3 O 5 planes of the spin lad- der is the existence of edge-sharing CuO 4 squares in the ladders instead of corner-sharing CuO 4 squares in the 2D planes of all superconductors. So far superconductivity has not been observed in the pure spin ladder compounds. An- other low-dimensional magnetic compound Refs. 2 and 3 Sr 14 Cu 24 O 41 has become a subject of intense research re- cently. This compound contains planes of two-leg ladders alternating with ribbonlike planes of edge-sharing CuO 2 squares. A spontaneously dimerized spin ground state resem- bling a spin-Peierls instability has been reported. This state is suggested to be of different origin from that of a standard spin-Peierls transition found in 1D AFM chains of spins S = 1 2 with nearest-neighbor interactions only where the dimer- ization occurs as a result of a spontaneous lattice distortion. In Sr 14 Cu 24 O 41 the dimerized state is a purely quantum phe- nomenon originating from competing nearest-neighbor NN ( J 1 ) and next-nearest-neighbor NNN exchange spin inter- actions ( J 2 ) and does not involve any lattice distortion. It has been shown 4,5 theoretically that in such a case a spontane- ously dimerized ground state exists in a wide range of pa- rameters, J 2 / J 1 . The dimerized state is a superposition of two states in one of which a spin forms a dimer with the spin on its right and in the other with the spin on its left. The microscopic model of magnetic ordering in Sr 14 Cu 24 O 41 is, however, still controversial. Early susceptibility measure- ments on a single crystal 6 suggested that a broad peak around 60 K originates from an AFM long-range ordering. Suscep- tibility measurements on a powder sample 7 reported a spin gap of 87 K that is supposed to originate from a ladder chain. Other susceptibility measurements and electron-spin reso- nance ESR on single crystals 8 have been interpreted with a model based on dimerized S = 1 2 spins that probably form in the simple CuO 2 chains. Spin ordering appears as a result of the superexchange bonding of magnetic Cu 2+ ( S = 1 2 ) ions via the oxygen atoms and the interplay with the nonmagnetic Cu 3+ ( S =0) ions the formal valence of copper in the stoichiometric Sr 14 Cu 24 O 41 is Cu +2.25 . The microscopic na- ture of magnetic ordering however remains unclear due to the simultaneous presence of Cu atoms in both the simple CuO 2 chains and the Cu 2 O 3 two-leg ladders. The respective role of the ladders and chains can be better understood if these two structural elements were separated from each other. Recently, new members of the copper oxide family have been synthesized that contain only the simple CuO 2 chains. These are the calcium cuprate Ca 0.85 CuO 2 Refs. 9 and 10 and the strontium cuprate Sr 0.73 CuO 2 Ref. 11 compounds where the CuO 2 chains are separated by pure Ca or Sr planes. The CuO 2 ribbons show slight modulation along the chain direction with the periodicity incommensu- rably related to that of the Ca or Sr sublattices. Sr 0.73 CuO 2 appears to be an insulator with an AFM exchange coupling. At 10 K it exhibits a sharp transition in the magnetic suscep- tibility at a magnetic-field-independent temperature probably due to a structural instability. In this paper we report the study of magnetic ordering in calcium cuprate Ca 0.85 CuO 2 by means of magnetic susceptibility and ESR measurements. PHYSICAL REVIEW B 1 APRIL 1998-I VOLUME 57, NUMBER 13 57 0163-1829/98/5713/77986/$15.00 7798 © 1998 The American Physical Society