Reduction of Ordered Moment and Néel Temperature of Quasi-One-Dimensional AntiferromagnetsSr2CuO3and

Abstract

We report elastic neutron diffraction and muon spin relaxation (µSR) measurements of the quasi one-dimensional antiferromagnets Sr2CuO3 and Ca2CuO3, which have extraordinarily reduced TN/J ratios.We observe almost resolution-limited antiferromagnetic Bragg reflections in Sr2CuO3 and obtain a reduced ordered moment size of ∼0.06µB. We find that the ratio of ordered moment size µ(Ca2CuO3)/µ(Sr2CuO3) = 1.5(1) roughly scales with their Néel temperatures, which suggests that the ordered moment size of quasi one-dimensional antiferromagnets decreases continuously in the limit of vanishing inter-chain interactions.PACS numbers: 76.75.+i, 75.25.+z, 75.10.Jm One-dimensional spin systems with antiferromagnetic interactions have received considerable attention because of their pronounced quantum mechanical effects. In the absence of inter-chain interactions, both integer and halfodd integer spin-chain systems have spin-singlet ground states, rather than an antiferromagnetically ordered Néel state [1][2][3]. Yet, for half odd-integer spin-chains, the spin-excitations are gap-less at momentum k = 0 and π [4]; this indicates that the ground state of a half-odd integer spin-chain is closer to the Néel ordered state than the integer spin systems, which have a so-called Haldane gap [3].Because of the gap-less feature of half-odd integer spinchains, one interesting question is whether the ground state is ordered or disordered when inter-chain interactions are introduced. Previously, it was proposed that there is a non-zero critical coupling ratio (J ′ /J = R c ), below which the system retains a singlet ground-state [5]. Recent renormalization group calculations however suggest that the ground state may depend on microscopic details of the model which describes the spin-spin interactions [6,7]. Numerical studies of the Heisenberg model suggested a vanishing critical coupling ratio (R c ∼ 0); namely, for infinitesimally small inter-chain couplings, half odd-integer spin-chains should exhibit Néel order [7].Experimentally, KCuF 3 is the most investigated quasione-dimensional S=1/2 antiferromagnet. Unfortunately, this material has relatively large coupling ratio R = J ′ /J ∼ 2 K/203 K = 1.0 × 10 −2 , as shown from neutron inelastic scattering measurements [8]. Probably reflecting the large coupling ratio R, the T N /J ratio (∼ 39 K/203 K = 0.2) and the ordered moment size (= 0.49(7)µ B [9]) were also found to be relatively large.To investigate the regime of the critical coupling ratio, model materials with smaller inter-chain couplings are needed; the quasi one-dimensional S=1/2 antiferromagnets Sr 2 CuO 3 and Ca 2 CuO 3 are suitable candidates. The intra-chain interaction (2J ∼ 2600 K) of these materials have been estimated from susceptibility [10,11] and infrared light absorption [12]. Néel ordering of these compounds was first observed in µSR measurements [13], with a significantly reduced T N /J ratio of ∼ 5 K/1300 K = 4 × 10 −3 for Sr 2 CuO 3 and T N /J ∼ 11 K/1300 K = 8 × 10 −3 for Ca 2 CuO 3 . Since T N /J is a measure of ...

Anisotropic Spin Freezing in theS=1/2Zigzag Chain CompoundSrCuO2

“…Spin-1/2 chains on the other hand, are predicted to order even for vanishingly small non-frustrated interchain interactions. This is consistent with the low temperature long range order among weakly coupled spin-1/2 chains found in Sr 2 CuO 3 and Ca 2 CuO 3 [8].In this letter we describe a different low temperature phase in closely related SrCuO 2 , which contains linear spin-1/2 chains assembled pairwise in an array of weakly interacting zigzag ladders [9][10][11][12][13][14][15]. Though weak static sublattice magnetization does develop for T < 5.0(4)K ≈ 2.8 · 10 −3 J/k B , three dimensional long range order is absent for T > ∼ 10 −4 J/k B .…”

“…Spin-1/2 chains on the other hand, are predicted to order even for vanishingly small non-frustrated interchain interactions. This is consistent with the low temperature long range order among weakly coupled spin-1/2 chains found in Sr 2 CuO 3 and Ca 2 CuO 3 [8].…”

“…While the peak position along the chain, l 0 , is temperature independent, both the incommensurability and the Half Width at Half Maximum (HWHM), κ h = a/(πξ a ) along a * , decrease by approximately a factor 2 below T c2 . It is useful to compare our results to those for Sr 2 CuO 3 [8]. The critical temperature (T N = 5 K) and intra-chain exchange constant (J = 220 meV) are similar for the two materials.…”

“…Nonetheless, the moment in SrCuO 2 is almost twice smaller than in Sr 2 CuO 3 . There is also a qualitative distinction between ordering in these materials; the frozen phase in SrCuO 2 is incommensurate along a, while Sr 2 CuO 3 has commensurate order [8]. Magnetic disorder along b is also unique to SrCuO 2 .…”

Heat transport by lattice and spin excitations in the spin-chain compoundsSrCuO2andSr2Sologubenko1, Giannò2, Ott3 et al. 2001Phys. Rev. B209242555View full textAdd to dashboardCite

“…At room temperature, this excess conductivity for the double-chain material SrCuO 2 exceeds that for the single-chain compound Sr 2 CuO 3 by almost a factor of two. In the temperature region between 1 and 6 K, where magnetic phase transitions have been reported for both materials, [35][36][37][38] no anomalous features in κ(T ) have been observed. Since these transitions are indicated by anomalies of the magnetic specific heat, 33,37 the interaction between the spin and the lattice systems as well as the spin contribution to the measured thermal conductivity must be negligible in this temperature region for both materials.…”