Magnetism and superconductivity in La1.875Ba0.125-ySryCuO4+delta and La1.6-yNd0.4SryCuO4

Hyperfine Interactions 105 (1997) 101–106

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Magnetism and superconductivity in La1.875Ba0.125−y Sry CuO4+δ and La1.6−y Nd0.4Sry CuO4 A. Lappas a , L. Cristofolini a , K. Prassides a , K. Vavekis a , A. Amato b , F.N. Gygax b , M. Pinkpank b and A. Schenck b a

School of Chemistry and Molecular Sciences, University of Sussex, Brighton BN1 9QJ, UK b Institute for Particle Physics, ETH Z¨urich, CH-5232 Villigen PSI, Switzerland

µ+ SR and neutron diffraction measurements show that at low temperatures in La1.875 Ba0.125 CuO4 , there is phase separation into a long range ordered magnetic component of SDW-like nature, identified with the LTT phase and a superconducting component, identified with the LTO phase. Partial substitution of Ba by Sr at a fixed doping level of 1/8 leads to gradual decrease of the magnetic freezing temperature, accompanied by a decrease in the fraction of the magnetic component. The La1.875 Ba0.025 Sr0.100 CuO4 composition exhibits freezing of the Cu electronic moments into a disordered arrangement. Excess interstitial oxygen leads to a rapid recovery of superconductivity and a marked depression of the magnetic volume fraction. Study of the LTT phase – after stabilisation with partial substitution of La by Nd – away from the 1/8 doping level, reveals no evidence of long range magnetic ordering with magnetic fluctuations surviving down to 3 K.

1. Introduction The relationship between structural instabilities and superconductivity in the highTc cuprates continues to be intriguing. In general, for any family of superconducting cuprates, Tc goes through a maximum with increasing hole doping and then monotonically decreases to zero. In La2−x Bax CuO4 , however, besides the expected maximum in Tc near x = 0.15, a deep, narrow minimum (below 4 K) exists at x = 1/8. This remarkable suppression of superconductivity is only weakly evident in the analogous Sr-doped series and has been associated with the occurrence of a low-temperature (at Td2 ) first-order structural phase transition from orthorhombic (LTO, Bmab) to tetragonal (LTT, P42/ncm) symmetry [1]. Electronic [2], spin–orbit coupling [3] and microstructural [4] models have been proposed to account for the significant changes in the superconducting properties of the Ba-doped phases that accompany the LTO→LTT phase transition. Very importantly however, early ZF-µ+ SR measurements on La1.875 Ba0.125 CuO4 also firmly established the occurrence of long range antiferromagnetic order below a freezing temperature, Tf [5].  J.C. Baltzer AG, Science Publishers

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In an effort to correlate the structural and magnetic changes with the detrimental effect on the superconducting properties, we have performed a systematic study of the structural, conducting and magnetic properties of the La1.875 Ba0.125−y Sry CuO4 (0 6 y 6 0.125) double-doped systems, combining neutron scattering and µ+ SR measurements [6,7]. Here Tc is found to increase monotonically with increasing Sr content, y . Both TF- and ZF-µ+ SR reveal multicomponent behaviour in the µ+ spin polarisation and spatially inhomogeneous features in all samples. Below about Td2 , the LTO→LTT transition temperature, a magnetic volume fraction appears and grows with decreasing T , while the rest of the sample volume, of non-magnetic origin, shrinks with decreasing T . The deduced magnetic and non-magnetic (superconducting) fractions from µ+ SR and their temperature evolution are in excellent agreement with those deduced from high-resolution neutron diffraction for the LTT and LTO phases, respectively, allowing an unambiguous identification of the LTT phase as nonsuperconducting, with pair-breaking associated with the existence of long range AF order. The evolution of the three characteristic critical temperatures ( Tc , Td2 and Tf ) with Sr content, y are shown in fig. 1 [8]. For compositions with y 6 0.075, longlived oscillations in the µ+ spin polarisation are present, implying an ordered AF state, whose volume fraction and Tf decrease smoothly with y ; the spontaneous µ+ precession frequency, however, saturates to the same value (≈ 3.5 MHz, cf. νµ = 5.6 MHz

Fig. 1. Evolution of the LTO→LTT phase transition temperature, Td2 (measured by neutron diffraction), the magnetic freezing temperature, Tf (measured by ZF-µ+ SR), and the superconducting critical temperature, Tc (measured by SQUID susceptometry), as a function of the Sr content, y in the La1.875 Ba0.125−y Sry CuO4 series. Inset: The spontaneous µ+ precession frequency, νµ for y = 0.0, 0.025, 0.050, and 0.075 (filled triangles, open circles, open triangles, and filled squares, respectively).

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in La2 CuO4 ) for all compositions (inset of fig. 1). The long range order of the Cu moments disappears at y ≈ 0.100; for this composition, ZF and LF measurements down to 68 mK show the existence of a frozen, but randomly oriented, spin state below Tf = 10 K [8]. In this contribution, we present details of some of our µ+ SR investigations of cuprates with the LTT structure, focusing on two specific examples: (i) La1.875 Ba0.125 CuO4+δ , where excess interstitial oxygen has been introduced into the structure, and (ii) La1.6−y Nd0.4 Sry CuO4 , where the doping level y is no longer equal to 1/8.

2. Experimental The La1.875 Ba0.125−y Sry CuO4 and La1.6−y Nd0.4 Sry CuO4 samples were prepared by high-temperature reactions of stoichiometric amounts of the appropriate rare earth and copper oxides and alkaline earth carbonates in flowing O2 . Repeated cycles of grinding, pelletising and heating were performed until phase purity was confirmed by X-ray diffraction. Reaction with sodium hypobromite solutions afforded superoxygenated compositions. Additional characterisation was performed by SQUID and/or ac susceptometry and, for selected samples, by high-resolution powder neutron diffraction. ZF, LF and TF µ+ SR data were collected on pelletised samples at the Paul Scherrer Institute, Villigen, Switzerland, with the GPS spectrometer on the PSI 600 MeV proton accelerator between 2.9 and 300 K.

3. Results and discussion 3.1. Excess interstitial oxygen: La1.875 Ba0.125 CuO4+δ At high temperature, the ZF-µ+ spin polarisation is best described by a Kubo– Toyabe relaxation function with a temperature-independent relaxation rate, ∆ ≈ 0.16 µs−1 , appropriate for a Gaussian distribution of static random fields, arising from Cu nuclear moments. Below about 70 K for the δ = 0 and 50 K for the δ > 0 systems, a second fast exponentially relaxing component appears which grows with decreasing temperature (fig. 2). On further cooling, well-resolved oscillations appear below 27.6 K for δ = 0 and 23.6 K for δ > 0, accompanied by a rapid increase of the relaxation rate (λ ≈ 10 µs−1) and providing unambiguous evidence for the long range ordered character of the Cu2+ localised electronic moments. Of particular importance is the fact that, even though a simple oscillating term of the form: exp(−λt) cos(2πνµ t + ϕ) can describe well component #2, the phase ϕ assumes large unphysical values. Consequently, we tried successfully an alternative fit, involving an exponentially-damped Bessel function: exp(−λt)J0 (2πνµ t) (theoretically expected for an incommensurate magnetic structure described by one q wavevector [9]). The

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Fig. 2. Temperature evolution of the relative volume fraction of magnetic component #2 in La1.875 Ba0.125 CuO4+δ [δ = 0 (open symbols), δ > 0 (closed symbols)]. Inset: temperature dependence of the spontaneous precession frequency, νµ .

temperature dependence of the precession frequencies, νµ , is shown in the inset of fig. 2. The magnitude of the internal fields strength at the µ+ site, Bµ , is 258(1) G (δ = 0, T = 3.1 K) and 252(4) G (δ > 0, T = 2.9 K), identical with the other LTT phases studied. Below 10 K, a residual relaxation of ≈ 2 µs−1 remains and can be ascribed to static disorder or residual fluctuating moments. The major effect of the excess oxygen incorporation occurs in the drastic reduction of the magnetic volume fraction from ≈ 72% to ≈ 40%, presumably arising from the suppression of the LTO → LTT transition. 3.2. Changing the doping level y : La1.6−y Nd0.4 Sry CuO4 The LTT phase field can be extended by partial substitution of La by Nd [10]. We focused on samples La1.6−y Nd0.4 Sry CuO4 with y = 0.175, 0.200 and 0.225 in order to probe the magnetic behaviour of the LTT phase away from the optimal doping level y = 1/8 and as the tilting distortion of the CuO6 octahedra diminishes with increasing y . The ZF-µ+ SR spectra at high temperature are again described by a Kubo–Toyabe relaxation function (∆ = 0.15 µs−1 ). Below about 80 K, a second non-oscillating component, best modelled as an exponentially damped Kubo–Toyabe function, presumably associated with the LTO→LTT phase transition, appears and continues to grow with decreasing temperature. However, in contrast to the y = 1/8 cases, no oscillations appear down to 2.8 K, implying the absence of long range magnetic order. Even though the relaxation rate, λ, of the rapidly relaxing component

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Fig. 3. Temperature evolution of the depolarisation rate of component #2 in La1.6−y Nd0.4 Sry CuO4 (y = 0.225, diamonds; 0.200, circles; 0.175, squares). Inset: Time dependence of the ZF and LF (6 kG) µ+ spin polarisation at 3.2 K for the y = 0.200 composition.

#2 gradually increases on cooling for all three compositions (fig. 3), a further sharp increase, signalling the slowing down of the magnetic fluctuations, is observed below about 20 K for y = 0.175 (λ → 15.2(7) µs−1 at T = 3.1 K) and below about 10 K for y = 0.200 (λ → 16.6(1.0) µs−1 at T = 3.2 K). LF measurements at 6 kG at the base temperature reveal that the origin of the fast depolarisation of the µ+ spin is not purely static in origin, and a fluctuating field component (with λ = 2.7(2) µs−1 at T = 3.2 K) still survives (inset of fig. 3). This contrasts with the case of the La1.875 Ba0.025 Sr0.100 CuO4 sample in which, even though long range magnetic order is not present, the application of a 5 kG LF leads to a complete recovery of the asymmetry, signifying the quasi-static nature of the local field distribution.

4. Conclusions Our combined µ+ SR and neutron studies clearly establish that the anomaly associated with the 1/8 doping level in the 214 cuprate superconductors is due to the development of static magnetic order of SDW-like type in the LTT phase. The coexisting superconducting fraction is restricted to the LTO domains which survive the first order structural phase transition. The partial substitution of Ba by isoelectronic

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Sr gradually suppresses the LTO→LTT transition and the magnetic component whose nature eventually changes to random static disorder. The magnetic component can be also suppressed by oxygen doping. The magnetic properties of the LTT phase away from the 1/8 doping level – as studied in the Nd stabilised systems with doping levels in the range 0.175–0.225 – show pronounced changes, as the magnetic correlations now retain some dynamic character down to base temperature. Our results are in excellent agreement with recent neutron scattering measurements on La1.48 Nd0.4 Sr0.12 CuO4 [11] that show the presence of magnetic stripe correlations which are commensurately pinned in the LTT phase only when y is a simple fractional number. Such experimental observations are reminiscent of theoretical models [12] which invoke frustrated phase separation into hole-rich (metallic/superconducting) and hole-poor (insulating/magnetic) regions in the high-Tc oxides. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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