Muon spin rotation study of magnetism in multilayer HgBa2Ca4Cu5Oy superconductor

Muon spin rotation study of magnetism in multilayer HgBa2Ca4Cu5Oy superconductor

Physica C 460–462 (2007) 892–895 www.elsevier.com/locate/physc Muon spin rotation study of magnetism in multilayer HgBa2Ca4Cu5Oy superconductor Kazuy...

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Physica C 460–462 (2007) 892–895 www.elsevier.com/locate/physc

Muon spin rotation study of magnetism in multilayer HgBa2Ca4Cu5Oy superconductor Kazuyasu Tokiwa a,*, Satoshi Mikusu a, Wataru Higemoto b, Kusuo Nishiyama c, Akira Iyo d, Yasumoto Tanaka d, Hisashi Kotegawa e, Hidekazu Mukuda f, Yoshio Kitaoka f, Tsuneo Watanabe a a Tokyo University of Science, Chiba, Noda 278-8510, Japan Japan Atomic Energy Research Institute, Ibaraki, Tokai 319-1195, Japan c High Energy Accelerator Research Organization, Ibaraki, Tsukuba 305-0801, Japan d National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan e Okayama University, Okayama, Okayama 700-8530, Japan f Osaka University, Osaka, Toyonaka 560-8531, Japan b

Available online 28 March 2007

Abstract Zero- and longitudinal-field muon-spin rotation (ZF-lSR and LF-lSR) measurements were carried out on a multi-layered HgBa2Ca4Cu5Oy (Hg-1245) superconductor with Tc of 108 K. With decreasing temperature, the observed ZF-lSR function changed from a Gaussian-type to an exponential-type below 60 K and zero field muon precession was also observed below 45 K. These lSR signals are attributed to the development of the antiferromagnetic ordering in inner CuO2 planes with a low carrier concentration. These results seem to indicate clear evidence that antiferromagnetism microscopically coexists with superconductivity in the Hg-1245 superconductor below 60 K. Ó 2007 Elsevier B.V. All rights reserved. Keywords: HgBa2Ca4Cu5Oy; lSR measurement; Coexistence of superconductivity and antiferromagnetism

1. Introduction A common feature of high Tc cuprate superconductors is to possess two-dimensional CuO2 planes stacked along the c-axis in the structural unit. The electric and magnetic properties of high Tc cuprate superconductors are strongly dependent on the carrier concentration in the CuO2 planes. It is well known that the properties of CuO2 planes change from the antiferromagnetic (AFM) state to a superconducting (SC) state as the carrier doping in the CuO2 planes is increased. The carriers are distributed homogeneously to each CuO2 plane in bi-layer compounds such as YBa2Cu3Oy, *

Corresponding author. Tel./fax: +81 4 7122 9718. E-mail address: [email protected] (K. Tokiwa).

0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.03.187

because all the CuO2 plane is crystallographically equivalent. A multi-layered superconductor with more than three CuO2 planes, however, has crystallographically inequivalent CuO2 planes in the unit cell, that is, outer planes (OP) with five oxygen coordination and inner planes (IP) with four oxygen coordination. The inhomogeneity of the hole carrier distribution among different kinds of CuO2 planes in the multi-layered superconductors has been postulated from results of the calculation of Madelung energy using point charge or sheet charge models [1–5]. It was shown that the imbalance of distributed carriers to IP and OP becomes larger by increasing the number of CuO2 layers (n) and the total number of doped holes. Kotegawa et al. separately evaluated carrier concentrations in IP and OP using empirical rules in relationship to the hole density and Knight (K) shift from 63Cu-NMR measurements on

K. Tokiwa et al. / Physica C 460–462 (2007) 892–895

2. Experimental Polycrystalline samples of Hg-1245 for lSR measurements were synthesized using a high-pressure technique. The mixtures of precursor materials of Ba2Ca4Cu3.5Oy, HgO, Cu2O, and CuO were used for starting reagents. The precursor of Ba2Ca4Cu3.5Oy was prepared from BaCO3, CaCO3 and CuO by heat treatment at 880 °C for 12 h in oxygen flow with intermediate grinding. The oxygen contents in starting mixtures for high-pressure synthesis were controlled by changing the Cu2O/CuO ratio. The mixture was pressed into a pellet and sealed in a gold capsule, and then finally heated at 1050 °C for 2 h under a pressure of 4.5 GPa using a cubic anvil type high pressure apparatus. The final products were characterized to be nearly single phase Hg-1245 by XRD measurement using Cu-Ka radiation. Small amounts of CaO and Hg-1234 phases were detected as impurity phases (less than 10%). Temperature dependence of magnetic susceptibility and resistivity were measured for determination of Tc in these samples. The Tc value obtained from these measurements was 108 K which was nearly equal to the highest Tc in Hg-1245 as reported in Ref. [9]. ZF-lSR measurements were carried out at surface positive muon channel pA at Meson Science Laboratory, Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK-MSL) in Japan. A spin-polarized pulsed positive surface-muon beam (50 ns width and 20 Hz) with a momentum of 27 MeV/c was implanted into the sample. 4He gas-flow-type cryostat

was used for measurements at temperatures above 4 K. The time variation of the asymmetry parameter A(t) is obtained as A(t) = [F(t)  B(t)]/[F(t) + aB(t)], where F(t) and B(t) are the numbers of muon decayed positrons counted by forward and backward counters at time t, respectively, and a is a calibration factor reflecting the relative counting efficiencies of the forward and backward counters. The initial asymmetry was defined as A(0). The time variation of the asymmetry parameter (the lSR time spectrum) was measured in zero (ZF) and longitudinal (LF) magnetic fields. The local magnetic field Bl is estimated from the muon spin precession frequency m using the relation 2pm = clBl, where cl = 851.4 MHz/T is the gyromagnetic ratio of the muon. 3. Results and discussion Fig. 1 shows the ZF-lSR time spectra of the Hg-1245 sample between 150 K and 4 K. The observed function at 150 K shows Gaussian-type depolarization behavior, and no significant differences are observed in the ZF-lSR time spectra down to 60 K. No distinctive variation was also seen at around Tc. This implies that implanted muons feel no local field except the almost static nuclear dipolar field originating from the copper. The drastic change in ZFlSR time spectra was observed below 60 K. With decreasing temperature, the observed ZF-lSR function changed from a Gaussian-type to an exponential type below 60 K. This change indicates the variation of the internal magnetic field surrounding at the muon site. Since the magnetic properties of this system come from Cu spin correlations,

Normalized asymmetry

various high-Tc cuprates [6,7]. They pointed out that the difference in the doping level between IP and OP (DNH) increases with increasing total carrier density and n in Tl-, Hg-, and Cu-compounds. Furthermore, from the results of NMR measurements, Tokunaga et al. found a notable phenomenon in that (Cu0.6C0.4)Ba2Ca3Cu4O12+y (Cu1234) possesses two different Tc’s in each IP and OP, namely, the SC gap in IP opens at 117 K, whereas that in OP does not fully develop below Tc = 117 K and opens below Tc2 = 60 K [8]. Thus, it seems that the different intrinsic properties for IP and OP are separately brought out by controlling the DNH in multi-layered superconductors. In this paper, we report on a lSR study of a multi-layered HgBa2Ca4Cu5Oy (Hg-1245) superconductor, including two OP’s and three IP’s. The zero field muon spin relaxation (ZF-lSR) technique is especially suited for the study of weak and short/long range magnetic correlations, since the positive muon acts as an extremely sensitive local probe to detect an internal magnetic field as small as 0.01 mT. With decreasing temperature, we have observed a remarkable change in the ZF-lSR time spectra which undergoes a change from Gaussian-type to exponentialtype and a muon spin precession below 45 K. These results show the coexistence of both superconductivity and magnetism (likely to be antiferromagnetism) below Tc in the Hg-1245 superconductor.

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1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0 1.0 0.8 0.6 0.4 0.2 0.0

150K

60 K

50 K

45 K

4K

0

1

2

3

4

5

Time [sec] Fig. 1. ZF-lSR time spectra of Hg-1245 samples at 150 K, 60 K, 50 K, 45 K and 4 K. Solid lines show the best fit for our analysis. ZF-lSR time spectra change from Gaussian-type to exponential-type below 60 K.

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it is believed that the additional spin correlations between Cu sites have developed dynamically or statically below 60 K. The l spin precession was also observed below 45 K. This gives clear evidence that the stopped muons feel the static internal field due to three-dimensional (3D) AFM ordering of Cu moments, namely, this means that a longrange ordered AFM state coexists with superconductivity in Hg-1245. The temperature dependence of the frequency of oscillating components below 45 K is shown in Fig. 2. The observed precession frequency m, which is proportional to the internal fields at muon sites, increases with decreasing temperature. The temperature dependence is similar to that observed in AFM phases of other cuprate superconductors [10–12]. The average internal field estimated from the value of m at 4 K is about 70 G. As seen in the spectra below 60 K in Fig. 1, we also observed a loss of initial asymmetry that indicates a fastrelaxing component in the ZF-lSR time spectra. The initial asymmetry at t = 0 is plotted against temperature in Fig. 3. It decreased with decreasing temperature below 60 K. However, no definite l spin precession was seen down to 45 K, indicating no development of a clear three-dimensional long-range magnetic order of Cu spin between 60 K and 45 K. Since the time resolution of the muon beam is 50 ns for this facility and it is hard to detect lSR signals in the initial time range, we could not distinguish whether a ‘‘dynamical’’ spin correlation or ‘‘static’’ one contributes to the loss of initial asymmetry in the present experiments. If the injected muons are depolarized by dynamic effects, then

1.0

ν [MHz]

0.8 0.6 0.4 0.2 0.0 0

10

20

30

40

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Temperature [K] Fig. 2. The temperature dependence of observed muon precession frequency m at muon sites is plotted. At a temperature below 45 K, the muon precessions are observed.

we should think of the suppression of Cu-spin fluctuations with decreasing temperature, so-called ‘‘slowing down’’ behavior. On the other hand, if they are depolarized by static effects of the Cu spins, it means that the internal fields in Hg-1245 are not homogenously distributed at muon sites. The two possibilities may be discerned by means of longitudinal magnetic field (LF). We applied a LF up to 1 kG parallel to the initial spin direction of the implanted muon in field-cool condition at 50 K. With increasing the LF, the lost initial asymmetry was almost recovered at a field up to 500 G. This decoupling behavior means that positive muons relating to this recovery feel a magnetic field of a few hundred gauss and the losses of initial asymmetry are not caused by dynamic effects of magnetic electrons on the time scale of lSR measurement (order of ls). Considering the appearance of coherent muon spin oscillation below 45 K, we suppose that the internal fields between 60 K and 45 K are due to the inhomogeneous distribution of static local fields rather than that of the spin-glass like phase. The magnitude of this static internal field relating to the lost initial asymmetry was roughly estimated by LF dependence of residual muon spin polarization at t = 0Pz as shown in Fig. 4. Since the specimen is polycrystalline sample, we assumed the polarization of missing initial asymmetry at ZF as 1/3. The LF dependence of polarization is analyzed using the function, 2

P Z ð1Þ ¼

ð x 2  1Þ ð x þ 1Þ 2 3 1  2þ log ; 2 3 4 4x 16x ð x  1Þ

ð1Þ

where x = HLF/Hlocal Æ Hlocal is the local internal field at the muon site. The solid line through the data in Fig. 4 is the best fit of Eq. (1). The fitted value for Hlocal is 162 ± 17G. This value is obviously larger than that of the internal field estimated from the results of precession signals. Thus, our data show that there exist more than two different muon sites which feel the static internal field in Hg-1245. These magnetic behavior observed in Hg-1245 are thought to come from its unique crystal structure. Hg-1245 has five CuO2 layers (two OP’s and three IP’s) in the unit cell. We think that coexistence of SC and 1.0

Polarization Pz

Initial Asymmetry

1.00 0.95 0.90

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0.6

0.4

0.85

50 K 0.2

0.80 40

60

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Temperature [K] Fig. 3. The initial asymmetry, indicating the loss of asymmetry at t = 0 is plotted against temperature above 45 K. The solid line is to guide the eye.

0

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400

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Longitudinal Field HLF [Gauss] Fig. 4. Longitudinal-field dependence of the residual muon spin polarization of Hg-1245 sample at 50 K. The solid line is the best fit of Eq. (1).

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magnetism in Hg-1245 are explained as the appearance of intrinsic features of each CuO2 plane caused by the large difference of carrier concentration between IP and OP. In fact, it has been found that the hole carrier concentration in OP’s is greater than that in IP’s, and IP’s are recognized to be in a fairly under-doped state from the results of NMR measurement [13]. Also, observation of NMR signals signifies the appearance of the internal field generated by AFM ordering in IP’s at 1.4 K. Therefore, it seems reasonable to suppose that this magnetic effect appearing below 60 K is attributed to the low carrier concentration in IP’s. The existence of different internal field components, namely missing asymmetry component, muon spin precession component and exponential-type component, suggests that there are two or more muon sites in the Hg-1245 sample. In cuprate superconductors, it is known that positive muons are attracted by negative charged oxygen ions [14,15]. For Hg-1245, there exist many kinds of possible oxygen sites because of its complex crystal structure. Unfortunately, we could not identify exact muon sites for each magnetic signal. However, if both of the internal fields were caused by the same magnetic origin, i.e., magnetic ordering of IP’s, the muons feeling the inhomogeneous internal fields should be located at closer site to IP’s. The development of magnetic order between 60 K and 45 K showing no coherent muon precession is thought to indicate that short range 2D-like AFM correlations developed between three IP’s, and these magnetic correlations are almost confined in these planes. The magnetic coupling between Cu spins along the c-direction is weak because three IP’s are separated from ones of other units by two SC outer planes and charge reservoir blocks. At a temperature below 45 K, where coherent oscillation was observed, a part of the muons feel the homogeneous internal field. It is thought that muons feeling such homogeneous local field are located at sites far from magnetically ordered IP’s and the oxygen sites of apex or Hg–O plane are probable as possible sites. Therefore, we consider that the observed l spin precession originated from the effect of the complete development of the 3D long range AF ordered state beyond the charge reservoir block in the Hg-1245 superconductor. In that case, it is expected that magnetic spin correlations are brought about in extent between IP and OP. In fact, the internal field at Cu site in the OP was observed as well as in IP at 1.4 K by NMR measurement [13]. The experimental value of the internal field in OP is considerably larger than the expected value from the calculation using the experimental value of Cu moments of IP’s. It may be possible that AFM order state is realized even in OP by the development of 3D magnetic long range order.

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In conclusion, we have measured the ZF- and LF-lSR for the multi-layered Hg-1245 superconductor, showing superconducting transition at 108 K. We have observed that the Gasussian-type ZF-lSR time spectrum gradually changed to exponential-type below 60 K together with loss of initial asymmetry, and that muon precession signals are observed below 45 K. These results indicate that the antiferromagnetic correlation of Cu moments develops below Tc in Hg-1245. This means that both superconductivity and antiferromagnetism coexist below 60 K. This phenomenon seems to be caused by a large difference of carrier concentration between IP and OP. From the temperature dependence of the ZF-lSR time spectrum, we suppose that the dimensionality of magnetic spin correlation changes from 2D to 3D at 45 K. It is believed that the coexistence of SC and AFM in Hg-1245 is the result of the different doping levels in each of the five CuO2 layers. This may be a different case from the magnetic superconductor including magnetic ions in the structure [16,17]. Acknowledgements We would like to thank Professor K. Nagamine of KEK-MSL for arranging special beam time for this study. We would also like to acknowledge Dr. M. Mori and Dr. N. Miyakawa for stimulating discussions, and S. Itoh and K. Yuasa for their experimental supports. This work was supported by MEXT. HAITEKU. We are grateful to S. Shiota and T. Ogashiwa (Tanaka Kikinzoku K.K.) for supporting our experiments. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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