μSR studies on layered cobalt oxides

Physica B 326 (2003) 518–521

mSR studies on layered cobalt oxides J. Sugiyamaa,*, J.H. Brewerb, E.J. Ansaldoc, H. Itaharaa, M. Bayerb, T. Tania b

a Toyota Central R&D Labs. Inc., Nagakute, Aichi 480-1192, Japan CIAR, University of British Columbia and TRIUMF, Vancouver, BC, Canada V6T 2A6 c University of Saskatchewan, Saskatoon, Saskatchewan, S7N 0W0 Canada

Abstract To investigate the role of magnetism on transport properties of ‘good’ thermoelectric oxides, mSR spectroscopy has been used for polycrystalline Ca3 Co4 O9 and Na0:7 CoO2 samples in the temperature range between 300 and 2:5 K: It was found that Ca3 Co4 O9 exhibits a magnetic transition below Tconset B100 K with a transition width of DTc ¼ 70 K; below Tconset ; two types of relaxation were observed using a weak transverse-field mSR technique; one exhibits a fast relaxation rate on the order of 10 ms1 and the other a slow one of about 0:1 ms1 : Furthermore, ZF-mSR measurements suggested the existence of an incommensurate spin density wave state below Tconset (i.e. Tconset ¼ TSDW ). Since the resistivity vs. T curve exhibited a broad minimum around 80 K; the SDW transition was found to be associated with the change in the transport properties of Ca3 Co4 O9 : Similar measurements on Na0:7 CoO2 indicated no magnetic transitions below 250 K: Considering the difference between the both crystal structures, it is concluded that the Co ions in the rocksalt-type layers of Ca3 Co4 O9 are likely to play the dominant role in inducing the observed magnetic transitions below 100 K: r 2002 Elsevier Science B.V. All rights reserved. Keywords: Spin density wave; Thermoelectrics; Layered cobalt oxides

1. Introduction Two cobalt oxides, Ca3 Co4 O9 [1–3] and Nax CoO2 [4,5], were reported to exhibit metallic conductivities and extraordinary large Seebeck coefficients (above þ100 mV=K at 300 K) for reasons currently unknown; because of their large thermoelectric figure of merits (ZTB1 at 1000 K), both compounds are considered to be promising candidates for the p-type material of thermoelectric power conversion elements. *Corresponding author. Tel.: +81-561-63-6196; fax: +81561-63-6498. E-mail address: [email protected] (J. Sugiyama).

The crystal structure of Ca3 Co4 O9 was assigned to be alternating stacks of two monoclinic subsystems along the c-axis [2,3,6]. The two subsystems, i.e., triple Ca2 CoO3 layers of the rocksalt-type (first subsystem) and single CoO2 sheets of the CdI2 -type (second subsystem), had identical a; c and b parameters but different b parameters. According to Miyazaki et al. [3,6], monoclinic Ca3 Co4 O9 was represented to ½Ca2 CoO3 x ½CoO2  with x ¼ b2 =b1 ¼ 0:62; this meant an incommensurate structure along the b-axis caused by a misfit between the two subsystems. In this paper, we abbreviate ½Ca2 CoO3 0:62 ½CoO2  as Ca3 Co4 O9 for simplicity. On the other hand, the crystal structure of Nax CoO2 with x ¼ 0:7 was reported to be a

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 8 0 - 0

J. Sugiyama et al. / Physica B 326 (2003) 518–521

bronze-type hexagonal system of space group P63 22 [7]; in this structure, single CoO2 sheets and single disordered Na planes form alternating stacks along the hexagonal c-axis. For both compounds, the CoO2 sheets, in which a two-dimensional triangular lattice of Co ions is formed by a network of edge-sharing CoO6 octahedra, are considered to contribute to the transport of charge carriers. The temperature dependence of resistivity r of Ca3 Co4 O9 exhibits a broad minimum at B80 K and a broad maximum at B400 K [2,3]. On the other hand, susceptibility w measurements indicate two magnetic transitions at 19 and 390 K [8,2]; the former is a ferrimagnetic transition and a latter a probable spin-state transition. It should be noted that the w vs. T curve shows no clear anomaly at B80 K; while the r vs. T curve exhibits a broad minimum. In order to solve this puzzle and know magnetism of these thermoelectric oxides, we measured both weak transverse field ðwTF-Þ mSR and ZF-mSR spectra using polycrystalline Ca3x Mx Co4 O9 (xp0:5; M ¼ Sr and Y) and Na0:7 CoO2 samples in the temperature range between 300 and 2:5 K:

519

The data were fitted in the time domain with a combination of a slowly relaxing precessing signal and a fast relaxing, non-oscillatory signal: A0 PðtÞ ¼ AS elS t cos ðom t þ fÞ þ AF elF t ;

ð1Þ

where A0 is the initial asymmetry, PðtÞ is the muon spin polarization function, om is the muon Larmor frequency, f is the initial phase of the precession and An and ln (n ¼ S; F ) are the asymmetries and exponential relaxation rates of the two components. This form was chosen because the local magnetic fields in the ordered phase (giving the ‘F ’ signal) are much larger than the applied field. Fig. 1(a) shows the temperature dependences of An in Ca3 Co4 O9 obtained by the fitting of the wTF-mSR spectra. As temperature decreases from 300 K; AS is nearly independent of temperature down to 100 K; then AS decreases suddenly with further lowering temperature, and AS seems to level off to a constant value below 30 K: This 0.25 0.2

An

0.15

0.05 0 (a)

10

Ca 3 Co4 O9- λS λ n (10 6 s-1)

Polycrystalline samples of Ca3x Mx Co4 O9 (xp0:5; M ¼ Sr and Y) and Na0:7 CoO2 were synthesized by a solid state reaction technique using reagent-grade CaCO3 ; Na2 CO3 ; Co3 O4 ; SrCO3 and Y2 O3 powders as starting materials. The preparation and characterization of the samples were reported in detail elsewhere [8]. The mSR experiments were performed on the M20 surface muon beam line at TRIUMF. The experimental setup is described elsewhere [9].

3. Results and discussion

Ca 3 Co4 O9 -λF

1

Na 0.7 CoO2 0.1

0.01 0

3.1. Below 300 K The wTF-mSR time spectra of Ca3 Co4 O9 in the magnetic field of H ¼ 104 Oe exhibited a clear decay of muon precession amplitude below 100 K:

Ca3 Co4 O9 -A S Ca3 Co4 O9 -A F Na0.7 CoO 2

0.1

2. Experimental

(b)

50

100

150

200

250

300

TEMPERATURE (K)

Fig. 1. Temperature dependences of: (a) asymmetry An ; and (b) exponential relaxation rate ln in Ca3 Co4 O9 (J: fast F, K: slow S) and Na0:7 CoO2 ; both parameters were obtained by the fitting of the wTF-mSR data using Eq. (1).

J. Sugiyama et al. / Physica B 326 (2003) 518–521

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indicates the existence of a magnetic transition with onset around 100 K: Moreover, AF has a finite value below 100 K and exhibits a maximum around 50 K: Fig. 1(b) shows the temperature dependences of ln ; both lS and lF exhibit broad maxima at 50 K: The highest value of lF ðB7  106 s1 Þ is 40 times larger than that of lS ðB0:17  106 s1 Þ: On the other hand, wTF-mSR measurements on Na0:7 CoO2 suggests neither magnetic transitions nor the fast relaxation component below 250 K (see Fig. 1), though both AS and lS seem to change slightly below 50 K: Considering the difference between the both crystal structures, it is concluded that Co ions in the rocksalt-type ½Ca2 CoO3  subsystem in Ca3 Co4 O9 are likely to play a dominant role in inducing the observed magnetic transitions and the fast relaxation component below B100 K: To investigate the internal magnetic field in Ca3 Co4 O9 below 100 K; Fig. 2 shows ZF-mSR time spectra at 100, 50, 30, 10 and 2:5 K; a clear oscillation due to a quasi-static internal field is Ca3 Co4 O9

ZF-µSR 0.25 0.2 0.15

100 K ~ ~

~ ~

ASYMMETRY

0.1 ~ ~ 0.15

~ ~

50 K

0.1 0.25 0.2

~ ~

observed below 30 K: These ZF-mSR spectra were well fitted using the phenomenological function for the IC-SDW state (see Fig. 2), as in the cases of the organic conductor ðTMTSFÞ2 X (TMTSF: tetramethyl-tetraselena-fulvalene, X ¼ PF6 NO3 and ClO4 ) [9] and La1:6x Nd0:4 Srx CuO4 [10] KT ðt; DÞ A0 PðtÞ ¼ ASDW sin ðom tÞ=ðom tÞ þ AKT Gzz

þ Atail eltail t ;

where A0 is the empirical maximum muon decay asymmetry, ASDW ; AKT and Atail are the asymmeKT tries associated with the three signals, Gzz ðt; DÞ is the static Gaussian Kubo–Toyabe function, D is the static width of the local frequencies at the disordered sites and ltail is the slow relaxation rate of the ‘tail’ (not shown in Fig. 2). Fig. 3 shows the temperature dependence of the muon precession frequency nm ¼ om =2p: As temperature decreases from 100 K; nm increases down to 10 K; then seems to be level off to a constant value of B55 MHz; i.e., Hint B4:1 kOe: Therefore, Ca3 Co4 O9 was found to show a magnetic transition from a high-temperature paramagnetic state to a low-temperature IC-SDW state; the onset on temperature of the transition ð¼ TSDW Þ was mid B100 K; the midpoint ð¼ TSDW Þ B65 K; and the end endpoint ð¼ TSDW Þ B30 K: Also, Fig. 3 shows the temperature dependence of nm in the Ca3x Mx Co4 O9 (xp0:5; M ¼ Sr and Y) samples. According to the analyses of the on wTF-mSR spectra of these samples, TSDW was increased by 30 K due to the Y-doping, while unaffected by the Sr-doping [8]. Actually, the slope

~ ~

30 K

80 ~ ~

~ ~

10 K

2.5 K 0

0.05

0.1

0.15

TIME (µs) Fig. 2. ZF-mSR time spectra of Ca3 Co4 O9 obtained at 100, 50, 30, 10 and 2:5 K; the bold lines represent the results of the fitting using Eq. (2).

νµ (MHz)

0.15

Ca3Co4O9 Ca2.8Sr0.2Co4O9 Ca2.5Sr0.5Co4O9 Ca2.7Y0.3Co4O9 Ca2.5Y0.5Co4O9

60

0.2

0.1

ð2Þ

40 20 0

0

50

100

150

TEMPERATURE (K) Fig. 3. Temperature dependence of the muon precession frequency nm for the pure and doped Ca3 Co4 O9 samples.

J. Sugiyama et al. / Physica B 326 (2003) 518–521

521

of the nm vs. T curve of the Y-doped samples seems to be more gentle than those of the pure and Sr-doped samples. Nevertheless, all the samples show the approximately same nm at zero temperature. This suggests that the local magnetic field Hint ð0 KÞ is independent of dopant and its content, if we assume the muon sites are essentially same in these compounds.

temperature range between 390 and 550 K [2]. Therefore, the change in l around 500 K is considered to be induced by the spin state transition of Co3þ ions in Ca3 Co4 O9 : In order to investigate magnetism around 500 K in detail, further mSR experiment at high temperatures is in progress.

3.2. Above 300 K

Acknowledgements

A preliminary wTF-mSR measurement on Ca3 Co4 O9 up to 700 K showed that the slope of the exponential relaxation rate l changed around 500 and 600 K with increasing temperature, as shown in Fig. 4. This suggests the existence of another magnetic transition around 500 K: Indeed, the w1 vs. T curve exhibits a small jump at 390 K (see Fig. 4(b)), probably due to a change in the spin state of Co3þ ions, as in the case of LaCoO3 [11]. Furthermore, r vs. T curve was reported to exhibit a broad maximum in the

This work was partially supported by the CIAR, the Natural Sciences and Engineering Research Council, and the National Research Council of Canada. We would like to thank Drs. S.R. Kreitzman, B. Hitti and D.J. Arseneau of TRIUMF for their help in carrying mSR experiment. Also, we thank Drs. S. Hirano and R. Asahi of Toyota Labs. for their discussions.

0.2 λ (106 s-1)

Ca 3 Co 4 O 9

(a)

wTF-µSR H=104 Oe

0.1

0

χ-1 (10 3 g/emu)

200

100 50 0

(b)

FC H=10 kOe

150

0

200 400 TEMPERATURE (K)

600

Fig. 4. Temperature dependence of: (a) the exponential relaxation rate l; and (b) the reciprocal DC susceptibility w1 of Ca3 Co4 O9 up to 700 K: In (a), the data of lS below 300 K are also plotted for comparison.

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