- Email: [email protected]
μ+SR studies on thermoelectric oxides
Physica B 329–333 (2003) 902–903
mþSR studies on thermoelectric oxides Jun Sugiyamaa,*, Jess H. Brewerb, Eduardo J. Ansaldoc, Hiroshi Itaharaa, Shingo Hiranoa, Toshihiko Tania a
Frontier Research Groups, Toyota Central R&D Labs, Inc., Nagukute, Aichi 480-1192, Japan b CIAR, University of British Columbia and TRIUMF, Vancouver, BC, Canada V6T 1Z1 c University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A5
Abstract To investigate the role of magnetism on transport properties of thermoelectric oxides, mþ SR spectroscopy has been used for polycrystalline Ca3 Co4 O9 ; Na0:7 CoO2 and Sr0:92y Cay Gd0:08 MnO3 samples in the temperature range between 300 and 2:5 K: It was found that Ca3 Co4 O9 exhibits an incommensurate spin density wave transition around 100 K; though similar measurements on Na0:7 CoO2 indicated no magnetic transitions below 250 K: The Sr0:92y Cay Gd0:08 MnO3 samples showed a ferro- or an antiferromagnetic transition below 250 K; while a muon precession was not observed even at 2:5 K probably due to an inhomogeneity of an internal magnetic field. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Spin density wave; Thermoelectrics; Layered cobaltites; Perovskite manganites
1. Introduction
2. Experimental
Both p-type Ca3 Co4 O9 [1,2] and n-type perovskite manganites Sr1xy Cay Gdx MnO3 [3] are known to exhibit large thermoelectric figure of merits ZT (the former’s ZTB1 and the latter’s B0:2 at 1000 K), for reasons currently unknown. In these compounds, interactions between 3d electrons are considered to be strongly correlated with their ‘good’ thermoelectric properties. Since magnetism is also affected by such interactions, magnetic properties are worth investigating to understand the mechanism of their thermoelectric properties. A positive muon spin rotation and relaxation mþ SR is a powerful technique to detect local magnetic order and/or disorder in zero applied field; here, we report both weak-transverse-field wTF-mþ SR and zero-field ZF-mþ SR results on polycrystalline samples to know magnetism of these thermoelectric oxides.
Polycrystalline samples of Ca3 Co4 O9 ; Na0:7 CoO2 and Sr0:92y Cay Gd0:08 MnO3 were synthesized by a solidstate reaction technique [3,4]. The mþ SR experiments were performed at TRIUMF. The experimental setup is described elsewhere [5].
*Corresponding author. Tel.: +81-561-63-6196: fax: +81561-63-6498. E-mail address: [email protected] (J. Sugiyama).
3. Results and discussion The wTF-mþ SR spectra in Ca3 Co4 O9 (which is represented as ½Ca2 CoO3 0:62 ½CoO2 ) exhibit a clear reduction of the mþ precession amplitude below 100 K: The data were fitted in the time domain with a combination of two relaxing signals. The large decrease in the slow asymmetry AS below 100 K (and the accompanying increase in lS ) indicate the existence of a magnetic transition with an onset temperature Tcon B100 K and a transition width DTB70 K (see Fig. 1). The related material ½Na0:7 ½CoO2 showed no magnetic transitions below 250 K: Considering the difference between the crystal structures of the two cobaltites,
0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02583-8
J. Sugiyama et al. / Physica B 329–333 (2003) 902–903 10
300
An 0
AS AF
wTF-µSR
λS
H=104 Oe
λF
100 200 TEMPERATURE (K)
1
λn (106 s-1)
0.1
Ca3Co4O9
0.1
ASYMMETRY
0.25
0
AF
0.8
0.6 0.4 Ca CONTENT y
0.2
0
Fig. 3. Magnetic phase diagram of Sr0:92y Cay Gd0:08 MnO3 determined by resistivity and susceptibility measurements.
10 K
on TSDW B100 K: Since the resistivity vs. T curve exhibited a broad minimum around 80 K; the SDW transition is considered to be associated with the change in the transport properties of Ca3 Co4 O9 : Both resistivity and susceptibility measurements suggested that a low-temperature phase of Sr0:92y Cay Gd0:08 MnO3 changes from an antiferromagnetic phase to a ferromagnetic one around yB0:6 with increasing y [3], as seen in Fig. 3. Nevertheless, ZFmþ SR spectra of the samples with y ¼ 0:1 and 0.92 at 2:5 K showed no muon precessions but probably very fast relaxations. This suggests an inhomogeneous or a fast fluctuating distribution of an internal magnetic field.
2.5 K
Acknowledgements
ZF-SR ~ ~
100
F
Ca3Co4O9
0.1
200
0.01 300
Fig. 1. Temperature dependence of asymmetry An and relaxation rate ln for Ca3 Co4 O9 : The data were obtained by fitting the wTF-mþ SR spectrum in a magnetic field H ¼ 104 Oe using the combination of two relaxing signals; AS expðlS tÞ cosðom t þ fÞ þ AF expðlF tÞ; where om is the muon Larmor frequency, f is the initial phase of the precession and An and ln (n ¼ S and F) are the asymmetries and exponential relaxation rates of the two signals. The fast signals ðn ¼ FÞ have finite amplitudes below B100 K: 0.15
TEMPERATURE (K)
Sr0.92-yCayGd0.08MnO3
0.2
0
903
30 K
0.2 ~ ~
~ ~
~ ~
0.2
0.15
0.1
0
0.05
0.1
0.15
TIME (s) þ
Fig. 2. ZF-m SR time spectra of Ca3 Co4 O9 obtained at 30, 10 and 2:5 K; the bold lines represent the results of the fitting using the phenomenological function for the IC-SDW state. The muon precession frequency is estimated to be B55 MHz at 2:5 K: Probably due to the fast relaxing signal (see Fig. 1), the initial asymmetry at 30 K is smaller than those at 10 and 2:5 K:
We would like to thank Drs. S.R. Kreitzman, B. Hitti and D.J. Arseneau of TRIUMF for their help in carrying on the mþ SR experiment. This work was partially supported by the CIAR, the Natural Sciences and Engineering Research Council of Canada and the National Research Council of Canada.
References we suggest that Co ions in the ½Ca2 CoO3 layers of Ca3 Co4 O9 are likely to play a significant role in inducing the magnetic transition around 100 K: Fig. 2 shows ZF-mþ SR time spectra of Ca3 Co4 O9 at 30, 10 and 2:5 K: Probably due to the large transition width (B70 K), a clear oscillation caused by a quasistatic internal field was observed only below 30 K: These spectra were fitted using the phenomenological function for the incommensurate spin density wave IC-SDW state [5]. Therefore, Ca3 Co4 O9 was found to show a magnetic transition from a high-temperature paramagnetic state to a low-temperature IC-SDW state at
[1] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, Jpn. J. Appl. Phys. 39 (2000) L1127. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [3] S. Hirano, unpublished. [4] J. Sugiyama, H. Itahara, T. Tani, J.H. Brewer, E.J. Ansaldo, Phys. Rev. B 66 (2002) 134413. [5] L.P. Le, A. Keren, G.M. Luke, B.J. Sternlieb, W.D. Wu, Y.J. Uemura, J.H. Brewer, T.M. Riseman, R.V. Upasani, L.Y. Chiang, W. Kang, P.M. Chaikin, T. Csiba, G. Gruner, . Phys. Rev. B 48 (1993) 7284.
mþSR studies on thermoelectric oxides Jun Sugiyamaa,*, Jess H. Brewerb, Eduardo J. Ansaldoc, Hiroshi Itaharaa, Shingo Hiranoa, Toshihiko Tania a
Frontier Research Groups, Toyota Central R&D Labs, Inc., Nagukute, Aichi 480-1192, Japan b CIAR, University of British Columbia and TRIUMF, Vancouver, BC, Canada V6T 1Z1 c University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A5
Abstract To investigate the role of magnetism on transport properties of thermoelectric oxides, mþ SR spectroscopy has been used for polycrystalline Ca3 Co4 O9 ; Na0:7 CoO2 and Sr0:92y Cay Gd0:08 MnO3 samples in the temperature range between 300 and 2:5 K: It was found that Ca3 Co4 O9 exhibits an incommensurate spin density wave transition around 100 K; though similar measurements on Na0:7 CoO2 indicated no magnetic transitions below 250 K: The Sr0:92y Cay Gd0:08 MnO3 samples showed a ferro- or an antiferromagnetic transition below 250 K; while a muon precession was not observed even at 2:5 K probably due to an inhomogeneity of an internal magnetic field. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Spin density wave; Thermoelectrics; Layered cobaltites; Perovskite manganites
1. Introduction
2. Experimental
Both p-type Ca3 Co4 O9 [1,2] and n-type perovskite manganites Sr1xy Cay Gdx MnO3 [3] are known to exhibit large thermoelectric figure of merits ZT (the former’s ZTB1 and the latter’s B0:2 at 1000 K), for reasons currently unknown. In these compounds, interactions between 3d electrons are considered to be strongly correlated with their ‘good’ thermoelectric properties. Since magnetism is also affected by such interactions, magnetic properties are worth investigating to understand the mechanism of their thermoelectric properties. A positive muon spin rotation and relaxation mþ SR is a powerful technique to detect local magnetic order and/or disorder in zero applied field; here, we report both weak-transverse-field wTF-mþ SR and zero-field ZF-mþ SR results on polycrystalline samples to know magnetism of these thermoelectric oxides.
Polycrystalline samples of Ca3 Co4 O9 ; Na0:7 CoO2 and Sr0:92y Cay Gd0:08 MnO3 were synthesized by a solidstate reaction technique [3,4]. The mþ SR experiments were performed at TRIUMF. The experimental setup is described elsewhere [5].
*Corresponding author. Tel.: +81-561-63-6196: fax: +81561-63-6498. E-mail address: [email protected] (J. Sugiyama).
3. Results and discussion The wTF-mþ SR spectra in Ca3 Co4 O9 (which is represented as ½Ca2 CoO3 0:62 ½CoO2 ) exhibit a clear reduction of the mþ precession amplitude below 100 K: The data were fitted in the time domain with a combination of two relaxing signals. The large decrease in the slow asymmetry AS below 100 K (and the accompanying increase in lS ) indicate the existence of a magnetic transition with an onset temperature Tcon B100 K and a transition width DTB70 K (see Fig. 1). The related material ½Na0:7 ½CoO2 showed no magnetic transitions below 250 K: Considering the difference between the crystal structures of the two cobaltites,
0921-4526/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02583-8
J. Sugiyama et al. / Physica B 329–333 (2003) 902–903 10
300
An 0
AS AF
wTF-µSR
λS
H=104 Oe
λF
100 200 TEMPERATURE (K)
1
λn (106 s-1)
0.1
Ca3Co4O9
0.1
ASYMMETRY
0.25
0
AF
0.8
0.6 0.4 Ca CONTENT y
0.2
0
Fig. 3. Magnetic phase diagram of Sr0:92y Cay Gd0:08 MnO3 determined by resistivity and susceptibility measurements.
10 K
on TSDW B100 K: Since the resistivity vs. T curve exhibited a broad minimum around 80 K; the SDW transition is considered to be associated with the change in the transport properties of Ca3 Co4 O9 : Both resistivity and susceptibility measurements suggested that a low-temperature phase of Sr0:92y Cay Gd0:08 MnO3 changes from an antiferromagnetic phase to a ferromagnetic one around yB0:6 with increasing y [3], as seen in Fig. 3. Nevertheless, ZFmþ SR spectra of the samples with y ¼ 0:1 and 0.92 at 2:5 K showed no muon precessions but probably very fast relaxations. This suggests an inhomogeneous or a fast fluctuating distribution of an internal magnetic field.
2.5 K
Acknowledgements
ZF-SR ~ ~
100
F
Ca3Co4O9
0.1
200
0.01 300
Fig. 1. Temperature dependence of asymmetry An and relaxation rate ln for Ca3 Co4 O9 : The data were obtained by fitting the wTF-mþ SR spectrum in a magnetic field H ¼ 104 Oe using the combination of two relaxing signals; AS expðlS tÞ cosðom t þ fÞ þ AF expðlF tÞ; where om is the muon Larmor frequency, f is the initial phase of the precession and An and ln (n ¼ S and F) are the asymmetries and exponential relaxation rates of the two signals. The fast signals ðn ¼ FÞ have finite amplitudes below B100 K: 0.15
TEMPERATURE (K)
Sr0.92-yCayGd0.08MnO3
0.2
0
903
30 K
0.2 ~ ~
~ ~
~ ~
0.2
0.15
0.1
0
0.05
0.1
0.15
TIME (s) þ
Fig. 2. ZF-m SR time spectra of Ca3 Co4 O9 obtained at 30, 10 and 2:5 K; the bold lines represent the results of the fitting using the phenomenological function for the IC-SDW state. The muon precession frequency is estimated to be B55 MHz at 2:5 K: Probably due to the fast relaxing signal (see Fig. 1), the initial asymmetry at 30 K is smaller than those at 10 and 2:5 K:
We would like to thank Drs. S.R. Kreitzman, B. Hitti and D.J. Arseneau of TRIUMF for their help in carrying on the mþ SR experiment. This work was partially supported by the CIAR, the Natural Sciences and Engineering Research Council of Canada and the National Research Council of Canada.
References we suggest that Co ions in the ½Ca2 CoO3 layers of Ca3 Co4 O9 are likely to play a significant role in inducing the magnetic transition around 100 K: Fig. 2 shows ZF-mþ SR time spectra of Ca3 Co4 O9 at 30, 10 and 2:5 K: Probably due to the large transition width (B70 K), a clear oscillation caused by a quasistatic internal field was observed only below 30 K: These spectra were fitted using the phenomenological function for the incommensurate spin density wave IC-SDW state [5]. Therefore, Ca3 Co4 O9 was found to show a magnetic transition from a high-temperature paramagnetic state to a low-temperature IC-SDW state at
[1] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, Jpn. J. Appl. Phys. 39 (2000) L1127. [2] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [3] S. Hirano, unpublished. [4] J. Sugiyama, H. Itahara, T. Tani, J.H. Brewer, E.J. Ansaldo, Phys. Rev. B 66 (2002) 134413. [5] L.P. Le, A. Keren, G.M. Luke, B.J. Sternlieb, W.D. Wu, Y.J. Uemura, J.H. Brewer, T.M. Riseman, R.V. Upasani, L.Y. Chiang, W. Kang, P.M. Chaikin, T. Csiba, G. Gruner, . Phys. Rev. B 48 (1993) 7284.