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Magnetic transition in Na0.5CoO2 at 88 K
ARTICLE IN PRESS
Physica B 378–380 (2006) 861–862 www.elsevier.com/locate/physb
Magnetic transition in Na0.5CoO2 at 88 K B. Pedrini, S. Weyeneth, J.L. Gavilano, J. Hinderer, M. Weller, H.R. Ott, S.M. Kazakov, J. Karpinski Labor fu¨r Festko¨rperphysik, ETH-Ho¨nggerberg, 8093 Zu¨rich, Switzerland
Abstract Previous investigations of the compound Na0.5CoO2 gave evidence for a metal–insulator transition at T MI ¼ 53 K upon cooling, and another transition at T X ¼ 88 K whose nature has not been clarified yet. We report the results of NMR measurements on a powder sample of Na0.5CoO2. They include the mapping of 23Na and 59Co spectra and the evaluation of the 23Na NMR spin-lattice relaxation rate T 1 1 ðTÞ in the temperature range between 10 and 305 K. The NMR data reflect the transition at T X very well but give little evidence 23 of the metal–insulator transition at T MI . A prominent peak in T 1 1 ðTÞ at T X is accompanied by an abrupt broadening of both the Na 59 and Co spectra upon decreasing temperature. The shape of the spectra below T X implies the formation of a staggered internal field below T X . r 2006 Elsevier B.V. All rights reserved. PACS: 71.27; 71.30; 75.10; 76.60 Keywords: Na0.5CoO2; Strongly correlated electrons; Metal–insulator transition; Magnetic ordering; NMR
1. Introduction Compounds of the family Nax CoO2 ð0:25oxo1Þ adopt a crystal structure consisting of alternating layers of Naþ ions and layers of CoO2 with a triangular arrangement of the Co sites [1]. They represent a physical realization of magnetic systems with planar triangular symmetry, in which metallicity is achieved by a controlled carrier injection. The paramagnetic metallic phase ðxo0:5Þ is separated from the Curie–Weiss metallic phase ðx40:5Þ by the insulating phase occurring at x ¼ 0:5 [2]. Very recently, Nax CoO2 has been the subject of intensive theoretical investigations, aiming at explaining the richness of the phase diagram [3–6], which is argued to be the result of a subtle interplay between electronic and structural degrees of freedom. Upon cooling, Na0.5CoO2 exhibits a transition from a bad metallic state to an insulating phase at T MI ¼ 53 K [7]. The formation of the insulating ground state has been suggested to be accompanied by a charge disproportionaCorresponding author.
E-mail address: [email protected] (B. Pedrini). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.318
tion 2Co3:5þ ! Co3þ þ Co4þ [8]. The temperature dependence of the magnetic susceptibility wðTÞ exhibits a small peak at T MI . Another anomaly in wðTÞ appears at T X ¼ 88 K, and was suggested to reflect a structural transition [7]. Electron diffraction studies [9] have indicated that the Na ions are ordered in chains at room temperature, but did not provide additional insights about the transition at T X . 2. Experimental results Our results on the magnetic susceptibility wðTÞ and the electrical resistivity rðTÞ of a polycrystalline sample of Na0.5CoO2 are in good agreement with previously published data [7]. Examples of 23Na NMR spectra are shown in Fig. 1. Above T X , we observed a single NMR line (labelled P), with a Knight shift KðTÞ proportional to the magnetic susceptibility wðTÞ, indicating a paramagnetic state of the sample. At T X , we note an abrupt change of the spectra with decreasing temperature. Below T X , we identified two NMR signals: a peak labelled A, and a rectangular-shaped signal in the range B1–B2, with corresponding frequencies
ARTICLE IN PRESS B. Pedrini et al. / Physica B 378–380 (2006) 861–862
862
Echo intensity (arb. units)
H = 7.0 T
factor of 500 between 300 and 30 K, exhibiting a prominent peak at T X but only a small cusp at T MI . Fig. 2 shows examples of 59Co NMR spectra at different temperatures between 50 and 100 K. We note a clear change in the signal shape between 100 and 65 K, but not between 65 K and temperatures below T MI ¼ 53 K (not shown). Two distinct signals with different spin–spin relaxation rates are identified below T X and are labelled X (central peak) and Z1–Z2 (rectangle-shaped signal). The width of the signal Z1–Z2, reflecting 2H int , the internal magnetic field, was found to be the same for the irradiation frequencies f ¼ 53:444 MHz and f ¼ 66:444 MHz, respectively.
P
T = 100 K A B1
B2
T = 56.0 K
78.50
79.00
3. Discussion
f (MHZ) Fig. 1. 23Na-NMR spectra at two different temperatures. The measurements were performed in a magnetic field of H ¼ 7:0 T. The echo intensity is plotted as a function of the irradiation frequency.
Echo intensity (arb. units)
f = 53.444 MHz 23Na
All the prominent anomalies in the NMR data occur at T X , and not at T MI , as might a priori be expected. In particular, it turns out that the transition at T X must have a magnetic component. The temperature evolution of both the 23Na and the 59Co spectra exhibit field independent features which most likely reflect the formation of an internal staggered field. The internal field H int at the Conuclei is approximately 0.8 T, and this small value suggests that this NMR-signal arises from non-magnetic Co-sites.
T = 100 K X
4. Conclusion Z2
Z1 T = 65 K 2.Hint 9.00
8.00
7.00
6.00 5.00 H (T)
4.00
3.00
2.00
Fig. 2. 59Co-NMR spectra at two different temperatures. The measurements were performed with a fixed irradiation frequency of 53.444 MHz. The echo intensity is plotted as a function of the magnetic field H.
f B1 ðTÞ and f B2 ðTÞ. The width of the peak A increases abruptly below T X upon cooling. The width Df B ðTÞ ¼ f B2 ðTÞ f B1 ðTÞ also exhibits a sudden extension below T X with decreasing temperature, and reaches the saturation value Df B;0 ¼ 0:5 MHz at T ¼ 0 K. Moreover, Df B is found to be unchanged upon varying the external magnetic field from H ¼ 7 to 4.2 T. Quite surprisingly, we detected no anomaly in the temperature evolution of the 23Na spectra around T MI ¼ 39 K. The 23Na spin lattice relaxation rate T 1 1 ðTÞ (not shown here) in total decreases by a
Our results presented above imply that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, itself leading to an insulating ground state. Acknowledgements We acknowledge useful discussions with M. Indergand and M. Sigrist. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Q. Huang, et al., Phys. Rev. B 70 (2004) 184110. M.L. Foo, et al., Phys. Rev. Lett. 92 (2004) 247001. M. Indergand, et al., cond-mat/0502116 (2005). P. Zhang, et al., Phys. Rev. B 71 (2005) 153102. Z. Li, et al., Phys. Rev. B 71 (2005) 024502. S. Zhou, et al., cond-mat/0503346 (2005). Q. Huang, et al., J. Phys.: Condens. Matter 16 (2004) 5803. K.-W. Lee, et al., Phys. Rev. Lett. 94 (2005) 026403. H.W. Zandbergen, et al., Phys. Rev. B 70 (2004) 024101.
Physica B 378–380 (2006) 861–862 www.elsevier.com/locate/physb
Magnetic transition in Na0.5CoO2 at 88 K B. Pedrini, S. Weyeneth, J.L. Gavilano, J. Hinderer, M. Weller, H.R. Ott, S.M. Kazakov, J. Karpinski Labor fu¨r Festko¨rperphysik, ETH-Ho¨nggerberg, 8093 Zu¨rich, Switzerland
Abstract Previous investigations of the compound Na0.5CoO2 gave evidence for a metal–insulator transition at T MI ¼ 53 K upon cooling, and another transition at T X ¼ 88 K whose nature has not been clarified yet. We report the results of NMR measurements on a powder sample of Na0.5CoO2. They include the mapping of 23Na and 59Co spectra and the evaluation of the 23Na NMR spin-lattice relaxation rate T 1 1 ðTÞ in the temperature range between 10 and 305 K. The NMR data reflect the transition at T X very well but give little evidence 23 of the metal–insulator transition at T MI . A prominent peak in T 1 1 ðTÞ at T X is accompanied by an abrupt broadening of both the Na 59 and Co spectra upon decreasing temperature. The shape of the spectra below T X implies the formation of a staggered internal field below T X . r 2006 Elsevier B.V. All rights reserved. PACS: 71.27; 71.30; 75.10; 76.60 Keywords: Na0.5CoO2; Strongly correlated electrons; Metal–insulator transition; Magnetic ordering; NMR
1. Introduction Compounds of the family Nax CoO2 ð0:25oxo1Þ adopt a crystal structure consisting of alternating layers of Naþ ions and layers of CoO2 with a triangular arrangement of the Co sites [1]. They represent a physical realization of magnetic systems with planar triangular symmetry, in which metallicity is achieved by a controlled carrier injection. The paramagnetic metallic phase ðxo0:5Þ is separated from the Curie–Weiss metallic phase ðx40:5Þ by the insulating phase occurring at x ¼ 0:5 [2]. Very recently, Nax CoO2 has been the subject of intensive theoretical investigations, aiming at explaining the richness of the phase diagram [3–6], which is argued to be the result of a subtle interplay between electronic and structural degrees of freedom. Upon cooling, Na0.5CoO2 exhibits a transition from a bad metallic state to an insulating phase at T MI ¼ 53 K [7]. The formation of the insulating ground state has been suggested to be accompanied by a charge disproportionaCorresponding author.
E-mail address: [email protected] (B. Pedrini). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.318
tion 2Co3:5þ ! Co3þ þ Co4þ [8]. The temperature dependence of the magnetic susceptibility wðTÞ exhibits a small peak at T MI . Another anomaly in wðTÞ appears at T X ¼ 88 K, and was suggested to reflect a structural transition [7]. Electron diffraction studies [9] have indicated that the Na ions are ordered in chains at room temperature, but did not provide additional insights about the transition at T X . 2. Experimental results Our results on the magnetic susceptibility wðTÞ and the electrical resistivity rðTÞ of a polycrystalline sample of Na0.5CoO2 are in good agreement with previously published data [7]. Examples of 23Na NMR spectra are shown in Fig. 1. Above T X , we observed a single NMR line (labelled P), with a Knight shift KðTÞ proportional to the magnetic susceptibility wðTÞ, indicating a paramagnetic state of the sample. At T X , we note an abrupt change of the spectra with decreasing temperature. Below T X , we identified two NMR signals: a peak labelled A, and a rectangular-shaped signal in the range B1–B2, with corresponding frequencies
ARTICLE IN PRESS B. Pedrini et al. / Physica B 378–380 (2006) 861–862
862
Echo intensity (arb. units)
H = 7.0 T
factor of 500 between 300 and 30 K, exhibiting a prominent peak at T X but only a small cusp at T MI . Fig. 2 shows examples of 59Co NMR spectra at different temperatures between 50 and 100 K. We note a clear change in the signal shape between 100 and 65 K, but not between 65 K and temperatures below T MI ¼ 53 K (not shown). Two distinct signals with different spin–spin relaxation rates are identified below T X and are labelled X (central peak) and Z1–Z2 (rectangle-shaped signal). The width of the signal Z1–Z2, reflecting 2H int , the internal magnetic field, was found to be the same for the irradiation frequencies f ¼ 53:444 MHz and f ¼ 66:444 MHz, respectively.
P
T = 100 K A B1
B2
T = 56.0 K
78.50
79.00
3. Discussion
f (MHZ) Fig. 1. 23Na-NMR spectra at two different temperatures. The measurements were performed in a magnetic field of H ¼ 7:0 T. The echo intensity is plotted as a function of the irradiation frequency.
Echo intensity (arb. units)
f = 53.444 MHz 23Na
All the prominent anomalies in the NMR data occur at T X , and not at T MI , as might a priori be expected. In particular, it turns out that the transition at T X must have a magnetic component. The temperature evolution of both the 23Na and the 59Co spectra exhibit field independent features which most likely reflect the formation of an internal staggered field. The internal field H int at the Conuclei is approximately 0.8 T, and this small value suggests that this NMR-signal arises from non-magnetic Co-sites.
T = 100 K X
4. Conclusion Z2
Z1 T = 65 K 2.Hint 9.00
8.00
7.00
6.00 5.00 H (T)
4.00
3.00
2.00
Fig. 2. 59Co-NMR spectra at two different temperatures. The measurements were performed with a fixed irradiation frequency of 53.444 MHz. The echo intensity is plotted as a function of the magnetic field H.
f B1 ðTÞ and f B2 ðTÞ. The width of the peak A increases abruptly below T X upon cooling. The width Df B ðTÞ ¼ f B2 ðTÞ f B1 ðTÞ also exhibits a sudden extension below T X with decreasing temperature, and reaches the saturation value Df B;0 ¼ 0:5 MHz at T ¼ 0 K. Moreover, Df B is found to be unchanged upon varying the external magnetic field from H ¼ 7 to 4.2 T. Quite surprisingly, we detected no anomaly in the temperature evolution of the 23Na spectra around T MI ¼ 39 K. The 23Na spin lattice relaxation rate T 1 1 ðTÞ (not shown here) in total decreases by a
Our results presented above imply that in Na0.5CoO2, an unusual type of magnetic ordering in the metallic phase precedes the onset of charge ordering, itself leading to an insulating ground state. Acknowledgements We acknowledge useful discussions with M. Indergand and M. Sigrist. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Q. Huang, et al., Phys. Rev. B 70 (2004) 184110. M.L. Foo, et al., Phys. Rev. Lett. 92 (2004) 247001. M. Indergand, et al., cond-mat/0502116 (2005). P. Zhang, et al., Phys. Rev. B 71 (2005) 153102. Z. Li, et al., Phys. Rev. B 71 (2005) 024502. S. Zhou, et al., cond-mat/0503346 (2005). Q. Huang, et al., J. Phys.: Condens. Matter 16 (2004) 5803. K.-W. Lee, et al., Phys. Rev. Lett. 94 (2005) 026403. H.W. Zandbergen, et al., Phys. Rev. B 70 (2004) 024101.