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Electronic structures of CMR pyrochlore Tl2−xScxMn2O7
Physica B 281&282 (2000) 528}530
Electronic structures of CMR pyrochlore Tl Sc Mn O 2~x x 2 7 S.K. Kwon, J.H. Park, B.I. Min* Department of Physics, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract We have performed electronic structure calculations on the colossal magnetoresistive (CMR) pyrochlore Tl Sc Mn O , using the linearized mu$n-tin orbital (LMTO) band method within the local density approximation 2~x x 2 7 (LDA). We have found that Tl Sc Mn O is almost half-metallic for x"0 and 0.5, while, for x"1.5 and 2.0, it 2~x x 2 7 becomes an insulator. The Mn spin magnetic moments vary from 2.84&2.91 l , indicating single valency of Mn4` in B these compounds. The half-metallic nature increases from x"0 to 0.5, which is consistent with experimental results that the magnetoresistance is larger for x"0.5. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Electronic structure; Half-metal; Magnetoresistance
After the discovery of the CMR phenomena in La Ca MnO [1], perovskite manganites have 1~x x 3 gained renewed interest. Extensive studies have been done to reveal the origin of the CMR and to understand rich states in the phase diagram. It is known that the ground state of doped manganites is a ferromagnetic half-metal [2] and there is Mn3`/Mn4` mixture. Because of the mixed valency, the double exchange mechanism lowers the total energy through hopping of Mn e electrons and makes the ferromagnetic ordering stable. ' In addition to the double exchange, the singly occupied Mn e electron is strongly coupled to phonon to induce ' lattice distortion, the so-called Jahn}Teller e!ect. The cooperation between the double exchange and the strong electron}phonon interaction is believed to be the working mechanism of magnetotransport in this system [3]. Pyrochlore Tl Sc Mn O , however, is found to be 2~x x 2 7 another kind of the CMR material where all Mn ions are tetra-valent Mn4` [4]. For Mn4`, t levels are 2' fully occupied by three d electrons and e levels are ' empty. Measurement of Hall coe$cients for Tl Mn O 2 2 7 indicates that the number of charge carrier is very small (&0.005 electrons/formula unit) [5]. In this case,
* Corresponding author. Tel.: #82-562-279-2074; fax: #82562-279-3099. E-mail address: [email protected] (B.I. Min)
the double exchange may not be the conducting mechanism, and the superexchange is thought to induce the ferromagnetic ground state. In contrast to La Ca MnO , the magnetism and the conduction 1~x x 3 seem to occur at separated lattice sites in Tl Sc Mn O . That is, the local moment of Mn 3d 2~x x 2 7 electrons is the source of the ferromagnetism, and Tl 6s band which is hybridized with Mn t band is responsible 2' for the conduction. Therefore, the transport of Tl 6s electrons will su!er from strong scattering by localized Mn 3d moments. It has been recently reported that the magnetoresistance (MR) becomes larger as x increases in Tl Sc Mn O for small x [6]. To understand the 2~x x 2 7 e!ect of Sc substitution, a systematic investigation of electronic structures is necessary. To accomplish this, we have carried out the LDA-LMTO calculations for Tl Sc Mn O with x"0, 0.5, 1.5, and 2.0. For 2~x x 2 7 x"0.5(1.5) calculation, we replaced one(three) Tl atom(s) by Sc among four Tl atoms in a unit cell, varying the lattice constant. It is found that the ferromagnetic ground state is metallic for x"0 and 0.5, while, for x"1.5 and 2.0, it is insulating. In Fig. 1, we show the density of states (DOS) of ferromagnetic Tl Mn O . The majority spin band edge 2 2 7 is 0.08 eV above the Fermi energy (E ), whereas the F minority spin band is broadly located above and below E . The states of the minority spin band are composed of F Tl 6s which are hybridized with Mn 3d. These results are
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 9 9 4 - 1
S.K. Kwon et al. / Physica B 281&282 (2000) 528}530
529
Fig. 1. The DOS of ferromagnetic Tl Mn O . The minority spin DOS at E is not zero, albeit very small. 2 2 7 F
Fig. 2. The DOS of ferromagnetic Sc Mn O . The Sc 3s band 2 2 7 is inactive in contrast to the Tl 6s band in Tl Mn O . 2 2 7
in agreement with existing band calculations [7,8]. The DOS of Sc Mn O is shown in Fig. 2. Interestingly, the 2 2 7 Sc 3s bands are inactive in contrast to the Tl 6s, which results in an insulating ground state. Due to its smaller radius of the Sc ion than the Tl ion, the Sc substitution in Tl Mn O has a role to play in the reduction of chem2 2 7 ical pressure. For Tl Sc Mn O , which has larger MR than 1.5 0.5 2 7 Tl Mn O , the majority spin band edge is only 0.01 eV 2 2 7 above E . On the other hand, the minority spin band F begins at 0.05 eV below E . This means that, with small F defects or excess charge, the majority spin band edge is easily shifted while keeping the minority spin band metallic. Once this situation is realized, Tl Sc Mn O 1.5 0.5 2 7 becomes a half-metal. The spin magnetic moment of Mn is 2.88 and 2.91 l B for x"0 and 0.5, respectively. These values are close to
3 l for a Mn4` con"guration. The neutron scattering B data in Tl Mn O are 2.91 l [10] consistent with our 2 2 7 B result. For Sc-rich compounds, we have obtained slightly smaller values, 2.84 l for x"1.5 and 2.87 l for x"2, B B still close to that of Mn4`. This suggests that the Mn3`/Mn4` mixture is absent, and so the double exchange and the Jahn}Teller distortion mechanism would be ruled out for the CMR phenomena in Tl Sc Mn O . 2~x x 2 7 Although the scattering of the minority spin electrons by Mn 3d moments is one reason for the observed CMR in the considered system, it is thought that this e!ect alone is insu$cient for MR &90%. The relation between the half-metallic nature and the CMR has not been clari"ed yet. It is, however, obvious that the half-metallic nature can enhance the MR through a tunneling mechanism at grain or magnetic domain boundaries [9]. In conclusion, we have obtained electronic structures of Tl Sc Mn O for x"0, 0.5, 1.5 and 2.0. The 2~x x 2 7 ground state is metallic for x"0 and insulating for x"1.5 and 2.0. Inbetween them, the almost half-metallic ground state is found for x"0.5, which will enhance the MR as compared to that for x"0.
Acknowledgements This work was supported by the Korean MOST fund, and in part by the POSTECH-BSRI program (BSRI-982438).
References [1] S. Jin et al., Science 264 (1994) 413.
530
S.K. Kwon et al. / Physica B 281&282 (2000) 528}530
[2] J.-H. Park et al., Nature 392 (1998) 794. [3] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [4] G.H. Kwei, C.H. Booth, F. Bridges, M.A. Subramanian, Phys. Rev. B 55 (1997) R688. [5] Y. Shimakawa, Y. Kubo, T. Manako, Nature 379 (1996) 53. [6] M.A. Subramanian, A.P. Ramirez, G.H. Kwei, Solid State Ionics 108 (1998) 185.
[7] D.J. Singh, Phys. Rev. B 55 (1997) 313. [8] D.-K. Seo, M.-H. Whangbo, M.A. Subramanian, Solid State Commun. 101 (1997) 417. [9] H.Y. Hwang, S.-W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [10] J.W. Lynn, L. Vasiliu-Doloc, M.A. Subramanian, Phys. Rev. Lett. 80 (1998) 4582.
Electronic structures of CMR pyrochlore Tl Sc Mn O 2~x x 2 7 S.K. Kwon, J.H. Park, B.I. Min* Department of Physics, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract We have performed electronic structure calculations on the colossal magnetoresistive (CMR) pyrochlore Tl Sc Mn O , using the linearized mu$n-tin orbital (LMTO) band method within the local density approximation 2~x x 2 7 (LDA). We have found that Tl Sc Mn O is almost half-metallic for x"0 and 0.5, while, for x"1.5 and 2.0, it 2~x x 2 7 becomes an insulator. The Mn spin magnetic moments vary from 2.84&2.91 l , indicating single valency of Mn4` in B these compounds. The half-metallic nature increases from x"0 to 0.5, which is consistent with experimental results that the magnetoresistance is larger for x"0.5. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Electronic structure; Half-metal; Magnetoresistance
After the discovery of the CMR phenomena in La Ca MnO [1], perovskite manganites have 1~x x 3 gained renewed interest. Extensive studies have been done to reveal the origin of the CMR and to understand rich states in the phase diagram. It is known that the ground state of doped manganites is a ferromagnetic half-metal [2] and there is Mn3`/Mn4` mixture. Because of the mixed valency, the double exchange mechanism lowers the total energy through hopping of Mn e electrons and makes the ferromagnetic ordering stable. ' In addition to the double exchange, the singly occupied Mn e electron is strongly coupled to phonon to induce ' lattice distortion, the so-called Jahn}Teller e!ect. The cooperation between the double exchange and the strong electron}phonon interaction is believed to be the working mechanism of magnetotransport in this system [3]. Pyrochlore Tl Sc Mn O , however, is found to be 2~x x 2 7 another kind of the CMR material where all Mn ions are tetra-valent Mn4` [4]. For Mn4`, t levels are 2' fully occupied by three d electrons and e levels are ' empty. Measurement of Hall coe$cients for Tl Mn O 2 2 7 indicates that the number of charge carrier is very small (&0.005 electrons/formula unit) [5]. In this case,
* Corresponding author. Tel.: #82-562-279-2074; fax: #82562-279-3099. E-mail address: [email protected] (B.I. Min)
the double exchange may not be the conducting mechanism, and the superexchange is thought to induce the ferromagnetic ground state. In contrast to La Ca MnO , the magnetism and the conduction 1~x x 3 seem to occur at separated lattice sites in Tl Sc Mn O . That is, the local moment of Mn 3d 2~x x 2 7 electrons is the source of the ferromagnetism, and Tl 6s band which is hybridized with Mn t band is responsible 2' for the conduction. Therefore, the transport of Tl 6s electrons will su!er from strong scattering by localized Mn 3d moments. It has been recently reported that the magnetoresistance (MR) becomes larger as x increases in Tl Sc Mn O for small x [6]. To understand the 2~x x 2 7 e!ect of Sc substitution, a systematic investigation of electronic structures is necessary. To accomplish this, we have carried out the LDA-LMTO calculations for Tl Sc Mn O with x"0, 0.5, 1.5, and 2.0. For 2~x x 2 7 x"0.5(1.5) calculation, we replaced one(three) Tl atom(s) by Sc among four Tl atoms in a unit cell, varying the lattice constant. It is found that the ferromagnetic ground state is metallic for x"0 and 0.5, while, for x"1.5 and 2.0, it is insulating. In Fig. 1, we show the density of states (DOS) of ferromagnetic Tl Mn O . The majority spin band edge 2 2 7 is 0.08 eV above the Fermi energy (E ), whereas the F minority spin band is broadly located above and below E . The states of the minority spin band are composed of F Tl 6s which are hybridized with Mn 3d. These results are
0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 0 9 9 4 - 1
S.K. Kwon et al. / Physica B 281&282 (2000) 528}530
529
Fig. 1. The DOS of ferromagnetic Tl Mn O . The minority spin DOS at E is not zero, albeit very small. 2 2 7 F
Fig. 2. The DOS of ferromagnetic Sc Mn O . The Sc 3s band 2 2 7 is inactive in contrast to the Tl 6s band in Tl Mn O . 2 2 7
in agreement with existing band calculations [7,8]. The DOS of Sc Mn O is shown in Fig. 2. Interestingly, the 2 2 7 Sc 3s bands are inactive in contrast to the Tl 6s, which results in an insulating ground state. Due to its smaller radius of the Sc ion than the Tl ion, the Sc substitution in Tl Mn O has a role to play in the reduction of chem2 2 7 ical pressure. For Tl Sc Mn O , which has larger MR than 1.5 0.5 2 7 Tl Mn O , the majority spin band edge is only 0.01 eV 2 2 7 above E . On the other hand, the minority spin band F begins at 0.05 eV below E . This means that, with small F defects or excess charge, the majority spin band edge is easily shifted while keeping the minority spin band metallic. Once this situation is realized, Tl Sc Mn O 1.5 0.5 2 7 becomes a half-metal. The spin magnetic moment of Mn is 2.88 and 2.91 l B for x"0 and 0.5, respectively. These values are close to
3 l for a Mn4` con"guration. The neutron scattering B data in Tl Mn O are 2.91 l [10] consistent with our 2 2 7 B result. For Sc-rich compounds, we have obtained slightly smaller values, 2.84 l for x"1.5 and 2.87 l for x"2, B B still close to that of Mn4`. This suggests that the Mn3`/Mn4` mixture is absent, and so the double exchange and the Jahn}Teller distortion mechanism would be ruled out for the CMR phenomena in Tl Sc Mn O . 2~x x 2 7 Although the scattering of the minority spin electrons by Mn 3d moments is one reason for the observed CMR in the considered system, it is thought that this e!ect alone is insu$cient for MR &90%. The relation between the half-metallic nature and the CMR has not been clari"ed yet. It is, however, obvious that the half-metallic nature can enhance the MR through a tunneling mechanism at grain or magnetic domain boundaries [9]. In conclusion, we have obtained electronic structures of Tl Sc Mn O for x"0, 0.5, 1.5 and 2.0. The 2~x x 2 7 ground state is metallic for x"0 and insulating for x"1.5 and 2.0. Inbetween them, the almost half-metallic ground state is found for x"0.5, which will enhance the MR as compared to that for x"0.
Acknowledgements This work was supported by the Korean MOST fund, and in part by the POSTECH-BSRI program (BSRI-982438).
References [1] S. Jin et al., Science 264 (1994) 413.
530
S.K. Kwon et al. / Physica B 281&282 (2000) 528}530
[2] J.-H. Park et al., Nature 392 (1998) 794. [3] A.J. Millis, P.B. Littlewood, B.I. Shraiman, Phys. Rev. Lett. 74 (1995) 5144. [4] G.H. Kwei, C.H. Booth, F. Bridges, M.A. Subramanian, Phys. Rev. B 55 (1997) R688. [5] Y. Shimakawa, Y. Kubo, T. Manako, Nature 379 (1996) 53. [6] M.A. Subramanian, A.P. Ramirez, G.H. Kwei, Solid State Ionics 108 (1998) 185.
[7] D.J. Singh, Phys. Rev. B 55 (1997) 313. [8] D.-K. Seo, M.-H. Whangbo, M.A. Subramanian, Solid State Commun. 101 (1997) 417. [9] H.Y. Hwang, S.-W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041. [10] J.W. Lynn, L. Vasiliu-Doloc, M.A. Subramanian, Phys. Rev. Lett. 80 (1998) 4582.