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La-substitution effects on double perovskite compound Ba2CoMoO6
ARTICLE IN PRESS
Physica B 359–361 (2005) 1336–1338 www.elsevier.com/locate/physb
La-substitution effects on double perovskite compound Ba2CoMoO6 T. Hiramaa,b, N. Tsujiia,, H. Kitazawaa, G. Kidoa,b a National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Department of Physics, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
b
Abstract Carrier doping effect on the antiferromagnetic insulator Ba2CoMoO6 has been investigated. Single-phased polycrystalline samples of Ba2x Lax CoMoO6 have been prepared in the range of 0pxp0:5: Magnetic susceptibility suggests the suppression of antiferromagnetic interaction with increasing x. Electrical resistivity shows a drastic decrease in its absolute value at 300 K with increasing x, indicating that carriers are effectively doped by the La-substitution. Temperature dependence of resistivity has been explained by the variable range hopping mechanism for all x. r 2005 Elsevier B.V. All rights reserved. PACS: 72.80.r; 75.47.De; 75.50.Ee Keywords: Double perovskite; Ba2CoMoO6; Carrier doping
Perovskite-related oxides have been the central issues for the research of strongly correlated electron systems due to their rich variety of phenomena [1]. Recently, the double-perovskitetype oxides also attract much attention since the discovery of giant magnetoresistance in Sr2FeMoO6 [2]. In this compound, Mo ions are considered to be in the Mo5þ (4d1 ) state. The existence of 4d electrons is considered to be crucial Corresponding author. Tel.: +81 29 859 2817;
fax: +81 29 859 2801. E-mail address: [email protected] (N. Tsujii).
for the metallic conductivity and the ferromagnetic interaction between Fe ions. In this respect, interesting properties are also expected for other insulating double perovskite compounds if carrier electrons are introduced. We have examined such electron-doping effect on Ba2CoMoO6. In this system, the electron configuration of Co2þ (3d7 ) and Mo6þ (4d0 ) is proposed [3]. By substituting La3þ for Ba2þ ; electrons are expected to enter the 4d-site and behave as carriers. Polycrystalline samples of Ba2x Lax CoMoO6 were prepared by a solid-state reaction. Samples were reacted at 1200 1C under Ar-gas or Ar/H2
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.403
ARTICLE IN PRESS T. Hirama et al. / Physica B 359– 361 (2005) 1336–1338
mixture atmosphere. For x ¼ 0; sample was also annealed in air at 300 1C for 3 days. X-ray diffraction shows that samples of xp0:5 are in a single-phase with the ordered double-perovskite structure. In Fig. 1, the inverse magnetic susceptibility w1 of Ba2x Lax CoMoO6 for x ¼ 0 (air-annealed sample) and x ¼ 0:25 is shown. For both the systems, w1 decreases linearly with decreasing T, corresponding to a Curie–Weiss behavior. Linear fit yields effective magnetic moments for x ¼ 0 and 0.25 to be peff ¼ 5:3 and 5:6mB per formula unit, respectively. These values are significantly larger than 3:87mB ; expected for a Co2þ ion (S ¼ 32 ; highspin), while they agree with the value 5:2mB ; expected for the total moment of Co3þ (S ¼ 2; high-spin) and Mo5þ (S ¼ 12). However, this electron configuration is not plausible. In such a configuration, metallic conductivity is expected like in Sr2FeMoO6, which is not the case as is shown later. Instead, this large peff is attributed to the unquenched orbital contribution, which can lead the magnetic moment of Co2þ ion to as large as 5:20mB for the case of non-distorted octahedral symmetry [4]. We therefore conclude that the configuration Co2þ Mo6þ is stable in these systems. For x ¼ 0:25; peff ¼ 5:6mB is slightly larger than 0.10
χ (emu/Co-mol)
Ba2CoMoO6 (Ar)
1/ (Co-mol/emu)
150
100
FC 100G ZFC 100G FC 0.1T ZFC 0.1T
0.05
0.00 0
50
100 T (K)
150
x=0 (air-annealed)
200
50
x = 0.25 Ba2-xLaxCoMoO6
0 0
50
100
150 T (K)
200
250
300
Fig. 1. Inverse magnetic susceptibility w1 of Ba2x Lax CoMoO6 for x ¼ 0 (air-annealed sample) and x ¼ 0:25: Inset shows w of x ¼ 0 prepared under Ar-atmosphere. ZFC and FC represent ‘zero-field cooled’ and ‘field-cooled’, respectively.
1337
that of x ¼ 0 (5:3mB ). This value, however, depends on the fitting-temperature range, thereby the contribution of Mo5þ ions is unclear which is possibly brought by the electron-doping. w1 for x ¼ 0 shows a minimum at 24 K, consistent with the antiferromagnetic ordering with T N ¼ 27 K [3]. For x ¼ 0:25; this minimum shifts to a lower temperature of 8 K, suggesting the decrease of T N : The Weiss-temperature also decreases from y ¼ 110 K for x ¼ 0 to 37 K for x ¼ 0:25: These results indicate the suppression of the antiferromagnetic interaction with the increase of x, possibly due to the carrier-mediated ferromagnetic interaction. It is noteworthy that the values of T N are small; only 14 of the respective y values. Here, we note that the Co ions are stacked to form a face-centeredcubic lattice, which leads to a geometrical frustration between Co ions and can suppress the antiferromagnetic ordering. The effect of frustration is also seen in the inset of Fig. 1, which shows w vs. T of the Ba2CoMoO6 sample prepared under Ar-atmosphere without air-annealing. w shows a hysteresis between zero-field-cooled and fieldcooled data. This anomaly vanishes completely in air-annealed sample. These data indicate that magnetic properties are sensitive to small amount of defects, implying the effect of geometrical frustration. Double-perovskite compounds are hence attractive as model substances for geometrical frustration. In Fig. 2, electrical resistivity r of Ba2x Lax CoMoO6 are shown. For all the x, r shows semiconducting behavior. The absolute value of r at 300 K decreases almost 7 orders of magnitude upon La-substitution; from 106 O cm from the air-annealed sample of x ¼ 0 to 101 O cm for x ¼ 0:5: This remarkable decrease of r indicates that carriers are doped effectively. However, metallic conductivity has not been observed even for x ¼ 0:5: In addition, Fig. 2 indicates that r–T curves are well described by the formula: rðTÞ ¼ r0 exp½ðT 0 =TÞ1=4 ; suggesting the three-dimensional variable-range-hopping mechanism for whole the x. In summary, Ba2x Lax CoMoO6 has been prepared as single-phased polycrystals for 0pxp0:5: Magnetic susceptibility data suggests
ARTICLE IN PRESS T. Hirama et al. / Physica B 359– 361 (2005) 1336–1338
1338 300 10 10 10
ρ (Ωcm)
10 10 10 10 10
100
50
6
metallic conductivity has not been observed even in the heavily doped x ¼ 0:5 system. This may be due to the electron configuration of this system: since Co1þ state is not likely to be stable in this crystal structure, doped electrons cannot hop to Co2þ sites. This limits the electron conduction only via the Mo6þ sites, resulting in the throughout hopping-like conduction. For metallic conductivity, valence-fluctuation of not only the 4d-site (Mo) but also the 3d-site (Co, Fe, etc.) should be important.
5
10
T (K)
7
x=0 (air-annealed)
5
4
x=0 (Ar)
3
0.4 2
x = 0.25
1
0.5 0
-1
10
0.2
0.3
0.4 T
0.5 -1/4
(K
-1/4
0.6
0.7
References
)
Fig. 2. Electrical resistivity r of Ba2x Lax CoMoO6 as a function of T 1=4 :
the suppression of antiferromagnetic interaction with increasing x. The effect of geometrical frustration is also prominent. Electrical resistivity shows semiconducting behavior for all the x, and
[1] Y. Tokura, Phys. Today 56 (2003) 50. [2] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [3] M.J. Martı´ nez-Lope, J.A. Alonso, M.T. Casais, M.T. Ferna´ndez-Dı´ az, Eur. J. Inorg. Chem. 2002 (2002) 2463. [4] M.C. Viola, M.J. Martı´ nez-Lope, J.A. Alonso, P. Velasco, J.L. Martı´ nez, J.C. Pedregosa, R.E. Carbonio, M.T. Ferna´ndez-Dı´ az, Chem. Mater. 14 (2002) 812.
Physica B 359–361 (2005) 1336–1338 www.elsevier.com/locate/physb
La-substitution effects on double perovskite compound Ba2CoMoO6 T. Hiramaa,b, N. Tsujiia,, H. Kitazawaa, G. Kidoa,b a National Institute for Materials Science, Sengen 1-2-1, Tsukuba, Ibaraki 305-0047, Japan Department of Physics, Faculty of Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
b
Abstract Carrier doping effect on the antiferromagnetic insulator Ba2CoMoO6 has been investigated. Single-phased polycrystalline samples of Ba2x Lax CoMoO6 have been prepared in the range of 0pxp0:5: Magnetic susceptibility suggests the suppression of antiferromagnetic interaction with increasing x. Electrical resistivity shows a drastic decrease in its absolute value at 300 K with increasing x, indicating that carriers are effectively doped by the La-substitution. Temperature dependence of resistivity has been explained by the variable range hopping mechanism for all x. r 2005 Elsevier B.V. All rights reserved. PACS: 72.80.r; 75.47.De; 75.50.Ee Keywords: Double perovskite; Ba2CoMoO6; Carrier doping
Perovskite-related oxides have been the central issues for the research of strongly correlated electron systems due to their rich variety of phenomena [1]. Recently, the double-perovskitetype oxides also attract much attention since the discovery of giant magnetoresistance in Sr2FeMoO6 [2]. In this compound, Mo ions are considered to be in the Mo5þ (4d1 ) state. The existence of 4d electrons is considered to be crucial Corresponding author. Tel.: +81 29 859 2817;
fax: +81 29 859 2801. E-mail address: [email protected] (N. Tsujii).
for the metallic conductivity and the ferromagnetic interaction between Fe ions. In this respect, interesting properties are also expected for other insulating double perovskite compounds if carrier electrons are introduced. We have examined such electron-doping effect on Ba2CoMoO6. In this system, the electron configuration of Co2þ (3d7 ) and Mo6þ (4d0 ) is proposed [3]. By substituting La3þ for Ba2þ ; electrons are expected to enter the 4d-site and behave as carriers. Polycrystalline samples of Ba2x Lax CoMoO6 were prepared by a solid-state reaction. Samples were reacted at 1200 1C under Ar-gas or Ar/H2
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.403
ARTICLE IN PRESS T. Hirama et al. / Physica B 359– 361 (2005) 1336–1338
mixture atmosphere. For x ¼ 0; sample was also annealed in air at 300 1C for 3 days. X-ray diffraction shows that samples of xp0:5 are in a single-phase with the ordered double-perovskite structure. In Fig. 1, the inverse magnetic susceptibility w1 of Ba2x Lax CoMoO6 for x ¼ 0 (air-annealed sample) and x ¼ 0:25 is shown. For both the systems, w1 decreases linearly with decreasing T, corresponding to a Curie–Weiss behavior. Linear fit yields effective magnetic moments for x ¼ 0 and 0.25 to be peff ¼ 5:3 and 5:6mB per formula unit, respectively. These values are significantly larger than 3:87mB ; expected for a Co2þ ion (S ¼ 32 ; highspin), while they agree with the value 5:2mB ; expected for the total moment of Co3þ (S ¼ 2; high-spin) and Mo5þ (S ¼ 12). However, this electron configuration is not plausible. In such a configuration, metallic conductivity is expected like in Sr2FeMoO6, which is not the case as is shown later. Instead, this large peff is attributed to the unquenched orbital contribution, which can lead the magnetic moment of Co2þ ion to as large as 5:20mB for the case of non-distorted octahedral symmetry [4]. We therefore conclude that the configuration Co2þ Mo6þ is stable in these systems. For x ¼ 0:25; peff ¼ 5:6mB is slightly larger than 0.10
χ (emu/Co-mol)
Ba2CoMoO6 (Ar)
1/ (Co-mol/emu)
150
100
FC 100G ZFC 100G FC 0.1T ZFC 0.1T
0.05
0.00 0
50
100 T (K)
150
x=0 (air-annealed)
200
50
x = 0.25 Ba2-xLaxCoMoO6
0 0
50
100
150 T (K)
200
250
300
Fig. 1. Inverse magnetic susceptibility w1 of Ba2x Lax CoMoO6 for x ¼ 0 (air-annealed sample) and x ¼ 0:25: Inset shows w of x ¼ 0 prepared under Ar-atmosphere. ZFC and FC represent ‘zero-field cooled’ and ‘field-cooled’, respectively.
1337
that of x ¼ 0 (5:3mB ). This value, however, depends on the fitting-temperature range, thereby the contribution of Mo5þ ions is unclear which is possibly brought by the electron-doping. w1 for x ¼ 0 shows a minimum at 24 K, consistent with the antiferromagnetic ordering with T N ¼ 27 K [3]. For x ¼ 0:25; this minimum shifts to a lower temperature of 8 K, suggesting the decrease of T N : The Weiss-temperature also decreases from y ¼ 110 K for x ¼ 0 to 37 K for x ¼ 0:25: These results indicate the suppression of the antiferromagnetic interaction with the increase of x, possibly due to the carrier-mediated ferromagnetic interaction. It is noteworthy that the values of T N are small; only 14 of the respective y values. Here, we note that the Co ions are stacked to form a face-centeredcubic lattice, which leads to a geometrical frustration between Co ions and can suppress the antiferromagnetic ordering. The effect of frustration is also seen in the inset of Fig. 1, which shows w vs. T of the Ba2CoMoO6 sample prepared under Ar-atmosphere without air-annealing. w shows a hysteresis between zero-field-cooled and fieldcooled data. This anomaly vanishes completely in air-annealed sample. These data indicate that magnetic properties are sensitive to small amount of defects, implying the effect of geometrical frustration. Double-perovskite compounds are hence attractive as model substances for geometrical frustration. In Fig. 2, electrical resistivity r of Ba2x Lax CoMoO6 are shown. For all the x, r shows semiconducting behavior. The absolute value of r at 300 K decreases almost 7 orders of magnitude upon La-substitution; from 106 O cm from the air-annealed sample of x ¼ 0 to 101 O cm for x ¼ 0:5: This remarkable decrease of r indicates that carriers are doped effectively. However, metallic conductivity has not been observed even for x ¼ 0:5: In addition, Fig. 2 indicates that r–T curves are well described by the formula: rðTÞ ¼ r0 exp½ðT 0 =TÞ1=4 ; suggesting the three-dimensional variable-range-hopping mechanism for whole the x. In summary, Ba2x Lax CoMoO6 has been prepared as single-phased polycrystals for 0pxp0:5: Magnetic susceptibility data suggests
ARTICLE IN PRESS T. Hirama et al. / Physica B 359– 361 (2005) 1336–1338
1338 300 10 10 10
ρ (Ωcm)
10 10 10 10 10
100
50
6
metallic conductivity has not been observed even in the heavily doped x ¼ 0:5 system. This may be due to the electron configuration of this system: since Co1þ state is not likely to be stable in this crystal structure, doped electrons cannot hop to Co2þ sites. This limits the electron conduction only via the Mo6þ sites, resulting in the throughout hopping-like conduction. For metallic conductivity, valence-fluctuation of not only the 4d-site (Mo) but also the 3d-site (Co, Fe, etc.) should be important.
5
10
T (K)
7
x=0 (air-annealed)
5
4
x=0 (Ar)
3
0.4 2
x = 0.25
1
0.5 0
-1
10
0.2
0.3
0.4 T
0.5 -1/4
(K
-1/4
0.6
0.7
References
)
Fig. 2. Electrical resistivity r of Ba2x Lax CoMoO6 as a function of T 1=4 :
the suppression of antiferromagnetic interaction with increasing x. The effect of geometrical frustration is also prominent. Electrical resistivity shows semiconducting behavior for all the x, and
[1] Y. Tokura, Phys. Today 56 (2003) 50. [2] K.I. Kobayashi, T. Kimura, H. Sawada, K. Terakura, Y. Tokura, Nature 395 (1998) 677. [3] M.J. Martı´ nez-Lope, J.A. Alonso, M.T. Casais, M.T. Ferna´ndez-Dı´ az, Eur. J. Inorg. Chem. 2002 (2002) 2463. [4] M.C. Viola, M.J. Martı´ nez-Lope, J.A. Alonso, P. Velasco, J.L. Martı´ nez, J.C. Pedregosa, R.E. Carbonio, M.T. Ferna´ndez-Dı´ az, Chem. Mater. 14 (2002) 812.