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The origin of Curie-Weiss susceptibility in Ba2YCu3Oy
PhysicaC 165 (1990) 139-142 North-Holland
THE ORIGIN OF CURIE-WEISS Takao ISHII and Tomoaki
SUSCEPTIBILITY
IN Ba2YCu,0,,
YAMADA
NTT Basic Research Laboratories, Musashino-shi, Tokyo 180, Japan
Kiyohiro
SUGIYAMA,
Hiroyuki
FUKE and Muneyuki
DATE
Department of Physics, Faculty of Science, Osaka University, Osaka 560, Japan
Received 16 November 1989
The high field magnetization of oxygen-depleted BazYCu,O, (y=6.34) is measured up to 350 kOe using a pulse magnet to clarify the origin of the Curie-Weiss term of its magnetic susceptibility. Analysis of the high field magnetization curve reveals that the Curie-Weiss term is a result of the isolated Cu ‘+ (S= l/2). On the basis of experimental results, the valence states of Cu ions in the Cu-0 chain in the BazYCu30, system are discussed.
1. Introduction
To obtain information about the magnetism of Cu atoms of a high temperature superconductor Ba2YCuj0,, the magnetic properties in normal state have been investigated in terms of characteristics such as static magnetic susceptibility [ l-3 1, neutron diffraction [ 4-5 1, nuclear magnetic resonance [ 61. One of the most important findings is the strong antiferromagnetic ordering of Cu2+ (II) ions in the Cu0 plane in Ba2YCu306 [4-61. It is probable that a strong antiferromagnetic correlation between Cu2+ ions, similar to that of Sr-doped La2Cu04 [ 7 1, exists in the superconducting phase. The static magnetic susceptibility behavior of BazYCuJO,, varies from Pauli-para like temperature independent susceptibility ( y > 6.8 ) to Curie-Weiss type susceptibility (y< 6.4) [ 31. Recently, Nakazawa and Ishikawa [ 8 ] extensively investigated the relationship between oxygen stoichiometry and its magnetic susceptibility in a Ba2YCu30, system and they proposed a model for the origin of the CurieWeiss term in the magnetic susceptibility. According to their model, the Cu-0 chain consists of three kinds of Cu( I) ions with different oxygen coordination. They assume that three-fold coordinated Cu (I) ions are divalent and that four- and two-fold coordinated Cu (I ) ions are non-magnetic trivalent and monov0921-4534/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland )
alent, respectively. The Curie-Weiss term is attributed to the three-fold coordinated Cu2+ ions. However, to date there are no experimental reports to verify this model. In the present study, we synthesized Ba2YCu,0, with various oxygen deficiencies and measured their static magnetic susceptibilities and high field magnetization curves to clarify the origin of the CurieWeiss term. Analysis of the high field magnetization curve revealed that the Curie-Weiss term is the result of the isolated Cu2+ (S= l/2) ion without magnetic interaction. This result is consistent with a model proposed by Nakazawa and Ishikawa [ 81. On the basis of the experimental results, the valence state of Cu ions in a Cu-0 chain in the Ba2YCuJ0, system is discussed.
2. Sample preparation and magnetic measurement Polycrystalline samples of Ba2YCu30, with various oxygen deficiencies were prepared using the following solid-state reaction. High purity (4N) Y20j, BaCO, and CuO powders of appropriate weights were mixed and fired at 900 ’ C for 48 h. The fired powder was crushed and refired. The obtained powder sample was sintered at 920°C for 72 h in an O2 atmosphere and annealed by successive step-by-step cool-
7: Ishii et al. / The origin of Curie- Weiss susceptlhitv in Ba2 YCu,O,
140
ing. Samples with various oxygen deficiencies were obtained by quenching them in liquid nitrogen at different steps of the annealing. The annealing time of 24 h was used at each step to attain equilibrium state at the annealing temperature. The obtained samples were confirmed to be single phase with an oxygen-deficient perovskite structure by an X-ray diffraction analysis. No impurity phase such as BaCuOz was detected. The oxygen content of the samples was determined by chemical analysis. Magnetic susceptibility (x) was measured by a superconducting quantum interference device (SQUID) magnetometer in a magnetic field of 5 kOe in a temperature range between 5 K and 300 K. Magnetization under a high magnetic field was measured using a pulse magnet with a temperature variable cryostat system in the Research Center for Extreme Materials, Osaka University. The details of generation of the pulsed field and magnetization measurement are described elsewhere [9-lo].
3. Results and discussion Fig. 1 shows the temperature dependence of magnetic susceptibility of the samples with various ox-
ygen deficiencies. The temperature dependence of susceptibility of BazYCu30,,,,. which is in an orthorhombic phase and is superconductive at -90 K, shows temperature independent Pauli-para like behavior and x=4.0x lo-’ emu/g. This value is in good agreement with the previously reported one [ 1,3]. With decreasing oxygen content, the value of magnetic susceptibility at 300 K decreases continuously. In contrast, low temperature magnetic susceptibility increases with decreasing temperature. Such an upturn of susceptibility at low temperature has been observed in most experiments to date. Fig. 2 shows the temperature dependence of reciprocal magnetic susceptibility of BazYCu30, 34, which is in a tetragonal phase and shows a variable range hopping type conduction down to 4.2 K. The paramagnetic Curie point is zero, implying that there is no magnetic interaction between localized moments. This result suggests that these Cu ions probably exist as free ions in Ba,YCu,0b3,. The effective Bohr magneton deduced from the Curie constant is 0.36-0.40 ,&mol, however. this value is small in comparison with the theoretical value of a localized Cu ion. Fig. 3 shows the magnetic field dependence of magnetization per formula unit of Ba2YCu30,,, up
lo------0
y=6.34
0
J
’ 0
100 TEMPERATURE
200
300 TEMPERATURE
(K)
Fig. 1. Temperature dependence of the magnetic BazYCu,O, with various oxygen deficiencies.
susceptibility
of
Fig. 2. Temperature of Ba2YCu306.,.,.
dependences
(K)
of the reciprocal
susceptibility
i? Ishii et al. / The origin of Curie- Weiss susceptibity in Ba,YCu30,,
I
01
0
loo
200
3ccl
400
MAGNETIC FIELD (kOe) Fig. 3. Magnetic field Ba2YCu306.34 at 4.2 K.
dependence
of the magnetization
of
to 350 kOe at 4.2 K (curve A). As seen in this figure, high field magnetic susceptibility was observed up to 350 kOe. Since the antiferromagnetic ordering of Cu( II) ions in the tetragonal phase (~~6.4) is confirmed on the basis of neutron diffraction experiments [ 51, one possible explanation of this small high field susceptibility is the strong antiferromagnetic coupling of Cu (II ) spins in the Cu-0 plane. Therefore, we subtract this high field susceptibility from the raw data (curve B). This magnetization curve in fig. 3 is analyzed using the Brillouin function. If the Curie-Weiss term is due to divalent Cu free ions (S= l/2), the Brillouin function is expressed as A41 A4,,= tanh (@,H/k,T), where ,& is the Bohr magneton and H is the external magnetic field. The calculated values are represented by open circles in fig. 3. Good agreement between experimental and calculated values suggests that the high field magnetization behavior is attributed to the Cu*+ free ions. The experimental fact that the Cu(I1) ions in the Cu-0 plane have antiferromagnetic ordering [ 51 leads to the consideration that the Cu*+ free ions are due to the Cu( I ) ions in the Cu-0 chain. Since the saturated magnetic moment at 4.2 K is 0.05 pn/mol, the concentration of Cu*+ ions is estimated to be about 5% of the total Cu (I) ions. In such a case, the effective Bohr magneton is calculated from p,n= (4S(S+ 1 )x0.05)“* (S= l/2), and the result, ,LL,~=0.39 fin, is in good agreement with the observed value. The low concentration of Cu*+ ions suggests that these ions probably exist in an isolated state in
141
Cu-0 chains. The isolated state for Cu*+ ions is also consistent with the zero paramagnetic Curie point deduced from the temperature dependence of reciprocal susceptibility. The existence of isolated Cu2+ ions supports a model proposed by Nakazawa and Ishikawa to explain the origin of the Curie-Weiss term in low temperature susceptibility. Since the resolution in phase separation by the Xray diffraction method is not especially high, it may be possible to explain this Curie-Weiss term as a contribution from a small amount of the impurity phase such as BaCu02. In such a case, considering that the concentration of Cu*+ free ions is 5% of total Cu( I) ions, it is assumed that the amount of the impurity phase is equal to 0.05 mole of BaCuO*. Then, the magnitude of magnetic susceptibility at 100 K is estimated to be about 1.5~ 10e6 emu/g, when referring to the susceptibility data of BaCu02 [ 111. The estimated value is five times larger than the observed one. Therefore, we feel that the origin of the Curie-Weiss term at low temperatures is not an impurity phase such as BaCu02 but an isolated Cu*+ ion in the Cu-0 chain. It is expected that the Ba2YCuJOh phase contains only two-folded coordinated Cu(I) ions in a Cu-0 chain and its valency is monovalent. In this case, the Cu( I ) ion is in the d” state and is non-magnetic. With increasing oxygen in Ba2YCu30,, three- and four-fold coordinated Cu (I ) ions increase gradually in the Cu-0 chain. In Ba2YCu307, the Cu-0 chain consists of only four-fold coordinated Cu (I) ions and their nominal valence states are Cu3+. However, recent photoemission spectra of Ba2YCu30, (y- 6.9) [ 121 suggest that the valence state of Cu ions is divalent and Cu3+ has not been observed. Moreover, it is revealed that holes are located at oxygen sites [ 13 1. Considering that four-fold coordinated Cu (I ) ions have magnetic moments, it is supposed that the Cu (I ) magnetic moments are probably quenched by making a singlet spin pair between Cu2+ and Ospins. In the intermediate oxygen concentration region, there is a high probability of finding three-fold coordinated Cu (I ) ions in a Cu-0 chain. Taking this oxygen coordination around the Cu ion into account, we hypothesize that the isolated Cu*+ ions result from three-folded oxygen coordination. To confirm our hypothesis, we examined the annealing effect on magnetic properties of Ba2YCu306.5
T. Ishii et al. / The origin qf Curie- Weiss susceptibity in Ba2 YCu,O,.
142
because this phase has a superlattice structure when oxygen vacancies are ordered by annealing [ 141. In the superlattice structure, the isolated Cu*+ ions with three-folded coordination (magnetic) change to twoor four-folded coordination (non-magnetic ). The quenched sample of Ba,YCu,06., was annealed at 600°C for 200 h in an evacuated quartz tube. The temperature dependences of magnetic susceptibility of quenched and annealed samples are shown in fig. 4. The Curie-Weiss term below 150 K decreases in the annealed sample. The small remaining CurieWeiss term suggests that perfect oxygen ordering can not be achieved in the present annealing. In conclusion, high field magnetization measurement of Ba,YCu306.,, up to 350 kOe using a pulse magnet revealed that the Curie-Weiss term is a result of Cu2+ free ions. Its concentration is estimated to be about 5% of total ions in the Cu-0 chain using the saturated magnetic moment value. These experimental results support a model proposed by Nakazawa and Ishikawa to explain the origin of the Curie-Weiss term in low temperature susceptibility.
lo/ G 6 3 5
quenched
P
0
100 TEMPERATURE
200
300
(K)
Fig 4. Temperature dependence of the magnetic quenched and annealed BazYCu,0b,5.
susceptibility
of
Acknowledgements The authors would like to thank Prof. K. Okuda, University of Osaka Prefecture and Dr. A. Matsuda for helpful discussions.
References [ I ] Y. Nakazawa, M. Ishikawa. T. Takabatake, K. Koga and K. Terakura, Jpn. J. Appl. Phys. 26 ( 1987) L790. [2] D.C. Johnston, A.J. Jacobson, J.M. Newsam. J.T. Lewandowski, D.P. Goshorn, D. Xie and W.B. Yelon, in: Chemistry of High Temperature Superconductors, eds. D.L. Nelson, MS. Whittingham and T.F. George (American Chemical Society, Washington, DC, 1987) p. 136. [3 ] T. Kawagoe. T. Mizoguchi, K. Kanoda, T. Takahashi, M. Hasumi and S. Kagoshima, J. Phys. Sot. Jpn. 57 (I 988) 2272. [4 ] J.M. Tranquada, DE. Cox, W. Kunnmann, A.H. Moudden, G. Shirane, M. Suenaga, P. Zolliker, D. Vaknin, SK. Sinha. M.S. Alvarez, A.J. Jacobson and D.C. Johnston, Phys. Rev. Lett.60(1988) 156. [ 5 ] J.M. Tranquada, A.H. Moudden, A.I. Goldman, P. Zolliker. D.E. Cox, G. Shirane. SK. Sinha, D. Vaknin, D.C. Johnston, M.S. Alvarez and A.J. Jacobson, Phys. Rev. B 38 (1988) 2477. [ 61 Y. Kitaoka, S. Hiramatsu, K.lshida, K. Asayama, H. Takagi. H. Iwabuchi, S. Uchida and S. Tanaka, J. Phys. Sot. Jpn. 57 (1988) 737; H. Yasuoka, T. Shimizu, Y. Ueda and K Kosuge, J. Phys. Sot. Jpn. 57 (1988) 2659. [ 71 Y. Endoh, K. Yamada, R.J. Birgeueau, D.R. Gabbe, H.P. Jenssen, M.A. Kaster, C.J. Peters, P.J. Picone, T.R. Thurston, J.M. Tranquada, G. Shirane, Y. Hidaka, M. Oda. Y. Enomoto, M. Suzuki and T. Murakami, Phys. Rev. B 37 (1988) 7443. [8] Y. Nakazawaand M. Ishikawa, Physica C 158 (1989) 381. [9] A. Yamagishi and M. Date, Physica B 155 (1989) 91. [IO] H. Mollymoto, M. Motokawa and M. Date, J. Phys. Sot. Jpn. 49 (1980) 108. [ I1 ] R. Trot. Z. Bukowski, R. Horyn and J. Klamut, Phys. Lett. A 125 (1987) 222. [ 121 A. Bianconi, M. De Santis, A. Di Cicco, A. Clozza, A. Congiu Castellano, S. Della Longa, A. Gargano, P. Delogu, T. Dikonimos Makris, R. Giorgi, A.M. Flank, A. Fontaine, P. Lagarde and A. Marcelli, Physica C 153-l 55 ( 1988) 115. [ I3 ] N. Nucker, J. Fink, J.C. Fuggle, P.J. Durham and W.M. Temmerman, Phys. Rev. B 37 ( 1988) 5 158. [ 141 T. Takabatake, M. Ishikawa, Y. Nakazawa and K. Koga. Physica C I52 ( 1988) 424.
THE ORIGIN OF CURIE-WEISS Takao ISHII and Tomoaki
SUSCEPTIBILITY
IN Ba2YCu,0,,
YAMADA
NTT Basic Research Laboratories, Musashino-shi, Tokyo 180, Japan
Kiyohiro
SUGIYAMA,
Hiroyuki
FUKE and Muneyuki
DATE
Department of Physics, Faculty of Science, Osaka University, Osaka 560, Japan
Received 16 November 1989
The high field magnetization of oxygen-depleted BazYCu,O, (y=6.34) is measured up to 350 kOe using a pulse magnet to clarify the origin of the Curie-Weiss term of its magnetic susceptibility. Analysis of the high field magnetization curve reveals that the Curie-Weiss term is a result of the isolated Cu ‘+ (S= l/2). On the basis of experimental results, the valence states of Cu ions in the Cu-0 chain in the BazYCu30, system are discussed.
1. Introduction
To obtain information about the magnetism of Cu atoms of a high temperature superconductor Ba2YCuj0,, the magnetic properties in normal state have been investigated in terms of characteristics such as static magnetic susceptibility [ l-3 1, neutron diffraction [ 4-5 1, nuclear magnetic resonance [ 61. One of the most important findings is the strong antiferromagnetic ordering of Cu2+ (II) ions in the Cu0 plane in Ba2YCu306 [4-61. It is probable that a strong antiferromagnetic correlation between Cu2+ ions, similar to that of Sr-doped La2Cu04 [ 7 1, exists in the superconducting phase. The static magnetic susceptibility behavior of BazYCuJO,, varies from Pauli-para like temperature independent susceptibility ( y > 6.8 ) to Curie-Weiss type susceptibility (y< 6.4) [ 31. Recently, Nakazawa and Ishikawa [ 8 ] extensively investigated the relationship between oxygen stoichiometry and its magnetic susceptibility in a Ba2YCu30, system and they proposed a model for the origin of the CurieWeiss term in the magnetic susceptibility. According to their model, the Cu-0 chain consists of three kinds of Cu( I) ions with different oxygen coordination. They assume that three-fold coordinated Cu (I) ions are divalent and that four- and two-fold coordinated Cu (I ) ions are non-magnetic trivalent and monov0921-4534/90/$03.50 0 Elsevier Science Publishers B.V. (North-Holland )
alent, respectively. The Curie-Weiss term is attributed to the three-fold coordinated Cu2+ ions. However, to date there are no experimental reports to verify this model. In the present study, we synthesized Ba2YCu,0, with various oxygen deficiencies and measured their static magnetic susceptibilities and high field magnetization curves to clarify the origin of the CurieWeiss term. Analysis of the high field magnetization curve revealed that the Curie-Weiss term is the result of the isolated Cu2+ (S= l/2) ion without magnetic interaction. This result is consistent with a model proposed by Nakazawa and Ishikawa [ 81. On the basis of the experimental results, the valence state of Cu ions in a Cu-0 chain in the Ba2YCuJ0, system is discussed.
2. Sample preparation and magnetic measurement Polycrystalline samples of Ba2YCu30, with various oxygen deficiencies were prepared using the following solid-state reaction. High purity (4N) Y20j, BaCO, and CuO powders of appropriate weights were mixed and fired at 900 ’ C for 48 h. The fired powder was crushed and refired. The obtained powder sample was sintered at 920°C for 72 h in an O2 atmosphere and annealed by successive step-by-step cool-
7: Ishii et al. / The origin of Curie- Weiss susceptlhitv in Ba2 YCu,O,
140
ing. Samples with various oxygen deficiencies were obtained by quenching them in liquid nitrogen at different steps of the annealing. The annealing time of 24 h was used at each step to attain equilibrium state at the annealing temperature. The obtained samples were confirmed to be single phase with an oxygen-deficient perovskite structure by an X-ray diffraction analysis. No impurity phase such as BaCuOz was detected. The oxygen content of the samples was determined by chemical analysis. Magnetic susceptibility (x) was measured by a superconducting quantum interference device (SQUID) magnetometer in a magnetic field of 5 kOe in a temperature range between 5 K and 300 K. Magnetization under a high magnetic field was measured using a pulse magnet with a temperature variable cryostat system in the Research Center for Extreme Materials, Osaka University. The details of generation of the pulsed field and magnetization measurement are described elsewhere [9-lo].
3. Results and discussion Fig. 1 shows the temperature dependence of magnetic susceptibility of the samples with various ox-
ygen deficiencies. The temperature dependence of susceptibility of BazYCu30,,,,. which is in an orthorhombic phase and is superconductive at -90 K, shows temperature independent Pauli-para like behavior and x=4.0x lo-’ emu/g. This value is in good agreement with the previously reported one [ 1,3]. With decreasing oxygen content, the value of magnetic susceptibility at 300 K decreases continuously. In contrast, low temperature magnetic susceptibility increases with decreasing temperature. Such an upturn of susceptibility at low temperature has been observed in most experiments to date. Fig. 2 shows the temperature dependence of reciprocal magnetic susceptibility of BazYCu30, 34, which is in a tetragonal phase and shows a variable range hopping type conduction down to 4.2 K. The paramagnetic Curie point is zero, implying that there is no magnetic interaction between localized moments. This result suggests that these Cu ions probably exist as free ions in Ba,YCu,0b3,. The effective Bohr magneton deduced from the Curie constant is 0.36-0.40 ,&mol, however. this value is small in comparison with the theoretical value of a localized Cu ion. Fig. 3 shows the magnetic field dependence of magnetization per formula unit of Ba2YCu30,,, up
lo------0
y=6.34
0
J
’ 0
100 TEMPERATURE
200
300 TEMPERATURE
(K)
Fig. 1. Temperature dependence of the magnetic BazYCu,O, with various oxygen deficiencies.
susceptibility
of
Fig. 2. Temperature of Ba2YCu306.,.,.
dependences
(K)
of the reciprocal
susceptibility
i? Ishii et al. / The origin of Curie- Weiss susceptibity in Ba,YCu30,,
I
01
0
loo
200
3ccl
400
MAGNETIC FIELD (kOe) Fig. 3. Magnetic field Ba2YCu306.34 at 4.2 K.
dependence
of the magnetization
of
to 350 kOe at 4.2 K (curve A). As seen in this figure, high field magnetic susceptibility was observed up to 350 kOe. Since the antiferromagnetic ordering of Cu( II) ions in the tetragonal phase (~~6.4) is confirmed on the basis of neutron diffraction experiments [ 51, one possible explanation of this small high field susceptibility is the strong antiferromagnetic coupling of Cu (II ) spins in the Cu-0 plane. Therefore, we subtract this high field susceptibility from the raw data (curve B). This magnetization curve in fig. 3 is analyzed using the Brillouin function. If the Curie-Weiss term is due to divalent Cu free ions (S= l/2), the Brillouin function is expressed as A41 A4,,= tanh (@,H/k,T), where ,& is the Bohr magneton and H is the external magnetic field. The calculated values are represented by open circles in fig. 3. Good agreement between experimental and calculated values suggests that the high field magnetization behavior is attributed to the Cu*+ free ions. The experimental fact that the Cu(I1) ions in the Cu-0 plane have antiferromagnetic ordering [ 51 leads to the consideration that the Cu*+ free ions are due to the Cu( I ) ions in the Cu-0 chain. Since the saturated magnetic moment at 4.2 K is 0.05 pn/mol, the concentration of Cu*+ ions is estimated to be about 5% of the total Cu (I) ions. In such a case, the effective Bohr magneton is calculated from p,n= (4S(S+ 1 )x0.05)“* (S= l/2), and the result, ,LL,~=0.39 fin, is in good agreement with the observed value. The low concentration of Cu*+ ions suggests that these ions probably exist in an isolated state in
141
Cu-0 chains. The isolated state for Cu*+ ions is also consistent with the zero paramagnetic Curie point deduced from the temperature dependence of reciprocal susceptibility. The existence of isolated Cu2+ ions supports a model proposed by Nakazawa and Ishikawa to explain the origin of the Curie-Weiss term in low temperature susceptibility. Since the resolution in phase separation by the Xray diffraction method is not especially high, it may be possible to explain this Curie-Weiss term as a contribution from a small amount of the impurity phase such as BaCu02. In such a case, considering that the concentration of Cu*+ free ions is 5% of total Cu( I) ions, it is assumed that the amount of the impurity phase is equal to 0.05 mole of BaCuO*. Then, the magnitude of magnetic susceptibility at 100 K is estimated to be about 1.5~ 10e6 emu/g, when referring to the susceptibility data of BaCu02 [ 111. The estimated value is five times larger than the observed one. Therefore, we feel that the origin of the Curie-Weiss term at low temperatures is not an impurity phase such as BaCu02 but an isolated Cu*+ ion in the Cu-0 chain. It is expected that the Ba2YCuJOh phase contains only two-folded coordinated Cu(I) ions in a Cu-0 chain and its valency is monovalent. In this case, the Cu( I ) ion is in the d” state and is non-magnetic. With increasing oxygen in Ba2YCu30,, three- and four-fold coordinated Cu (I ) ions increase gradually in the Cu-0 chain. In Ba2YCu307, the Cu-0 chain consists of only four-fold coordinated Cu (I) ions and their nominal valence states are Cu3+. However, recent photoemission spectra of Ba2YCu30, (y- 6.9) [ 121 suggest that the valence state of Cu ions is divalent and Cu3+ has not been observed. Moreover, it is revealed that holes are located at oxygen sites [ 13 1. Considering that four-fold coordinated Cu (I ) ions have magnetic moments, it is supposed that the Cu (I ) magnetic moments are probably quenched by making a singlet spin pair between Cu2+ and Ospins. In the intermediate oxygen concentration region, there is a high probability of finding three-fold coordinated Cu (I ) ions in a Cu-0 chain. Taking this oxygen coordination around the Cu ion into account, we hypothesize that the isolated Cu*+ ions result from three-folded oxygen coordination. To confirm our hypothesis, we examined the annealing effect on magnetic properties of Ba2YCu306.5
T. Ishii et al. / The origin qf Curie- Weiss susceptibity in Ba2 YCu,O,.
142
because this phase has a superlattice structure when oxygen vacancies are ordered by annealing [ 141. In the superlattice structure, the isolated Cu*+ ions with three-folded coordination (magnetic) change to twoor four-folded coordination (non-magnetic ). The quenched sample of Ba,YCu,06., was annealed at 600°C for 200 h in an evacuated quartz tube. The temperature dependences of magnetic susceptibility of quenched and annealed samples are shown in fig. 4. The Curie-Weiss term below 150 K decreases in the annealed sample. The small remaining CurieWeiss term suggests that perfect oxygen ordering can not be achieved in the present annealing. In conclusion, high field magnetization measurement of Ba,YCu306.,, up to 350 kOe using a pulse magnet revealed that the Curie-Weiss term is a result of Cu2+ free ions. Its concentration is estimated to be about 5% of total ions in the Cu-0 chain using the saturated magnetic moment value. These experimental results support a model proposed by Nakazawa and Ishikawa to explain the origin of the Curie-Weiss term in low temperature susceptibility.
lo/ G 6 3 5
quenched
P
0
100 TEMPERATURE
200
300
(K)
Fig 4. Temperature dependence of the magnetic quenched and annealed BazYCu,0b,5.
susceptibility
of
Acknowledgements The authors would like to thank Prof. K. Okuda, University of Osaka Prefecture and Dr. A. Matsuda for helpful discussions.
References [ I ] Y. Nakazawa, M. Ishikawa. T. Takabatake, K. Koga and K. Terakura, Jpn. J. Appl. Phys. 26 ( 1987) L790. [2] D.C. Johnston, A.J. Jacobson, J.M. Newsam. J.T. Lewandowski, D.P. Goshorn, D. Xie and W.B. Yelon, in: Chemistry of High Temperature Superconductors, eds. D.L. Nelson, MS. Whittingham and T.F. George (American Chemical Society, Washington, DC, 1987) p. 136. [3 ] T. Kawagoe. T. Mizoguchi, K. Kanoda, T. Takahashi, M. Hasumi and S. Kagoshima, J. Phys. Sot. Jpn. 57 (I 988) 2272. [4 ] J.M. Tranquada, DE. Cox, W. Kunnmann, A.H. Moudden, G. Shirane, M. Suenaga, P. Zolliker, D. Vaknin, SK. Sinha. M.S. Alvarez, A.J. Jacobson and D.C. Johnston, Phys. Rev. Lett.60(1988) 156. [ 5 ] J.M. Tranquada, A.H. Moudden, A.I. Goldman, P. Zolliker. D.E. Cox, G. Shirane. SK. Sinha, D. Vaknin, D.C. Johnston, M.S. Alvarez and A.J. Jacobson, Phys. Rev. B 38 (1988) 2477. [ 61 Y. Kitaoka, S. Hiramatsu, K.lshida, K. Asayama, H. Takagi. H. Iwabuchi, S. Uchida and S. Tanaka, J. Phys. Sot. Jpn. 57 (1988) 737; H. Yasuoka, T. Shimizu, Y. Ueda and K Kosuge, J. Phys. Sot. Jpn. 57 (1988) 2659. [ 71 Y. Endoh, K. Yamada, R.J. Birgeueau, D.R. Gabbe, H.P. Jenssen, M.A. Kaster, C.J. Peters, P.J. Picone, T.R. Thurston, J.M. Tranquada, G. Shirane, Y. Hidaka, M. Oda. Y. Enomoto, M. Suzuki and T. Murakami, Phys. Rev. B 37 (1988) 7443. [8] Y. Nakazawaand M. Ishikawa, Physica C 158 (1989) 381. [9] A. Yamagishi and M. Date, Physica B 155 (1989) 91. [IO] H. Mollymoto, M. Motokawa and M. Date, J. Phys. Sot. Jpn. 49 (1980) 108. [ I1 ] R. Trot. Z. Bukowski, R. Horyn and J. Klamut, Phys. Lett. A 125 (1987) 222. [ 121 A. Bianconi, M. De Santis, A. Di Cicco, A. Clozza, A. Congiu Castellano, S. Della Longa, A. Gargano, P. Delogu, T. Dikonimos Makris, R. Giorgi, A.M. Flank, A. Fontaine, P. Lagarde and A. Marcelli, Physica C 153-l 55 ( 1988) 115. [ I3 ] N. Nucker, J. Fink, J.C. Fuggle, P.J. Durham and W.M. Temmerman, Phys. Rev. B 37 ( 1988) 5 158. [ 141 T. Takabatake, M. Ishikawa, Y. Nakazawa and K. Koga. Physica C I52 ( 1988) 424.