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Single crystal growth of superconducting La2-xBaxCuO4 by TSFZ method
Physica C 209 ( 1993) 442-448 North-Holland
Single crystal growth of superconducting La2_,BaxCu04 method Jianding Yu, Yoshitaka TetsurZ, Nakamura
Yanagida,
Research Laboratory ofEngineering Received
25 November
Hiroshi
Takashima,
Yoshiyuki
Inaguma,
by TSFZ
Mitsuru
Itoh ’ and
Materials, Tokyo Institute of Technology, 4259 Nagatsuta. Midori-ku, Yokohama 227, Japan
1992
Single crystals of La2_,Ba,Cu0, with 5 mm diameter and 25 mm length have been grown by the traveling solvent floating zone (TSFZ) method. The composition of the single crystal containing maximum Ba content and showing a superconducting transition at 30 K was determined to be La ,.935Ba0.065Cu04. Electrical resistivities along the c-axis and in the ab-plane reveal significant anisotropy. Magnetic susceptibilities have been measured under a field of 50 Oe from 5 to 50 K. Under fields Hlab-plane and HIlab-plane, the single crystal demonstrates 3.6% and 0.7% Meissner fractions, respectively, at 5 K.
1. Introduction Since the discovery of high-T, superconductivity in the La-Ba-Cu-0 system by Bednorz and Miiller [ 11, the properties of the superconducting Ln-MCu-0 (M = Ca, Sr, Ba, Ln = lanthanide ions) system have been extensively investigated. For polycrystalline Laz_,BaXCuO, (LBCO), the T, changes with Ba content and reveals a local minimum at x= 0.125 [ 2 1. The significant anomalous behavior has been understood in terms of a structural phase transition between the orthorhombic mid-temperature (OMT) phase and the tetragonal low-temperature (TLT) phase occurring at a low temperature of about 50 K [ 3,4]. It is of particular interest to study the relation between T, and composition x and to investigate the anisotropy of the magnetic and electrical properties for a single crystal with homogeneity. It is well known that the phase diagram of the LazCu04-CuO system [ 5,6] is favorable for the growth of single crystals of Ba- or Sr-doped La2Cu04. Single crystals of LBCO have been grown by the flux growth and top-seeded solution growth (TSSG) methods [ 6- 111. Single crystals of LBCO obtained so far have not shown superconductivity in the as’ Correspondence 81-45-922-1111 0921-4534/93/$06.00
should be addressed to: Dr. Mitsuru Itoh, Tel. ex.2626, FAX 81-45-922-5169. 0 1993 Elsevier Science Publishers
grown state [ 6- 111, possibly due to low concentration of Ba or flux inclusions. Large single crystals of Laz_,SrXCuOd (LSCO) have been grown by the traveling solvent floating zone (TSFZ) method [ 12,13 1. However, to our knowledge, single crystals of LBCO grown by TSFZ have not yet been reported. In this paper, we describe the growth of LBCO single crystals with superconductivity by the TSFZ method.
2. Experimental
procedure
For preparing the feed rods and solvent rods, powders of 99.9% pure Laz03, Ba(N03)2 and 99.99% pure CuO were mixed in ethanol with stoichiometric composition of LBCO followed by calcining at 900°C for 12 h in air. After grinding, they were recalcined at 1050°C for 12 h in air. The obtained powder was put into a thin-wall rubber tube and compressed into rods with about 5 mm diameter and 70 mm length under hydrostatic pressure of 250 MPa. Finally, the rods used as feed rods for crystal growth were sintered at 1080°C for 100 h in oxygen and the rods used as solvent, containing the rich composition of CuO in a ratio of Laz_XBaXCu04: CuO=22:56 (mol%), were fired at 800” for 8 h in air. The crystal growth apparatus is an infrared radiation furnace (Nichiden Machinery Ltd.) equipped
B.V. All rights reserved.
J. Yu et al. / Single crystal growth of La~_,Ba,Cu~~ by EW.Z
with an ellipsoidal mirror and a 700 W halogen lamp as the heat source. After a small portion of solvent phase was attached to the top of the feed rod by melting, the feed rod was suspended at the bottom of the upper shaft and the seed made from a grow boule was held at the top of the lower shaft. Each shaft was counter-rotated at a rate of 30 rpm. The growth rate of 1 mm/h was chosen, and oxygen gas under 2 atm pressure was supplied as growth atmosphere to prevent the evaporation of CuO from the melt. The compositions of the crystals were analyzed by the inductively coupled plasma (ICP, SEIKO model SPS 1500) and electron probe microanalyzer (EPMA, SHIMADZU model 8705) techniques. The lattice parameters of the crystals were determined by the single crystal X-ray diffractometer (MAC.Science). Crystallinity and crystallographic orientation were determined by the back refraction Laue X-ray technique. The electrical resistivities of the growth crystals were measured by the four-probe method. The electrical leads were soldered with indium on the single crystals of typical dimension 3x 3x2 mm3. The magnetization measurements were performed under a magnetic field of 50 Oe using a Quantum Design MPMS-2 SQUID magnetometer in the temperature range from 5 to 50 K. The measured magnetism of samples was corrected by subtracting the orbital diamagnetism of relevant ions from the raw data.
3. Results and discussion The growth of a LBCO single crystal by the TSFZ method is similar to the growth of LSCO from consideration of the Laz03-CuO phase diagram [ 14,15 1.
443
LazCuOd melts incongruently over a composition range of 75 mol% to 95 mol% CuO. According to Tanaka and Kojima [ 13 1, in the growth of LSCO single crystals, a solvent of 78 mol% CuO had to be adopted to prevent precipitation of La203. Single crystals of La2_,Sr,Cu04 from x=0.06 up to 0.30 have been successfully grown by the TSFZ method [ 16 1. However, the Ba contents of single crystals of Laz_,Ba,Cu04 grown by many groups did not exceed x= 0.10 [6-lo]. The Ba concentration of a best crystal grown from a melt containing 15 mol% La1.8Ba,,2Cu04+ 85 mol% CuO by the TSSG method was x= 0.05 [ 11,17 1. This suggests that the Ba solubility limit in La2Cu04 in equilibrium with the composition of the liquidus line at the crystal growth temperature is much lower than the Sr solubility limit in La2Cu04. In order to determine the Ba solubility, a preliminary crystal growth experiment of La*_,Ba,CuO, was performed. A 30 mm long feed rod of x= 0.125 and solvent of 78 mol% CuO were used for crystal growth by the TSFZ method at a rate of 1 mm/h. The Ba contents of the solvents were changed from x=0.125 to 0.225. From this experiment, it was known that the Ba content varied along the direction of the growth boule and became nearly constant when the growth distance from the seed was in the range of 20-25 mm. Table 1 shows the Ba contents in the growth boule in the portion after about 20 mm growth, analyzed by ICP. It indicates that the Ba solubility at high temperature did not exceed x= 0.070. The final molten zone remaining on the top of the feed rod was ground and analyzed by a powder Xray diffractometer. As shown in fig. 1, La4BaCu50,3 phase has been observed in the final molten zone grown using the feed of x= 0.125 and the solvent of
Table 1 The results of the preliminary crystal growth experiment of La2_,BaXCu04 Ba content in starting feed rod of L.a_,Ba,CuO,
Ba content in solvent (78 mol% CuO)
Ba content of La_,BaXCu04 boule in the portion after about 20 mm growth
Dimensions in as-grown boule
x=0.125 x=0.125 x=0.125 x=0.125
x=0.125 x=0.150 x=0.180 x=0.225
x=0.052 + 0.003 x=0.060&0.005 x=0.060&0.005 ‘) crystal cannot grow
05mmx25mm 05mmx25mm 05mmx15mm
‘) Taken from the portion after about 10 mm growth.
444
J. Yu et al. /Single crystal growth of LaZ_,BaXCuO, by TSFZ
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phase of solvents and Ba contents of the precipitating LBCO phases. It also indicates that the Ba solubility at the high temperature of crystal growth by the TSFZ method is less than x=0.070. During the crystal growth process, the Ba content in the liquid solvent should be less than x=0.180 to prevent the precipitation of the La4BaCu50i3 phase. By using the information obtained from the preliminary experiment, the feed rod of x = 0.1 O-O.125 and solvent rod of x= 0.150 were found to be optimum for crystal growth. An as-grown boule of La,_,Ba,CuO, was obtained as shown in fig. 2. The crystal with black metallic luster has the dimensions of 5-6 mm diameter and 35 mm length. The initial portion of the as-grown boule contained some inclusions, became white powder in air a few days later, and is similar to that of the LSCO as-grown boule [ 13 1. From the part grown in the last stage, a single crystal of 5 mm diameter by 15 mm length was obtained. Figure 3 shows a back reflection Laue X-ray photograph of a cleaved surface parallel to the crystal growth direction. The diffraction spots reveal a (00 1) crystallographic plane and identify the crystal as being a single crystal. Although the Ba content of the growth boule varied along the direction of crystal growth in the initial growth stage, as mentioned above, over the growth distance of 20 mm from the seed the composition of the single crystal was homogeneous within a radius of 2 mm and a length of about 10 mm as analyzed by EPMA and ICP. The Ba content of the La2_,Ba,Cu04 single crystal in the homogeneous portion was determined to be x=0.065 f.O.003, which was lower than that of the feed rod. By using EPMA, we have not found any inclusion larger than 1 urn existing in the single crystal. The single crystal had an orthorhombic symmetry and the lattice pa-
60
Cu Ka 28 I deg. Fig. I. X-ray diffraction pattern for La2_,BaxCu04 as-grown boule grown using the feed of x=0.125 and the solvent of x=0.180.
x = 0.180. It is clear that the Ba content in the liquid solvent has been concentrated during crystal growth and become sufftcient to precipitate the La4BaCu50i3 phase. In fact, we once found La4BaCu50,3 phase in the sample using the solvent of x= 0.150 and the feed rod of x= 0.125 which was 6.5 mm in diameter. In order to prevent the precipitation of the La4BaCu5013 phase, we adopted a feed rod of 5 mm diameter. When the Ba content of the solvent exceeds x= 0.180, the crystal growth process is too difficult to be continued throughout the feed rod. By using the infrared heating furnace, the solvent rods of 78 mol% CuO with different Ba contents were melted and quenched to investigate the equilibrium precipitating phases in the liquid of solvents. The molten zone of the solvent rod was mechanically separated from the solvent rod after rapid freezing and submitted to composition analysis by EPMA using the polished face of the molten zone section. Table 2 shows the precipitating phases from the liquid Table 2 The precipitates
from the molten zone of the 78 mol% CuO solvent rod Ba content in starting solvent (78 mol% CuO)
Precipitates from solvent after quenching
Ba content in La,_,Ba,CuO, from solvent after quenching
x=0.100 x=0.150 x=0.180 x=0.225
Laz_,BaxCu04 LaZ_,BaxCu04 La,_,Ba,CuO,, La,_,Ba,CuO,,
x= 0.050 f. 0.005 x=0.055 kO.005 x=0.060 AI0.005 x=O.O65kO.O05
La4BaCuS0r3 La4BaCuSOL3
J. Yu et al. /Single crystalgrowth ofLa,_,Ba,CuO,
Fig. 2. La2_,Ba,CuO,
as-grown
445
by TSFZ
boule grown by TSFZ method.
rameters were determined to be a= 5.3604 f. 0.0006 A, b=5.3828k0.0006A and c=13.178*0.002 A, of sintered close to the lattice parameters Lai.9J3ao.&u04. Figure 4 shows the results of resistivity measurements within the ab-plane, &b, and along the c-axis, pC. A sharp superconducting transition is observed from the resistivity versus temperature curves. The T, determined from the mid-point temperature of the superconducting transition is 30 IL In the nonsuperconducting state, the temperature dependence of pab shows metallic behavior. On the other hand,
12,.
,
.,
,
,
,
,
(1.2
1
- 0.2 LaI.d%wFu04
0
50
100
150
200
250
_ o
300
350
T/K Fig. 3. Back-reflection Laue X-ray photograph face of the La2_,BaxCu0, grown crystal.
of a cleaved
sur-
Fig. 4. Temperature dependence of electrical La,.935Ba~.~65Cu04 as-grown boule.
resistivity
of the
J. Yu et al. /Single crystal growth of La2_,BaxCuOd
446
the temperature dependence of pc shows an anomalous behavior. Above T,,,,, the pc behavior changes from semiconductive to metallic at about 65 K and changes again from metallic to semiconductive at about 240 K as temperature increases. The pc anomalous behavior seems to confirm that the appearance of superconductivity is independent of the metallicity along the c-axis, which was also observed in an LSCO single crystal [ 16 1. The resistivity anisotropy ratio defined as pc/pab is about 200 at 50 K, which was much smaller than that of Laz_,Sr,Cu04 (x=0.06) and approaches that of Laz_,Sr,Cu04 (x= 0.15) [ 161. It should be mentioned that the change of (dp/dT) occurring in the resistivity curves at about 65 K may be considered as a sign of the OMT to TLT phase transition. However, the OMTTLT transition temperature determined by the X-ray diffraction method for polycrystalline La,_,Ba,CuO, (x=0.08) was much lower than 65 K, and was 45 K as pointed out by Suzuki and Fujita [ 41. In order to clarify the relationship between the anomalous behavior of the resistivity and the structure phase transition, we are preparing to carry out a low-temperature X-ray diffraction experiment on a single crystal of La,_,Ba,CuOd. Figure 5 shows the temperature dependence of the electrical resistivities of a La,.9~sBao.o&u04 single crystal after annealing at 773 K for 2 days in oxygen atmosphere. The p& kept its metallic behavior, but the superconducting transition was not observed down to 10 K. The pc shows complete semiconductive behavior. The same phenomenon of disappear-
by TSFZ
ing superconducting transition was observed in an LSCO single crystal grown by the TSFZ method. After annealing at 973 K for 2 days in oxygen atthe superconducting transition of mosphere, Laz_,Sr,Cu04 (x= 0.15 ) single crystals disappeared. The annealed samples were examined by Xray diffraction and ICP analysis and no significant evidence of decomposition or composition shift was obtained within the accuracy of our X-ray diffraction and ICP analysis. Further studies should be done to determine the reason for the disappearance of the superconducting transition. Figure 6 shows the temperature dependence of the susceptibility x of a volume magnetic La 1,935Ba0.065Cu04 single crystal under a field of 50 Oe, normalized to the value (- 1/4x) for perfect diamagnetism. As shown in fig. 6 (a), under the field HI&plane, the single crystal demonstrates a
lo(a)
H=50Oe
100
I
I
I
I
I
I
142
ab-plane
HI L6l.p3s
Ba 0.065 CuO4
-I
,....y--.... FC
1201
AA
p@----
0o
(h)
0
35
H=SOOe H/lab-plane
Lal.p3s na
0.065 CuO4
0 ZFC
Fig. 5. Temperature dependence of electrical resistivity of the La,,9ssBa0,0&u04 single crystal annealed in oxygen atmosphere at 773 K for 2 days.
Fig. 6. Temperature dependence of volume magnetic susceptibility x of La,.935Baee6sCu04 under a field of 50 Oe. Field-cooling (FC) and zero-field-cooling (ZFC) data are shown. (a) Measured under a magnetic field of HI ab-plane; (b) measured under a magnetic field of HIlab-plane.
J. Yu et al. /Single crystalgrowth of Laz_,BaxCuO,
shielding fraction of 65% of ( - 1/47r) for the zerofield-cooling (ZFC) case and a 3.6% Meissner fraction for the field-cooling (FC) case at 5 K. Under HIjab-plane, the shielding fraction and Meissner fraction shown by fig. 6 (b) are 11% and 0.7%, respectively, at 5 K. The magnetic susceptibility anisotropy ratio defined as x(H_I_ ab) /~&Hllab) is about 6 at 5 K. Figure 7 shows the temperature dependence of the magnetic susceptibilities of a La1.935B&.065C~04 single crystal from 20 to 50 K. The Tconset temperature of the superconducting transition obtained from the magnetic susceptibility is 30 magnetic suscepK. Above T,..,,, the normal-state tibilities show ~(Hl.ab) >~~~(Hllab), which is a common feature for all quasi-two-dimensional highT, cuprates. The magnetic susceptibilities difference
*O[0
-20 A
-40
. -
H=50Oe
A
F
A HI
A
-60
A
-9. ‘S
ab-plane
A H/lab-plane
.
-40
G
l
-60
HI
by TSFZ
447
Ax=x,(Hlub) -~,~(Hllub) is independent of temperature and equals 2.9 x 10m4 emu/mol from T,,,,, to 50 K. On the other hand, below TConset,the magnetic susceptibilities show x(HI ub) <~,~(Hllub) and the absolute value of Ax increases with decreasing temperature, which confirms that the superconductivity was mainly in relation to the &plane ( CuOz plane).
4. Conclusions ( 1) The preliminary crystal growth experiment of Laz_,BaXCu04 was carried out to determine the Ba solubility. It was confirmed that the Ba solubility limit in La&uO, in equilibrium with the composition of the liquidus line at the temperature of crystal growth by the TSFZ method did not exceed x=0.070. (2) Single crystals of La2_,BaXCu04 with 5 mm diameter and 25 mm length have been grown by the traveling solvent floating zone method. The composition of the single crystal containing maximum Ba content was determined to be La,.93,Ba0.0&u04. The onset temperature of the superconducting transition obtained from the magnetic susceptibility is 30 K. The superconducting transition disappeared after annealing in oxygen atmosphere at 773 K for 2 days. (3) The electrical resistivities along the c-axis and in the &plane have revealed significant anisotropy. Magnetic susceptibilities have been measured under a field of 50 Oe from 5 to 50 K. Under fields HI ubplane and HIlab-plane, the single crystal demonstrates 3.6% and 0.7% Meissner fractions, respectively, at 5 K.
ab-plane
0 H/lab-plane
Acknowledgements 20
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T/K Fig. 7. Temperature dependence of volume magnetic susceptibility x of La1.935B~.065C~04 under a field of 50 Oe from 20 to 50 K. Closed triangle and circle represent magnetic susceptibility xc(HI ab) for ZFC and FC cases, respectively. Open triangle and circle represent magnetic susceptibility ~.&Illab) for ZFC and FC cases, respectively. (a) Measured for ZFC case; (b) measured for FC case. The r,,,, temperature of the superconducting transition is 30 K.
The authors would like to thank Katsuyuki Yanagihara and Toshio Maruyama of the Metallurgy Department, Tokyo Institute of Technology for permission to use the EPMA. Thank are also due to Liquan Chen, guest professor of Tokyo Institute of Technology, and H. Kojima and I. Tanaka of Yamanashi University for useful discussions. This work was supported by a Grant-in-Aid for Scientific Re-
448
search from the Ministry Culture, Japan.
J. Yu et al. /Single crystal growth of La2_,BaxCu0,
of Education,
Science and
References [ 1] J.G. Bednorz and K.A. Milller, 2. Phys. B 64 ( 1986) 189. [2] A.R. Moodenbaugh, Y. Xu and M. Suenaga, Phys. Rev. B 38 (1988) 4596. [ 3 ] J.D. Axe, A.H. Moudden, D.E. Cox, K.M. Mohanty and A.R. Moodenbaugh, Phys. Rev. Lett. 63 (1989) 275 1. [ 41 T. Suzuki and T. Fujita, Physica C 159 ( 1989) 111. [ 5 ] P.J. Picone, H.P. Jenssen and D.R. Gabbe, J. Crystal Growth 91 (1988) 463. [ 61 K. Oka and H. Unoki, Jpn. J. Appl. Phys. 26 ( 1987) L1590. [7] Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T. Murakami, J. Crystal Growth 85 (1987) 581. [8] S. Shamoto, S. Hosoya and M. Sato, Solid State Commun. 66 (1988) 195.
by TSFZ
[ 91 M. Sato, S. Hosoya, S. Shamoto, M. Onoda, K. Imaeda and H. Inokuchi, Solid State Commun. 62 (1988) 85. [IO] S. Shamoto, M. Onoda and M. Sato, Solid State Commun. 62 (1987) 479. [ 111 D. Rytz, B.A. Wechsler, C.C. Nelson and K.W. Kirby, J. Crystal Growth 99 (1990) 864. [ 121 I. Tanakaand H. Kojima, Nature 337 (1989) 21. [13]I.TanakaandH.Kojima,J.CrystalGrowth96(1989)711. [ 141 C.Chen, B.E. Watts,B.M. Wanklyn,P.A.ThomasandP.W. Haycock, J. Crystal Growth 91 (1988) 659. [ 151A.B. Bykov, L.N. Zibrov, G.V. Kanunnikov, O.K. Melnikov and S.M. Stishov, J. Crystal Growth 91 (1988) 302. [ 161 T. Kimura, K. Kishio, T. Kobayashi, Y. Nakayama, N. Motohira, K. Kitazawa and K. Yamafuji, Physica C 192 (1992) 247. [ 171 G. Aeppli, S.M. Hayden, H.A. Mook, Z. Fisk, S.W. Cheong, D. Rytz, J.P. Remeika, G.P. Espinosa and AS. Cooper, Phys. Rev. Lett. 62 ( 1989) 2052.
Single crystal growth of superconducting La2_,BaxCu04 method Jianding Yu, Yoshitaka TetsurZ, Nakamura
Yanagida,
Research Laboratory ofEngineering Received
25 November
Hiroshi
Takashima,
Yoshiyuki
Inaguma,
by TSFZ
Mitsuru
Itoh ’ and
Materials, Tokyo Institute of Technology, 4259 Nagatsuta. Midori-ku, Yokohama 227, Japan
1992
Single crystals of La2_,Ba,Cu0, with 5 mm diameter and 25 mm length have been grown by the traveling solvent floating zone (TSFZ) method. The composition of the single crystal containing maximum Ba content and showing a superconducting transition at 30 K was determined to be La ,.935Ba0.065Cu04. Electrical resistivities along the c-axis and in the ab-plane reveal significant anisotropy. Magnetic susceptibilities have been measured under a field of 50 Oe from 5 to 50 K. Under fields Hlab-plane and HIlab-plane, the single crystal demonstrates 3.6% and 0.7% Meissner fractions, respectively, at 5 K.
1. Introduction Since the discovery of high-T, superconductivity in the La-Ba-Cu-0 system by Bednorz and Miiller [ 11, the properties of the superconducting Ln-MCu-0 (M = Ca, Sr, Ba, Ln = lanthanide ions) system have been extensively investigated. For polycrystalline Laz_,BaXCuO, (LBCO), the T, changes with Ba content and reveals a local minimum at x= 0.125 [ 2 1. The significant anomalous behavior has been understood in terms of a structural phase transition between the orthorhombic mid-temperature (OMT) phase and the tetragonal low-temperature (TLT) phase occurring at a low temperature of about 50 K [ 3,4]. It is of particular interest to study the relation between T, and composition x and to investigate the anisotropy of the magnetic and electrical properties for a single crystal with homogeneity. It is well known that the phase diagram of the LazCu04-CuO system [ 5,6] is favorable for the growth of single crystals of Ba- or Sr-doped La2Cu04. Single crystals of LBCO have been grown by the flux growth and top-seeded solution growth (TSSG) methods [ 6- 111. Single crystals of LBCO obtained so far have not shown superconductivity in the as’ Correspondence 81-45-922-1111 0921-4534/93/$06.00
should be addressed to: Dr. Mitsuru Itoh, Tel. ex.2626, FAX 81-45-922-5169. 0 1993 Elsevier Science Publishers
grown state [ 6- 111, possibly due to low concentration of Ba or flux inclusions. Large single crystals of Laz_,SrXCuOd (LSCO) have been grown by the traveling solvent floating zone (TSFZ) method [ 12,13 1. However, to our knowledge, single crystals of LBCO grown by TSFZ have not yet been reported. In this paper, we describe the growth of LBCO single crystals with superconductivity by the TSFZ method.
2. Experimental
procedure
For preparing the feed rods and solvent rods, powders of 99.9% pure Laz03, Ba(N03)2 and 99.99% pure CuO were mixed in ethanol with stoichiometric composition of LBCO followed by calcining at 900°C for 12 h in air. After grinding, they were recalcined at 1050°C for 12 h in air. The obtained powder was put into a thin-wall rubber tube and compressed into rods with about 5 mm diameter and 70 mm length under hydrostatic pressure of 250 MPa. Finally, the rods used as feed rods for crystal growth were sintered at 1080°C for 100 h in oxygen and the rods used as solvent, containing the rich composition of CuO in a ratio of Laz_XBaXCu04: CuO=22:56 (mol%), were fired at 800” for 8 h in air. The crystal growth apparatus is an infrared radiation furnace (Nichiden Machinery Ltd.) equipped
B.V. All rights reserved.
J. Yu et al. / Single crystal growth of La~_,Ba,Cu~~ by EW.Z
with an ellipsoidal mirror and a 700 W halogen lamp as the heat source. After a small portion of solvent phase was attached to the top of the feed rod by melting, the feed rod was suspended at the bottom of the upper shaft and the seed made from a grow boule was held at the top of the lower shaft. Each shaft was counter-rotated at a rate of 30 rpm. The growth rate of 1 mm/h was chosen, and oxygen gas under 2 atm pressure was supplied as growth atmosphere to prevent the evaporation of CuO from the melt. The compositions of the crystals were analyzed by the inductively coupled plasma (ICP, SEIKO model SPS 1500) and electron probe microanalyzer (EPMA, SHIMADZU model 8705) techniques. The lattice parameters of the crystals were determined by the single crystal X-ray diffractometer (MAC.Science). Crystallinity and crystallographic orientation were determined by the back refraction Laue X-ray technique. The electrical resistivities of the growth crystals were measured by the four-probe method. The electrical leads were soldered with indium on the single crystals of typical dimension 3x 3x2 mm3. The magnetization measurements were performed under a magnetic field of 50 Oe using a Quantum Design MPMS-2 SQUID magnetometer in the temperature range from 5 to 50 K. The measured magnetism of samples was corrected by subtracting the orbital diamagnetism of relevant ions from the raw data.
3. Results and discussion The growth of a LBCO single crystal by the TSFZ method is similar to the growth of LSCO from consideration of the Laz03-CuO phase diagram [ 14,15 1.
443
LazCuOd melts incongruently over a composition range of 75 mol% to 95 mol% CuO. According to Tanaka and Kojima [ 13 1, in the growth of LSCO single crystals, a solvent of 78 mol% CuO had to be adopted to prevent precipitation of La203. Single crystals of La2_,Sr,Cu04 from x=0.06 up to 0.30 have been successfully grown by the TSFZ method [ 16 1. However, the Ba contents of single crystals of Laz_,Ba,Cu04 grown by many groups did not exceed x= 0.10 [6-lo]. The Ba concentration of a best crystal grown from a melt containing 15 mol% La1.8Ba,,2Cu04+ 85 mol% CuO by the TSSG method was x= 0.05 [ 11,17 1. This suggests that the Ba solubility limit in La2Cu04 in equilibrium with the composition of the liquidus line at the crystal growth temperature is much lower than the Sr solubility limit in La2Cu04. In order to determine the Ba solubility, a preliminary crystal growth experiment of La*_,Ba,CuO, was performed. A 30 mm long feed rod of x= 0.125 and solvent of 78 mol% CuO were used for crystal growth by the TSFZ method at a rate of 1 mm/h. The Ba contents of the solvents were changed from x=0.125 to 0.225. From this experiment, it was known that the Ba content varied along the direction of the growth boule and became nearly constant when the growth distance from the seed was in the range of 20-25 mm. Table 1 shows the Ba contents in the growth boule in the portion after about 20 mm growth, analyzed by ICP. It indicates that the Ba solubility at high temperature did not exceed x= 0.070. The final molten zone remaining on the top of the feed rod was ground and analyzed by a powder Xray diffractometer. As shown in fig. 1, La4BaCu50,3 phase has been observed in the final molten zone grown using the feed of x= 0.125 and the solvent of
Table 1 The results of the preliminary crystal growth experiment of La2_,BaXCu04 Ba content in starting feed rod of L.a_,Ba,CuO,
Ba content in solvent (78 mol% CuO)
Ba content of La_,BaXCu04 boule in the portion after about 20 mm growth
Dimensions in as-grown boule
x=0.125 x=0.125 x=0.125 x=0.125
x=0.125 x=0.150 x=0.180 x=0.225
x=0.052 + 0.003 x=0.060&0.005 x=0.060&0.005 ‘) crystal cannot grow
05mmx25mm 05mmx25mm 05mmx15mm
‘) Taken from the portion after about 10 mm growth.
444
J. Yu et al. /Single crystal growth of LaZ_,BaXCuO, by TSFZ
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50
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phase of solvents and Ba contents of the precipitating LBCO phases. It also indicates that the Ba solubility at the high temperature of crystal growth by the TSFZ method is less than x=0.070. During the crystal growth process, the Ba content in the liquid solvent should be less than x=0.180 to prevent the precipitation of the La4BaCu50i3 phase. By using the information obtained from the preliminary experiment, the feed rod of x = 0.1 O-O.125 and solvent rod of x= 0.150 were found to be optimum for crystal growth. An as-grown boule of La,_,Ba,CuO, was obtained as shown in fig. 2. The crystal with black metallic luster has the dimensions of 5-6 mm diameter and 35 mm length. The initial portion of the as-grown boule contained some inclusions, became white powder in air a few days later, and is similar to that of the LSCO as-grown boule [ 13 1. From the part grown in the last stage, a single crystal of 5 mm diameter by 15 mm length was obtained. Figure 3 shows a back reflection Laue X-ray photograph of a cleaved surface parallel to the crystal growth direction. The diffraction spots reveal a (00 1) crystallographic plane and identify the crystal as being a single crystal. Although the Ba content of the growth boule varied along the direction of crystal growth in the initial growth stage, as mentioned above, over the growth distance of 20 mm from the seed the composition of the single crystal was homogeneous within a radius of 2 mm and a length of about 10 mm as analyzed by EPMA and ICP. The Ba content of the La2_,Ba,Cu04 single crystal in the homogeneous portion was determined to be x=0.065 f.O.003, which was lower than that of the feed rod. By using EPMA, we have not found any inclusion larger than 1 urn existing in the single crystal. The single crystal had an orthorhombic symmetry and the lattice pa-
60
Cu Ka 28 I deg. Fig. I. X-ray diffraction pattern for La2_,BaxCu04 as-grown boule grown using the feed of x=0.125 and the solvent of x=0.180.
x = 0.180. It is clear that the Ba content in the liquid solvent has been concentrated during crystal growth and become sufftcient to precipitate the La4BaCu50i3 phase. In fact, we once found La4BaCu50,3 phase in the sample using the solvent of x= 0.150 and the feed rod of x= 0.125 which was 6.5 mm in diameter. In order to prevent the precipitation of the La4BaCu5013 phase, we adopted a feed rod of 5 mm diameter. When the Ba content of the solvent exceeds x= 0.180, the crystal growth process is too difficult to be continued throughout the feed rod. By using the infrared heating furnace, the solvent rods of 78 mol% CuO with different Ba contents were melted and quenched to investigate the equilibrium precipitating phases in the liquid of solvents. The molten zone of the solvent rod was mechanically separated from the solvent rod after rapid freezing and submitted to composition analysis by EPMA using the polished face of the molten zone section. Table 2 shows the precipitating phases from the liquid Table 2 The precipitates
from the molten zone of the 78 mol% CuO solvent rod Ba content in starting solvent (78 mol% CuO)
Precipitates from solvent after quenching
Ba content in La,_,Ba,CuO, from solvent after quenching
x=0.100 x=0.150 x=0.180 x=0.225
Laz_,BaxCu04 LaZ_,BaxCu04 La,_,Ba,CuO,, La,_,Ba,CuO,,
x= 0.050 f. 0.005 x=0.055 kO.005 x=0.060 AI0.005 x=O.O65kO.O05
La4BaCuS0r3 La4BaCuSOL3
J. Yu et al. /Single crystalgrowth ofLa,_,Ba,CuO,
Fig. 2. La2_,Ba,CuO,
as-grown
445
by TSFZ
boule grown by TSFZ method.
rameters were determined to be a= 5.3604 f. 0.0006 A, b=5.3828k0.0006A and c=13.178*0.002 A, of sintered close to the lattice parameters Lai.9J3ao.&u04. Figure 4 shows the results of resistivity measurements within the ab-plane, &b, and along the c-axis, pC. A sharp superconducting transition is observed from the resistivity versus temperature curves. The T, determined from the mid-point temperature of the superconducting transition is 30 IL In the nonsuperconducting state, the temperature dependence of pab shows metallic behavior. On the other hand,
12,.
,
.,
,
,
,
,
(1.2
1
- 0.2 LaI.d%wFu04
0
50
100
150
200
250
_ o
300
350
T/K Fig. 3. Back-reflection Laue X-ray photograph face of the La2_,BaxCu0, grown crystal.
of a cleaved
sur-
Fig. 4. Temperature dependence of electrical La,.935Ba~.~65Cu04 as-grown boule.
resistivity
of the
J. Yu et al. /Single crystal growth of La2_,BaxCuOd
446
the temperature dependence of pc shows an anomalous behavior. Above T,,,,, the pc behavior changes from semiconductive to metallic at about 65 K and changes again from metallic to semiconductive at about 240 K as temperature increases. The pc anomalous behavior seems to confirm that the appearance of superconductivity is independent of the metallicity along the c-axis, which was also observed in an LSCO single crystal [ 16 1. The resistivity anisotropy ratio defined as pc/pab is about 200 at 50 K, which was much smaller than that of Laz_,Sr,Cu04 (x=0.06) and approaches that of Laz_,Sr,Cu04 (x= 0.15) [ 161. It should be mentioned that the change of (dp/dT) occurring in the resistivity curves at about 65 K may be considered as a sign of the OMT to TLT phase transition. However, the OMTTLT transition temperature determined by the X-ray diffraction method for polycrystalline La,_,Ba,CuO, (x=0.08) was much lower than 65 K, and was 45 K as pointed out by Suzuki and Fujita [ 41. In order to clarify the relationship between the anomalous behavior of the resistivity and the structure phase transition, we are preparing to carry out a low-temperature X-ray diffraction experiment on a single crystal of La,_,Ba,CuOd. Figure 5 shows the temperature dependence of the electrical resistivities of a La,.9~sBao.o&u04 single crystal after annealing at 773 K for 2 days in oxygen atmosphere. The p& kept its metallic behavior, but the superconducting transition was not observed down to 10 K. The pc shows complete semiconductive behavior. The same phenomenon of disappear-
by TSFZ
ing superconducting transition was observed in an LSCO single crystal grown by the TSFZ method. After annealing at 973 K for 2 days in oxygen atthe superconducting transition of mosphere, Laz_,Sr,Cu04 (x= 0.15 ) single crystals disappeared. The annealed samples were examined by Xray diffraction and ICP analysis and no significant evidence of decomposition or composition shift was obtained within the accuracy of our X-ray diffraction and ICP analysis. Further studies should be done to determine the reason for the disappearance of the superconducting transition. Figure 6 shows the temperature dependence of the susceptibility x of a volume magnetic La 1,935Ba0.065Cu04 single crystal under a field of 50 Oe, normalized to the value (- 1/4x) for perfect diamagnetism. As shown in fig. 6 (a), under the field HI&plane, the single crystal demonstrates a
lo(a)
H=50Oe
100
I
I
I
I
I
I
142
ab-plane
HI L6l.p3s
Ba 0.065 CuO4
-I
,....y--.... FC
1201
AA
p@----
0o
(h)
0
35
H=SOOe H/lab-plane
Lal.p3s na
0.065 CuO4
0 ZFC
Fig. 5. Temperature dependence of electrical resistivity of the La,,9ssBa0,0&u04 single crystal annealed in oxygen atmosphere at 773 K for 2 days.
Fig. 6. Temperature dependence of volume magnetic susceptibility x of La,.935Baee6sCu04 under a field of 50 Oe. Field-cooling (FC) and zero-field-cooling (ZFC) data are shown. (a) Measured under a magnetic field of HI ab-plane; (b) measured under a magnetic field of HIlab-plane.
J. Yu et al. /Single crystalgrowth of Laz_,BaxCuO,
shielding fraction of 65% of ( - 1/47r) for the zerofield-cooling (ZFC) case and a 3.6% Meissner fraction for the field-cooling (FC) case at 5 K. Under HIjab-plane, the shielding fraction and Meissner fraction shown by fig. 6 (b) are 11% and 0.7%, respectively, at 5 K. The magnetic susceptibility anisotropy ratio defined as x(H_I_ ab) /~&Hllab) is about 6 at 5 K. Figure 7 shows the temperature dependence of the magnetic susceptibilities of a La1.935B&.065C~04 single crystal from 20 to 50 K. The Tconset temperature of the superconducting transition obtained from the magnetic susceptibility is 30 magnetic suscepK. Above T,..,,, the normal-state tibilities show ~(Hl.ab) >~~~(Hllab), which is a common feature for all quasi-two-dimensional highT, cuprates. The magnetic susceptibilities difference
*O[0
-20 A
-40
. -
H=50Oe
A
F
A HI
A
-60
A
-9. ‘S
ab-plane
A H/lab-plane
.
-40
G
l
-60
HI
by TSFZ
447
Ax=x,(Hlub) -~,~(Hllub) is independent of temperature and equals 2.9 x 10m4 emu/mol from T,,,,, to 50 K. On the other hand, below TConset,the magnetic susceptibilities show x(HI ub) <~,~(Hllub) and the absolute value of Ax increases with decreasing temperature, which confirms that the superconductivity was mainly in relation to the &plane ( CuOz plane).
4. Conclusions ( 1) The preliminary crystal growth experiment of Laz_,BaXCu04 was carried out to determine the Ba solubility. It was confirmed that the Ba solubility limit in La&uO, in equilibrium with the composition of the liquidus line at the temperature of crystal growth by the TSFZ method did not exceed x=0.070. (2) Single crystals of La2_,BaXCu04 with 5 mm diameter and 25 mm length have been grown by the traveling solvent floating zone method. The composition of the single crystal containing maximum Ba content was determined to be La,.93,Ba0.0&u04. The onset temperature of the superconducting transition obtained from the magnetic susceptibility is 30 K. The superconducting transition disappeared after annealing in oxygen atmosphere at 773 K for 2 days. (3) The electrical resistivities along the c-axis and in the &plane have revealed significant anisotropy. Magnetic susceptibilities have been measured under a field of 50 Oe from 5 to 50 K. Under fields HI ubplane and HIlab-plane, the single crystal demonstrates 3.6% and 0.7% Meissner fractions, respectively, at 5 K.
ab-plane
0 H/lab-plane
Acknowledgements 20
25
30
35
40
45
50
T/K Fig. 7. Temperature dependence of volume magnetic susceptibility x of La1.935B~.065C~04 under a field of 50 Oe from 20 to 50 K. Closed triangle and circle represent magnetic susceptibility xc(HI ab) for ZFC and FC cases, respectively. Open triangle and circle represent magnetic susceptibility ~.&Illab) for ZFC and FC cases, respectively. (a) Measured for ZFC case; (b) measured for FC case. The r,,,, temperature of the superconducting transition is 30 K.
The authors would like to thank Katsuyuki Yanagihara and Toshio Maruyama of the Metallurgy Department, Tokyo Institute of Technology for permission to use the EPMA. Thank are also due to Liquan Chen, guest professor of Tokyo Institute of Technology, and H. Kojima and I. Tanaka of Yamanashi University for useful discussions. This work was supported by a Grant-in-Aid for Scientific Re-
448
search from the Ministry Culture, Japan.
J. Yu et al. /Single crystal growth of La2_,BaxCu0,
of Education,
Science and
References [ 1] J.G. Bednorz and K.A. Milller, 2. Phys. B 64 ( 1986) 189. [2] A.R. Moodenbaugh, Y. Xu and M. Suenaga, Phys. Rev. B 38 (1988) 4596. [ 3 ] J.D. Axe, A.H. Moudden, D.E. Cox, K.M. Mohanty and A.R. Moodenbaugh, Phys. Rev. Lett. 63 (1989) 275 1. [ 41 T. Suzuki and T. Fujita, Physica C 159 ( 1989) 111. [ 5 ] P.J. Picone, H.P. Jenssen and D.R. Gabbe, J. Crystal Growth 91 (1988) 463. [ 61 K. Oka and H. Unoki, Jpn. J. Appl. Phys. 26 ( 1987) L1590. [7] Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T. Murakami, J. Crystal Growth 85 (1987) 581. [8] S. Shamoto, S. Hosoya and M. Sato, Solid State Commun. 66 (1988) 195.
by TSFZ
[ 91 M. Sato, S. Hosoya, S. Shamoto, M. Onoda, K. Imaeda and H. Inokuchi, Solid State Commun. 62 (1988) 85. [IO] S. Shamoto, M. Onoda and M. Sato, Solid State Commun. 62 (1987) 479. [ 111 D. Rytz, B.A. Wechsler, C.C. Nelson and K.W. Kirby, J. Crystal Growth 99 (1990) 864. [ 121 I. Tanakaand H. Kojima, Nature 337 (1989) 21. [13]I.TanakaandH.Kojima,J.CrystalGrowth96(1989)711. [ 141 C.Chen, B.E. Watts,B.M. Wanklyn,P.A.ThomasandP.W. Haycock, J. Crystal Growth 91 (1988) 659. [ 151A.B. Bykov, L.N. Zibrov, G.V. Kanunnikov, O.K. Melnikov and S.M. Stishov, J. Crystal Growth 91 (1988) 302. [ 161 T. Kimura, K. Kishio, T. Kobayashi, Y. Nakayama, N. Motohira, K. Kitazawa and K. Yamafuji, Physica C 192 (1992) 247. [ 171 G. Aeppli, S.M. Hayden, H.A. Mook, Z. Fisk, S.W. Cheong, D. Rytz, J.P. Remeika, G.P. Espinosa and AS. Cooper, Phys. Rev. Lett. 62 ( 1989) 2052.