- Email: [email protected]
Eutectic systems containing the superconductor Sr2RuO4
PmcA , ELSEVIER
Physica C 341-348 (2000) 787-788 www.elsevier.nl/Iocate/physc
Eutectic systems containing the superconductor Sr2Ru04 Z. Q. Mao"b and Y. Maeno~'b 'Department of Physics, Kyoto University, Kyoto 606-8502, Japan bCore Research for Evolutional Science and Technology, Japan Science and Technology Corporation (CRESTJST), Kawaguchi, Saitama 332-0012, Japan We have successfully prepared Sr2RuO4 crystals with To = 1.49 K by optimizing crystal growth conditions with a floating-zone method. In that process, we observed two intriguing eutectic solidification systems: (1) Ru metal embedded in the Sr2RuO4 matrix, and (2) epitaxial-like intergrowth of ferromagnetic SrRuO3 on Sr2RuO4. 1. INTRODUCTION Successive experiments [1-3] supported that is a spin-triplet p-wave superconductor [4]. Its superconducting transition temperature To is extremely sensitive to nonmagnetic impurities or defects [5,6]. The intrinsic To is -1.5 K, which is deduced by the fit of modified Abrikosov Gor'kov equation to the experimental curve of go (residual resistivity) vs. To [5]. We have successfully grown crystals with To up to 1.49 K (~T,0) by optimizing crystal growth conditions with a floating-zone (FZ) method. While searching for the optimal growth condition, we observed two intriguing eutoctic solidification systems: (1) Ru metal embedded in the primary phase of Sr2RuO4, and (2) epitaxial-like intergrowth of ferromagnetic SrRuO3 with the Curie temperature Tc = 160 K on Sr2RuO4. In this paper, we report on the characteristics of these two solidification systems. Sr2RuO 4
2. EXPERIMENTAL METHODS
Single crystals of Sr2RuO4 were grown by a FZ teclmique with Ru self-flux using a commercial image furnace. Tile quality of Sr2RuO4 crystals sensitively depend on the amount of excessive Ru in the feed rod serving as flux, as well as other growth conditions (especially on the crystal growth speed), file details of which will be published elsewhere [7]. The best crystal with To= 1.49 K we prepared so far was obtained with 15% excess Ru, the growth speed of 45 mm/h and the gap speed (the difference
between the crystal growth speed and the feed speed of ceramic rod) o f - 20 mm/h. When the amount of excess Ru is increased to 20~35°/~ a second phase, Ru metal, easily appears and forms an interesting composite structure with Sr2RuO4. Furthermore, we found that the size of Ru metal can be controlled by changing the crystal growth speed. On the other hand, when the gap speed is reduced to < 10mm/h for the crystal growth with 15% excess Ru in feed rod and at higher oxygen partial pressure P(O2), 0.3 bar (0.2 < / ( 0 2 ) <0.3 bar often used for pure Sr2RuO4 growth), another k i n d of two-phase composite structure, Sr2RuO4- SrRuO3, occurs. 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1. Sr3RIIO4-Rueutectic system
For crystals containing Sr2RuO4-Ru composite structure grown by a faster speed, 45 nun/h, with the gap speed of ~10 mnd~ Ru metal appears as particles with typical size of ~l/an. Figure l(a) displays a typical optical microscopy picture of a polished (010) surface of such a sample. It dearly indicates that Ru particles are embedded in the Sr2RuO4 main phase, typical of a eutectic solidificatiorL It is worthwhile to mention that such a Sr2RuO4-Ru composite structure occurs only in the core region of crystal rod. This indicates that because of the active evaporation of Ru from the surface of molten-zone, only the liquid inside can reach the eutectic concentration.
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. Pll S0921-4534(00)00693-6
ZQ. Mao, Y Maeno/Physica C 341-348 (2000) 787 7~'8
788
.
,
•
,
,
-.
•
h
and a weak peak from the tiny amount of Ru metal. These additional peaks are in good agreement with the expected (00/) diffraction of SrRuO3. Moreover, the analysis of Lane pictures on such a cleavage surface containing SrRuO3 reveals that the crystal lattice of Sr2RuO4 matches well with that of SrRuO3
0
10
20
30
40
50
60
20 (deg)
Figure 1. Optical microscopy pictures of polished cross section of crystal rod grown with speeds of (a) -45 nun/h and (b) -20 nun/h. The bright regions are Ru metal and the black regions Sr2RuO4.
Figure 2. XRD pattern of the cleaved (001) surface of Sr2gnO4 containing SrRuOa. The indexed peaks with and without the markers %' correspond to SrRuO3 and Sr2RuO4respectively.
However, if the crystal growth speed is reduced to -20 ram/b, with the zero gap speed, the precipitat6d Ru metal appears as lamellae with thickness of I/.an and .typical area of 10/an x 30/an. This observation has been introduced in our earlier report [8]. For comparison, here we also give an optical microscopy picture of (010) plane reflecting such Ru lamellae precipitation in Fig. l(b). In ref. [8], we have reported that embedding Ru lamellae in the primary phase of Sr2RuO4 induces an To enhancement up to 3 K. Our ac susceptibility measurements for the Sr2RuO4 sample embedded by Ru particles, as shown in Fig. l(a), indicate the same To enhancement. This suggests that the shape of Ru metal does not have substantial influence on the generation of superconductivity at 3 K. This information should be important in clarifying the reason for such a To enhancement.
at their interface along the (001) plane. This configuration may prove interesting in the study of tunnelling effect along the c-axis between a spintriplet superconductor and a ferromagnetic metal.
3.2. El)itaxial-likc growth of SrRuOa on Sr2RuO4 For a crystal grown by much smaller gap speed and at higher P(O2) as mentioned above, we observed a substantial amount of SrRuO3 on the cleavage surface of Sr2RuO4. Figure 2 shows the xray diffraction (XRD) pattern of the cleaved (001) surface of Sr2RuO4 which contains SrRuO3. An additional set of diffraction peaks (marked with '.') is detected besides the (00/) diffraction of Sr2RuO4
4. CONCLUSION In summary, we observed two types of eutectic solidification systems in the crystal growth of Sr2RuO4: Sr2RuO4=Ruand Sr2RuO4-SrRuO3. For the Sr2RuO4-Ru system, Ru metal can have either particle or lamellae shape with different growth speeds. But both cases lead to the Tc enhancement upto3 K. REFERENCES
1. K. Ishida et al., Nature (London), 396 (1998) 658. 2. G. M. Luke et al., Nature (London), 394 (1998) 558. 3. R. Jin et aL, Phys. Rev. B, 59 (1999) 4433. 4. T. M. Rice and M. Sigrist, J. Phys. Condens., 7 (1995) L643. 5. A. P. Mackenzie et al., Phys. Rev. Lett., 80 (1998) 161. 6. Z. Q. Mao et al., Phys. Rev. B, 60 (1999) 610. 7. Z. Q. Mao et al., submitted to Mater. Res. Bull. 8. Y. Maeno etaL, Phys. Rev. Lett., 81 (1998) 3765.
Physica C 341-348 (2000) 787-788 www.elsevier.nl/Iocate/physc
Eutectic systems containing the superconductor Sr2Ru04 Z. Q. Mao"b and Y. Maeno~'b 'Department of Physics, Kyoto University, Kyoto 606-8502, Japan bCore Research for Evolutional Science and Technology, Japan Science and Technology Corporation (CRESTJST), Kawaguchi, Saitama 332-0012, Japan We have successfully prepared Sr2RuO4 crystals with To = 1.49 K by optimizing crystal growth conditions with a floating-zone method. In that process, we observed two intriguing eutectic solidification systems: (1) Ru metal embedded in the Sr2RuO4 matrix, and (2) epitaxial-like intergrowth of ferromagnetic SrRuO3 on Sr2RuO4. 1. INTRODUCTION Successive experiments [1-3] supported that is a spin-triplet p-wave superconductor [4]. Its superconducting transition temperature To is extremely sensitive to nonmagnetic impurities or defects [5,6]. The intrinsic To is -1.5 K, which is deduced by the fit of modified Abrikosov Gor'kov equation to the experimental curve of go (residual resistivity) vs. To [5]. We have successfully grown crystals with To up to 1.49 K (~T,0) by optimizing crystal growth conditions with a floating-zone (FZ) method. While searching for the optimal growth condition, we observed two intriguing eutoctic solidification systems: (1) Ru metal embedded in the primary phase of Sr2RuO4, and (2) epitaxial-like intergrowth of ferromagnetic SrRuO3 with the Curie temperature Tc = 160 K on Sr2RuO4. In this paper, we report on the characteristics of these two solidification systems. Sr2RuO 4
2. EXPERIMENTAL METHODS
Single crystals of Sr2RuO4 were grown by a FZ teclmique with Ru self-flux using a commercial image furnace. Tile quality of Sr2RuO4 crystals sensitively depend on the amount of excessive Ru in the feed rod serving as flux, as well as other growth conditions (especially on the crystal growth speed), file details of which will be published elsewhere [7]. The best crystal with To= 1.49 K we prepared so far was obtained with 15% excess Ru, the growth speed of 45 mm/h and the gap speed (the difference
between the crystal growth speed and the feed speed of ceramic rod) o f - 20 mm/h. When the amount of excess Ru is increased to 20~35°/~ a second phase, Ru metal, easily appears and forms an interesting composite structure with Sr2RuO4. Furthermore, we found that the size of Ru metal can be controlled by changing the crystal growth speed. On the other hand, when the gap speed is reduced to < 10mm/h for the crystal growth with 15% excess Ru in feed rod and at higher oxygen partial pressure P(O2), 0.3 bar (0.2 < / ( 0 2 ) <0.3 bar often used for pure Sr2RuO4 growth), another k i n d of two-phase composite structure, Sr2RuO4- SrRuO3, occurs. 3. EXPERIMENTAL RESULTS AND DISCUSSIONS 3.1. Sr3RIIO4-Rueutectic system
For crystals containing Sr2RuO4-Ru composite structure grown by a faster speed, 45 nun/h, with the gap speed of ~10 mnd~ Ru metal appears as particles with typical size of ~l/an. Figure l(a) displays a typical optical microscopy picture of a polished (010) surface of such a sample. It dearly indicates that Ru particles are embedded in the Sr2RuO4 main phase, typical of a eutectic solidificatiorL It is worthwhile to mention that such a Sr2RuO4-Ru composite structure occurs only in the core region of crystal rod. This indicates that because of the active evaporation of Ru from the surface of molten-zone, only the liquid inside can reach the eutectic concentration.
0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. Pll S0921-4534(00)00693-6
ZQ. Mao, Y Maeno/Physica C 341-348 (2000) 787 7~'8
788
.
,
•
,
,
-.
•
h
and a weak peak from the tiny amount of Ru metal. These additional peaks are in good agreement with the expected (00/) diffraction of SrRuO3. Moreover, the analysis of Lane pictures on such a cleavage surface containing SrRuO3 reveals that the crystal lattice of Sr2RuO4 matches well with that of SrRuO3
0
10
20
30
40
50
60
20 (deg)
Figure 1. Optical microscopy pictures of polished cross section of crystal rod grown with speeds of (a) -45 nun/h and (b) -20 nun/h. The bright regions are Ru metal and the black regions Sr2RuO4.
Figure 2. XRD pattern of the cleaved (001) surface of Sr2gnO4 containing SrRuOa. The indexed peaks with and without the markers %' correspond to SrRuO3 and Sr2RuO4respectively.
However, if the crystal growth speed is reduced to -20 ram/b, with the zero gap speed, the precipitat6d Ru metal appears as lamellae with thickness of I/.an and .typical area of 10/an x 30/an. This observation has been introduced in our earlier report [8]. For comparison, here we also give an optical microscopy picture of (010) plane reflecting such Ru lamellae precipitation in Fig. l(b). In ref. [8], we have reported that embedding Ru lamellae in the primary phase of Sr2RuO4 induces an To enhancement up to 3 K. Our ac susceptibility measurements for the Sr2RuO4 sample embedded by Ru particles, as shown in Fig. l(a), indicate the same To enhancement. This suggests that the shape of Ru metal does not have substantial influence on the generation of superconductivity at 3 K. This information should be important in clarifying the reason for such a To enhancement.
at their interface along the (001) plane. This configuration may prove interesting in the study of tunnelling effect along the c-axis between a spintriplet superconductor and a ferromagnetic metal.
3.2. El)itaxial-likc growth of SrRuOa on Sr2RuO4 For a crystal grown by much smaller gap speed and at higher P(O2) as mentioned above, we observed a substantial amount of SrRuO3 on the cleavage surface of Sr2RuO4. Figure 2 shows the xray diffraction (XRD) pattern of the cleaved (001) surface of Sr2RuO4 which contains SrRuO3. An additional set of diffraction peaks (marked with '.') is detected besides the (00/) diffraction of Sr2RuO4
4. CONCLUSION In summary, we observed two types of eutectic solidification systems in the crystal growth of Sr2RuO4: Sr2RuO4=Ruand Sr2RuO4-SrRuO3. For the Sr2RuO4-Ru system, Ru metal can have either particle or lamellae shape with different growth speeds. But both cases lead to the Tc enhancement upto3 K. REFERENCES
1. K. Ishida et al., Nature (London), 396 (1998) 658. 2. G. M. Luke et al., Nature (London), 394 (1998) 558. 3. R. Jin et aL, Phys. Rev. B, 59 (1999) 4433. 4. T. M. Rice and M. Sigrist, J. Phys. Condens., 7 (1995) L643. 5. A. P. Mackenzie et al., Phys. Rev. Lett., 80 (1998) 161. 6. Z. Q. Mao et al., Phys. Rev. B, 60 (1999) 610. 7. Z. Q. Mao et al., submitted to Mater. Res. Bull. 8. Y. Maeno etaL, Phys. Rev. Lett., 81 (1998) 3765.