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La1−xSrxMnO3 heterostructures
Physica C 351 (2001) 13±16
www.elsevier.nl/locate/physc
Spin-polarized quasiparticle injection eect in MOCVD-grown YBa2Cu3O7/SrTiO3/La1 x Srx MnO3 heterostructures V. Plausinaitiene a,b, A. Abrutis a,*, B. Vengalis b, R. Butkute b, J.P. Senateur c, Z. Saltyte a, V. Kubilius a a
Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-2006 Vilnius, Lithuania b Semiconductor Physics Institute, A.Gostauto 11, LT-2600 Vilnius, Lithuania c LMGP-ENSPG-BP46-Saint-Martin d'Heres, France
Abstract Metal oxide heterostructures consisting of the ferromagnetic La1 x Srx MnO3 x 0:3 underlayer, thin d 5±10 nm SrTiO3 barrier layer and YBa2 Cu3 O7 cap layer were grown on single crystal LaAlO3 (1 0 0) substrates at 825°C under oxygen pressure of about 200 Pa by single source pulsed injection MOCVD method. The technological conditions were optimized in order to improve crystallinity and surface quality of the constituent layers. Wet etching was applied to form tape-like superconducting ®lms for investigation of both longitudinal and transverse electrical transport in the heterostructures. Signi®cant suppression of supercurrent has been found by injecting tunneling spin-polarized current from the underlying FM layer. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin oxide ®lms; MOCVD; YBCO; Colossal magnetoresistance manganites; FM/I/HTS heterostructures
1. Introduction During the last few years, increasing attention was attributed to various metal oxide heterostructures containing high-Tc superconductors (HTS). Combination of HTS and colossal magnetoresistance (CMR) manganites such as La1 x Srx MnO3 (LSMO) in the heterostructures provides great interest both for basic research of nonequilibrium superconductivity and novel applications. CMR manganites exhibiting almost 100% of spinpolarized carriers in the ferromagnetic (FM) state
* Corresponding author. Tel.: +370-2-331004; fax: +370-2330987. E-mail address: [email protected] (A. Abrutis).
oer promising possibility to inject spin-polarized quasiparticles into a superconductor in order to suppress superconductivity by breaking Cooper pairs. Signi®cant pair breaking eect has been demonstrated by injecting spin-polarized electrons into the YBCO superconductor through a thin d 1± 5 nm intermediate insulating (I) layer inhibiting proximity eect between HTS and FM [1±4]. A key issue for spin-polarized quasiparticle injection devices (SPQID) based on HTS/I/FM heterostructures is perfect crystalline structure of all constituent layers ensuring certain electrical and magnetic properties. In addition, high quality (¯at and sharp) interfaces are highly appreciated. The HTS/I/FM heterostructures reported up to now have been prepared mainly by pulsed laser deposition. In this work, we demonstrate ®rstly similar
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 1 - 6
14
V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
SPQID based on high quality YBCO/I/FM heterostructures deposited by cost-eective pulsed injection MOCVD technique oering wider possibilities for industrial applications.
2. Experimental The La1 x Srx MnO3 (150±200 nm)/SrTiO3 (5±10 nm)/YBa2 Cu3 O7 (200±250 nm) heterostructures were grown on LaAlO3 (1 0 0) substrates 10 6 mm2 at 825°C by applying pulsed injection MOCVD technique [5]. A vertical hot wall CVD reactor contained three computer driven injectors allowing in-situ deposition of a heterostructure consisting of three dierent layers. Precise microdoses (few microliters) of an organic solution containing a mixture of organometallic precursors were injected into an evaporator. After ¯ash evaporation, the resulting vapor mixture was transported by Ar O2 gas towards heated substrate. La(tmhd)3 , Sr(tmhd)2 , Mn(tmhd)3 , Y(tmhd)3 , Cu(tmhd)2 (tmhd-2,2,6,6-tetramethylheptane-3,5-dione), Ba(tmhd)2 (phen)2 and Ti(OEt)4 synthesized in Vilnius University were used as precursor materials for the ®lm deposition. High crystalline quality single phase LSMO, STO and YBCO ®lms have been grown by using the precursorsÕ solutions (in monoglyme) of the compositions: La0:37 Sr0:24 Mn0:39 , Sr0:58 Ti0:42 and Y0:19 Ba0:39 Cu0:42 . Composition of the grown LSMO layers was close to La0:7 Sr0:3 MnO3 . The LSMO ground layer was grown on the whole substrate surface. In following, it was masked partly by thin MgO diaphragms to grow the SrTiO3 and YBCO overlayers on an unmasked central part of the manganite ®lm. The grown heterostructures were cooled down slowly to room temperature in an oxygen atmosphere. Thin Ag coatings d 0:3 lm used as electrodes for electrical measurements were magnetron sputtered at T 300 K. Finally, lithographic process and wet etching were applied to form a tape-like YBCO layers for investigation of longitudinal and transverse electrical transport in the heterostructures. Microstructure of the layers was investigated by measuring X-ray diraction (XRD) spectra
Fig. 1. XRD study of epitaxy in YBa2 Cu3 O7 /SrTiO3 /La1 x Srx MnO3 /LaAlO3 heterostructure h=2h scans (a,c) and u-scans (b,d) measured for bottom La1 x Srx MnO3 and top YBCO layers.
in Bragg±Brentano and Schultz geometry using Siemens D5000 texture diractometer. Surface morphology of the ®lms was studied by SEM operating with EDX analysis. Epitaxial quality with ``cube-on-cube'' orientation has been found for all the constituent layers (see XRD data in Fig. 1). SEM investigations revealed smooth surface of the LSMO and STO/LSMO ®lms. However, Curich precipitates were observed on a rather smooth surface of the YBCO top layers.
3. Electrical properties Electrical resistance measurements of individual YBCO and LSMO layers were carried out in a conventional four-probe con®guration using AC current I of about 1 lA. The critical temperature (Tc ) and critical current density (jc ) of the YBCO layers were evaluated either from AC susceptibility and electrical transport measurements. Fig. 2 shows temperature dependencies of the AC susceptibility measured for two dierent YBCO/ STO/LSMO heterostructures (dI 5 and 10 nm). Resistance RYBCO U25 =I34 vs temperature dependence of the patterned tape-like YBCO layer of
V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
Fig. 2. AC susceptability vs temperature of the YBCO top layer.
the dimensions 5 mm 0.5 mm 0.25 lm showed zero resistance state at T 92 K DTc < 0:5 K in good accordance with the susceptibility data (see inset and curve 1 in Fig. 3). The temperature dependent resistance of the LSMO layers (curve 2 in Fig. 3) showed ferromagnetic transition at T 315 K. Resistivity of the layers was found to be 1000 lX cm and about 20 lX cm, at 300 and 100 K, respectively. The transverse electrical transport in the heterostructure is characterized by the eective barrier resistance Rbar de®ned by the relationship
Fig. 3. (1) Resistance vs temperature of the YBCO top layer, (2) LSMO bottom layer and (3) barrier resistance of the YBCO/ STO/LSMO heterostructure multiplied by a factor of 1:2 104 .
15
Rbar U36 =I12 where U36 is the voltage drop between points 3 and 6 if passing electrical current through 1 and 2 (see curve 3 in Fig. 3). The area resistivity of the STO barrier layer d 5 nm of about 5 10 5 X cm2 at 100 K has been estimated. Following the measured Rbar T dependence we point out lower peak temperature of the barrier resistance compared to that of the LSMO layer. Taking into account that tunneling current should depend on density of states of the adjacent YBCO and LSMO layers we conclude that the LSMO layer exhibits depressed FM properties in a close vicinity to the LSMO/STO boundary: Fig. 4 displays the critical current density of the patterned tape-like YBCO overlayer measured by passing simultaneously DC I 0±100 mA and small AC I 1 lA current through the same pair of Ag electrodes. The AC current was used in this case to test the resistive transition in the superconductive ®lm induced either by the transport I or by the injection Iinj current. The as measured jc values were lower compared to those determined from the AC susceptibility measurements before wet etching >106 A=cm2 at 78 K. The Ic vs Iinj dependencies shown in Fig. 5 for the prepared three terminal device (see inset in Fig. 4) with the I layer thickness of 5 nm were measured at various temperatures in a close vicinity to Tc .
Fig. 4. Critical current density vs temperature of tape-like YBCO top layer.
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V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
tive pulsed injection MOCVD technique. The as grown YBCO/STO/LSMO heterostructure with 5 nm thick barrier layer was found to exhibit signi®cant supercurrent suppression eect of the YBCO superconductor by injecting spin-polarized quasiparticles from the ferromagnetic LSMO underlayer. Acknowledgements The authors acknowledge partial ®nancial support from European Community (FP5, MULTIMETOX network, contract C5RT-CT-199905001) and from Lithuanian Science and Study Foundation (contract 405). Fig. 5. Critical current of YBCO layer vs injection current at various temperatures. Full and open points show dierent Iinj polarities.
References
Fig. 5 shows signi®cant supercurrent suppression eect for the prepared three terminal device by injecting spin-polarized current from the underlying LSMO layer at dierent temperatures. Reduced eect resulting signi®cantly lower current gain G DIc =Iinj of less than unity has been indicated for the heterostructure with dI 10 nm. In conclusion, we have demonstrated promising possibility to grow high quality HTS/I/FM metal oxide heterostructures by applying the cost-eec-
[1] Z.W. Dong, R. Remesh, T. Venkatesan, M. Johnson, Z.Y. Chen, S.P. Pai, V. Talyansky, R.P. Sharma, R. Shrekala, C.J. Lob, Appl. Phys. Lett. 71 (1997) 1718. [2] N.-C. Yeh, R.P. Vasquez, C.C. Fu, A.V. Samoilov, Y. Li, K. Vakii, Phys. Rev. B 60 (1999) 10522. [3] V.A. VasÕko, V.A. Larkin, P.A. Kraus, K.R. Nikolaev, D.E. Grupp, C.A. Nordman, A.M. Goldman, Phys. Rev. Lett. 78 (1977) 1134. [4] L. Fabrega, R. Rubi, V. Vrtik, C. Ferrater, F. Sachez, M. Varela, J. Fontcuberta, J. Magn. Magn. Mater. 211 (2000) 180. [5] A. Abrutis, V. Kubilius, V. Bigelyte, A. Teiserskis, Z. Saltyte, J.P. Senateur, F. Weiss, Mater. Lett. 31 (1997) 201.
www.elsevier.nl/locate/physc
Spin-polarized quasiparticle injection eect in MOCVD-grown YBa2Cu3O7/SrTiO3/La1 x Srx MnO3 heterostructures V. Plausinaitiene a,b, A. Abrutis a,*, B. Vengalis b, R. Butkute b, J.P. Senateur c, Z. Saltyte a, V. Kubilius a a
Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-2006 Vilnius, Lithuania b Semiconductor Physics Institute, A.Gostauto 11, LT-2600 Vilnius, Lithuania c LMGP-ENSPG-BP46-Saint-Martin d'Heres, France
Abstract Metal oxide heterostructures consisting of the ferromagnetic La1 x Srx MnO3 x 0:3 underlayer, thin d 5±10 nm SrTiO3 barrier layer and YBa2 Cu3 O7 cap layer were grown on single crystal LaAlO3 (1 0 0) substrates at 825°C under oxygen pressure of about 200 Pa by single source pulsed injection MOCVD method. The technological conditions were optimized in order to improve crystallinity and surface quality of the constituent layers. Wet etching was applied to form tape-like superconducting ®lms for investigation of both longitudinal and transverse electrical transport in the heterostructures. Signi®cant suppression of supercurrent has been found by injecting tunneling spin-polarized current from the underlying FM layer. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin oxide ®lms; MOCVD; YBCO; Colossal magnetoresistance manganites; FM/I/HTS heterostructures
1. Introduction During the last few years, increasing attention was attributed to various metal oxide heterostructures containing high-Tc superconductors (HTS). Combination of HTS and colossal magnetoresistance (CMR) manganites such as La1 x Srx MnO3 (LSMO) in the heterostructures provides great interest both for basic research of nonequilibrium superconductivity and novel applications. CMR manganites exhibiting almost 100% of spinpolarized carriers in the ferromagnetic (FM) state
* Corresponding author. Tel.: +370-2-331004; fax: +370-2330987. E-mail address: [email protected] (A. Abrutis).
oer promising possibility to inject spin-polarized quasiparticles into a superconductor in order to suppress superconductivity by breaking Cooper pairs. Signi®cant pair breaking eect has been demonstrated by injecting spin-polarized electrons into the YBCO superconductor through a thin d 1± 5 nm intermediate insulating (I) layer inhibiting proximity eect between HTS and FM [1±4]. A key issue for spin-polarized quasiparticle injection devices (SPQID) based on HTS/I/FM heterostructures is perfect crystalline structure of all constituent layers ensuring certain electrical and magnetic properties. In addition, high quality (¯at and sharp) interfaces are highly appreciated. The HTS/I/FM heterostructures reported up to now have been prepared mainly by pulsed laser deposition. In this work, we demonstrate ®rstly similar
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 6 8 1 - 6
14
V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
SPQID based on high quality YBCO/I/FM heterostructures deposited by cost-eective pulsed injection MOCVD technique oering wider possibilities for industrial applications.
2. Experimental The La1 x Srx MnO3 (150±200 nm)/SrTiO3 (5±10 nm)/YBa2 Cu3 O7 (200±250 nm) heterostructures were grown on LaAlO3 (1 0 0) substrates 10 6 mm2 at 825°C by applying pulsed injection MOCVD technique [5]. A vertical hot wall CVD reactor contained three computer driven injectors allowing in-situ deposition of a heterostructure consisting of three dierent layers. Precise microdoses (few microliters) of an organic solution containing a mixture of organometallic precursors were injected into an evaporator. After ¯ash evaporation, the resulting vapor mixture was transported by Ar O2 gas towards heated substrate. La(tmhd)3 , Sr(tmhd)2 , Mn(tmhd)3 , Y(tmhd)3 , Cu(tmhd)2 (tmhd-2,2,6,6-tetramethylheptane-3,5-dione), Ba(tmhd)2 (phen)2 and Ti(OEt)4 synthesized in Vilnius University were used as precursor materials for the ®lm deposition. High crystalline quality single phase LSMO, STO and YBCO ®lms have been grown by using the precursorsÕ solutions (in monoglyme) of the compositions: La0:37 Sr0:24 Mn0:39 , Sr0:58 Ti0:42 and Y0:19 Ba0:39 Cu0:42 . Composition of the grown LSMO layers was close to La0:7 Sr0:3 MnO3 . The LSMO ground layer was grown on the whole substrate surface. In following, it was masked partly by thin MgO diaphragms to grow the SrTiO3 and YBCO overlayers on an unmasked central part of the manganite ®lm. The grown heterostructures were cooled down slowly to room temperature in an oxygen atmosphere. Thin Ag coatings d 0:3 lm used as electrodes for electrical measurements were magnetron sputtered at T 300 K. Finally, lithographic process and wet etching were applied to form a tape-like YBCO layers for investigation of longitudinal and transverse electrical transport in the heterostructures. Microstructure of the layers was investigated by measuring X-ray diraction (XRD) spectra
Fig. 1. XRD study of epitaxy in YBa2 Cu3 O7 /SrTiO3 /La1 x Srx MnO3 /LaAlO3 heterostructure h=2h scans (a,c) and u-scans (b,d) measured for bottom La1 x Srx MnO3 and top YBCO layers.
in Bragg±Brentano and Schultz geometry using Siemens D5000 texture diractometer. Surface morphology of the ®lms was studied by SEM operating with EDX analysis. Epitaxial quality with ``cube-on-cube'' orientation has been found for all the constituent layers (see XRD data in Fig. 1). SEM investigations revealed smooth surface of the LSMO and STO/LSMO ®lms. However, Curich precipitates were observed on a rather smooth surface of the YBCO top layers.
3. Electrical properties Electrical resistance measurements of individual YBCO and LSMO layers were carried out in a conventional four-probe con®guration using AC current I of about 1 lA. The critical temperature (Tc ) and critical current density (jc ) of the YBCO layers were evaluated either from AC susceptibility and electrical transport measurements. Fig. 2 shows temperature dependencies of the AC susceptibility measured for two dierent YBCO/ STO/LSMO heterostructures (dI 5 and 10 nm). Resistance RYBCO U25 =I34 vs temperature dependence of the patterned tape-like YBCO layer of
V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
Fig. 2. AC susceptability vs temperature of the YBCO top layer.
the dimensions 5 mm 0.5 mm 0.25 lm showed zero resistance state at T 92 K DTc < 0:5 K in good accordance with the susceptibility data (see inset and curve 1 in Fig. 3). The temperature dependent resistance of the LSMO layers (curve 2 in Fig. 3) showed ferromagnetic transition at T 315 K. Resistivity of the layers was found to be 1000 lX cm and about 20 lX cm, at 300 and 100 K, respectively. The transverse electrical transport in the heterostructure is characterized by the eective barrier resistance Rbar de®ned by the relationship
Fig. 3. (1) Resistance vs temperature of the YBCO top layer, (2) LSMO bottom layer and (3) barrier resistance of the YBCO/ STO/LSMO heterostructure multiplied by a factor of 1:2 104 .
15
Rbar U36 =I12 where U36 is the voltage drop between points 3 and 6 if passing electrical current through 1 and 2 (see curve 3 in Fig. 3). The area resistivity of the STO barrier layer d 5 nm of about 5 10 5 X cm2 at 100 K has been estimated. Following the measured Rbar T dependence we point out lower peak temperature of the barrier resistance compared to that of the LSMO layer. Taking into account that tunneling current should depend on density of states of the adjacent YBCO and LSMO layers we conclude that the LSMO layer exhibits depressed FM properties in a close vicinity to the LSMO/STO boundary: Fig. 4 displays the critical current density of the patterned tape-like YBCO overlayer measured by passing simultaneously DC I 0±100 mA and small AC I 1 lA current through the same pair of Ag electrodes. The AC current was used in this case to test the resistive transition in the superconductive ®lm induced either by the transport I or by the injection Iinj current. The as measured jc values were lower compared to those determined from the AC susceptibility measurements before wet etching >106 A=cm2 at 78 K. The Ic vs Iinj dependencies shown in Fig. 5 for the prepared three terminal device (see inset in Fig. 4) with the I layer thickness of 5 nm were measured at various temperatures in a close vicinity to Tc .
Fig. 4. Critical current density vs temperature of tape-like YBCO top layer.
16
V. Plausinaitiene et al. / Physica C 351 (2001) 13±16
tive pulsed injection MOCVD technique. The as grown YBCO/STO/LSMO heterostructure with 5 nm thick barrier layer was found to exhibit signi®cant supercurrent suppression eect of the YBCO superconductor by injecting spin-polarized quasiparticles from the ferromagnetic LSMO underlayer. Acknowledgements The authors acknowledge partial ®nancial support from European Community (FP5, MULTIMETOX network, contract C5RT-CT-199905001) and from Lithuanian Science and Study Foundation (contract 405). Fig. 5. Critical current of YBCO layer vs injection current at various temperatures. Full and open points show dierent Iinj polarities.
References
Fig. 5 shows signi®cant supercurrent suppression eect for the prepared three terminal device by injecting spin-polarized current from the underlying LSMO layer at dierent temperatures. Reduced eect resulting signi®cantly lower current gain G DIc =Iinj of less than unity has been indicated for the heterostructure with dI 10 nm. In conclusion, we have demonstrated promising possibility to grow high quality HTS/I/FM metal oxide heterostructures by applying the cost-eec-
[1] Z.W. Dong, R. Remesh, T. Venkatesan, M. Johnson, Z.Y. Chen, S.P. Pai, V. Talyansky, R.P. Sharma, R. Shrekala, C.J. Lob, Appl. Phys. Lett. 71 (1997) 1718. [2] N.-C. Yeh, R.P. Vasquez, C.C. Fu, A.V. Samoilov, Y. Li, K. Vakii, Phys. Rev. B 60 (1999) 10522. [3] V.A. VasÕko, V.A. Larkin, P.A. Kraus, K.R. Nikolaev, D.E. Grupp, C.A. Nordman, A.M. Goldman, Phys. Rev. Lett. 78 (1977) 1134. [4] L. Fabrega, R. Rubi, V. Vrtik, C. Ferrater, F. Sachez, M. Varela, J. Fontcuberta, J. Magn. Magn. Mater. 211 (2000) 180. [5] A. Abrutis, V. Kubilius, V. Bigelyte, A. Teiserskis, Z. Saltyte, J.P. Senateur, F. Weiss, Mater. Lett. 31 (1997) 201.