- Email: [email protected]
CMR superlattices
Physica C 408–410 (2004) 896–897 www.elsevier.com/locate/physc
Interface disorder and transport properties in HTC/CMR superlattices N. Haberkorn
a,b,* ,
J. Guimpel a, M. Sirena a, L.B. Steren a, G. Campillo c, W. Saldarriaga c, M.E. G omez c
a
b
Comision Nacional de Energıa Atomica, Centro Atomico Bariloche and Instituto Balseiro, Bustillo 92500, Bariloche, RN 8400, Argentina Depto. Quımica e Ingenierıa Quımica, Univ. Nacional del Sur, 8000 Bahıa Blanca, Argentina c Departamento de Fısica, Univ. del Valle A.A., 25360 Cali, Colombia
Abstract The physical properties of superlattices are affected by interface disorder, like roughness and interdiffusion. X-ray diffraction allows its measurement through modeling and structure refinement. The high-Tc RBa2 Cu3 O7 (RBCO) and colossal magnetoresistance Lax A1x MnO3 (LAMO) perovskites are interesting superlattice partners given their similar lattice parameters and because the combination of magnetic and superconducting properties is interesting for both basic and applied research. We have investigated the structural and transport properties of YBCO/La2=3 Ca1=3 MnO3 and GdBCO/La0:6 Sr0:04 MnO3 superlattices grown by sputtering on (1 0 0)MgO. We find a roughness of 1 RBCO unit cell and a 30% interdiffusion in the same length from the interfaces for all samples. The superconducting behavior is found strongly dependent on the LAMO layer thickness. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Superlattices; Interface; Disorder
The high-Tc (HTC) and colossal magnetoresistant perovskites (CMR) are interesting partners for construction of superlattices both from the basic and applied research points of view. The combination of superconducting and magnetic materials causes the appearance of proximity effects and magnetic interactions [1,2]. To correctly analyze the behavior of the superlattices, the interface structure should be studied, since it will have a direct effect on the physical properties. In these experiments we study YBa2 Cu3 O7 / La2=3 Ca1=3 MnO3 (YBCOM /LCMON )L and GdBa2 Cu3 O7 /La0:6 Sr0:4 MnO3 (GBCOM /LSMON )L superlattices, labeled in general as RBCO/LAMO, where M and N are
*
Corresponding author. Comisi on Nacional de Energıa At omica, Centro At omico Bariloche and Instituto Balseiro, Bustillo 92500, Bariloche, RN 8400, Argentina. Tel.: +54-2944445-171; fax: +54-2944-445-299. E-mail address: [email protected] (N. Haberkorn).
the layer thicknesses in unit cells (u.c.) of the respective materials and L is the number of periods of the superlattice. The samples were grown by DC magnetron sputtering as described elsewhere [2,3]. The interface structure was analyzed [4] by modeling the crystalline structure and refining the X-ray diffraction patterns, with a modified version of the SUPREX code [5]. A detailed discussion of the modeling and structural results can be found in [4]. The crystalline coupling of the two materials at the interface was modeled by matching a LAMO MnO2 plane to an RBCO BaO plane, following the results on the YBCO/ SrTiO3 system [6]. As is well known [5], disorder is present at the interface as both interdiffussion and layer thickness fluctuations, or roughness. The refinement accurately reproduces the experimental data, as shown in Fig. 1. The results show that: (a) there is 1 RBCO u.c. roughness at the interface, independently of the layer thickness; (b) the interdiffussion extends at 1 RBCO u.c. from the interface with a value of 30 ± 10% for both the
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.152
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N. Haberkorn et al. / Physica C 408–410 (2004) 896–897
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2θ [°] Fig. 1. Experimental and refined X-ray diffraction patterns for a [GBCO5 /LSMO25 ]9 superlattice. The indexing follows the GBCO structure. The MgO (2 0 0) substrate peak at 42.95° has been erased for clarity.
RBCO/LAMO and the LAMO/RBCO interfaces; (c) the material’s lattice parameters show the usual values for thin films. These numbers imply two facts regarding the RBCO layers. First, the probability of pinholes through the superconductive layer is non-negligible for thin layers, which could be an important point regarding the magnetic coupling between LAMO layers. Second, there is at least 1 non-superconducting unit cell at each interface due to interdiffussion with LAMO. This fact will affect the minimum RBCO layer thickness for which superconductivity is observed [2]. Fig. 2 shows a summary of the magnetotransport results. The main panel shows the T dependence of the resistivity, q, and the inset shows the magnetoresistance at H ¼ 2T , MR ¼ ½qð2T Þ qð0Þ=qð0Þ for three different
897
superlattices. The sample with thicker RBCO and thinner LAMO layers, i.e. [YBCO10 /LCMO09 ]10 , shows a wide superconducting transition with an onset at around 75 K and the expected positive MR. When the LAMO layer is thickened up to 30 u.c. in [YBCO10 /LCMO30 ]7 , the superconducting transition is only incipient and does not percolate. The negative MR dominates at high T , although the positive MR component survives at low T , indicating that superconductivity is still present in the sample. Finally, when the RBCO thickness is decreased for thick LAMO layers in [YBCO03 /LCMO30 ]10 , the q and MR curves are typical of a CMR material and the superconductivity, if present, is not obvious. This behavior of the transport properties of the superlattices can be understood in terms of the structural results. Given the 1 RBCO u.c. interdiffussion at each interface plus the 1 RBCO u.c. thickness fluctuation of the layers, superconductivity is not expected to be observable for thicknesses below 5 u.c. Thus, the observed maximum in the qT ) curve for [YBCO03 / LCMO30 ]10 should be ascribed to the CMR’s metal– insulator, M–I, transition, modified with respect to the film values due, again, to interface disorder. For [YBCO10 /LCMO09 ]10 the same argument applies now for the LAMO layers which will have a degraded magnetism. This, together with a thicker superconducting layer, causes the superconductivity to percolate. However, the effective thickness of the YBCO layer is reduced to approximately 6 u.c., thus the reduced transition temperature. The ‘‘semiconducting-like’’ T dependence of q probably originates from disorder introduced into the whole thickness of the RBCO layers from the interfaces. Finally, for [YBCO10 /LCMO30 ]7 percolation of superconductivity is not achieved, probably due to the enhanced magnetism of the thicker LAMO layers.
Acknowledgements 1
ρ [Ωcm]
10
0
-3
MR [%]
10
100
200
T [K]
0
-50
-7
References
10
0
100
Work partially supported by grant ANPCyT PICT99-6340, Fundaci on Balseiro, Fundaci on Antorchas in Argentina and by COLCIENCIAS project 110605-11458, CT46-2002 in Colombia. JG and LBS are CONICET, Argentina, fellows.
T [K]
200
300
Fig. 2. Resistivity, q, and magnetoresistance, MR, vs. temperature, T , for [YBCO10 /LCMO09 ]10 , lower curve and closed triangles; [YBCO10 /LCMO30 ]7 , middle curve and open circles; [YBCO03 /LCMO30 ]10 , upper curve and closed circles.
[1] [2] [3] [4] [5] [6]
G. Jakobs et al., Appl. Phys. Lett. 66 (1995) 2564. P. Prieto et al., J. Appl. Phys. 89 (2001) 8026. J. Guimpel et al., Phys. Rev. B 56 (1997) 3552. N. Haberkorn et al., J. Appl. Phys. 94 (2003) 3011. E.E. Fullerton et al., Phys. Rev. B 45 (1992) 9292. H.W. Zandbergen et al., J. Alloys Comp. 195 (1993) 82.
Interface disorder and transport properties in HTC/CMR superlattices N. Haberkorn
a,b,* ,
J. Guimpel a, M. Sirena a, L.B. Steren a, G. Campillo c, W. Saldarriaga c, M.E. G omez c
a
b
Comision Nacional de Energıa Atomica, Centro Atomico Bariloche and Instituto Balseiro, Bustillo 92500, Bariloche, RN 8400, Argentina Depto. Quımica e Ingenierıa Quımica, Univ. Nacional del Sur, 8000 Bahıa Blanca, Argentina c Departamento de Fısica, Univ. del Valle A.A., 25360 Cali, Colombia
Abstract The physical properties of superlattices are affected by interface disorder, like roughness and interdiffusion. X-ray diffraction allows its measurement through modeling and structure refinement. The high-Tc RBa2 Cu3 O7 (RBCO) and colossal magnetoresistance Lax A1x MnO3 (LAMO) perovskites are interesting superlattice partners given their similar lattice parameters and because the combination of magnetic and superconducting properties is interesting for both basic and applied research. We have investigated the structural and transport properties of YBCO/La2=3 Ca1=3 MnO3 and GdBCO/La0:6 Sr0:04 MnO3 superlattices grown by sputtering on (1 0 0)MgO. We find a roughness of 1 RBCO unit cell and a 30% interdiffusion in the same length from the interfaces for all samples. The superconducting behavior is found strongly dependent on the LAMO layer thickness. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Superlattices; Interface; Disorder
The high-Tc (HTC) and colossal magnetoresistant perovskites (CMR) are interesting partners for construction of superlattices both from the basic and applied research points of view. The combination of superconducting and magnetic materials causes the appearance of proximity effects and magnetic interactions [1,2]. To correctly analyze the behavior of the superlattices, the interface structure should be studied, since it will have a direct effect on the physical properties. In these experiments we study YBa2 Cu3 O7 / La2=3 Ca1=3 MnO3 (YBCOM /LCMON )L and GdBa2 Cu3 O7 /La0:6 Sr0:4 MnO3 (GBCOM /LSMON )L superlattices, labeled in general as RBCO/LAMO, where M and N are
*
Corresponding author. Comisi on Nacional de Energıa At omica, Centro At omico Bariloche and Instituto Balseiro, Bustillo 92500, Bariloche, RN 8400, Argentina. Tel.: +54-2944445-171; fax: +54-2944-445-299. E-mail address: [email protected] (N. Haberkorn).
the layer thicknesses in unit cells (u.c.) of the respective materials and L is the number of periods of the superlattice. The samples were grown by DC magnetron sputtering as described elsewhere [2,3]. The interface structure was analyzed [4] by modeling the crystalline structure and refining the X-ray diffraction patterns, with a modified version of the SUPREX code [5]. A detailed discussion of the modeling and structural results can be found in [4]. The crystalline coupling of the two materials at the interface was modeled by matching a LAMO MnO2 plane to an RBCO BaO plane, following the results on the YBCO/ SrTiO3 system [6]. As is well known [5], disorder is present at the interface as both interdiffussion and layer thickness fluctuations, or roughness. The refinement accurately reproduces the experimental data, as shown in Fig. 1. The results show that: (a) there is 1 RBCO u.c. roughness at the interface, independently of the layer thickness; (b) the interdiffussion extends at 1 RBCO u.c. from the interface with a value of 30 ± 10% for both the
0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.03.152
20
30
50
(009)
(006)
40
(008)
(002)
(004)
(007)
(005)
(003)
logI [a.u.]
N. Haberkorn et al. / Physica C 408–410 (2004) 896–897
60
70
2θ [°] Fig. 1. Experimental and refined X-ray diffraction patterns for a [GBCO5 /LSMO25 ]9 superlattice. The indexing follows the GBCO structure. The MgO (2 0 0) substrate peak at 42.95° has been erased for clarity.
RBCO/LAMO and the LAMO/RBCO interfaces; (c) the material’s lattice parameters show the usual values for thin films. These numbers imply two facts regarding the RBCO layers. First, the probability of pinholes through the superconductive layer is non-negligible for thin layers, which could be an important point regarding the magnetic coupling between LAMO layers. Second, there is at least 1 non-superconducting unit cell at each interface due to interdiffussion with LAMO. This fact will affect the minimum RBCO layer thickness for which superconductivity is observed [2]. Fig. 2 shows a summary of the magnetotransport results. The main panel shows the T dependence of the resistivity, q, and the inset shows the magnetoresistance at H ¼ 2T , MR ¼ ½qð2T Þ qð0Þ=qð0Þ for three different
897
superlattices. The sample with thicker RBCO and thinner LAMO layers, i.e. [YBCO10 /LCMO09 ]10 , shows a wide superconducting transition with an onset at around 75 K and the expected positive MR. When the LAMO layer is thickened up to 30 u.c. in [YBCO10 /LCMO30 ]7 , the superconducting transition is only incipient and does not percolate. The negative MR dominates at high T , although the positive MR component survives at low T , indicating that superconductivity is still present in the sample. Finally, when the RBCO thickness is decreased for thick LAMO layers in [YBCO03 /LCMO30 ]10 , the q and MR curves are typical of a CMR material and the superconductivity, if present, is not obvious. This behavior of the transport properties of the superlattices can be understood in terms of the structural results. Given the 1 RBCO u.c. interdiffussion at each interface plus the 1 RBCO u.c. thickness fluctuation of the layers, superconductivity is not expected to be observable for thicknesses below 5 u.c. Thus, the observed maximum in the qT ) curve for [YBCO03 / LCMO30 ]10 should be ascribed to the CMR’s metal– insulator, M–I, transition, modified with respect to the film values due, again, to interface disorder. For [YBCO10 /LCMO09 ]10 the same argument applies now for the LAMO layers which will have a degraded magnetism. This, together with a thicker superconducting layer, causes the superconductivity to percolate. However, the effective thickness of the YBCO layer is reduced to approximately 6 u.c., thus the reduced transition temperature. The ‘‘semiconducting-like’’ T dependence of q probably originates from disorder introduced into the whole thickness of the RBCO layers from the interfaces. Finally, for [YBCO10 /LCMO30 ]7 percolation of superconductivity is not achieved, probably due to the enhanced magnetism of the thicker LAMO layers.
Acknowledgements 1
ρ [Ωcm]
10
0
-3
MR [%]
10
100
200
T [K]
0
-50
-7
References
10
0
100
Work partially supported by grant ANPCyT PICT99-6340, Fundaci on Balseiro, Fundaci on Antorchas in Argentina and by COLCIENCIAS project 110605-11458, CT46-2002 in Colombia. JG and LBS are CONICET, Argentina, fellows.
T [K]
200
300
Fig. 2. Resistivity, q, and magnetoresistance, MR, vs. temperature, T , for [YBCO10 /LCMO09 ]10 , lower curve and closed triangles; [YBCO10 /LCMO30 ]7 , middle curve and open circles; [YBCO03 /LCMO30 ]10 , upper curve and closed circles.
[1] [2] [3] [4] [5] [6]
G. Jakobs et al., Appl. Phys. Lett. 66 (1995) 2564. P. Prieto et al., J. Appl. Phys. 89 (2001) 8026. J. Guimpel et al., Phys. Rev. B 56 (1997) 3552. N. Haberkorn et al., J. Appl. Phys. 94 (2003) 3011. E.E. Fullerton et al., Phys. Rev. B 45 (1992) 9292. H.W. Zandbergen et al., J. Alloys Comp. 195 (1993) 82.