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Ni multilayers
Journal of Magnetism and Magnetic Materials 240 (2002) 586–588
Superconducting and structural properties of Nb/Ni multilayers E. Navarroa,*, J.E. Villegasb, J.L. Vicentb a
Departamento de F!ısica Aplicada, Facultad de Veterinaria, Universidad Complutense, Av. Puerta de Hierro, 28040 Madrid, Spain b Departamento de F!ısica de Materiales, C. C. F!ısicas, Universidad Complutense, Av. Complutense, 28040 Madrid, Spain
Abstract Multilayers of Nb/Ni have been fabricated by magnetron sputtering technique. Diffraction patterns show a good multilayer structure. The Ni layers induce superconducting critical temperatures lower than the expected taking into account only metallic proximity effect. The upper critical magnetic fields measurements allow us to extract a ( r 2002 Elsevier Science B.V. All superconducting coherence length perpendicular to the layers of the order of 27 A. rights reserved. Keywords: Multilayers – metallic; Superconductivity; Sputtering; Transport properties
The study of competing long range order cooperative mechanisms, in particular the interaction of superconductivity and magnetism, is one of the most relevant topics in basic and applied research. For example, the proximity interactions across an interface separating a superconductor from a magnet produce exchange of Cooper pairs and spin polarized electrons that could lead to changes in the properties of both, superconductor and magnet, within the proximity length on either side of the interface [1,2]. These effects are crucial to nanoscale materials engineering, for example to the emerging superconducting/magnetic hybrid devices that could play a dominant role in magnetoelectronics or spintronics. In particular, superlattices allow us to explore many interesting phenomena. Multilayers, which are built with ferromagnetic/superconducting layers, could be used as a perfect tool in many fields. Metallic superlattices are ideal to study effects related with basic phenomena; for example one interesting subject is to play with the different length scales which govern many phenomena. Coupling and dimensionality problems could be easily addressed using the appropriate superlattice systems [3]. There are several superconducting/magnetic systems that have been studied in the literature, among them *Corresponding author. Fax: +34-19-1394-3813. E-mail address: [email protected] (E. Navarro).
we can point out for instance V/Ni multilayers [4] that show a striking reversal of the critical field anisotropy close to the superconducting critical temperature Tsc : As far as we know this behavior is not understood at present. Concerning the layer thickness dependence of the superconducting Tsc ; a crucial parameter in this kind of system, the Tsc literature data span from steps or oscillations in Nb/Fe [5] and NbN/GdN [6] to monotonic decreases for instance in V/Fe [7] and Nb/Gd [8]. These two different behaviors could be explained in the frame of proximity effects [9] (the latter) or unusual phase switching models [10] (the former). In this work, we present preliminary results on the structural characterization and superconducting transport properties of the Nb/Ni system. Previous results on these multilayers were focused on Hall effect [11] and magnetic measurements [12]. Unfortunately, the poor base vacuum in the chamber during fabrication of the Nb/Ni multilayers [11] prevented the Nb from being a superconductor. Mattson et al. [12] found superconduc( of Ni in a Nb87 A( /Nix tivity up to a thickness of x ¼ 12 A multilayer. Multilayers have been grown on Si(1 0 0) substrate at ambient temperature by magnetron sputtering of 99.99% Nb and 99.98% Ni targets in 99.99% pure Ar gas (1 mTorr). The base pressure of the sputtering system was 6 10–8 Torr. Typical growth rates were ( s1 for Nb and 2 A ( s1 for Ni. The substrate-to1.8 A target distance was 3 cm.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 5 3 - 8
E. Navarro et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 586–588
Standard X-ray diffraction with the scattering vector perpendicular to the film surface was done with an X’Pert diffractometer using Cu Ka radiation. Measurements have been performed at low and high angle of incidence. Images of samples surfaces have been taken with an AFM model Park version 1.51b. The standard four-point setup was used to measure resistance. The four-point pattern was defined photolithographically using wet etching technique. Electrical measurements have been done in a commercial helium cryostat with superconducting solenoid of 9 T. The temperature was varied from 1.5 to 10 K with a stability better than 20 mK. Low-angle diffraction profiles show good layering quality (Fig. 1). For all samples, clear superlattice peaks are visible, resulting from Nb/Ni bilayer thickness. In between we find seven oscillations, the so-called Kiessing fringes which are related to the total thickness. The structural coherence length perpendicular to the layers, determined from full-width at half-maximum of y22y scan at the first-order Bragg peak position, is delimited by multilayer thickness. The relevant structural parameters of the multilayers such as superlattice period, thicknesses, roughness, and interdiffusion of the layers, were obtained from the low-angle X-ray profile using the Suprex [13] program. In Fig. 1 the simulated profiles are ( also presented. Typically, we find roughness of B2–3 A at the interfaces between Nb and Ni and negligible interdiffusion between them. The bilayer thicknesses are within 5% of their nominal values. In the inset of Fig. 1,
Fig. 1. Measured and simulated low-angle x-ray-diffraction profile of the (Nb100 A( /Ni22 A( )8 multilayer. The inset shows high-angle diffraction results of (Nb75 A( /Ni75 A( )13 multilayer.
587
y22y scan in the high-angle region is presented. As can be observed, for (Nb75/Ni75)13 the high-angle y22y measurements clearly show the Nb(1 1 0) Bragg reflection at 2y ¼ 38:81 and the Ni(1 1 1) reflection at 2y ¼ 44:41: The typical full-width at half-maximum values of the rocking curves of these peaks are 131 and 6.51 for Nb and Ni, respectively. Finally, we have to point out that a hint of high-angle superlattice satellite peaks appears between the main Ni and Nb peaks. It is worth noting that the Nb/Ni multilayer has a structural lattice mismatch of 14.8%, very close to the limit of 15% for which multilayers high-angle superlattice peaks are not expected [14]. Topography images have been developed in order to obtain information about the roughness of the surface multilayers, and Nb and Ni films. The films were grown ( in the same conditions as the multilayers and with 100 A thickness. From the study of the line profile of the topography images it can be obtained that the values of ( roughness for all the samples checked were below 3 A. The superconducting critical temperature Tsc ; and the temperature dependence of the upper critical field Hc2 are basic superconducting parameters to be affected by the superconducting/magnetic layered structure. Fig. 2 shows the resistivity vs. temperature in Nb/Ni multilayers. The effect of Ni layers on the superconductivity of Nb layers is illustrated in the inset of Fig. 2, where the Tsc of the multilayers, keeping constant Nb layer ( is plotted against tNi : On increasing thickness (100 A) the thickness of the Ni layers the Tsc drops and finally ( This sharp Tsc is completely suppressed when tNi > 22 A. decrease is around 50% more dramatic than in similar
Fig. 2. Temperature dependence of resistivity for (Nb100 A( / ( The insert shows the Tsc as a Nix)8 with x=10, 15, 18 and 22 A. ( thick function of tNi ; and the symbol (m) denotes Tsc of 2000 A Nb single film.
588
E. Navarro et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 586–588
continuously as the Ni thickness increases, ranging ( and 1.7 K for x ¼ 22 A. ( between 5.2 K for x ¼ 10 A This work was supported by Spanish CICYT (MAT99-0720) and European Science Foundation Vortex Program. J.E. Villegas acknowledges a fellowship from Comunidad de Madrid.
References
Fig. 3. Temperature dependence of parallel ðHc2II Þ and perpendicular (ðHc2> Þ upper critical fields as a function of temperature for (Nb100 A( /Ni18 A( )8 multilayer.
superconducting/non-magnetic superlattices [16]. The upper critical fields were measured using the resistivity transition values at constant temperature changing the applied magnetic field up to 9 T. Fig. 3 shows the thermal dependence of the parallel and perpendicular critical fields of (Nb100 A( /Ni18 A( )8 multilayer. From the parallel Hc2 ðTÞ data we can extract the superconducting coherence length, x0 ; [15] perpendicular to the layers. ( The values are around x0 ¼ 27 A. In summary, the structural characterization of (Nb/ Ni) multilayers shows high-quality samples with rough( at the interfaces between Nb and Ni, ness of B2–3 A and negligible interdiffusion. The interaction between superconductivity and magnetism has been studied by means of the temperature dependence of the resistance in order to know how Ni thickness affects the superconducting critical temperature of Nb. The critical superconductor temperature of multilayers decreases
[1] T. Kontos, M. Aprili, J. Lesueur, X. Grison, Phys. Rev. Lett. 86 (2001) 304. [2] M. Velez, M.C. Cyrille, S. Kim, J.L. Vicent, I.K. Schuller, Phys. Rev. B 59 (1999) 14659. [3] C.S.L. Chun, G.G. Zheng, J.L. Vicent, I.K. Schuller, Phys. Rev. B 29 (1984) 2879. [4] H. Homma, C.S.L. Chun, G.-G. Zheng, I.V. Schuller, Phys. Rev. B 33 (1986) 3562. [5] Th. Muhge, . N.N. Garif’yanov, Yu.V. Goryunov, G.G. Khaliullin, L.R. Tagirov, K. Westerholt, I.A. Garifullin, H. Zabel, Phys. Rev. Lett. 77 (1996) 1857. [6] J.Q. Xiao, C.L. Chien, Phys. Rev. Lett. 76 (1996) 1727. [7] J. Aarts, J.M.E. Geers, E. Bruck, . A.A. Golubov, R. Cehoorn, Phys. Rev. B 56 (1997) 2779. [8] L.V. Marcato, C. Attanasio, C. Coccorese, L. Maritato, S.L. Prischepa, M. Salvato, Phys. Rev. B 53 (1996) 14040. [9] J. Hauser, H.C. Theurer, N.R. Werthamer, Phys. Rev. 142 (1966) 118. [10] Z. Radovic, L. Dobrosavljevic-Grujic, A.I. Buzdin, J.R. Clem, Phys. Rev. B 44 (1991) 759. [11] M.T. Perez-Frias, J.L. Vicent, Phys. Rev. B 38 (1998) 9503. [12] J.E. Mattson, R.M. Osgood III, C.D. Potter, C.H. Sowers, S.D. Bader, J. Vac. Sci. Technol. A 15 (1997) 177. [13] E. Fullerton, I.K. Schuller, H. Vandestraeten, Y. Bruynseraede, Phys. Rev. B 45 (1992) 9292. [14] B.M. Clemens, J.G. Gay, Phys. Rev. B 35 (1987) 9337. [15] M. Tinkham, Introduction to Superconductivity. McGraw-Hill, New York, 1980, p. 129. [16] I. Banerjee, Q.S. Yang, C.M. Falco, I.K. Schuller, Solid State. Commun. 41 (1982) 805.
Superconducting and structural properties of Nb/Ni multilayers E. Navarroa,*, J.E. Villegasb, J.L. Vicentb a
Departamento de F!ısica Aplicada, Facultad de Veterinaria, Universidad Complutense, Av. Puerta de Hierro, 28040 Madrid, Spain b Departamento de F!ısica de Materiales, C. C. F!ısicas, Universidad Complutense, Av. Complutense, 28040 Madrid, Spain
Abstract Multilayers of Nb/Ni have been fabricated by magnetron sputtering technique. Diffraction patterns show a good multilayer structure. The Ni layers induce superconducting critical temperatures lower than the expected taking into account only metallic proximity effect. The upper critical magnetic fields measurements allow us to extract a ( r 2002 Elsevier Science B.V. All superconducting coherence length perpendicular to the layers of the order of 27 A. rights reserved. Keywords: Multilayers – metallic; Superconductivity; Sputtering; Transport properties
The study of competing long range order cooperative mechanisms, in particular the interaction of superconductivity and magnetism, is one of the most relevant topics in basic and applied research. For example, the proximity interactions across an interface separating a superconductor from a magnet produce exchange of Cooper pairs and spin polarized electrons that could lead to changes in the properties of both, superconductor and magnet, within the proximity length on either side of the interface [1,2]. These effects are crucial to nanoscale materials engineering, for example to the emerging superconducting/magnetic hybrid devices that could play a dominant role in magnetoelectronics or spintronics. In particular, superlattices allow us to explore many interesting phenomena. Multilayers, which are built with ferromagnetic/superconducting layers, could be used as a perfect tool in many fields. Metallic superlattices are ideal to study effects related with basic phenomena; for example one interesting subject is to play with the different length scales which govern many phenomena. Coupling and dimensionality problems could be easily addressed using the appropriate superlattice systems [3]. There are several superconducting/magnetic systems that have been studied in the literature, among them *Corresponding author. Fax: +34-19-1394-3813. E-mail address: [email protected] (E. Navarro).
we can point out for instance V/Ni multilayers [4] that show a striking reversal of the critical field anisotropy close to the superconducting critical temperature Tsc : As far as we know this behavior is not understood at present. Concerning the layer thickness dependence of the superconducting Tsc ; a crucial parameter in this kind of system, the Tsc literature data span from steps or oscillations in Nb/Fe [5] and NbN/GdN [6] to monotonic decreases for instance in V/Fe [7] and Nb/Gd [8]. These two different behaviors could be explained in the frame of proximity effects [9] (the latter) or unusual phase switching models [10] (the former). In this work, we present preliminary results on the structural characterization and superconducting transport properties of the Nb/Ni system. Previous results on these multilayers were focused on Hall effect [11] and magnetic measurements [12]. Unfortunately, the poor base vacuum in the chamber during fabrication of the Nb/Ni multilayers [11] prevented the Nb from being a superconductor. Mattson et al. [12] found superconduc( of Ni in a Nb87 A( /Nix tivity up to a thickness of x ¼ 12 A multilayer. Multilayers have been grown on Si(1 0 0) substrate at ambient temperature by magnetron sputtering of 99.99% Nb and 99.98% Ni targets in 99.99% pure Ar gas (1 mTorr). The base pressure of the sputtering system was 6 10–8 Torr. Typical growth rates were ( s1 for Nb and 2 A ( s1 for Ni. The substrate-to1.8 A target distance was 3 cm.
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 8 5 3 - 8
E. Navarro et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 586–588
Standard X-ray diffraction with the scattering vector perpendicular to the film surface was done with an X’Pert diffractometer using Cu Ka radiation. Measurements have been performed at low and high angle of incidence. Images of samples surfaces have been taken with an AFM model Park version 1.51b. The standard four-point setup was used to measure resistance. The four-point pattern was defined photolithographically using wet etching technique. Electrical measurements have been done in a commercial helium cryostat with superconducting solenoid of 9 T. The temperature was varied from 1.5 to 10 K with a stability better than 20 mK. Low-angle diffraction profiles show good layering quality (Fig. 1). For all samples, clear superlattice peaks are visible, resulting from Nb/Ni bilayer thickness. In between we find seven oscillations, the so-called Kiessing fringes which are related to the total thickness. The structural coherence length perpendicular to the layers, determined from full-width at half-maximum of y22y scan at the first-order Bragg peak position, is delimited by multilayer thickness. The relevant structural parameters of the multilayers such as superlattice period, thicknesses, roughness, and interdiffusion of the layers, were obtained from the low-angle X-ray profile using the Suprex [13] program. In Fig. 1 the simulated profiles are ( also presented. Typically, we find roughness of B2–3 A at the interfaces between Nb and Ni and negligible interdiffusion between them. The bilayer thicknesses are within 5% of their nominal values. In the inset of Fig. 1,
Fig. 1. Measured and simulated low-angle x-ray-diffraction profile of the (Nb100 A( /Ni22 A( )8 multilayer. The inset shows high-angle diffraction results of (Nb75 A( /Ni75 A( )13 multilayer.
587
y22y scan in the high-angle region is presented. As can be observed, for (Nb75/Ni75)13 the high-angle y22y measurements clearly show the Nb(1 1 0) Bragg reflection at 2y ¼ 38:81 and the Ni(1 1 1) reflection at 2y ¼ 44:41: The typical full-width at half-maximum values of the rocking curves of these peaks are 131 and 6.51 for Nb and Ni, respectively. Finally, we have to point out that a hint of high-angle superlattice satellite peaks appears between the main Ni and Nb peaks. It is worth noting that the Nb/Ni multilayer has a structural lattice mismatch of 14.8%, very close to the limit of 15% for which multilayers high-angle superlattice peaks are not expected [14]. Topography images have been developed in order to obtain information about the roughness of the surface multilayers, and Nb and Ni films. The films were grown ( in the same conditions as the multilayers and with 100 A thickness. From the study of the line profile of the topography images it can be obtained that the values of ( roughness for all the samples checked were below 3 A. The superconducting critical temperature Tsc ; and the temperature dependence of the upper critical field Hc2 are basic superconducting parameters to be affected by the superconducting/magnetic layered structure. Fig. 2 shows the resistivity vs. temperature in Nb/Ni multilayers. The effect of Ni layers on the superconductivity of Nb layers is illustrated in the inset of Fig. 2, where the Tsc of the multilayers, keeping constant Nb layer ( is plotted against tNi : On increasing thickness (100 A) the thickness of the Ni layers the Tsc drops and finally ( This sharp Tsc is completely suppressed when tNi > 22 A. decrease is around 50% more dramatic than in similar
Fig. 2. Temperature dependence of resistivity for (Nb100 A( / ( The insert shows the Tsc as a Nix)8 with x=10, 15, 18 and 22 A. ( thick function of tNi ; and the symbol (m) denotes Tsc of 2000 A Nb single film.
588
E. Navarro et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 586–588
continuously as the Ni thickness increases, ranging ( and 1.7 K for x ¼ 22 A. ( between 5.2 K for x ¼ 10 A This work was supported by Spanish CICYT (MAT99-0720) and European Science Foundation Vortex Program. J.E. Villegas acknowledges a fellowship from Comunidad de Madrid.
References
Fig. 3. Temperature dependence of parallel ðHc2II Þ and perpendicular (ðHc2> Þ upper critical fields as a function of temperature for (Nb100 A( /Ni18 A( )8 multilayer.
superconducting/non-magnetic superlattices [16]. The upper critical fields were measured using the resistivity transition values at constant temperature changing the applied magnetic field up to 9 T. Fig. 3 shows the thermal dependence of the parallel and perpendicular critical fields of (Nb100 A( /Ni18 A( )8 multilayer. From the parallel Hc2 ðTÞ data we can extract the superconducting coherence length, x0 ; [15] perpendicular to the layers. ( The values are around x0 ¼ 27 A. In summary, the structural characterization of (Nb/ Ni) multilayers shows high-quality samples with rough( at the interfaces between Nb and Ni, ness of B2–3 A and negligible interdiffusion. The interaction between superconductivity and magnetism has been studied by means of the temperature dependence of the resistance in order to know how Ni thickness affects the superconducting critical temperature of Nb. The critical superconductor temperature of multilayers decreases
[1] T. Kontos, M. Aprili, J. Lesueur, X. Grison, Phys. Rev. Lett. 86 (2001) 304. [2] M. Velez, M.C. Cyrille, S. Kim, J.L. Vicent, I.K. Schuller, Phys. Rev. B 59 (1999) 14659. [3] C.S.L. Chun, G.G. Zheng, J.L. Vicent, I.K. Schuller, Phys. Rev. B 29 (1984) 2879. [4] H. Homma, C.S.L. Chun, G.-G. Zheng, I.V. Schuller, Phys. Rev. B 33 (1986) 3562. [5] Th. Muhge, . N.N. Garif’yanov, Yu.V. Goryunov, G.G. Khaliullin, L.R. Tagirov, K. Westerholt, I.A. Garifullin, H. Zabel, Phys. Rev. Lett. 77 (1996) 1857. [6] J.Q. Xiao, C.L. Chien, Phys. Rev. Lett. 76 (1996) 1727. [7] J. Aarts, J.M.E. Geers, E. Bruck, . A.A. Golubov, R. Cehoorn, Phys. Rev. B 56 (1997) 2779. [8] L.V. Marcato, C. Attanasio, C. Coccorese, L. Maritato, S.L. Prischepa, M. Salvato, Phys. Rev. B 53 (1996) 14040. [9] J. Hauser, H.C. Theurer, N.R. Werthamer, Phys. Rev. 142 (1966) 118. [10] Z. Radovic, L. Dobrosavljevic-Grujic, A.I. Buzdin, J.R. Clem, Phys. Rev. B 44 (1991) 759. [11] M.T. Perez-Frias, J.L. Vicent, Phys. Rev. B 38 (1998) 9503. [12] J.E. Mattson, R.M. Osgood III, C.D. Potter, C.H. Sowers, S.D. Bader, J. Vac. Sci. Technol. A 15 (1997) 177. [13] E. Fullerton, I.K. Schuller, H. Vandestraeten, Y. Bruynseraede, Phys. Rev. B 45 (1992) 9292. [14] B.M. Clemens, J.G. Gay, Phys. Rev. B 35 (1987) 9337. [15] M. Tinkham, Introduction to Superconductivity. McGraw-Hill, New York, 1980, p. 129. [16] I. Banerjee, Q.S. Yang, C.M. Falco, I.K. Schuller, Solid State. Commun. 41 (1982) 805.