- Email: [email protected]
SmCo5 bilayers prepared by UHV – pulsed laser deposition
Physica C 460–462 (2007) 1390–1391 www.elsevier.com/locate/physc
Nb/SmCo5 bilayers prepared by UHV – pulsed laser deposition S. Haindl *, V. Neu, L. Schultz, B. Holzapfel Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany Available online 22 April 2007
Abstract Since the possibility of ‘‘magnetic pinning’’ of vortices was reported in literature many attempts were made to join the obvious antagonistic couple in the form of multilayers with a certain superconductor/ferromagnet sequence. Magnetic pinning describes the interaction between vortices of a superconductor with domain walls of a ferromagnet situated directly above or below the superconductor. So-called hybrid structures based on superconducting Nb and high coercive ferromagnetic SmCo5 films grown on MgO(1 0 0) substrates were fabricated by pulsed laser deposition (PLD) in UHV conditions. We examined thin film architectures of SmCo5 on Nb and the reversed system both, with and without intermediate Cr-spacer layer. Epitaxy and texture of the grown films were characterized by X-ray diffraction, lateral homogeneity was investigated by atomic force microscopy and electron microscopy. Magnetic properties were determined by VSM measurements, TC was obtained from four probe measurements. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ferromagnetic-superconducting hybrids; Thin films; Pulsed laser deposition
1. Introduction Within our experimental approach to superconductingferromagnetic hybrids we make use of highly coercive Sm–Co thin films. In previous work the epitaxial growth of SmCo5 on MgO substrates with a Cr buffer layer by pulsed laser deposition was shown [1]. Although the easy axis of magnetization of SmCo5, [0 0 1], lies in the film plane, the small grain size and hence, a fine scaled domain structure leads to out-of-plane stray field components, which influence the superconducting properties of Nb. 2. Experimental All thin films were prepared by pulsed laser deposition under UHV conditions (109 mbar) using a KrF excimer laser (Lambda Physik LPX 305, k = 248 nm), which was operating at 5 Hz (Sm–Co) and 9 Hz (Nb) repetition rate. Elemental targets of at least 99.95% purity and MgO(1 0 0) single crystal substrates with a thickness of *
Corresponding author. Tel.: +49 0 531 4659 543. E-mail address: [email protected] (S. Haindl).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.171
0.15 mm were used for ablation and growth. In this study, attention will be focused on two different film architectures (i) a MgO–Cr–Nb–Cr–SmCo5–Cr (= series A) and (ii) the reverse, a MgO–Cr–SmCo5–Cr–Nb–Cr layer sequence (= series B). In both cases the thickness of the Cr-spacer layer was varied (0/2.5/5/10 and 20 nm), which gives us two sets of samples with variable distance between the superconductor and the ferromagnet. The thicknesses of the Nb and SmCo5 layers were 50 nm for each sample. A Cr buffer and a Cr cover layer with 20 nm in thickness were added to avoid oxidation. The specified thicknesses above are nominal and were obtained via rate measurements before deposition using an Inficon XTM/2 rate gauge monitor. During sample preparation (Cr, Nb and SmCo5 deposition), the substrate was heated to 400 °C, 500 °C and 550 °C, respectively. Phase identification was performed by X-ray diffraction (Co–Ka radiation) using a h/2h Bragg Brentano geometry. Additional in-plane textures were analyzed using a Philips X-Pert diffractometer (Cu–Ka radiation). Surface morphology of the samples was examined by atomic force microscopy (AFM). The superconducting transition temperatures and coercivities of SmCo5 were measured using a quantum design PPMS/VSM system.
S. Haindl et al. / Physica C 460–462 (2007) 1390–1391
3. Results and discussion
intensities (arb. units)
Cr(200)
5
SmCo (220)
5
SmCo (110)
Nb(200)
Examining the XRD patterns (Figs. 1 and 2), the following observations can be summarized: (1) In combination with results of texture measurements epitaxial growth of Nb on Cr and SmCo5 on Cr is obtained. The directional relations are given by MgO(0 0 1)[1 0 0]kCr(0 0 1)[1 1 0]kNb(0 0 1) [1 1 0] and MgO(0 0 1)[1 0 0]kCr(0 0 1)[1 1 0]kSmCo5(1 1 0) [0 0 1]. Despite the large natural misfit of 13.5%, Nb grows cube on cube on a Cr surface. (2) With successive reduction of the thickness of the intermediate Cr-layer, the SmCo5 peaks vanish in the series A (Fig. 1), whereas the Nb peak vanishes in case of the series B (Fig. 2). (3) Additional phases (marked by arrows) appear around 2h = 90.9° (Fig. 1) and around 2h = 44.4°, 52.0°, 90.9° and 96.9° (Fig. 2). Their peak positions are also slightly shifted within a series. Reference
no Cr
2.5 nm
5 nm
10 nm
20 nm
30
40
50
60
70
80
90
100
110
2θ (°)
intensities (arb. units)
SmCo (220)
5
Cr(200)
5
Nb(200)
SmCo (110)
Fig. 1. XRD pattern of series A: MgO–Cr–Nb–Cr–SmCo5–Cr.
no Cr
1391
Table 1 Superconducting and magnetic properties for samples with different nominal superconductor-ferromagnet spacing Spacing
0 nm
2.5 nm
5 nm
10 nm
20 nm
Series A
TC l0HC
6.64 K 0.14 T
< 2K 0.64 T
2.92 K 0.39 T
6.12 K 0.64 T
6.71 K 0.59 T
Series B
TC l0HC
6.03 K 3.23 T
2.13 K 2.54 T
6.48 K 0.30 T
6.14 K 0.30 T
5.05 K 0.30 T
samples were produced to identify the phases responsible for the peaks. From MgO–Cr–SmCo5–Cr and MgO–Cr– Nb–Cr references Nb–Cr and Co–Cr phase formation can be excluded. On the other hand, a MgO–Cr–Co–Nb–Cr sample showed peaks not far from the mentioned positions above. It is therefore possible that a Co–Nb alloyed interface forms also in the presence of a intermediate Cr layer. However, due to the decrease in the intensity of the peak at 2h = 90.9° with decreasing thickness of the Cr intermediate layer, it is also possible that this peak appears due to a formation of the ternary CrCoNb. TEM investigations are under way to clarify the structure at the interfaces, but at this point one can already conclude that (i) the layer sequence of both series is of different quality and (ii) intermixture between Nb and Co seems to occur if the thickness of the intermediate Cr layer is below 20 nm. Determination of the coercive field, l0HC, (in plane with l0HkSmCo5[0 0 1]) from hysteresis at 300 K support the assumption that the occurring phase around 2h = 90.9° reduces the coercivity. Table 1 summarizes coercive fields at 300 K and transition temperatures for all samples. 4. Summary The fabricated two sets of samples show asymmetric structural features which can be seen easily by means of X-ray diffraction. Within the technical aspect concerning the fabrication of trilayers (e.g. Nb–SmCo5–Nb) or larger multilayers, texture relations and homogeneous magnetic properties are not guaranteed. There is a clear variation in superconducting transition temperature with variable spacing between the superconductor and the ferromagnet. However, due to an impurity phase, the magnetic properties vary drastically within the sample series.
2.5 nm
Acknowledgement
5 nm
We would like to acknowledge the financial support of the Deutsche Forschungsgemeinschaft, SFB 463.
10 nm
20 nm
30
40
50
60
70
80
90
100
110
2θ (°)
Fig. 2. XRD pattern of series B: MgO–Cr–SmCo5–Cr–Nb–Cr.
References [1] A. Singh, R. Tamm, V. Neu, S. Fa¨hler, C.-G. Oertel, W. Skrotzki, L. Schultz, B. Holzapfel, J. Appl. Phys. 97 (2005) 093902.
Nb/SmCo5 bilayers prepared by UHV – pulsed laser deposition S. Haindl *, V. Neu, L. Schultz, B. Holzapfel Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany Available online 22 April 2007
Abstract Since the possibility of ‘‘magnetic pinning’’ of vortices was reported in literature many attempts were made to join the obvious antagonistic couple in the form of multilayers with a certain superconductor/ferromagnet sequence. Magnetic pinning describes the interaction between vortices of a superconductor with domain walls of a ferromagnet situated directly above or below the superconductor. So-called hybrid structures based on superconducting Nb and high coercive ferromagnetic SmCo5 films grown on MgO(1 0 0) substrates were fabricated by pulsed laser deposition (PLD) in UHV conditions. We examined thin film architectures of SmCo5 on Nb and the reversed system both, with and without intermediate Cr-spacer layer. Epitaxy and texture of the grown films were characterized by X-ray diffraction, lateral homogeneity was investigated by atomic force microscopy and electron microscopy. Magnetic properties were determined by VSM measurements, TC was obtained from four probe measurements. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Ferromagnetic-superconducting hybrids; Thin films; Pulsed laser deposition
1. Introduction Within our experimental approach to superconductingferromagnetic hybrids we make use of highly coercive Sm–Co thin films. In previous work the epitaxial growth of SmCo5 on MgO substrates with a Cr buffer layer by pulsed laser deposition was shown [1]. Although the easy axis of magnetization of SmCo5, [0 0 1], lies in the film plane, the small grain size and hence, a fine scaled domain structure leads to out-of-plane stray field components, which influence the superconducting properties of Nb. 2. Experimental All thin films were prepared by pulsed laser deposition under UHV conditions (109 mbar) using a KrF excimer laser (Lambda Physik LPX 305, k = 248 nm), which was operating at 5 Hz (Sm–Co) and 9 Hz (Nb) repetition rate. Elemental targets of at least 99.95% purity and MgO(1 0 0) single crystal substrates with a thickness of *
Corresponding author. Tel.: +49 0 531 4659 543. E-mail address: [email protected] (S. Haindl).
0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.171
0.15 mm were used for ablation and growth. In this study, attention will be focused on two different film architectures (i) a MgO–Cr–Nb–Cr–SmCo5–Cr (= series A) and (ii) the reverse, a MgO–Cr–SmCo5–Cr–Nb–Cr layer sequence (= series B). In both cases the thickness of the Cr-spacer layer was varied (0/2.5/5/10 and 20 nm), which gives us two sets of samples with variable distance between the superconductor and the ferromagnet. The thicknesses of the Nb and SmCo5 layers were 50 nm for each sample. A Cr buffer and a Cr cover layer with 20 nm in thickness were added to avoid oxidation. The specified thicknesses above are nominal and were obtained via rate measurements before deposition using an Inficon XTM/2 rate gauge monitor. During sample preparation (Cr, Nb and SmCo5 deposition), the substrate was heated to 400 °C, 500 °C and 550 °C, respectively. Phase identification was performed by X-ray diffraction (Co–Ka radiation) using a h/2h Bragg Brentano geometry. Additional in-plane textures were analyzed using a Philips X-Pert diffractometer (Cu–Ka radiation). Surface morphology of the samples was examined by atomic force microscopy (AFM). The superconducting transition temperatures and coercivities of SmCo5 were measured using a quantum design PPMS/VSM system.
S. Haindl et al. / Physica C 460–462 (2007) 1390–1391
3. Results and discussion
intensities (arb. units)
Cr(200)
5
SmCo (220)
5
SmCo (110)
Nb(200)
Examining the XRD patterns (Figs. 1 and 2), the following observations can be summarized: (1) In combination with results of texture measurements epitaxial growth of Nb on Cr and SmCo5 on Cr is obtained. The directional relations are given by MgO(0 0 1)[1 0 0]kCr(0 0 1)[1 1 0]kNb(0 0 1) [1 1 0] and MgO(0 0 1)[1 0 0]kCr(0 0 1)[1 1 0]kSmCo5(1 1 0) [0 0 1]. Despite the large natural misfit of 13.5%, Nb grows cube on cube on a Cr surface. (2) With successive reduction of the thickness of the intermediate Cr-layer, the SmCo5 peaks vanish in the series A (Fig. 1), whereas the Nb peak vanishes in case of the series B (Fig. 2). (3) Additional phases (marked by arrows) appear around 2h = 90.9° (Fig. 1) and around 2h = 44.4°, 52.0°, 90.9° and 96.9° (Fig. 2). Their peak positions are also slightly shifted within a series. Reference
no Cr
2.5 nm
5 nm
10 nm
20 nm
30
40
50
60
70
80
90
100
110
2θ (°)
intensities (arb. units)
SmCo (220)
5
Cr(200)
5
Nb(200)
SmCo (110)
Fig. 1. XRD pattern of series A: MgO–Cr–Nb–Cr–SmCo5–Cr.
no Cr
1391
Table 1 Superconducting and magnetic properties for samples with different nominal superconductor-ferromagnet spacing Spacing
0 nm
2.5 nm
5 nm
10 nm
20 nm
Series A
TC l0HC
6.64 K 0.14 T
< 2K 0.64 T
2.92 K 0.39 T
6.12 K 0.64 T
6.71 K 0.59 T
Series B
TC l0HC
6.03 K 3.23 T
2.13 K 2.54 T
6.48 K 0.30 T
6.14 K 0.30 T
5.05 K 0.30 T
samples were produced to identify the phases responsible for the peaks. From MgO–Cr–SmCo5–Cr and MgO–Cr– Nb–Cr references Nb–Cr and Co–Cr phase formation can be excluded. On the other hand, a MgO–Cr–Co–Nb–Cr sample showed peaks not far from the mentioned positions above. It is therefore possible that a Co–Nb alloyed interface forms also in the presence of a intermediate Cr layer. However, due to the decrease in the intensity of the peak at 2h = 90.9° with decreasing thickness of the Cr intermediate layer, it is also possible that this peak appears due to a formation of the ternary CrCoNb. TEM investigations are under way to clarify the structure at the interfaces, but at this point one can already conclude that (i) the layer sequence of both series is of different quality and (ii) intermixture between Nb and Co seems to occur if the thickness of the intermediate Cr layer is below 20 nm. Determination of the coercive field, l0HC, (in plane with l0HkSmCo5[0 0 1]) from hysteresis at 300 K support the assumption that the occurring phase around 2h = 90.9° reduces the coercivity. Table 1 summarizes coercive fields at 300 K and transition temperatures for all samples. 4. Summary The fabricated two sets of samples show asymmetric structural features which can be seen easily by means of X-ray diffraction. Within the technical aspect concerning the fabrication of trilayers (e.g. Nb–SmCo5–Nb) or larger multilayers, texture relations and homogeneous magnetic properties are not guaranteed. There is a clear variation in superconducting transition temperature with variable spacing between the superconductor and the ferromagnet. However, due to an impurity phase, the magnetic properties vary drastically within the sample series.
2.5 nm
Acknowledgement
5 nm
We would like to acknowledge the financial support of the Deutsche Forschungsgemeinschaft, SFB 463.
10 nm
20 nm
30
40
50
60
70
80
90
100
110
2θ (°)
Fig. 2. XRD pattern of series B: MgO–Cr–SmCo5–Cr–Nb–Cr.
References [1] A. Singh, R. Tamm, V. Neu, S. Fa¨hler, C.-G. Oertel, W. Skrotzki, L. Schultz, B. Holzapfel, J. Appl. Phys. 97 (2005) 093902.