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Co trilayers
Journal of Magnetism and Magnetic Materials 209 (2000) 231}233
Properties of Nb/Co trilayers S.F. Lee*, C. Yu, W.T. Shih, Y. Liou, Y.D. Yao Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, ROC
Abstract The magnetic and electric properties of Nb/Co/Nb and Co/Nb/Co trilayers were studied experimentally. At 10 K, Co thickness larger than 0.4 nm showed hysteresis and ferromagnetism was well established in the trilayers. Below superconducting transition temperature (¹ ), magnetic moments versus "eld curves showed a mixture of superconducC tivity and ferromagnetism. The linear dependence of upper critical "eld with temperature close to ¹ showed C superconductive coupling across 1 nm of Co. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; Proximity e!ect
1. Introduction The interplay between superconductivity and magnetism has long been an interesting topic. To date, studies on superconductor(SC)/ferromagnet(FM) multilayers about n-junction, proximity e!ect, and magnetic coupling across SC are all under investigation experimentally and theoretically [1,2]. For bulk SC, the upper critical "eld close to transition temperature (¹ ) has 3D behavC ior and goes to zero linearly, i.e., H 2, &(1!¹/¹ ). # C For thin SC "lms it was predicted and veri"ed to have 2D behavior H 2, &(1!¹/¹ )1@2 [3]. Thus, in a # C SC/FM/SC sandwich, if the FM layer is thick, SC will show individual properties. If FM layer is thin, Cooper pairs can tunnel through and two thin SC can couple together and show 3D behavior. Whether this coupling can happen across strong FM material remains to be veri"ed. Following some experimental hints, theory predicted the existence of n-phase di!erence of Cooper pair wave functions between neighboring SC across FM layers. For certain FM thickness ranges, such so-called n-junction is the preferred low-energy state and has higher ¹ C than the usual zero-phase junction. Thus, when FM
* Corresponding author. Tel.: #886-2-2789-6767; fax: 886-22783-4187. E-mail address: [email protected] (S.F. Lee)
thickness increases, the lowest energy state switches between zero and n phase and an oscillatory behavior of ¹ is expected. Recently, experimental results on Nb/Gd C multilayers and Nb/Gd/Nb trilayers both showed non-monotonic ¹ behavior with Gd thickness [4]. This C was explained as evidence of n-junction. However, experiments on Fe/Nb/Fe trilayers also showed similar ¹ behavior [5]. Since only one SC layer exists, nC junction can only be ful"lled if the SC wave function is re#ected at the outer surfaces of FM layers and interferes with itself. Whether n-junction exists in conventional SC junctions or the origin of the observed ¹ behavior is still C under debate. Since the discovery of magnetic coupling and giant magnetoresistance in FM/normal metal triple- and multilayers, coupling of FM layers across semiconductors and superconductors are considered. Experiments on FM/semiconductor multilayers have met with di$culty in preventing compound formation. For FM/superconductor, there is theoretical work [6]. Experimentally, it is hard to determine the coupling by conventional resistance and magnetic moment measurements because of the presence of superconductor. We chose to study Nb/Co to assure the well-established magnetism down to very thin Co layers and avoid large spin}orbit interaction, which can strongly suppress the superconducting order parameter when FM exchange "eld is present [2]. Here we present results on some properties and propose new ways to study the above-mentioned e!ects.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 6 9 6 - 4
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S.F. Lee et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 231}233
2. Experiments Samples were fabricated by DC sputtering onto Si(1 0 0) substrates. The base pressure of the growth chamber was 2]10~7 mbar or less. Up to 12 di!erent samples could be fabricated in one run. Substrate temperature during deposition was monitored to be between 203C and 303C. Magnetic measurements were made by MOKE at room temperature and by SQUID magnetometer at low temperatures. Resistance is measured by standard four-point measurement with a LR-700 AC-resistance bridge. Nb and Co can form alloys at high temperature, but they should be immiscible around room temperature [7]. Series of Nb/Co(t)/Nb trilayers with "xed Nb thickness and varying Co thickness were made. Nb layer thickness was chosen to be 56 and 150 nm which has ¹ of 5.8 and C 8.5 K for single "lm, respectively. Series of pure Nb "lm with variable thickness and sandwiched between 2 nm Co layers were also made. A bulk Nb has ¹ about 9.2 K C and BCS coherence length of 50 nm. X-ray results on these samples showed that Nb has BCC (1 1 0) structure and thicker Co has FCC (1 1 1) or HCP (0 0 0 1), not distinguishable from h}2h scan. Atomic force microscopy images on selected samples showed a #at surface with roughness about 0.3 nm rms.
broadened. Thus, we de"ne ¹ as the temperature where C resistance value is 90% of the resistance just above transition. For series of pure Nb "lms and Co(2 nm)/Nb(t)/Co(2 nm) samples, ¹ s show reasonable C decrease with decreasing Nb thickness ("gure not shown) with a little scattering. However, for Nb/Co/Nb samples with various Co thickness, ¹ data are scattered with C a trend that ¹ decreases when Co thickness increases. C We infer that ¹ of SC/FM samples is very sensitive to C the preparation conditions. Only samples prepared in the same run and under strict conditions should be compared to ¹ s quantitatively. C Magnetic moments response to external magnetic "eld above and below ¹ is shown in Fig. 2 for sample C Nb(56 nm)/Co(4 nm)/Nb(56 nm). Magnetic hysteresis loop of the Co is clearly shown for samples with Co
3. Results and discussion Fig. 1 shows resistance versus temperature curve of Nb(56 nm)/Co(2.5 nm)/Nb(56 nm) sample. The inset shows the same sample near superconducting transition together with a single-Nb "lm 500 nm thick and one other sample of Nb(150 nm)/Co(1.3 nm)/Nb(150 nm). We see that with the reduction of Nb thickness and insertion of Co, ¹ is reduced and the transition becomes C
Fig. 1. Resistance versus temperature of Nb(56 nm)/Co(2.5 nm)/ Nb(56 nm) sample. The same sample are plotted with a single Nb "lm 500 nm and a Nb(150 nm)/Co(1.3 nm)/Nb(150 nm) sample in the inset.
Fig. 2. Moment per area versus "eld of sample Nb(56 nm)/ Co(4 nm)/Nb(56 nm) at (a) 10 K, (b) 5 K, and (c) sample Co(2 nm)/Nb(120 nm)/Co(2 nm) at 5 K. Sample sizes are about 2 mm wide and 4 mm long.
S.F. Lee et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 231}233
Fig. 3. H 1, and H 2, of sample Nb(56 nm)/Co(1 nm)/Nb(56 nm) # # versus reduced temperature.
233
e!ect of 1 nm Co do not suppress the superconducting order parameter completely. We propose that magnetic coupling of FM materials through SC can be studied by element speci"c X-ray magnetic circular dichroism (XMCD). For samples like FM /SC/FM /SC multilayers, XMCD can measure the 1 2 individual hysteresis curves of FM and FM and deter1 2 mine the coupling between them. Theory predicted [6] that this coupling can happen if an indirect exchange coupling exists when the SC layer is in its normal state. For SC layer to persist its superconductivity down to small thickness, its coherence length must not be too big. Thus a dirty SC should be a good candidate.
4. Summary thickness larger than 0.4 nm. Below ¹ , superconductive C response of the Nb dominates the curve. The maximum moment is 20 times larger than Co saturation moment. When compared with sample Co(2 nm)/Nb(120 nm)/ Co(2 nm), which has similar amount of total thickness for both Co and Nb, we see clearly from Fig. 2(b) and (c) that when Co are put on both the sides of Nb, the superconductive response is suppressed and the curve starts to show a mixture of superconducting and ferromagnetic behavior. With magnetic "eld applied in the layer plane, H 1, and H 2, of sample Nb(56 nm)/Co(1 nm)/Nb(56 nm) # # close to transition are measured and shown in Fig. 3. It has been well established that H 2, of 2D thin SC layers # has non-linear dependence close to ¹ . Straight-line beC havior as shown in the "gure indicates that the sample has three-dimensional response. We explain this by stating that as the Cooper pairs on two sides of 1 nm Co are coupled together the FM exchange "eld and spin}orbit
We have fabricated Nb/Co layered structures to investigate the interplay between superconductivity and magnetism. Co layer as thin as 0.4 nm sandwiched in Nb showed ferromagnetic response. In Co/Nb/Co, superconductivity is strongly suppressed. In Nb/Co/Nb sandwich, Nb layers couple through 1 nm of Co and showed 3D behavior.
References [1] [2] [3] [4]
C. Strunk et al., Phys. Rev. B 49 (1994) 4053. E.A. Demler, Phys. Rev. B 55 (1997) 15174. Z. Radovic et al., Phys. Rev. B 38 (1988) 2388. C.L. Chien et al., J. Appl. Phys. 81 (1997) 5358 and references therein. [5] Th. MuK hge et al., Phys. Rev. Lett. 77 (1996) 1857. [6] C.A.R. SaH de Melo, Phys. Rev. Lett. 79 (1997) 1933. [7] R. Krishnan, J. Magn. Magn. Mater. 50 (1985) 189.
Properties of Nb/Co trilayers S.F. Lee*, C. Yu, W.T. Shih, Y. Liou, Y.D. Yao Institute of Physics, Academia Sinica, Nankang, Taipei 115, Taiwan, ROC
Abstract The magnetic and electric properties of Nb/Co/Nb and Co/Nb/Co trilayers were studied experimentally. At 10 K, Co thickness larger than 0.4 nm showed hysteresis and ferromagnetism was well established in the trilayers. Below superconducting transition temperature (¹ ), magnetic moments versus "eld curves showed a mixture of superconducC tivity and ferromagnetism. The linear dependence of upper critical "eld with temperature close to ¹ showed C superconductive coupling across 1 nm of Co. ( 2000 Elsevier Science B.V. All rights reserved. Keywords: Superconductivity; Proximity e!ect
1. Introduction The interplay between superconductivity and magnetism has long been an interesting topic. To date, studies on superconductor(SC)/ferromagnet(FM) multilayers about n-junction, proximity e!ect, and magnetic coupling across SC are all under investigation experimentally and theoretically [1,2]. For bulk SC, the upper critical "eld close to transition temperature (¹ ) has 3D behavC ior and goes to zero linearly, i.e., H 2, &(1!¹/¹ ). # C For thin SC "lms it was predicted and veri"ed to have 2D behavior H 2, &(1!¹/¹ )1@2 [3]. Thus, in a # C SC/FM/SC sandwich, if the FM layer is thick, SC will show individual properties. If FM layer is thin, Cooper pairs can tunnel through and two thin SC can couple together and show 3D behavior. Whether this coupling can happen across strong FM material remains to be veri"ed. Following some experimental hints, theory predicted the existence of n-phase di!erence of Cooper pair wave functions between neighboring SC across FM layers. For certain FM thickness ranges, such so-called n-junction is the preferred low-energy state and has higher ¹ C than the usual zero-phase junction. Thus, when FM
* Corresponding author. Tel.: #886-2-2789-6767; fax: 886-22783-4187. E-mail address: [email protected] (S.F. Lee)
thickness increases, the lowest energy state switches between zero and n phase and an oscillatory behavior of ¹ is expected. Recently, experimental results on Nb/Gd C multilayers and Nb/Gd/Nb trilayers both showed non-monotonic ¹ behavior with Gd thickness [4]. This C was explained as evidence of n-junction. However, experiments on Fe/Nb/Fe trilayers also showed similar ¹ behavior [5]. Since only one SC layer exists, nC junction can only be ful"lled if the SC wave function is re#ected at the outer surfaces of FM layers and interferes with itself. Whether n-junction exists in conventional SC junctions or the origin of the observed ¹ behavior is still C under debate. Since the discovery of magnetic coupling and giant magnetoresistance in FM/normal metal triple- and multilayers, coupling of FM layers across semiconductors and superconductors are considered. Experiments on FM/semiconductor multilayers have met with di$culty in preventing compound formation. For FM/superconductor, there is theoretical work [6]. Experimentally, it is hard to determine the coupling by conventional resistance and magnetic moment measurements because of the presence of superconductor. We chose to study Nb/Co to assure the well-established magnetism down to very thin Co layers and avoid large spin}orbit interaction, which can strongly suppress the superconducting order parameter when FM exchange "eld is present [2]. Here we present results on some properties and propose new ways to study the above-mentioned e!ects.
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 6 9 6 - 4
232
S.F. Lee et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 231}233
2. Experiments Samples were fabricated by DC sputtering onto Si(1 0 0) substrates. The base pressure of the growth chamber was 2]10~7 mbar or less. Up to 12 di!erent samples could be fabricated in one run. Substrate temperature during deposition was monitored to be between 203C and 303C. Magnetic measurements were made by MOKE at room temperature and by SQUID magnetometer at low temperatures. Resistance is measured by standard four-point measurement with a LR-700 AC-resistance bridge. Nb and Co can form alloys at high temperature, but they should be immiscible around room temperature [7]. Series of Nb/Co(t)/Nb trilayers with "xed Nb thickness and varying Co thickness were made. Nb layer thickness was chosen to be 56 and 150 nm which has ¹ of 5.8 and C 8.5 K for single "lm, respectively. Series of pure Nb "lm with variable thickness and sandwiched between 2 nm Co layers were also made. A bulk Nb has ¹ about 9.2 K C and BCS coherence length of 50 nm. X-ray results on these samples showed that Nb has BCC (1 1 0) structure and thicker Co has FCC (1 1 1) or HCP (0 0 0 1), not distinguishable from h}2h scan. Atomic force microscopy images on selected samples showed a #at surface with roughness about 0.3 nm rms.
broadened. Thus, we de"ne ¹ as the temperature where C resistance value is 90% of the resistance just above transition. For series of pure Nb "lms and Co(2 nm)/Nb(t)/Co(2 nm) samples, ¹ s show reasonable C decrease with decreasing Nb thickness ("gure not shown) with a little scattering. However, for Nb/Co/Nb samples with various Co thickness, ¹ data are scattered with C a trend that ¹ decreases when Co thickness increases. C We infer that ¹ of SC/FM samples is very sensitive to C the preparation conditions. Only samples prepared in the same run and under strict conditions should be compared to ¹ s quantitatively. C Magnetic moments response to external magnetic "eld above and below ¹ is shown in Fig. 2 for sample C Nb(56 nm)/Co(4 nm)/Nb(56 nm). Magnetic hysteresis loop of the Co is clearly shown for samples with Co
3. Results and discussion Fig. 1 shows resistance versus temperature curve of Nb(56 nm)/Co(2.5 nm)/Nb(56 nm) sample. The inset shows the same sample near superconducting transition together with a single-Nb "lm 500 nm thick and one other sample of Nb(150 nm)/Co(1.3 nm)/Nb(150 nm). We see that with the reduction of Nb thickness and insertion of Co, ¹ is reduced and the transition becomes C
Fig. 1. Resistance versus temperature of Nb(56 nm)/Co(2.5 nm)/ Nb(56 nm) sample. The same sample are plotted with a single Nb "lm 500 nm and a Nb(150 nm)/Co(1.3 nm)/Nb(150 nm) sample in the inset.
Fig. 2. Moment per area versus "eld of sample Nb(56 nm)/ Co(4 nm)/Nb(56 nm) at (a) 10 K, (b) 5 K, and (c) sample Co(2 nm)/Nb(120 nm)/Co(2 nm) at 5 K. Sample sizes are about 2 mm wide and 4 mm long.
S.F. Lee et al. / Journal of Magnetism and Magnetic Materials 209 (2000) 231}233
Fig. 3. H 1, and H 2, of sample Nb(56 nm)/Co(1 nm)/Nb(56 nm) # # versus reduced temperature.
233
e!ect of 1 nm Co do not suppress the superconducting order parameter completely. We propose that magnetic coupling of FM materials through SC can be studied by element speci"c X-ray magnetic circular dichroism (XMCD). For samples like FM /SC/FM /SC multilayers, XMCD can measure the 1 2 individual hysteresis curves of FM and FM and deter1 2 mine the coupling between them. Theory predicted [6] that this coupling can happen if an indirect exchange coupling exists when the SC layer is in its normal state. For SC layer to persist its superconductivity down to small thickness, its coherence length must not be too big. Thus a dirty SC should be a good candidate.
4. Summary thickness larger than 0.4 nm. Below ¹ , superconductive C response of the Nb dominates the curve. The maximum moment is 20 times larger than Co saturation moment. When compared with sample Co(2 nm)/Nb(120 nm)/ Co(2 nm), which has similar amount of total thickness for both Co and Nb, we see clearly from Fig. 2(b) and (c) that when Co are put on both the sides of Nb, the superconductive response is suppressed and the curve starts to show a mixture of superconducting and ferromagnetic behavior. With magnetic "eld applied in the layer plane, H 1, and H 2, of sample Nb(56 nm)/Co(1 nm)/Nb(56 nm) # # close to transition are measured and shown in Fig. 3. It has been well established that H 2, of 2D thin SC layers # has non-linear dependence close to ¹ . Straight-line beC havior as shown in the "gure indicates that the sample has three-dimensional response. We explain this by stating that as the Cooper pairs on two sides of 1 nm Co are coupled together the FM exchange "eld and spin}orbit
We have fabricated Nb/Co layered structures to investigate the interplay between superconductivity and magnetism. Co layer as thin as 0.4 nm sandwiched in Nb showed ferromagnetic response. In Co/Nb/Co, superconductivity is strongly suppressed. In Nb/Co/Nb sandwich, Nb layers couple through 1 nm of Co and showed 3D behavior.
References [1] [2] [3] [4]
C. Strunk et al., Phys. Rev. B 49 (1994) 4053. E.A. Demler, Phys. Rev. B 55 (1997) 15174. Z. Radovic et al., Phys. Rev. B 38 (1988) 2388. C.L. Chien et al., J. Appl. Phys. 81 (1997) 5358 and references therein. [5] Th. MuK hge et al., Phys. Rev. Lett. 77 (1996) 1857. [6] C.A.R. SaH de Melo, Phys. Rev. Lett. 79 (1997) 1933. [7] R. Krishnan, J. Magn. Magn. Mater. 50 (1985) 189.