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Fabrication and characterisation of Ni nanocontacts
Journal of Magnetism and Magnetic Materials 242–245 (2002) 492–494
Fabrication and characterisation of Ni nanocontacts O. Ce! spedesa,*, M.A. Baria, C. Dennisb, J.J. Versluijsa, G. Jana, J. O’Sullivanc, J.F. Greggb, J.M.D. Coeya a
Physics Department, Trinity College, Dublin 2, Ireland Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK c Trikon Technologies, Inc. Ringland Way, Newport, South Wales, NP18 2TA, UK
b
Abstract We propose a simple method to make metallic nanocontacts. A gap is electrochemically etched in a Ni track. The gap resistance is monitored as material is redeposited from the solution to bridge the gap. I:V characteristics were compared with those of Ni nanocontacts fabricated using a focussed ion beam (FIB). The I:V characteristic is asymmetric above 200 K in both cases. r 2002 Published by Elsevier Science B.V. Keywords: Sputtering; Ferromagnetic films; Ferromagnetic-nanoscale: electrodeposition
1. Introduction The application of reliable nanofabrication techniques will be required to control and duplicate nanostructures easily in new electronic systems. The study of spin transport in ferromagnetic nanoconstrictions where a domain wall can be trapped [1] leads to ideas for new magnetic devices [2,3]. Here we propose a novel technique to make magnetic nanostructures using standard lithographic and electrochemical methods. We present atomic force microscopy (AFM) and scanning electronic microscopy images, as well as I:V curves of nickel nanocontacts fabricated using this technique. We compare the data for electrodeposited Ni nanostructures with those for nanostructures fabricated by focussed ion beam (FIB) etching.
2. Fabrication of magnetic nanocontacts by electrochemical nanodeposition (ELENA) The thin film of Ni deposited on a SiO2 wafer (supplied by Trikon (UK)) was prepared by physical *Corresponding author. Tel: +353-1-608-2171; fax: +353-1671-1759. E-mail address: [email protected] (O. C!espedes).
vapour deposition. The Ni films have a thickness of 60 nm with uniformity better than 5% over a 200 mm wafer. For the preparation of the nanocontact we first used the technique described by Morpurgo et al. [4] for gold. Here, conventional e-beam lithography is used to pattern two pads connected by a track with a gap of B100 nm, then, by using one of them as working electrode of an electrochemical cell, they proceeded to fill the gap. By monitoring the resistance between the two pads with a lock-in amplifier it is possible to stop the electrodeposition at the very moment the resistance matches a desired value (1–10 kO). The counter electrode used was a solution of nickel sulphate from 0.01 to 0.02 M and 0.1 to 0.3 M Boric acid [5]. We have introduced three modifications to the method described by Morpurgo et al. 1. After UV lithography and etching to define the track structure, the photoresist layer is not removed. This allows us to etch and deposit in only two dimensions. 2. We then deposit a second layer of photoresist with opposite behaviour to the first one, leaving by lithography a gap over the zone of the track where we want to define the nanocontact. In that gap only the top of the track is protected, but not the sides where the electrochemical processing takes place. 3. The track is electrochemically etched until the desired nanocontact resistance is reached or until a gap is
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 5 3 - 8
O. C!espedes et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 492–494
formed, which may be subsequently refilled by electrodeposition. We have no need of e-beam lithography to etch the small gap, on the order of 100 nm or smaller. Fig. 1 describes the process. In Fig. 2a an AFM image of such a gap is shown.
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order of 1000 s for a track 50 mm wide, the residual photoresist that was over the track slows down the process.
3. Results With the concentrations specified above we use an etching current of 1–20 mA and a deposition current of 10–100 mA. This gives us etching-deposition times of the
The track resistance is controllable from 100 O to 100 kO and we have measured the I:V characteristics of
Fig. 1. Schematic of the electrochemical process.
Fig. 2. (a) AFM of a gap structure in a Ni track, the picture has been modified by a grain size average (210 nm in a 160 nm thick film). (b) SEM of filled nanocontact. (c) AFM of FIB nanocontact.
Fig. 3. (a) I:V measurements of Ni nanostructures fabricated by FIB at different temperatures (up triangle at 250 K, circle at 200 K and square at 300 K) and (b) RðTÞ in the FIB sample.
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O. C!espedes et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 492–494
This might explain the reduction of the asymmetry at low temperatures, when the wall remains trapped, and the need for a minimum voltage to reduce the resistance at high temperatures (Fig. 4). No magnetoresistance was observed in an applied field of 100 mT at RT:
4. Conclusions
Fig. 4. Switching voltage in ELENA-2.
three samples with resistances of 14, 3.5 and 1 kO (ELENA-1, 2 and 3). Fig. 2b shows a SEM picture of ELENA-1. Due to the nature of the process the track is not as well defined as for Ni samples prepared by FIB, so we can only estimate the size of the contact that corresponds to a particular resistance. The Ni nanocontact fabricated by FIB is shown in Fig. 2c. Non-ohmic and asymmetric I:V curves (Figs. 3a and b) were measured at different temperatures between 200 and 300 K for the FIB nanocontact and from 12 to 298 K for the ELENA samples. The asymmetry is independent of the contact configuration. It becomes larger as we increase the temperature and not when we decrease it as other authors found [6], and it is negligible below 200 K. This behaviour is common to all four samples and cannot be explained by the presence of magnetic impurities, as some authors did for gold contacts [7], as Ni is the only metal present. The resistance of the FIB contact at room temperature was B1 kO. At 250 K the resistance drops by a factor of 2.5, like that of the film itself; this behaviour contrasts with the dependence RðTÞ for the ELENA samples, where the resistance increases by a factor of the order of 10 when we lower the temperature from RT to 12 K. It is possible that the non-linearity and the temperature dependence of the asymmetry might be associated with spin pressure on a domain wall trapped [3] in an asymmetric well with barrier height in the order of kT:
Our technique to make magnetic nanocontacts uses only standard UV lithography and electrochemical methods, and it is simple and reproducible. The I:V curves of three samples made using this technique are non-ohmic and asymmetric at high temperature. Their I:V curves resemble that of nickel nanocontact fabricated by FIB. We have no definitive explanation of the origin of the unusual I:V curves but we suggest that they reflect the effect of spin pressure on a narrow domain wall trapped at the contact.
Acknowledgements This work was partly supported by the Science Foundation of Ireland. It forms part of the MAGNOISE project, supported by EU growth program. The authors would also like to thank M. Venkatesan, C. Fitzgerald and G. Hinds, members of the Group D in the Physics Department at Trinity College Dublin, and Martin Thornton in the Clarendon Laboratory, Oxford University.
References [1] P. Brunno, Phys. Rev. Lett. 83 (1999) 2425. [2] N. Garcia, I.G. Saveliev, Y.-W. Zhao, A. Zlatkine, J. Magn. Magn. Mater. 214 (2000) 7. [3] J.J. Versluijs, et al., Phys. Rev. Lett. 87 (2001) 26601. [4] F. Morpurgo, et al., Appl. Phys. Lett. 74 (1999) 2084. [5] A. Fert, L. Piraux, J. Magn. Magn. Mater. 200 (1999) 38. [6] J. Eom, et al., Phys. Rev. Lett. 77 (1996) 2276. [7] A.D. Benoit, et al., Phys. Rev. Lett. 57 (1986) 1765.
Fabrication and characterisation of Ni nanocontacts O. Ce! spedesa,*, M.A. Baria, C. Dennisb, J.J. Versluijsa, G. Jana, J. O’Sullivanc, J.F. Greggb, J.M.D. Coeya a
Physics Department, Trinity College, Dublin 2, Ireland Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK c Trikon Technologies, Inc. Ringland Way, Newport, South Wales, NP18 2TA, UK
b
Abstract We propose a simple method to make metallic nanocontacts. A gap is electrochemically etched in a Ni track. The gap resistance is monitored as material is redeposited from the solution to bridge the gap. I:V characteristics were compared with those of Ni nanocontacts fabricated using a focussed ion beam (FIB). The I:V characteristic is asymmetric above 200 K in both cases. r 2002 Published by Elsevier Science B.V. Keywords: Sputtering; Ferromagnetic films; Ferromagnetic-nanoscale: electrodeposition
1. Introduction The application of reliable nanofabrication techniques will be required to control and duplicate nanostructures easily in new electronic systems. The study of spin transport in ferromagnetic nanoconstrictions where a domain wall can be trapped [1] leads to ideas for new magnetic devices [2,3]. Here we propose a novel technique to make magnetic nanostructures using standard lithographic and electrochemical methods. We present atomic force microscopy (AFM) and scanning electronic microscopy images, as well as I:V curves of nickel nanocontacts fabricated using this technique. We compare the data for electrodeposited Ni nanostructures with those for nanostructures fabricated by focussed ion beam (FIB) etching.
2. Fabrication of magnetic nanocontacts by electrochemical nanodeposition (ELENA) The thin film of Ni deposited on a SiO2 wafer (supplied by Trikon (UK)) was prepared by physical *Corresponding author. Tel: +353-1-608-2171; fax: +353-1671-1759. E-mail address: [email protected] (O. C!espedes).
vapour deposition. The Ni films have a thickness of 60 nm with uniformity better than 5% over a 200 mm wafer. For the preparation of the nanocontact we first used the technique described by Morpurgo et al. [4] for gold. Here, conventional e-beam lithography is used to pattern two pads connected by a track with a gap of B100 nm, then, by using one of them as working electrode of an electrochemical cell, they proceeded to fill the gap. By monitoring the resistance between the two pads with a lock-in amplifier it is possible to stop the electrodeposition at the very moment the resistance matches a desired value (1–10 kO). The counter electrode used was a solution of nickel sulphate from 0.01 to 0.02 M and 0.1 to 0.3 M Boric acid [5]. We have introduced three modifications to the method described by Morpurgo et al. 1. After UV lithography and etching to define the track structure, the photoresist layer is not removed. This allows us to etch and deposit in only two dimensions. 2. We then deposit a second layer of photoresist with opposite behaviour to the first one, leaving by lithography a gap over the zone of the track where we want to define the nanocontact. In that gap only the top of the track is protected, but not the sides where the electrochemical processing takes place. 3. The track is electrochemically etched until the desired nanocontact resistance is reached or until a gap is
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 5 3 - 8
O. C!espedes et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 492–494
formed, which may be subsequently refilled by electrodeposition. We have no need of e-beam lithography to etch the small gap, on the order of 100 nm or smaller. Fig. 1 describes the process. In Fig. 2a an AFM image of such a gap is shown.
493
order of 1000 s for a track 50 mm wide, the residual photoresist that was over the track slows down the process.
3. Results With the concentrations specified above we use an etching current of 1–20 mA and a deposition current of 10–100 mA. This gives us etching-deposition times of the
The track resistance is controllable from 100 O to 100 kO and we have measured the I:V characteristics of
Fig. 1. Schematic of the electrochemical process.
Fig. 2. (a) AFM of a gap structure in a Ni track, the picture has been modified by a grain size average (210 nm in a 160 nm thick film). (b) SEM of filled nanocontact. (c) AFM of FIB nanocontact.
Fig. 3. (a) I:V measurements of Ni nanostructures fabricated by FIB at different temperatures (up triangle at 250 K, circle at 200 K and square at 300 K) and (b) RðTÞ in the FIB sample.
494
O. C!espedes et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 492–494
This might explain the reduction of the asymmetry at low temperatures, when the wall remains trapped, and the need for a minimum voltage to reduce the resistance at high temperatures (Fig. 4). No magnetoresistance was observed in an applied field of 100 mT at RT:
4. Conclusions
Fig. 4. Switching voltage in ELENA-2.
three samples with resistances of 14, 3.5 and 1 kO (ELENA-1, 2 and 3). Fig. 2b shows a SEM picture of ELENA-1. Due to the nature of the process the track is not as well defined as for Ni samples prepared by FIB, so we can only estimate the size of the contact that corresponds to a particular resistance. The Ni nanocontact fabricated by FIB is shown in Fig. 2c. Non-ohmic and asymmetric I:V curves (Figs. 3a and b) were measured at different temperatures between 200 and 300 K for the FIB nanocontact and from 12 to 298 K for the ELENA samples. The asymmetry is independent of the contact configuration. It becomes larger as we increase the temperature and not when we decrease it as other authors found [6], and it is negligible below 200 K. This behaviour is common to all four samples and cannot be explained by the presence of magnetic impurities, as some authors did for gold contacts [7], as Ni is the only metal present. The resistance of the FIB contact at room temperature was B1 kO. At 250 K the resistance drops by a factor of 2.5, like that of the film itself; this behaviour contrasts with the dependence RðTÞ for the ELENA samples, where the resistance increases by a factor of the order of 10 when we lower the temperature from RT to 12 K. It is possible that the non-linearity and the temperature dependence of the asymmetry might be associated with spin pressure on a domain wall trapped [3] in an asymmetric well with barrier height in the order of kT:
Our technique to make magnetic nanocontacts uses only standard UV lithography and electrochemical methods, and it is simple and reproducible. The I:V curves of three samples made using this technique are non-ohmic and asymmetric at high temperature. Their I:V curves resemble that of nickel nanocontact fabricated by FIB. We have no definitive explanation of the origin of the unusual I:V curves but we suggest that they reflect the effect of spin pressure on a narrow domain wall trapped at the contact.
Acknowledgements This work was partly supported by the Science Foundation of Ireland. It forms part of the MAGNOISE project, supported by EU growth program. The authors would also like to thank M. Venkatesan, C. Fitzgerald and G. Hinds, members of the Group D in the Physics Department at Trinity College Dublin, and Martin Thornton in the Clarendon Laboratory, Oxford University.
References [1] P. Brunno, Phys. Rev. Lett. 83 (1999) 2425. [2] N. Garcia, I.G. Saveliev, Y.-W. Zhao, A. Zlatkine, J. Magn. Magn. Mater. 214 (2000) 7. [3] J.J. Versluijs, et al., Phys. Rev. Lett. 87 (2001) 26601. [4] F. Morpurgo, et al., Appl. Phys. Lett. 74 (1999) 2084. [5] A. Fert, L. Piraux, J. Magn. Magn. Mater. 200 (1999) 38. [6] J. Eom, et al., Phys. Rev. Lett. 77 (1996) 2276. [7] A.D. Benoit, et al., Phys. Rev. Lett. 57 (1986) 1765.