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NiFe spin valve
PERGAMON
Solid State Communications 120 (2001) 459±462
www.elsevier.com/locate/ssc
In¯uence of Ga 1 ion irradiation on magnetoresistance and exchange bias of IrMn/CoFe/Cu/CoFe/NiFe spin valve Z.B. Guo a,*, D. You a, J.J. Qiu a, K.B. Li a, Y.H. Wu a,b b
a Data Storage Institute, 5 Engineering Drive 1, National University of Singapore, Singapore 117608 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260
Received 16 July 2001; received in revised form 11 September 2001; accepted 26 September 2001 by H. Akai
Abstract The spin valve with the structure of IrMn/CoFe/Cu/CoFe/NiFe has been patterned to be a wire with four current±voltage probes. The detailed study of the in¯uence of 30 KeV focused Ga 1 ion beam irradiation on the magnetoresistance and exchange bias on the patterned sample has been carried out. With an increase in the ion dose, magnetoresistance and exchange bias have been found to be decreasing, and resistance has been found to be increasing. At low doses #1:05 £ 1015 ions=cm2 ; the alternation in resistance is mainly attributed to atomic mixing in the interfacial regions induced by Ga 1 ion irradiation. However, at high doses $3:51 £ 1015 ions=cm2 ; the bulk defects generated by Ga 1 ion irradiation have a signi®cant effect on the increase of resistance. At the dose of 3.51 £ 10 15 ions/cm 2 both GMR and AMR behaviors have been observed. q 2001 Published by Elsevier Science Ltd. PACS: 75.70.Pa; 61.80.Jh Keywords: A. Magnetic ®lms and multilayers; B. Irradiation effects
Ion irradiation of magnetic multilayer structures is an excellent tool to modify their magnetic and electrical behaviors without modifying their topography. Depending on the ion mass, its energy, and the dosage, ion-irradiation induces displacement cascades resulting in atomic diffusion, the modi®cation of crystal structures, and the alteration of grain size [1±4]. Recently, ion irradiation has been used for the nano-scale patterning of CoPt multilayers, in which irradiation-induced reductions of the magnetic anisotropy have been observed. Such patterned magnetic ®lms are of great interest for application in high density magnetic recording [5±7]. On the other hand, ion irradiation has been used for the studies of magnetization and transport properties in relation to interfacial magnetization in ferromagnetic/antiferromagnetic bilayers and ferromagnetic/ non-magnetic multilayers. Local manipulation of the exchange bias has been achieved in FeNi/FeMn bilayers by He ion irradiation under an external magnetic ®eld [8]. Increases in magnetoresistance have been achieved in Fe/Cr multilayers by Xe ion irradiation [9]. * Corresponding author. Tel.: 165-8748200; fax: 165-7771349. E-mail address: [email protected] (Z.B. Guo).
The effects of Ni ion irradiation on a spin valve structure have been reported by Lin et al. [10]. Magnetoresistance has been found to be decreased after irradiation. In this paper, we report on Ga ion irradiation experiments on the top spin valve structure of IrMn/CoFe/Cu/CoFe/NiFe, the penetration depth of 30 KeV Ga ions is much less than that of 8 MeV Ni ions used by Lin et al. [10]. Two 1 inch £ 1 inch samples with the structure of 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/ 2 nm Ni81Fe19/10 nm Ta/thermally oxided Si(100) substrate were prepared simultaneously at room temperature by helicon sputtering deposition under a magnetic ®eld of 100 Oe. The base pressure of the system is 5 £ 10210 Tor r: Their as-grown microstructures of the two samples are believed to be the same. The functions of the magnetic ®eld are to induce an easy axis in the ®eld direction and at the same time to set exchange bias in the same direction. One of the samples was for the magnetization measurements with a vibrating sample magnetometer, and the other one was patterned to be a wire with four current±voltage probes by using photolithography and ion beam etching for accurate measurements of resistance. The width of the wire is 125 mm and the length of the wire
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00427-6
460
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
Fig. 1. The dependence of magnetization (a) and resistance (b) as a function of magnetic ®eld for the annealed 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/2 nm Ni81Fe19/10 nm Ta.
between two voltage probes is 700 mm. The wire is along the direction of the applied magnetic ®eld during ®lm deposition. The patterned and non-patterned samples were then annealed at 2308C for 1 h under a vacuum of 1 £ 1027 Torr with a magnetic ®eld of 1T parallel to the magnetic ®eld applied during ®lm deposition. For such a spin valve structure, annealing can reduce interfacial diffusion resulting in a increasing of giant magnetoresistance (GMR) [11]. Fig. 1a and b shows the dependence of magnetization (M) and resistance (R) of the annealed samples, respectively. Exchange bias in the samples is 395 Oe, magnetoresistance ratio Rmax 2 Rmin =Rmin is 7.2%. The annealed patterned sample was irradiated with a 30 Kev Ga 1 focused ion beam system (FEI corporation 800 THP). The irradiations were performed at room temperature with the ion beam normal to the sample surface. The protective layer of 14 nm Ta is to ensure that the ions did not sputter through to the underlying magnetic ®lm. By using in situ end-point detection and atomic force microscopy (AFM) measurements, we found that a dose of 3:2 £ 1016 ions=cm2 sputtered away the entire 14 nm Ta protective layer. The surface roughness of the ®lms was measured by AFM. The three dimensional AFM images of the samples before and after ion irradiation with the dose of 2:93 £ 1016 ions=cm2 are shown in Fig. 2a and b, corresponding to the root-mean-square roughness Rrms , 0:23 and 0.12 nm,
respectively. The roughness was decreased after ion irradiation. The dose of 2:93 £ 1016 ions=cm2 is the maximum dose we have used in the experiment, therefore, ion induced pinholes in the protective layer should be negligible. The effects of ion irradiation on magnetoresistance (MR) was measured at room temperature, the representative R±H loops being shown in Fig. 3. The resistance was found to increase with increasing ion dose. On the other hand, the exchange bias between the IrMn(6 nm) and CoFe(2 nm) layers was found to decrease with ion doses up to 1:05 £ 1015 ions=cm2 (Fig. 3a and b). The resistance at the ®eld of 1 750 Oe and MR ratio as a function of ion dose are shown in Fig. 4. MR ratio exhibits a much sharper decrease at low doses #1:05 £ 1015 ions=cm2 ; where the resistance exhibits a less sharper increase. This characteristic implies that at low doses the alternation in resistance is mainly attributed to atomic mixing in the interfacial regions induced by Ga 1 ion irradiation, in particular the mixing in the interfaces of the layers above Cu layer, because only 0.21% of Ga 1 ions penetrate to the Cu layer according to computer simulations using the trim program [12]. At high doses $3:51 £ 1015 ions=cm2 ; Ga 1 ion irradiation generates bulk defects which have the signi®cant effect on the much sharper increase of resistance. With increasing ion dose, anisotropic magnetoresistance (AMR) in the free layer, i.e. the CoFe(1 nm)/NiFe(2 nm) layer, was observed at the dose of 3:51 £ 1015 ions=cm2
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
461
Fig. 2. The AFM images of the samples, where (a) is the surface before ion irradiation, its surface roughness Rrms is , 0.23 nm and (b) is after ion irradiation with the dose of 2:93 £ 1016 ions=cm2 ; its surface roughness Rrms is , 0.12 nm.
(Fig. 3b), which shall be attributed to the decrease of GMR to a value comparable to the AMR value. For a spin valve structure, a magnetization reversal in a free layer shall induce both AMR and GMR, however, the AMR effect is usually overwhelmed by a much larger GMR effect, and cannot be observed. For the ion doses larger than 8:42 £ 1015 ions=cm2 ; the AMR effect disappears. The shapes of these R±H loops are similar to that of a typical GMR structure, but the MR ratio is only ,0.12%. The vanish of the AMR can be attributed to the transition from ferromagnetic to paramagnetic in the free layer, due to the reduction of Curie temperature by ion irradiation [4,6]. However, the presence of exchange bias and GMR-type R±H curves, which ®rst exhibited in the sample with ion dose of 5:39 £ 1014 ions=cm2 and became more and more evident with increasing ion dose, can be attributed to the presence of a few ferromagnetic and/or superparamagnetic components of spin valve structures in the irradiated samples. Inset of Fig. 4 shows the dependence of the exchange bias of the ferromagnetic and/or superparamagnetic spin valve structures as a function of ion dose. With increasing ion dose the
Fig. 3. The representative results of R±H loops after 30 KeV Ga 1 ion irradiation on the patterned sample.
exchange bias is showing a decreasing tendency accompanied by a few ¯uctuations. Experimentally, it has been observed that exchange bias is ®rst increased and then decreased compared to its initial strength [8], which demonstrate that the ion irradiation can induce not only pinning
462
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
Acknowledgements The authors thank Dr G.C. Han, Dr Y.K. Zheng, and Z.Y. Liu for their assistance on some of the experiments.
References
Fig. 4. The dependence of the resistance at the ®eld of 1750 Oe and MR ratio as a function of ion dose. Inset: the dependence of exchange bias as a function of ion dose at high doses.
effect but also interfacial mixing. The former shall normally result in an increase of exchange bias, however, the latter has an opposite effect on the exchange bias. Therefore, there exist competitive behaviors for ion irradiation on exchange bias. In summary, the sample with structure of 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/2 nm Ni81Fe19/10 nm Ta/thermally oxided Si (100) substrate has been patterned to be a wire with four current±voltage probes. Ga 1 ion irradiation on the patterned sample was performed by focused ion beam. The surface roughness has been found to be improved by ion irradiation. With increasing ion dose, MR ratio and exchange bias have been found to be decreasing, and resistance has been found to be increasing. At the dose of 3:51 £ 1015 ions=cm2 both GMR and AMR behaviors have been observed. As increasing ion dose, the magnetoresistance understood by the presence of a few ferromagnetic and/or superparamagnetic spin-valve structures has been observed.
[1] D. Weller, J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Toney, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [2] L.M. Wang, S.X. Wang, R.C. Ewing, A. Meldrum, R.C. Birtcher, P.N. Provencio, W.J. Weber, J. Matzke, Mater. Sci. Engng, A 286 (2000) 72. [3] T. Veres, M. Cai, S. Germain, M. Rouabhi, F. Schiettekatte, S. Roorda, R.W. Cochrane, J. Appl. Phys. 87 (2000) 8513. [4] W.M. Kaminsky, G.A.C. Jones, N.K. Patel, W.E. Booij, M.G. Blamire, S.M. Gardiner, Y.B. Xu, J.A.C. Bland, Appl. Phys. Lett. 78 (2001) 1589. [5] C. Chappert, H. Bernas, J. FerreÂ, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, H. Launois, Science 280 (1998) 1919. [6] B.D. Terris, D. Weller, L. Folks, J.E.E. Baglin, A.J. Kellock, H. Rothuizen, P. Vettiger, J. Appl. Phys. 87 (2000) 7004. [7] C. Vieu, J. Gierak, H. Launois, T. Aign, P. Meyer, J.P. Jamet, J. FerreÂ, C. Chappert, V. Mathet, H. Bernas, Microelectron. Engng 53 (2000) 191. [8] A. Mougin, T. Mewes, M. Jung, D. Engel, A. Ehresmann, H. Schmoranzer, J. Fassbender, B. Hillebrands, Phys. Rev. B 63 (2001) 060409(R). [9] D.M. Kelly, I.K. Schuller, V. Korenivski, K.V. Rao, K.K. Larsen, J. Bottiger, E.M. Gyorgy, R.B. van Dover, Phys. Rev. B 50 (1994) 3481. [10] J.G. Lin, M.R. Wu, D.H. Ngu, C.Y. Huang, S. Mao, J. Magn. Magn. Mater. 209 (2000) 128. [11] A.M. Zeltser, K. PeÂntek, M. MenyhaÂrd, A. Sulyok, IEEE Trans. Magn. 34 (1998) 1417. [12] Ion ranges were calculated using trim, J.P. Biersack, L. Haggmark. Nucl. Instrum. Methods 174 (1980) pp. 257. (trim program for PCs available from J.F. Ziegler, [email protected]).
Solid State Communications 120 (2001) 459±462
www.elsevier.com/locate/ssc
In¯uence of Ga 1 ion irradiation on magnetoresistance and exchange bias of IrMn/CoFe/Cu/CoFe/NiFe spin valve Z.B. Guo a,*, D. You a, J.J. Qiu a, K.B. Li a, Y.H. Wu a,b b
a Data Storage Institute, 5 Engineering Drive 1, National University of Singapore, Singapore 117608 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 119260
Received 16 July 2001; received in revised form 11 September 2001; accepted 26 September 2001 by H. Akai
Abstract The spin valve with the structure of IrMn/CoFe/Cu/CoFe/NiFe has been patterned to be a wire with four current±voltage probes. The detailed study of the in¯uence of 30 KeV focused Ga 1 ion beam irradiation on the magnetoresistance and exchange bias on the patterned sample has been carried out. With an increase in the ion dose, magnetoresistance and exchange bias have been found to be decreasing, and resistance has been found to be increasing. At low doses #1:05 £ 1015 ions=cm2 ; the alternation in resistance is mainly attributed to atomic mixing in the interfacial regions induced by Ga 1 ion irradiation. However, at high doses $3:51 £ 1015 ions=cm2 ; the bulk defects generated by Ga 1 ion irradiation have a signi®cant effect on the increase of resistance. At the dose of 3.51 £ 10 15 ions/cm 2 both GMR and AMR behaviors have been observed. q 2001 Published by Elsevier Science Ltd. PACS: 75.70.Pa; 61.80.Jh Keywords: A. Magnetic ®lms and multilayers; B. Irradiation effects
Ion irradiation of magnetic multilayer structures is an excellent tool to modify their magnetic and electrical behaviors without modifying their topography. Depending on the ion mass, its energy, and the dosage, ion-irradiation induces displacement cascades resulting in atomic diffusion, the modi®cation of crystal structures, and the alteration of grain size [1±4]. Recently, ion irradiation has been used for the nano-scale patterning of CoPt multilayers, in which irradiation-induced reductions of the magnetic anisotropy have been observed. Such patterned magnetic ®lms are of great interest for application in high density magnetic recording [5±7]. On the other hand, ion irradiation has been used for the studies of magnetization and transport properties in relation to interfacial magnetization in ferromagnetic/antiferromagnetic bilayers and ferromagnetic/ non-magnetic multilayers. Local manipulation of the exchange bias has been achieved in FeNi/FeMn bilayers by He ion irradiation under an external magnetic ®eld [8]. Increases in magnetoresistance have been achieved in Fe/Cr multilayers by Xe ion irradiation [9]. * Corresponding author. Tel.: 165-8748200; fax: 165-7771349. E-mail address: [email protected] (Z.B. Guo).
The effects of Ni ion irradiation on a spin valve structure have been reported by Lin et al. [10]. Magnetoresistance has been found to be decreased after irradiation. In this paper, we report on Ga ion irradiation experiments on the top spin valve structure of IrMn/CoFe/Cu/CoFe/NiFe, the penetration depth of 30 KeV Ga ions is much less than that of 8 MeV Ni ions used by Lin et al. [10]. Two 1 inch £ 1 inch samples with the structure of 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/ 2 nm Ni81Fe19/10 nm Ta/thermally oxided Si(100) substrate were prepared simultaneously at room temperature by helicon sputtering deposition under a magnetic ®eld of 100 Oe. The base pressure of the system is 5 £ 10210 Tor r: Their as-grown microstructures of the two samples are believed to be the same. The functions of the magnetic ®eld are to induce an easy axis in the ®eld direction and at the same time to set exchange bias in the same direction. One of the samples was for the magnetization measurements with a vibrating sample magnetometer, and the other one was patterned to be a wire with four current±voltage probes by using photolithography and ion beam etching for accurate measurements of resistance. The width of the wire is 125 mm and the length of the wire
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00427-6
460
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
Fig. 1. The dependence of magnetization (a) and resistance (b) as a function of magnetic ®eld for the annealed 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/2 nm Ni81Fe19/10 nm Ta.
between two voltage probes is 700 mm. The wire is along the direction of the applied magnetic ®eld during ®lm deposition. The patterned and non-patterned samples were then annealed at 2308C for 1 h under a vacuum of 1 £ 1027 Torr with a magnetic ®eld of 1T parallel to the magnetic ®eld applied during ®lm deposition. For such a spin valve structure, annealing can reduce interfacial diffusion resulting in a increasing of giant magnetoresistance (GMR) [11]. Fig. 1a and b shows the dependence of magnetization (M) and resistance (R) of the annealed samples, respectively. Exchange bias in the samples is 395 Oe, magnetoresistance ratio Rmax 2 Rmin =Rmin is 7.2%. The annealed patterned sample was irradiated with a 30 Kev Ga 1 focused ion beam system (FEI corporation 800 THP). The irradiations were performed at room temperature with the ion beam normal to the sample surface. The protective layer of 14 nm Ta is to ensure that the ions did not sputter through to the underlying magnetic ®lm. By using in situ end-point detection and atomic force microscopy (AFM) measurements, we found that a dose of 3:2 £ 1016 ions=cm2 sputtered away the entire 14 nm Ta protective layer. The surface roughness of the ®lms was measured by AFM. The three dimensional AFM images of the samples before and after ion irradiation with the dose of 2:93 £ 1016 ions=cm2 are shown in Fig. 2a and b, corresponding to the root-mean-square roughness Rrms , 0:23 and 0.12 nm,
respectively. The roughness was decreased after ion irradiation. The dose of 2:93 £ 1016 ions=cm2 is the maximum dose we have used in the experiment, therefore, ion induced pinholes in the protective layer should be negligible. The effects of ion irradiation on magnetoresistance (MR) was measured at room temperature, the representative R±H loops being shown in Fig. 3. The resistance was found to increase with increasing ion dose. On the other hand, the exchange bias between the IrMn(6 nm) and CoFe(2 nm) layers was found to decrease with ion doses up to 1:05 £ 1015 ions=cm2 (Fig. 3a and b). The resistance at the ®eld of 1 750 Oe and MR ratio as a function of ion dose are shown in Fig. 4. MR ratio exhibits a much sharper decrease at low doses #1:05 £ 1015 ions=cm2 ; where the resistance exhibits a less sharper increase. This characteristic implies that at low doses the alternation in resistance is mainly attributed to atomic mixing in the interfacial regions induced by Ga 1 ion irradiation, in particular the mixing in the interfaces of the layers above Cu layer, because only 0.21% of Ga 1 ions penetrate to the Cu layer according to computer simulations using the trim program [12]. At high doses $3:51 £ 1015 ions=cm2 ; Ga 1 ion irradiation generates bulk defects which have the signi®cant effect on the much sharper increase of resistance. With increasing ion dose, anisotropic magnetoresistance (AMR) in the free layer, i.e. the CoFe(1 nm)/NiFe(2 nm) layer, was observed at the dose of 3:51 £ 1015 ions=cm2
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
461
Fig. 2. The AFM images of the samples, where (a) is the surface before ion irradiation, its surface roughness Rrms is , 0.23 nm and (b) is after ion irradiation with the dose of 2:93 £ 1016 ions=cm2 ; its surface roughness Rrms is , 0.12 nm.
(Fig. 3b), which shall be attributed to the decrease of GMR to a value comparable to the AMR value. For a spin valve structure, a magnetization reversal in a free layer shall induce both AMR and GMR, however, the AMR effect is usually overwhelmed by a much larger GMR effect, and cannot be observed. For the ion doses larger than 8:42 £ 1015 ions=cm2 ; the AMR effect disappears. The shapes of these R±H loops are similar to that of a typical GMR structure, but the MR ratio is only ,0.12%. The vanish of the AMR can be attributed to the transition from ferromagnetic to paramagnetic in the free layer, due to the reduction of Curie temperature by ion irradiation [4,6]. However, the presence of exchange bias and GMR-type R±H curves, which ®rst exhibited in the sample with ion dose of 5:39 £ 1014 ions=cm2 and became more and more evident with increasing ion dose, can be attributed to the presence of a few ferromagnetic and/or superparamagnetic components of spin valve structures in the irradiated samples. Inset of Fig. 4 shows the dependence of the exchange bias of the ferromagnetic and/or superparamagnetic spin valve structures as a function of ion dose. With increasing ion dose the
Fig. 3. The representative results of R±H loops after 30 KeV Ga 1 ion irradiation on the patterned sample.
exchange bias is showing a decreasing tendency accompanied by a few ¯uctuations. Experimentally, it has been observed that exchange bias is ®rst increased and then decreased compared to its initial strength [8], which demonstrate that the ion irradiation can induce not only pinning
462
Z.B. Guo et al. / Solid State Communications 120 (2001) 459±462
Acknowledgements The authors thank Dr G.C. Han, Dr Y.K. Zheng, and Z.Y. Liu for their assistance on some of the experiments.
References
Fig. 4. The dependence of the resistance at the ®eld of 1750 Oe and MR ratio as a function of ion dose. Inset: the dependence of exchange bias as a function of ion dose at high doses.
effect but also interfacial mixing. The former shall normally result in an increase of exchange bias, however, the latter has an opposite effect on the exchange bias. Therefore, there exist competitive behaviors for ion irradiation on exchange bias. In summary, the sample with structure of 14 nm Ta /6 nm Ir20Mn80/2 nm Co75Fe25/2.2 nm Cu/1 nm Co75Fe25/2 nm Ni81Fe19/10 nm Ta/thermally oxided Si (100) substrate has been patterned to be a wire with four current±voltage probes. Ga 1 ion irradiation on the patterned sample was performed by focused ion beam. The surface roughness has been found to be improved by ion irradiation. With increasing ion dose, MR ratio and exchange bias have been found to be decreasing, and resistance has been found to be increasing. At the dose of 3:51 £ 1015 ions=cm2 both GMR and AMR behaviors have been observed. As increasing ion dose, the magnetoresistance understood by the presence of a few ferromagnetic and/or superparamagnetic spin-valve structures has been observed.
[1] D. Weller, J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Toney, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [2] L.M. Wang, S.X. Wang, R.C. Ewing, A. Meldrum, R.C. Birtcher, P.N. Provencio, W.J. Weber, J. Matzke, Mater. Sci. Engng, A 286 (2000) 72. [3] T. Veres, M. Cai, S. Germain, M. Rouabhi, F. Schiettekatte, S. Roorda, R.W. Cochrane, J. Appl. Phys. 87 (2000) 8513. [4] W.M. Kaminsky, G.A.C. Jones, N.K. Patel, W.E. Booij, M.G. Blamire, S.M. Gardiner, Y.B. Xu, J.A.C. Bland, Appl. Phys. Lett. 78 (2001) 1589. [5] C. Chappert, H. Bernas, J. FerreÂ, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, H. Launois, Science 280 (1998) 1919. [6] B.D. Terris, D. Weller, L. Folks, J.E.E. Baglin, A.J. Kellock, H. Rothuizen, P. Vettiger, J. Appl. Phys. 87 (2000) 7004. [7] C. Vieu, J. Gierak, H. Launois, T. Aign, P. Meyer, J.P. Jamet, J. FerreÂ, C. Chappert, V. Mathet, H. Bernas, Microelectron. Engng 53 (2000) 191. [8] A. Mougin, T. Mewes, M. Jung, D. Engel, A. Ehresmann, H. Schmoranzer, J. Fassbender, B. Hillebrands, Phys. Rev. B 63 (2001) 060409(R). [9] D.M. Kelly, I.K. Schuller, V. Korenivski, K.V. Rao, K.K. Larsen, J. Bottiger, E.M. Gyorgy, R.B. van Dover, Phys. Rev. B 50 (1994) 3481. [10] J.G. Lin, M.R. Wu, D.H. Ngu, C.Y. Huang, S. Mao, J. Magn. Magn. Mater. 209 (2000) 128. [11] A.M. Zeltser, K. PeÂntek, M. MenyhaÂrd, A. Sulyok, IEEE Trans. Magn. 34 (1998) 1417. [12] Ion ranges were calculated using trim, J.P. Biersack, L. Haggmark. Nucl. Instrum. Methods 174 (1980) pp. 257. (trim program for PCs available from J.F. Ziegler, [email protected]).