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High flux 56Fe+ and 57Fe+ implantations for GMR applications
Nuclear Instruments and Methods in Physics Research B 139 (1998) 350±354
High ¯ux
56
Fe and
57
Fe implantations for GMR applications
L.M. Redondo a, C.M. de Jesus b, J.G. Marques b, M.F. da Silva a, J.C. Soares M.M. Pereira de Azevedo c, J.A. Mendes c, M.S. Rogalski c, J.B. Sousa c b
b,*
,
a Instituto Tecnol ogico e Nuclear, E.N. 10, P-2685 Sacav em, Portugal CFNUL, Universidade de Lisboa, Av. Prof. Gama Pinto 2, P-1699 Lisboa Codex, Portugal c IFIMUP, Universidade do Porto, P-4150 Porto, Portugal
Abstract The magnetoresistive behaviour of granular thin ®lms prepared by 56 Fe and 57 Fe ion implantation into Ag thin ®lms with ¯uences up to 8 ´ 1016 at./cm2 is reported. The implantation produced both small and large Fe clusters, with the large clusters being dominant for high ¯uences. A signi®cant magnetoresistive response of the ®lms was obtained for ¯uences above 6 ´ 1016 Fe/cm2 , reaching values of 1±2% at 10 K and low ®elds. Ó 1998 Elsevier Science B.V.
1. Introduction Giant magnetoresistance (GMR) was discovered in magnetic multilayers [1] and then observed in a variety of magnetic nanostructures, multilayers, and cluster-based materials [2]. Granular materials formed by small particles of a ferromagnetic metal dispersed in a nonmagnetic metallic matrix are particularly interesting systems where GMR is observed. The relevant parameters to optimize GMR in these materials are the size and concentration of the magnetic clusters, and their distribution in the matrix [3]. Several nonequilibrium preparation techniques have been used, e.g., sputtering co-deposition [4], melt spinning rapid quenching [5] or splat cooling [6] of two immiscible metals like Fe and Cu. However,
* Corresponding author. Tel.: +351/1/7904987; fax: +351/1/ 7954288; e-mail: soaresjc@al¯.cii.fc.ul.pt.
ion implantation has been overlooked so far. In this work we present ®rst results on the magnetoresistive behavior of granular thin ®lms prepared by Fe ion implantation into Ag ®lms. 2. Experimental details thickness were Ag thin ®lms of 1700±2800 A deposited by evaporation or pulsed laser ablation onto glass and silicon substrates from 99.99 at.% pure Ag. The ®lms were implanted with 57 Fe and 56 Fe ions at ¯uences of 1±8 ´ 1016 at./cm2 and 150±180 keV energies. The implantations were performed using the Danfysik 1090 High Current Ion Implanter of ITN, Sacavem. Fe ions were produced in a CHORDIS (Cold HOt Re¯ex Discharge Ion Source) model 920, using a sputter target. The obtained iron mass spectrum is shown in Fig. 1. Although 57 Fe is much less abundant than 56 Fe (2.2% vs. 91.7%) it was possible to im-
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 1 0 0 7 - 0
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
351
The thicknesses of the Ag ®lms and the retained Fe doses were determined by Rutherford Backscattering Spectroscopy (RBS), using a collimated 2.4 MeV He beam at the 3.1 MV van de Graa accelerator of ITN, Sacavem. The backscattered particles were detected at 140° and 180°, with respect to the beam direction, using silicon surface barrier detectors, located in the standard IBM geometry, with resolutions of 13 and 18 keV, respectively. The local structure of the implanted iron was evidenced by Conversion Electron M ossbauer Spectroscopy (CEMS) and the magnetic properties were measured with a Vibrating Sample Magnetometer (VMS) in the ®eld range of 0±0.5 T at room temperature. Magnetoresistance measurements (MR) were performed from 4 to 300 K with a standard electromagnet (0±1 T), under DC current using a four-point probe technique.
Fig. 1. Fe mass spectrum obtained in the CHORDIS ion source. The additional Cr and Mn peaks are due to sputtering of the steel in the ion source. Currents of 30 lA for 57 Fe and 1250 lA for 56 Fe are routinely obtained in the target chamber.
plant high ¯uences of 57 Fe due to the high current easily achieved with the CHORDIS ion source. Usually, currents of 30 lA 57 Fe and 1250 lA 56 Fe were obtained in the target chamber. The peaks corresponding to 57 Fe and 56 Fe are clearly resolved and it is possible to selectively implant 57 Fe with a contamination of less than 5% of 56 Fe. The additional Cr and Mn peaks shown in the mass spectrum come from the stainless steel in the ion source which is also sputtered.
3. Results and discussion Table 1 shows the nominal and retained doses, implantation energies and thickness of the ®lms as determined by the RBS measurements. The TRIM97 code [7] yields a projected range of 460 for 150 keV Fe ions in Ag, with a standard deA and a sputter coecient of 8 for viation of 230 A, Fe in Ag. Due to sputtering during implantation the thickness of the ®lms decreased about 200 A for each 1 ´ 1016 at./cm2 implanted. The estimated error in the thickness of the implanted ®lms due to the deviation of the density from the bulk material for all ®lms. The sputter coecient is about 70 A for Fe in Ag derived from the dierences in the thicknesses before and after implantation is
Table 1 Nominal and retained doses, implantation energies and thicknesses of the ®lms as determined by the RBS measurements Sample
Dep. thick. (A)
Isotope
Implanted ¯uence (´1016 at./cm2 ) Energy (keV)
Retained ¯uence (´1016 at./cm2 )
FeAg1 FeAg2 FeAg3 FeAg4 FeAg5
2570 1700 2200 2800 2000
57
1 1+5 1+5 6 + 1 + 0.5 8
5 5 6 7
a
Not measured.
Fe Fe+ 57 Fe+ 56 Fe 56 Fe 57
56
Fe 56 Fe
180 150 150 150,100,50 150
a
352
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
12(1), much higher than the value predicted by the TRIM97 code. This can be due to the surface enrichment on Fe due to the implantations. Fig. 2 shows the RBS spectrum taken in the ®lm implanted with 56 Fe at a ¯uence of 8 ´ 1016 at./cm2 and 150 keV energy (FeAg5). Fe is located mainly in the surface region, which can be explained by Fe sputtering in Fe, due to the high ¯uence of the implantation. The retained ¯uence was in all cases about 80% of the nominal implanted value. In order to check the possibility of producing an homogeneous Fe depth pro®le, an implantation with three dierent energies was performed for sample FeAg4, as detailed in Table 1. Contrary to our expectations, the 56 Fe pro®le showed essentially the same depth distribution. This can be explained by the radiation enhanced diusion of Fe to the highly damaged region created by the implantation. In fact, the observed Fe pro®le is in good agreement with the vacancy distribution predicted by TRIM97, suggesting that all Fe goes to the damaged region. Fig. 3 shows CEMS spectra recorded at room temperature using a He/CH4 gas ¯ow detector [8] for the ®lms FeAg1 and FeAg3, where we can separate two dierent states (structural/aggregated) of the implanted Fe. For the ®lm FeAg1 (implanted with 1 ´ 1016 Fe/cm2 ), the singlet with isomer shift IS 0.5 mm/s is likely to be due to individual Fe atoms dissolved in the fcc Ag matrix surrounded by 12 nearest neighbour Ag atoms. The large width indicates that the Fe atoms are sit-
Fig. 2. RBS spectrum taken in the ®lm implanted with 8 ´ 1016 Fe/cm2 (FeAg5) at 150 keV.
Fig. 3. CEMS spectra for the ®lms FeAg1 and FeAg3.
uated in damaged regions of the lattice, in agreement with the RBS results. The additional doublet is assigned to Fe atoms which have at least one Fe atom amongst their 12 nearest neighbours. The presence of both Fe and Ag in this shell destroys the cubic symmetry at the central Fe site, resulting in a large quadrupole splitting, QS 1.1 mm/s. In this ®lm we have 83% of Fe atoms dissolved in the Ag matrix (therefore in a non-ferromagnetic state), adding the contributions from the singlet (46%) and the doublet (37%). The singlet near the origin has IS 0.08 mm/s, which is close to the one of metallic Fe and hence is assigned to suciently large Fe clusters where each Fe has 12 Fe nearest neighbours as in the bcc structure of a-Fe. It is reasonable to assume that the expected six-line pattern is not seen here (room temperature measurement) because the Fe clusters exhibit superparamagnetic relaxation and the sixline pattern collapses into a singlet. This metallic phase has a relative intensity of 17%. A detailed discussion of the temperature and magnetic ®eld dependence of the MR for this ¯uence was recently made [9].
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
When the ¯uence was increased to 6 ´ 1016 Fe/ cm (®lm FeAg3) the presence of isolated Fe atoms is slightly diminished as evidenced by the amplitude of the singlet at IS 0.44 mm/s. The dominant contribution is now given by the singlets assigned to large Fe clusters (relative amplitude of 49%), while the contribution of the small clusters decreased to 13%. The CEMS spectrum for the sample FeAg2 (not shown here) is identical to the one for the sample FeAg3. After these initial CEMS experiments which showed the formation of Fe clusters only 56 Fe was used in the ®lms implanted with higher ¯uences. Fig. 4 shows the VSM results obtained at room temperature for the ®lms FeAg1 and FeAg4. In the case of the FeAg4 ®lm implanted with a higher ¯uence (7.5 ´ 1016 Fe/cm2 ) there is a slight enhancement of the coercive ®eld HC and a de®nite increase in the squareness of the hysteretic cycle, as is characteristic in ferromagnetic materials. Fig. 5 shows the MR for all ®lms at T 10 K and ®elds up to 1 T. We observe an increase of the MR response for the samples FeAg2, FeAg3 and FeAg4, relatively to the sample FeAg1, which can be mainly attributed to the enhancement of the implanted ¯uence. The concomitant increase in the concentration of the large Fe clusters and the reduction of the distance between them has direct consequence for the GMR eect, which depends on the ratio between the electron mean free path and the distance between clusters. The dierent be-
353
2
Fig. 5. Magnetoresistance for all ®lms at T 10 K and ®elds up to 1 T.
havior of the samples FeAg2 and FeAg3, implanted with the same ¯uence, is not clear and will be further investigated. The best result obtained for the MR is 2% at 10 K and 1 T ®eld. 4. Conclusions First results were reported on the magnetoresistive behaviour of granular thin ®lms prepared by high ¯uence ion implantation of Fe into Ag thin ®lms. The implantation produced both small and large clusters, with the latter being dominant for high ¯uences. The magnetization exhibits ferromagnetic behavior due to the large clusters, added to a superparamagnetic contribution due to the small clusters. Incipient GMR-like behavior was observed for ¯uences above 6 ´ 1016 Fe/cm2 at low ®elds (<1 T) and low temperatures. Further work is in progress to increase the MR response through an increase of the implanted ¯uence and annealing of the samples. Acknowledgements
Fig. 4. Magnetization data for the ®lms FeAg1 and FeAg4.
This work was supported by the PRAXIS XXI Programme, Portugal, through project PRAXIS/3/ 3.1/FIS/21/94 and individual grants (C.M.J., J.G.M., M.M.P.A. and M.S.R.).
354
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petro, P. Etienne, G. Creuzet, A. Friedrich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] B. Doudin, J.-Ph. Ansermet, Europhysics News 28 (1997) 14. [3] B. Dieny, S.R. Teixeira, B. Rodmacq, C. Cowache, S. Aufret, O. Redon, J. Pierre, J. Magn. Magn. Mater. 130 (1994) 197. [4] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745.
[5] J. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982. [6] M.S. Rogalski, M.M. Pereira de Azevedo, J.B. Sousa, J. Magn. Magn. Mater. 163 (1996) L256. [7] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York. [8] I. Bibicu, M.S. Rogalski, Meas. Sci. Tech. 7 (1996) 113. [9] M.M. Pereira de Azevedo, J.A. Mendes, M.S. Rogalski, J.B. Sousa, L.M. Redondo, C.M. de Jesus, J.G. Marques, M.F. da Silva, J.C. Soares, J. Magn. Magn. Mater. 173 (1997) 230.
High ¯ux
56
Fe and
57
Fe implantations for GMR applications
L.M. Redondo a, C.M. de Jesus b, J.G. Marques b, M.F. da Silva a, J.C. Soares M.M. Pereira de Azevedo c, J.A. Mendes c, M.S. Rogalski c, J.B. Sousa c b
b,*
,
a Instituto Tecnol ogico e Nuclear, E.N. 10, P-2685 Sacav em, Portugal CFNUL, Universidade de Lisboa, Av. Prof. Gama Pinto 2, P-1699 Lisboa Codex, Portugal c IFIMUP, Universidade do Porto, P-4150 Porto, Portugal
Abstract The magnetoresistive behaviour of granular thin ®lms prepared by 56 Fe and 57 Fe ion implantation into Ag thin ®lms with ¯uences up to 8 ´ 1016 at./cm2 is reported. The implantation produced both small and large Fe clusters, with the large clusters being dominant for high ¯uences. A signi®cant magnetoresistive response of the ®lms was obtained for ¯uences above 6 ´ 1016 Fe/cm2 , reaching values of 1±2% at 10 K and low ®elds. Ó 1998 Elsevier Science B.V.
1. Introduction Giant magnetoresistance (GMR) was discovered in magnetic multilayers [1] and then observed in a variety of magnetic nanostructures, multilayers, and cluster-based materials [2]. Granular materials formed by small particles of a ferromagnetic metal dispersed in a nonmagnetic metallic matrix are particularly interesting systems where GMR is observed. The relevant parameters to optimize GMR in these materials are the size and concentration of the magnetic clusters, and their distribution in the matrix [3]. Several nonequilibrium preparation techniques have been used, e.g., sputtering co-deposition [4], melt spinning rapid quenching [5] or splat cooling [6] of two immiscible metals like Fe and Cu. However,
* Corresponding author. Tel.: +351/1/7904987; fax: +351/1/ 7954288; e-mail: soaresjc@al¯.cii.fc.ul.pt.
ion implantation has been overlooked so far. In this work we present ®rst results on the magnetoresistive behavior of granular thin ®lms prepared by Fe ion implantation into Ag ®lms. 2. Experimental details thickness were Ag thin ®lms of 1700±2800 A deposited by evaporation or pulsed laser ablation onto glass and silicon substrates from 99.99 at.% pure Ag. The ®lms were implanted with 57 Fe and 56 Fe ions at ¯uences of 1±8 ´ 1016 at./cm2 and 150±180 keV energies. The implantations were performed using the Danfysik 1090 High Current Ion Implanter of ITN, Sacavem. Fe ions were produced in a CHORDIS (Cold HOt Re¯ex Discharge Ion Source) model 920, using a sputter target. The obtained iron mass spectrum is shown in Fig. 1. Although 57 Fe is much less abundant than 56 Fe (2.2% vs. 91.7%) it was possible to im-
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 1 0 0 7 - 0
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
351
The thicknesses of the Ag ®lms and the retained Fe doses were determined by Rutherford Backscattering Spectroscopy (RBS), using a collimated 2.4 MeV He beam at the 3.1 MV van de Graa accelerator of ITN, Sacavem. The backscattered particles were detected at 140° and 180°, with respect to the beam direction, using silicon surface barrier detectors, located in the standard IBM geometry, with resolutions of 13 and 18 keV, respectively. The local structure of the implanted iron was evidenced by Conversion Electron M ossbauer Spectroscopy (CEMS) and the magnetic properties were measured with a Vibrating Sample Magnetometer (VMS) in the ®eld range of 0±0.5 T at room temperature. Magnetoresistance measurements (MR) were performed from 4 to 300 K with a standard electromagnet (0±1 T), under DC current using a four-point probe technique.
Fig. 1. Fe mass spectrum obtained in the CHORDIS ion source. The additional Cr and Mn peaks are due to sputtering of the steel in the ion source. Currents of 30 lA for 57 Fe and 1250 lA for 56 Fe are routinely obtained in the target chamber.
plant high ¯uences of 57 Fe due to the high current easily achieved with the CHORDIS ion source. Usually, currents of 30 lA 57 Fe and 1250 lA 56 Fe were obtained in the target chamber. The peaks corresponding to 57 Fe and 56 Fe are clearly resolved and it is possible to selectively implant 57 Fe with a contamination of less than 5% of 56 Fe. The additional Cr and Mn peaks shown in the mass spectrum come from the stainless steel in the ion source which is also sputtered.
3. Results and discussion Table 1 shows the nominal and retained doses, implantation energies and thickness of the ®lms as determined by the RBS measurements. The TRIM97 code [7] yields a projected range of 460 for 150 keV Fe ions in Ag, with a standard deA and a sputter coecient of 8 for viation of 230 A, Fe in Ag. Due to sputtering during implantation the thickness of the ®lms decreased about 200 A for each 1 ´ 1016 at./cm2 implanted. The estimated error in the thickness of the implanted ®lms due to the deviation of the density from the bulk material for all ®lms. The sputter coecient is about 70 A for Fe in Ag derived from the dierences in the thicknesses before and after implantation is
Table 1 Nominal and retained doses, implantation energies and thicknesses of the ®lms as determined by the RBS measurements Sample
Dep. thick. (A)
Isotope
Implanted ¯uence (´1016 at./cm2 ) Energy (keV)
Retained ¯uence (´1016 at./cm2 )
FeAg1 FeAg2 FeAg3 FeAg4 FeAg5
2570 1700 2200 2800 2000
57
1 1+5 1+5 6 + 1 + 0.5 8
5 5 6 7
a
Not measured.
Fe Fe+ 57 Fe+ 56 Fe 56 Fe 57
56
Fe 56 Fe
180 150 150 150,100,50 150
a
352
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
12(1), much higher than the value predicted by the TRIM97 code. This can be due to the surface enrichment on Fe due to the implantations. Fig. 2 shows the RBS spectrum taken in the ®lm implanted with 56 Fe at a ¯uence of 8 ´ 1016 at./cm2 and 150 keV energy (FeAg5). Fe is located mainly in the surface region, which can be explained by Fe sputtering in Fe, due to the high ¯uence of the implantation. The retained ¯uence was in all cases about 80% of the nominal implanted value. In order to check the possibility of producing an homogeneous Fe depth pro®le, an implantation with three dierent energies was performed for sample FeAg4, as detailed in Table 1. Contrary to our expectations, the 56 Fe pro®le showed essentially the same depth distribution. This can be explained by the radiation enhanced diusion of Fe to the highly damaged region created by the implantation. In fact, the observed Fe pro®le is in good agreement with the vacancy distribution predicted by TRIM97, suggesting that all Fe goes to the damaged region. Fig. 3 shows CEMS spectra recorded at room temperature using a He/CH4 gas ¯ow detector [8] for the ®lms FeAg1 and FeAg3, where we can separate two dierent states (structural/aggregated) of the implanted Fe. For the ®lm FeAg1 (implanted with 1 ´ 1016 Fe/cm2 ), the singlet with isomer shift IS 0.5 mm/s is likely to be due to individual Fe atoms dissolved in the fcc Ag matrix surrounded by 12 nearest neighbour Ag atoms. The large width indicates that the Fe atoms are sit-
Fig. 2. RBS spectrum taken in the ®lm implanted with 8 ´ 1016 Fe/cm2 (FeAg5) at 150 keV.
Fig. 3. CEMS spectra for the ®lms FeAg1 and FeAg3.
uated in damaged regions of the lattice, in agreement with the RBS results. The additional doublet is assigned to Fe atoms which have at least one Fe atom amongst their 12 nearest neighbours. The presence of both Fe and Ag in this shell destroys the cubic symmetry at the central Fe site, resulting in a large quadrupole splitting, QS 1.1 mm/s. In this ®lm we have 83% of Fe atoms dissolved in the Ag matrix (therefore in a non-ferromagnetic state), adding the contributions from the singlet (46%) and the doublet (37%). The singlet near the origin has IS 0.08 mm/s, which is close to the one of metallic Fe and hence is assigned to suciently large Fe clusters where each Fe has 12 Fe nearest neighbours as in the bcc structure of a-Fe. It is reasonable to assume that the expected six-line pattern is not seen here (room temperature measurement) because the Fe clusters exhibit superparamagnetic relaxation and the sixline pattern collapses into a singlet. This metallic phase has a relative intensity of 17%. A detailed discussion of the temperature and magnetic ®eld dependence of the MR for this ¯uence was recently made [9].
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
When the ¯uence was increased to 6 ´ 1016 Fe/ cm (®lm FeAg3) the presence of isolated Fe atoms is slightly diminished as evidenced by the amplitude of the singlet at IS 0.44 mm/s. The dominant contribution is now given by the singlets assigned to large Fe clusters (relative amplitude of 49%), while the contribution of the small clusters decreased to 13%. The CEMS spectrum for the sample FeAg2 (not shown here) is identical to the one for the sample FeAg3. After these initial CEMS experiments which showed the formation of Fe clusters only 56 Fe was used in the ®lms implanted with higher ¯uences. Fig. 4 shows the VSM results obtained at room temperature for the ®lms FeAg1 and FeAg4. In the case of the FeAg4 ®lm implanted with a higher ¯uence (7.5 ´ 1016 Fe/cm2 ) there is a slight enhancement of the coercive ®eld HC and a de®nite increase in the squareness of the hysteretic cycle, as is characteristic in ferromagnetic materials. Fig. 5 shows the MR for all ®lms at T 10 K and ®elds up to 1 T. We observe an increase of the MR response for the samples FeAg2, FeAg3 and FeAg4, relatively to the sample FeAg1, which can be mainly attributed to the enhancement of the implanted ¯uence. The concomitant increase in the concentration of the large Fe clusters and the reduction of the distance between them has direct consequence for the GMR eect, which depends on the ratio between the electron mean free path and the distance between clusters. The dierent be-
353
2
Fig. 5. Magnetoresistance for all ®lms at T 10 K and ®elds up to 1 T.
havior of the samples FeAg2 and FeAg3, implanted with the same ¯uence, is not clear and will be further investigated. The best result obtained for the MR is 2% at 10 K and 1 T ®eld. 4. Conclusions First results were reported on the magnetoresistive behaviour of granular thin ®lms prepared by high ¯uence ion implantation of Fe into Ag thin ®lms. The implantation produced both small and large clusters, with the latter being dominant for high ¯uences. The magnetization exhibits ferromagnetic behavior due to the large clusters, added to a superparamagnetic contribution due to the small clusters. Incipient GMR-like behavior was observed for ¯uences above 6 ´ 1016 Fe/cm2 at low ®elds (<1 T) and low temperatures. Further work is in progress to increase the MR response through an increase of the implanted ¯uence and annealing of the samples. Acknowledgements
Fig. 4. Magnetization data for the ®lms FeAg1 and FeAg4.
This work was supported by the PRAXIS XXI Programme, Portugal, through project PRAXIS/3/ 3.1/FIS/21/94 and individual grants (C.M.J., J.G.M., M.M.P.A. and M.S.R.).
354
L.M. Redondo et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 350±354
References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petro, P. Etienne, G. Creuzet, A. Friedrich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [2] B. Doudin, J.-Ph. Ansermet, Europhysics News 28 (1997) 14. [3] B. Dieny, S.R. Teixeira, B. Rodmacq, C. Cowache, S. Aufret, O. Redon, J. Pierre, J. Magn. Magn. Mater. 130 (1994) 197. [4] A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745.
[5] J. Wang, G. Xiao, Phys. Rev. B 49 (1994) 3982. [6] M.S. Rogalski, M.M. Pereira de Azevedo, J.B. Sousa, J. Magn. Magn. Mater. 163 (1996) L256. [7] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York. [8] I. Bibicu, M.S. Rogalski, Meas. Sci. Tech. 7 (1996) 113. [9] M.M. Pereira de Azevedo, J.A. Mendes, M.S. Rogalski, J.B. Sousa, L.M. Redondo, C.M. de Jesus, J.G. Marques, M.F. da Silva, J.C. Soares, J. Magn. Magn. Mater. 173 (1997) 230.