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Cr multilayers
Nuclear Instruments and Methods in Physics Research B 156 (1999) 158±161
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Swift heavy ion irradiation eects on magnetoresistance of Fe/Cr multilayers Amitesh Paul
a,*
, Ajay Gupta a, S.M. Chaudhari a, D.M. Phase a, S. Ghosh b, D.K. Avasthi b
a
b
Inter-University Consortium for DAE Facilities, Khandwa Road, Indore 452017, India Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India
Abstract Eect of 200 MeV Ag ion irradiation on giant magnetoresistance in Fe/Cr multilayers has been studied. Irradiation is found to modify the interface structure without aecting the morphological features like grain size or texture. The GMR shows a monotonous decrease with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation ¯uence of 1.0 ´ 1013 ions/cm2 . It is suggested that intermixing at the interfaces along with changes in the interface roughness due to formation of asperities at the surface of the ¯oat glass substrate may be the possible cause of such a large change in GMR with irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 75.70; 42.88; 73.50.J Keywords: Magnetic multilayers; GMR; Ion irradiation; Interface roughness
1. Introduction Metallic multilayers exhibiting Giant magnetoresistance (GMR) have been studied extensively in recent years in order to develop an understanding of the basic processes involved e.g. interlayer magnetic coupling [1,2] and spin dependent electron scattering [3]. In the case of polycrystalline ®lms of Fe/Cr and Co/Cu large variation in the magnitude of GMR with preparation conditions like sputtering gas pressure [4],
* Corresponding author. Tel.: +91-0731-463913; fax: +910731-462294; e-mail: [email protected]
deposition rate, substrate temperature and also with post deposition treatments like thermal annealing [5] and ion irradiation [6] etc. suggests that various structural and morphological parameters like grain size [7], structural defects, [8], interface roughness, interdiusion [9], orientation and texture [10±12] of the ®lm in¯uence signi®cantly the GMR in multilayers. Eect of interface structure on GMR has been studied extensively in order to elucidate the role of interface scattering in GMR [4±6,13]. Kelly et al. irradiated sputtered Fe/Cr multilayers with 500 keV Xe ions with an aim to study the eect of interfacial roughness in a single specimen, when its structure is modi®ed by ion irra-
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 7 5 - X
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
diation. However in addition to the changes in the interface structure, irradiation also caused the structural coherence length to increase with dose, and thus it was not possible to separate the eects of interface roughness alone on GMR. The combined eect of the changes in the interface structure and the morphological changes was to vary the GMR in a non-monotonous manner with dose. Swift heavy ion beams in MeV energy range can be used to produce interfacial modi®cations via electronic energy loss (dE/dx)e unlike that of keV energy range where the mechanism is mainly nuclear energy loss, and the eects of the two types of irradiations may be quite dierent. Therefore, in the present work we have studied the eect of 200 MeV Ag ions irradiation on GMR in Fe/Cr multilayers. The aim of the studies is to elucidate the role of various parameters characterizing the state of interface on the GMR in Fe/Cr multilayers. 2. Experimental details Fe/Cr multilayers of composition (Fe (3.0 nm)/ Cr (1.2 nm)) ´ 20/Fe (5.0 nm) were deposited on a set of ¯oat glass substrates in a UHV chamber at a base pressure of 8.0 ´ 10ÿ10 mbar using e-beam evaporation at a rate of 0.01 nm/s. The temperature of the substrate was not controlled during the evaporation and might have risen by a few tens of degrees above the room temperature. Thicknesses of individual layers were controlled during deposition using a standard quartz crystal oscillator to an accuracy of 0.1 nm. Cr spacer layer thickness of 1.2 nm corresponds to ®rst peak in the antiferromagnetic coupling between Fe layers [4]. Four dierent samples were irradiated with 200 MeV Ag ions upto ¯uences of 5.0 ´ 1011 , 1.0 ´ 1012 , 5.0 ´ 1012 and 1.0 ´ 1013 ions/cm2 using 15 UD Pelletron at Nuclear Science Centre, New Delhi, India. Ion beam was scanned over an area of 10 ´ 10 mm2 by magnetic beam steerer. X-Ray diraction (XRD), X-Ray re¯ectivity (XRR), and Atomic Force microscopy (AFM) were used to characterize the multilayers before and after irradiation. For XRD and XRR measurements a powder X-Ray diractometer model D5000 of Siemens with CuKa radiation was used.
159
Magnetoresistance was measured at room temperature using standard four probe technique with a constant current source and a nanovoltmeter in an external ®eld upto 1 Tesla. 3. Results and discussion XRD measurements showed that the ®lms have a texture along [1 1 0] direction. Irradiation did not cause any change in the ®lm texture. Width of [1 1 0] re¯ection was used to determine the coherency of grains along the momentum transfer vector q using the Scherrer method. Fig. 1 shows the grain size as a function of irradiation dose. It may be noted that the grain size is several times the thickness of individual layers indicating a high degree of coherency between successive Fe and Cr layers. Further, the grain size does not vary appreciably with increasing irradiation dose. The grain size in the ®lm plane, as obtained from AFM measurements, was about 400 nm and did not change with irradiation. Fig. 2 shows the specular and o-specular re¯ectivity patterns of as-deposited and irradiated specimens with ¯uences of (a) 5.0 ´ 1011 (b) 1.0 ´ 1012 (c) 5.0 ´ 1012 and (d) 1.0 ´ 1013 ions/cm2 . The ®rst Bragg peak due to multilayer periodicity is visible in the specular re¯ectivity curves. However beyond the ®rst Bragg peak the re¯ectivity pattern becomes obscure due to strong diuse
Fig. 1. Coherent grain size in Fe/Cr multilayers as calculated from Scherrer method versus irradiation dose of 200 MeV Ag ions.
160
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
Fig. 2. Low angle XRR specular scans ( ± ) along with o-specular scans (± ± ±) of Fe/Cr multilayers before and after irradiation with 200 MeV Ag ions to ¯uences of (a) 5.0 ´ 1011 ions/cm2 ; (b) 1.0 ´ 1012 ions/cm2 ; (c) 5.0 ´ 1012 ions/cm2 and (d) 1.0 ´ 1013 ions/cm2 .
scattering. From the position of the Bragg peak, one can ®nd that the bilayer periodicity is 4.4 0.1 nm. It is seen from the reduced height of the Bragg peak in the case of the sample irradiated with 1.0 ´ 1013 ions/cm2 that the irradiation induces roughening and/or intermixing at the interfaces. Apart from the specular pattern, to have an idea of the nature of interface roughness correlations, ospecular scans were also performed with an oset in the incident angle Dx 0.1° [13]. The presence of a faint Bragg peak in the o-specular scans suggests that there exists a partial correlation between roughnesses of successive interfaces. The irradiation ¯uence dependence of the magnetoresistance of the irradiated specimens is shown in Fig. 3. Magnetoresistance decreases monotonically with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation dose of 1.0 ´ 1013 ions/cm2 . Since there is no change in the grain size or texture with irradiation, the observed change in GMR may be attributed solely to the interface modi®cations. In the earlier study of the eects of 500 keV Xe ion irradiation on Fe/Cr multilayers, GMR was found to increase with ¯uence upto 1.0 ´ 1013 ions/
cm2 followed by a decrease for still higher irradiation ¯uences [6]. In contrast to this, in the present work GMR decreases monotonically with irradiation dose. This qualitatively dierent result may at least partly be attributed to the fact that in the earlier study [6] irradiation resulted in both interface modi®cation as well as morphological changes, e.g., increase in the structural coherence length, and thus, the observed variation in GMR is
Fig. 3. Magnetoresistance of the irradiated specimens as a function of ¯uences of irradiation with 200 MeV Ag ions.
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
a combined eect of interface modi®cation and the morphological changes, while in the present case the variation in GMR is mainly due to interface modi®cations only. Thus, swift heavy ion irradiation allows one to selectively study the eect of interface structure on GMR. Total decrease in GMR after the highest ¯uence of 1.0 ´ 1013 ions/cm2 is about 70%, thus indicating a large change in the interface structure. It may be noted that the electronic energy loss (dE/dx)e for 200 MeV Ag ion as calculated using TRIM95 code are 32.5 and 30.8 keV/nm in Fe and Cr layers, respectively. From the literature one ®nds that the threshold (dE/dx)e value for damage creation in bulk Fe is 50 keV/nm [14]. In thin ®lms this threshold may be reduced to some extent because of the reduced mobility of conduction electrons [15]. Since the (dE/dx)e value is much below the threshold value, one does not expect large modi®cations to occur at the interfaces. However, in the present samples the bombarding particles lose most of their energy in the ¯oat glass substrate and the substrate being nonconducting, electronic energy loss may create extensive damage in the same. Studies have indicated that swift heavy ion irradiation of quartz causes a reduction in the mass density resulting in formation of asperities at the surface [16], on the other hand in amorphous silicon irradiation causes densi®cation resulting in the formation of craters at the surface [17]. In both the cases surface roughness is substantially increased as a result of swift heavy ion irradiation. In the present case, increase in the surface roughness of the glass substrate due to such eects would also cause the roughnesses of the subsequent interfaces to increase and can account for the observed magnitude of the decrease in GMR with irradiation dose. 4. Conclusions Eect of 200 MeV Ag ion irradiation on Fe/Cr multilayers has been studied. The electronic energy loss results in modi®cations at the interfaces, without modifying the morphology of the ®lms, thus allowing one to selectively study the eects of
161
interface modi®cations on GMR. The magnetoresistance decreases monotonically with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation ¯uence of 1.0 ´ 1013 ions/cm2 . Such large change in GMR with irradiation may be a combined eect of (i) the intermixing at the interfaces due to electronic energy loss in the multilayer and (ii) changes in the roughness of the interfaces of Fe and Cr layers as a result of the formation of asperities at the surface of the ¯oat glass substrate. 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] R.Q. Hood, L.M. Falicov, D.R. Penn, Phys. Rev. B 49 (1994) 368. [3] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [4] E.E. Fullerton, D.M. Kelly, J. Guimpel, I.K. Schuller, Y. Bruynserade, Phys. Rev. Lett. 68 (1992) 859. [5] L.H. Laider, B.J. Hickey, T.R.A. Hickey, T.R.A. Hase, B.K. Tanner, R. Schad, Y. Bruynserade, J. Magn. Magn. Mater. 156 (1996) 332. [6] D.M. Kelly, I. van Schuller, V. Koreniviski, K.V. Rao, K.K. Larsen, J. Bottiger, E.M. Gyorgy, R.B. Van Dover, Phys. Rev. B 50 (1994) 3481. [7] S.S.P. Parkin, B.R. York, Appl. Phys. Lett. 62 (1993) 1842. [8] A.R. Modak, David J. Smith, S.S.P. Parkin, Phys. Rev. B 50 (1994) 4232. [9] E.Yu. Tsymbal, D.G. Pettifor, Phys. Rev. B 54 (1996) 15314. [10] J. Kudrnovsk y, V. Drchal, I. Turek, M. Sob, P. Weinberger, Phys. Rev. B 53 (1996) 5152. [11] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers, S.D. Bader, Phys. Rev. B 48 (1993) 15755. [12] J.M. Colino, I.K. Schuller, R. schad, C.D. Potter, P. Belien, G. Verbanck, V.V. Moshchalokov, Y. Bruynserade, Phys. Rev. B 53 (1996) 766. [13] P. Belien, R. Schad, C.D. Potter, G. Verbanck, V.V. Moshchalkov, Y. Bruynserade, Phys. Rev. B 50 (1994) 9957. [14] A. Dunlop, D. Lesueur, P. Legrand, H. Dammak, J. Dural, Nucl. Instr. and Meth. B 90 (1994) 330. [15] A. Gupta, S. Pandita, D.K. Avasthi, G.S. Lodha, R.V. Nandedkar, Nucl. Instr. and Meth. B 146 (1998) 265. [16] I.H. Wilson, J.B. Xu, R.A.B. Devine, R.P. Webb, Nucl. Instr. and Meth. B 118 (1996) 473. [17] A.B. Devine, Nucl. Instr. and Meth. B 91 (1994) 378.
www.elsevier.nl/locate/nimb
Swift heavy ion irradiation eects on magnetoresistance of Fe/Cr multilayers Amitesh Paul
a,*
, Ajay Gupta a, S.M. Chaudhari a, D.M. Phase a, S. Ghosh b, D.K. Avasthi b
a
b
Inter-University Consortium for DAE Facilities, Khandwa Road, Indore 452017, India Nuclear Science Centre, Aruna Asaf Ali Marg, P.O. Box 10502, New Delhi 110067, India
Abstract Eect of 200 MeV Ag ion irradiation on giant magnetoresistance in Fe/Cr multilayers has been studied. Irradiation is found to modify the interface structure without aecting the morphological features like grain size or texture. The GMR shows a monotonous decrease with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation ¯uence of 1.0 ´ 1013 ions/cm2 . It is suggested that intermixing at the interfaces along with changes in the interface roughness due to formation of asperities at the surface of the ¯oat glass substrate may be the possible cause of such a large change in GMR with irradiation. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: 75.70; 42.88; 73.50.J Keywords: Magnetic multilayers; GMR; Ion irradiation; Interface roughness
1. Introduction Metallic multilayers exhibiting Giant magnetoresistance (GMR) have been studied extensively in recent years in order to develop an understanding of the basic processes involved e.g. interlayer magnetic coupling [1,2] and spin dependent electron scattering [3]. In the case of polycrystalline ®lms of Fe/Cr and Co/Cu large variation in the magnitude of GMR with preparation conditions like sputtering gas pressure [4],
* Corresponding author. Tel.: +91-0731-463913; fax: +910731-462294; e-mail: [email protected]
deposition rate, substrate temperature and also with post deposition treatments like thermal annealing [5] and ion irradiation [6] etc. suggests that various structural and morphological parameters like grain size [7], structural defects, [8], interface roughness, interdiusion [9], orientation and texture [10±12] of the ®lm in¯uence signi®cantly the GMR in multilayers. Eect of interface structure on GMR has been studied extensively in order to elucidate the role of interface scattering in GMR [4±6,13]. Kelly et al. irradiated sputtered Fe/Cr multilayers with 500 keV Xe ions with an aim to study the eect of interfacial roughness in a single specimen, when its structure is modi®ed by ion irra-
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 2 7 5 - X
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
diation. However in addition to the changes in the interface structure, irradiation also caused the structural coherence length to increase with dose, and thus it was not possible to separate the eects of interface roughness alone on GMR. The combined eect of the changes in the interface structure and the morphological changes was to vary the GMR in a non-monotonous manner with dose. Swift heavy ion beams in MeV energy range can be used to produce interfacial modi®cations via electronic energy loss (dE/dx)e unlike that of keV energy range where the mechanism is mainly nuclear energy loss, and the eects of the two types of irradiations may be quite dierent. Therefore, in the present work we have studied the eect of 200 MeV Ag ions irradiation on GMR in Fe/Cr multilayers. The aim of the studies is to elucidate the role of various parameters characterizing the state of interface on the GMR in Fe/Cr multilayers. 2. Experimental details Fe/Cr multilayers of composition (Fe (3.0 nm)/ Cr (1.2 nm)) ´ 20/Fe (5.0 nm) were deposited on a set of ¯oat glass substrates in a UHV chamber at a base pressure of 8.0 ´ 10ÿ10 mbar using e-beam evaporation at a rate of 0.01 nm/s. The temperature of the substrate was not controlled during the evaporation and might have risen by a few tens of degrees above the room temperature. Thicknesses of individual layers were controlled during deposition using a standard quartz crystal oscillator to an accuracy of 0.1 nm. Cr spacer layer thickness of 1.2 nm corresponds to ®rst peak in the antiferromagnetic coupling between Fe layers [4]. Four dierent samples were irradiated with 200 MeV Ag ions upto ¯uences of 5.0 ´ 1011 , 1.0 ´ 1012 , 5.0 ´ 1012 and 1.0 ´ 1013 ions/cm2 using 15 UD Pelletron at Nuclear Science Centre, New Delhi, India. Ion beam was scanned over an area of 10 ´ 10 mm2 by magnetic beam steerer. X-Ray diraction (XRD), X-Ray re¯ectivity (XRR), and Atomic Force microscopy (AFM) were used to characterize the multilayers before and after irradiation. For XRD and XRR measurements a powder X-Ray diractometer model D5000 of Siemens with CuKa radiation was used.
159
Magnetoresistance was measured at room temperature using standard four probe technique with a constant current source and a nanovoltmeter in an external ®eld upto 1 Tesla. 3. Results and discussion XRD measurements showed that the ®lms have a texture along [1 1 0] direction. Irradiation did not cause any change in the ®lm texture. Width of [1 1 0] re¯ection was used to determine the coherency of grains along the momentum transfer vector q using the Scherrer method. Fig. 1 shows the grain size as a function of irradiation dose. It may be noted that the grain size is several times the thickness of individual layers indicating a high degree of coherency between successive Fe and Cr layers. Further, the grain size does not vary appreciably with increasing irradiation dose. The grain size in the ®lm plane, as obtained from AFM measurements, was about 400 nm and did not change with irradiation. Fig. 2 shows the specular and o-specular re¯ectivity patterns of as-deposited and irradiated specimens with ¯uences of (a) 5.0 ´ 1011 (b) 1.0 ´ 1012 (c) 5.0 ´ 1012 and (d) 1.0 ´ 1013 ions/cm2 . The ®rst Bragg peak due to multilayer periodicity is visible in the specular re¯ectivity curves. However beyond the ®rst Bragg peak the re¯ectivity pattern becomes obscure due to strong diuse
Fig. 1. Coherent grain size in Fe/Cr multilayers as calculated from Scherrer method versus irradiation dose of 200 MeV Ag ions.
160
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
Fig. 2. Low angle XRR specular scans ( ± ) along with o-specular scans (± ± ±) of Fe/Cr multilayers before and after irradiation with 200 MeV Ag ions to ¯uences of (a) 5.0 ´ 1011 ions/cm2 ; (b) 1.0 ´ 1012 ions/cm2 ; (c) 5.0 ´ 1012 ions/cm2 and (d) 1.0 ´ 1013 ions/cm2 .
scattering. From the position of the Bragg peak, one can ®nd that the bilayer periodicity is 4.4 0.1 nm. It is seen from the reduced height of the Bragg peak in the case of the sample irradiated with 1.0 ´ 1013 ions/cm2 that the irradiation induces roughening and/or intermixing at the interfaces. Apart from the specular pattern, to have an idea of the nature of interface roughness correlations, ospecular scans were also performed with an oset in the incident angle Dx 0.1° [13]. The presence of a faint Bragg peak in the o-specular scans suggests that there exists a partial correlation between roughnesses of successive interfaces. The irradiation ¯uence dependence of the magnetoresistance of the irradiated specimens is shown in Fig. 3. Magnetoresistance decreases monotonically with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation dose of 1.0 ´ 1013 ions/cm2 . Since there is no change in the grain size or texture with irradiation, the observed change in GMR may be attributed solely to the interface modi®cations. In the earlier study of the eects of 500 keV Xe ion irradiation on Fe/Cr multilayers, GMR was found to increase with ¯uence upto 1.0 ´ 1013 ions/
cm2 followed by a decrease for still higher irradiation ¯uences [6]. In contrast to this, in the present work GMR decreases monotonically with irradiation dose. This qualitatively dierent result may at least partly be attributed to the fact that in the earlier study [6] irradiation resulted in both interface modi®cation as well as morphological changes, e.g., increase in the structural coherence length, and thus, the observed variation in GMR is
Fig. 3. Magnetoresistance of the irradiated specimens as a function of ¯uences of irradiation with 200 MeV Ag ions.
A. Paul et al. / Nucl. Instr. and Meth. in Phys. Res. B 156 (1999) 158±161
a combined eect of interface modi®cation and the morphological changes, while in the present case the variation in GMR is mainly due to interface modi®cations only. Thus, swift heavy ion irradiation allows one to selectively study the eect of interface structure on GMR. Total decrease in GMR after the highest ¯uence of 1.0 ´ 1013 ions/cm2 is about 70%, thus indicating a large change in the interface structure. It may be noted that the electronic energy loss (dE/dx)e for 200 MeV Ag ion as calculated using TRIM95 code are 32.5 and 30.8 keV/nm in Fe and Cr layers, respectively. From the literature one ®nds that the threshold (dE/dx)e value for damage creation in bulk Fe is 50 keV/nm [14]. In thin ®lms this threshold may be reduced to some extent because of the reduced mobility of conduction electrons [15]. Since the (dE/dx)e value is much below the threshold value, one does not expect large modi®cations to occur at the interfaces. However, in the present samples the bombarding particles lose most of their energy in the ¯oat glass substrate and the substrate being nonconducting, electronic energy loss may create extensive damage in the same. Studies have indicated that swift heavy ion irradiation of quartz causes a reduction in the mass density resulting in formation of asperities at the surface [16], on the other hand in amorphous silicon irradiation causes densi®cation resulting in the formation of craters at the surface [17]. In both the cases surface roughness is substantially increased as a result of swift heavy ion irradiation. In the present case, increase in the surface roughness of the glass substrate due to such eects would also cause the roughnesses of the subsequent interfaces to increase and can account for the observed magnitude of the decrease in GMR with irradiation dose. 4. Conclusions Eect of 200 MeV Ag ion irradiation on Fe/Cr multilayers has been studied. The electronic energy loss results in modi®cations at the interfaces, without modifying the morphology of the ®lms, thus allowing one to selectively study the eects of
161
interface modi®cations on GMR. The magnetoresistance decreases monotonically with irradiation dose, the maximum decrease in GMR being as large as 70% at an irradiation ¯uence of 1.0 ´ 1013 ions/cm2 . Such large change in GMR with irradiation may be a combined eect of (i) the intermixing at the interfaces due to electronic energy loss in the multilayer and (ii) changes in the roughness of the interfaces of Fe and Cr layers as a result of the formation of asperities at the surface of the ¯oat glass substrate. 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] R.Q. Hood, L.M. Falicov, D.R. Penn, Phys. Rev. B 49 (1994) 368. [3] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [4] E.E. Fullerton, D.M. Kelly, J. Guimpel, I.K. Schuller, Y. Bruynserade, Phys. Rev. Lett. 68 (1992) 859. [5] L.H. Laider, B.J. Hickey, T.R.A. Hickey, T.R.A. Hase, B.K. Tanner, R. Schad, Y. Bruynserade, J. Magn. Magn. Mater. 156 (1996) 332. [6] D.M. Kelly, I. van Schuller, V. Koreniviski, K.V. Rao, K.K. Larsen, J. Bottiger, E.M. Gyorgy, R.B. Van Dover, Phys. Rev. B 50 (1994) 3481. [7] S.S.P. Parkin, B.R. York, Appl. Phys. Lett. 62 (1993) 1842. [8] A.R. Modak, David J. Smith, S.S.P. Parkin, Phys. Rev. B 50 (1994) 4232. [9] E.Yu. Tsymbal, D.G. Pettifor, Phys. Rev. B 54 (1996) 15314. [10] J. Kudrnovsk y, V. Drchal, I. Turek, M. Sob, P. Weinberger, Phys. Rev. B 53 (1996) 5152. [11] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers, S.D. Bader, Phys. Rev. B 48 (1993) 15755. [12] J.M. Colino, I.K. Schuller, R. schad, C.D. Potter, P. Belien, G. Verbanck, V.V. Moshchalokov, Y. Bruynserade, Phys. Rev. B 53 (1996) 766. [13] P. Belien, R. Schad, C.D. Potter, G. Verbanck, V.V. Moshchalkov, Y. Bruynserade, Phys. Rev. B 50 (1994) 9957. [14] A. Dunlop, D. Lesueur, P. Legrand, H. Dammak, J. Dural, Nucl. Instr. and Meth. B 90 (1994) 330. [15] A. Gupta, S. Pandita, D.K. Avasthi, G.S. Lodha, R.V. Nandedkar, Nucl. Instr. and Meth. B 146 (1998) 265. [16] I.H. Wilson, J.B. Xu, R.A.B. Devine, R.P. Webb, Nucl. Instr. and Meth. B 118 (1996) 473. [17] A.B. Devine, Nucl. Instr. and Meth. B 91 (1994) 378.