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Cr epitaxial multilayers using swift heavy ion irradiation
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 244 (2006) 202–205 www.elsevier.com/locate/nimb
Interface modification in Fe/Cr epitaxial multilayers using swift heavy ion irradiation Ajay Gupta *, Dileep Kumar UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India Available online 20 December 2005
Abstract Effect of 100 MeV Au ions irradiation on the interface structure and magneto transport properties of Fe/Cr epitaxial multilayers: ˚ )/[Fe (30 A ˚ )/Cr (12 A ˚ )]20, have been studied. Interface structure is characterized by X-ray reflectivity and X-ray MgO(0 0 1)/Cr (100 A diffuse scattering measurements. In order to enhance the contrast between Fe and Cr layers, measurements are done at the X-ray energy just below the absorption edge of the iron. Diffuse scattering measurements show that the irradiation results in a substantial decrease in the in-plane correlation length of the roughness, while the roughness itself changes only by a small amount. Further, measurements at different values of qz suggest that the variations take place mainly in the uncorrelated part of the roughness. Giant magnetoresistance measurements in the irradiated multilayer suggest that it is mainly affected by the change in root mean square roughness and is influenced only weakly by variation in the in-plane correlation length. 2005 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.47.De; 61.10.Kw; 61.80.Jh Keywords: Magnetic multilayers; Interface roughness; Diffused scattering; GMR
1. Introduction Magnetic multilayers exhibiting GMR like Fe/Cr are important because of their application in magnetic storage devices as well as read–write heads. While the basic theory of GMR in idealized magnetic multilayers is well developed, the effects of various imperfections in real multilayers are not at all properly understood. Effect of interface roughness on giant magneto resistance (GMR) in Fe/Cr multilayers has been a subject of considerable interest. A number of studies have been done in recent years with an aim to study the effect of interface structure on GMR [1– 5]. It has been found that generally the GMR decreases with increasing interface roughness. The observed change in the GMR with roughness can be attributed to two different factors: (1) variation in the interlayer coupling and (2)
*
Corresponding author. E-mail address: [email protected] (A. Gupta).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.021
variation in the spin dependent scattering at the interfaces [6–8]. However, the interface roughness is not the only parameter for characterizing the interface structure in multilayers. The other parameters, which are important for complete characterization of interface structure, are interdiffusion and in-plane correlation length. In case of multilayers, another quantity of interest is the correlation between the height variations of successive interfaces. The total roughness r consists of two components; the correlated roughness rc and the uncorrelated roughness ru ðr2 ¼ r2c þ r2u Þ. As shown in Fig. 1, generally the uncorrelated part of the roughness has a smaller in-plane correlated length nu as compared to that of the correlation part of the roughness nc [9]. In the present work the interface structure of epitaxial Fe/Cr multilayers has been varied by irradiation with swift heavy ions and the resultant effect on the magneto transport properties has been studied. Effect of irradiation on the interface roughness as well as in-plane correlation length of both correlated and uncorrelated part of the
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
203
Z-axis
ξu
Reflectivity (arb. unit)
ξc
10
-1
10
-4
Experimental Fitted
Pristine 10
-7
-10
10
13
2
1x10 ions/cm -13
10
-16
Fig. 1. Schematics of the interface structure in multilayers. Z-axis is along the thickness of the multilayer. nc and nu are the in-plane correlation length of correlated and uncorrelated roughness, respectively.
roughness has been studied using X-ray reflectivity and X-ray diffuse scattering. 2. Experimental Fe/Cr multilayers having the structure: MgO(0 0 1) sub˚ )/[Fe (30 A ˚ )/Cr (12 A ˚ )]20 were prepared strate/Cr (100 A using electron beam evaporation in a UHV chamber. The base pressure in the chamber was better than ˚ Cr buffer layer has been depos5 · 109 mbar. The 100 A ited in order to provide a lattice matched template for the subsequent growth of the film [10]. Film was irradiated with 100 MeV Au ions using 15 UD pelletron of Nuclear Science Centre, New Delhi, with a fluence of 1 · 1013 ions/cm2. Interface structure of the multilayers was characterized using X-ray reflectivity (which gives information about the layer thicknesses and rms interface roughness) and X-ray diffuse scattering (which gives information about the in-plane correlation length), using ID32 beamline of ESRF, Grenoble, France. It may be noted that the electron density in Fe and Cr layers are very close to each other and therefore, in general, X-ray contrast between these two layers is very poor. In order to increase the contrast, measurements were done at energy of 7.111 keV (just below the absorption edge of Fe). Magnetoresistance at room temperature was measured using the standard four-probe technique in an external magnetic field in the plane of the film (CIP geometry). 3. Result and discussion Fig. 2 gives X-ray reflectivity data of the pristine and irradiated multilayer at 7.111 keV energy. Well-defined Bragg peaks up to second order corresponding to the multilayer periodicity are visible due to the large scattering contrast between Fe and Cr at this energy. The reflectivity patterns are fitted using Parratt’s formalism [11] with the bilayer thickness and the average rms interface roughnesses of Fe-on-Cr and Cr-on-Fe interfaces, taken as fitting vari-
10
0.05
0.10
0.15
0.20 0.25 -1 qz (Å)
0.30
0. 35
0. 40
Fig. 2. X-ray reflectivity of the pristine and irradiated (1 · 1013 ions/cm2) samples taken at the K edge of Iron.
ables. It is found that bilayer thicknesses are the same for both pristine and irradiated samples while interface roughness exhibits some variation. The results of fitting are given in Table 1. It may be noted that the roughness of Cr-on-Fe interface (rFe) is significantly more than that of the Fe-onCr interface (rCr) [12]. These results are in accordance with the conversion electron Mo¨ssbauer spectroscopy (CEMS) measurements, which suggest that Cr-on-Fe interface is more diffused than the Fe-on-Cr interface [13,14]. Table 1 also shows that the average interface roughness of both Cr-on-Fe and Fe-on-Cr interfaces increases upon irradiation (1 · 1013 ions/cm2). One may note that irradiation results in only a small increase in the interface roughness. It may be noted that the X-ray reflectivity measurements cannot differentiate between intermixing and interface roughness. Therefore it is possible that the observed increase in the roughness may be due the some intermixing at the interfaces upon irradiation. However in the existing theories in the literature on the effect of interface roughness on interlayer coupling/GMR no differentiation has been made between interface roughness or intermixing [8,15]. X-ray diffuse scattering measurements were done by keeping the scattering angle 2h fixed and varying the angle of incidence x from 0 to 2h. Measurements are reported for two different values of 2h: (1) Corresponding to the position of the first Bragg peak in the specular X-ray reflectivity and (2) at the adjacent minimum in the specular reflectivity. Fig. 3 gives the X-ray diffuse scattering results of both the pristine and irradiated samples. The data has been corrected for the variation in the illuminated volume of the sample with varying angle of incidence by multiplying with a factor sin(x)/sin(h), which has the effect of normalizing the data to constant sample volume [9]. The rocking curve taken at the first Bragg peak shows only a small variation with irradiation (Fig. 3(a)). In contrast to this, the rocking curve taken at the minimum adjacent to the Bragg peak exhibits considerable changes with
204
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
Table 1 Results of the XRR, diffuse scattering and magneto resistance measurements ˚] ˚] ˚ 2] rFe [A rCr [A vc (104) [A Fluence [ions/cm2]
˚ 2] vu (104) [A
GMR [%]
0 1 · 1013
10.1 25.7
11.62 ± 0.01 11.02 ± 0.01
12.0 ± 0.5 13.0 ± 0.5
6.0 ± 0.5 8.0 ± 0.5
15.6 17.8
rFe and rCr are the interface roughness of Cr-on-Fe and Fe-on-Cr interface, respectively, while vc and vu are the variance of diffuse scattering data taken at the first Bragg peak and the adjacent minimum, respectively.
Pristine
(a) 10
5
Irradiated
10 4
10 3
Intensity (arb.unit)
10 2
10 1 10 5
-1.5
-1.0
-0.5
0.0
0.5
(b)
1.0
1.5
Pristine Irradiated
10 4
10 3
10 2
10 1 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-1
qx (Å)
tered intensity would mean a smaller in-plane correlation length. From Table 1, it may be noted that, while the variance vc of diffused scattering data taken at the first Bragg ˚ 2 to peak, shows a small increase (from 15.6 · 104 A 4 ˚ 2 17.8 · 10 A ) upon irradiation, variance vu of the data taken at the adjacent minimum shows a large change (from ˚ 2 to 25.7 · 104 A ˚ 2). Thus the diffused 10.1 · 104 A scattering measurements show that the irradiation results in a substantial decrease in the in-plane correlation length of the uncorrelated part of the roughness, while that of the correlated part remains almost unchanged. This is as per expectation, since modification produced at the interfaces due to irradiation is not expected to be correlated to one interface to the other. Fig. 4 gives that resistance of pristine and irradiated samples as a function of applied external magnetic field in the film plane. The value of magneto resistance is s obtained using the relation: R0RR 100, where R0 and Rs s are the resistance value at zero and saturating magnetic field, respectively and is included in Table 1. It may be noted that influence of 1 · 1013 ions/cm2 results in a decrease in GMR from 11.62% to 11.02%. Thus an increase ˚ , results in a in average interface roughness by 1.5 A decrease in GMR by 0.6%. Present results are consistence with the earlier studies in terms of the dependence of change in GMR on change in roughness where a change ˚ to 12 A ˚ resulted in a change in rms roughness from 6 A in GMR by 2% [2]. However it may be noted that in the earlier studies the interface roughness was achieved
Fig. 3. Diffuse scattering curves at first Bragg peak (a) and at the adjacent minimum (b). The lines are guides to the eye.
Iðqx Þ dqx
may be noted that the larger variance of the diffused scat-
1.00
Pristine 1E13 ions/cm2
0.98 Resistance (arb.unit)
the irradiation fluence (Fig. 3(b)). It may be noted that for 2h corresponding to the Bragg peak, the diffuse scattering has much stronger contribution from the correlated part of the roughness as compared to that from the uncorrelated part, while at 2h values corresponding to a minimum in the X-ray reflectivity, diffuse scattering has stronger contribution from the uncorrelated part of the roughness [9]. Therefore the diffuse scattering measurements show that the irradiation essentially results in variation in the uncorrelated part of the roughness. Perusal of Fig. 3(b) shows that the main qualitative change in the rocking curve is that as a result of irradiation, the intensity near qx = 0 is pushed out to the wings indicating that the in-plane correlation length is decreased. The statistical variance of the diffuse scatterR Iðqx Þq2x dqx ing curve defined as: R , is also given in Table 1. It
0.96 0.94 0.92 0.90 0.88 -800
-400
0
400
800
Magnetic field (kA/m) Fig. 4. In plan resistance verses magnetic field curves of pristine and irradiated Fe/Cr multilayers at room temperature.
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
by simultaneous deposition of films on substrate of different surface roughness. Therefore the in-plane correlation length of the roughness was expected to be the same. On the other hand in the present case, the in-plane correlation length of uncorrelated part also got changed substantially. This suggests that the GMR is mainly affected by the change in rms roughness and is influenced only weakly by variation in the in-plane correlation length. In conclusion, the effect of swift heavy ion irradiation on Fe/Cr epitaxial multilayers has been studied. XRR results show that irradiation results in an increase in the average roughnesses of both interfaces rFe and rCr increases, while diffuse scattering measurements show that it causes a substantial decrease in the in-plane correlation length of uncorrelated part of the roughness, whereas in-plane correlation length of the correlated part changes only by a small amount. GMR measurements suggest that it is mainly affected by the change in root mean square roughness and is influenced only weakly by variation in the in-plane correlation length. Acknowledgement One of the authors Dileep Kumar is thankful to Council for scientific and Industrial research, New Delhi, India, for financial support in the form of senior research fellowship.
205
References [1] M. Kopcewicz, F. Stobiecki, J. Jagielski, B. Szymanski, M. Schmidt, J. Dubowik, J. Kalinowska, J. Appl. Phys. 93 (2003) 5514. [2] A. Gupta, A. Paul, S.M. Chaudhary, D.M. Phase, J. Phys. Soc. Jpn. 69 (2000) 2182. [3] A. Paul, J. Magn. Magn. Mater. 240 (2002) 497. [4] R. Schad, P. Belie¨n, G. Verbanck, C.D. Potter, H. Fischer, S. Lefebvre, M. Bessiere, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. B 57 (1998) 13692. [5] M. Velez, I.K. Schuller, J. Magn. Magn. Mater. 184 (1998) 275. [6] R.Q. Hood, L.M. Falicov, D.R. Penn, Phys. Rev. B 49 (1994) 368. [7] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [8] R. Schad, P. Belie¨n, G. Verbanck, V.V. Moshchalkov, Y. Bruynseraede, H.E. Fischer, S. Lefebvre, B. Bessiere, Phys. Rev. B 59 (1999) 1242. [9] D.E. Savage, J. Kleiner, N. Schimke, Y.H. Phang, T. Jankowski, J. Jacobs, R. Kariotis, M.G. Lagally, J. Appl. Phys. 69 (1991) 1411. [10] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers, S.D. Bader, Appl. Phys. Lett. 63 (1993) 1699. [11] L.G. Parratt, Phys. Rev. 95 (1954) 359. [12] A. Gupta, A. Paul, M. Gupta, C. Meneghini, U. Pietsch, K. Mibu, A. Maddalena, S.D. Toe, G. Principi, J. Magn. Magn. Mater. 272–276 (2004) 1219. [13] T. Shinjo, W. Keune, J. Magn. Magn. Mater. 200 (1999) 598. [14] V. Uzdin, W. Keune, M. Walterfang, J. Magn. Magn. Mater. 240 (2002) 504. [15] H. Ehrenreich, F. Spaepen, Solid State Phys. 56 (2001) 113.
Nuclear Instruments and Methods in Physics Research B 244 (2006) 202–205 www.elsevier.com/locate/nimb
Interface modification in Fe/Cr epitaxial multilayers using swift heavy ion irradiation Ajay Gupta *, Dileep Kumar UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India Available online 20 December 2005
Abstract Effect of 100 MeV Au ions irradiation on the interface structure and magneto transport properties of Fe/Cr epitaxial multilayers: ˚ )/[Fe (30 A ˚ )/Cr (12 A ˚ )]20, have been studied. Interface structure is characterized by X-ray reflectivity and X-ray MgO(0 0 1)/Cr (100 A diffuse scattering measurements. In order to enhance the contrast between Fe and Cr layers, measurements are done at the X-ray energy just below the absorption edge of the iron. Diffuse scattering measurements show that the irradiation results in a substantial decrease in the in-plane correlation length of the roughness, while the roughness itself changes only by a small amount. Further, measurements at different values of qz suggest that the variations take place mainly in the uncorrelated part of the roughness. Giant magnetoresistance measurements in the irradiated multilayer suggest that it is mainly affected by the change in root mean square roughness and is influenced only weakly by variation in the in-plane correlation length. 2005 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 75.47.De; 61.10.Kw; 61.80.Jh Keywords: Magnetic multilayers; Interface roughness; Diffused scattering; GMR
1. Introduction Magnetic multilayers exhibiting GMR like Fe/Cr are important because of their application in magnetic storage devices as well as read–write heads. While the basic theory of GMR in idealized magnetic multilayers is well developed, the effects of various imperfections in real multilayers are not at all properly understood. Effect of interface roughness on giant magneto resistance (GMR) in Fe/Cr multilayers has been a subject of considerable interest. A number of studies have been done in recent years with an aim to study the effect of interface structure on GMR [1– 5]. It has been found that generally the GMR decreases with increasing interface roughness. The observed change in the GMR with roughness can be attributed to two different factors: (1) variation in the interlayer coupling and (2)
*
Corresponding author. E-mail address: [email protected] (A. Gupta).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.11.021
variation in the spin dependent scattering at the interfaces [6–8]. However, the interface roughness is not the only parameter for characterizing the interface structure in multilayers. The other parameters, which are important for complete characterization of interface structure, are interdiffusion and in-plane correlation length. In case of multilayers, another quantity of interest is the correlation between the height variations of successive interfaces. The total roughness r consists of two components; the correlated roughness rc and the uncorrelated roughness ru ðr2 ¼ r2c þ r2u Þ. As shown in Fig. 1, generally the uncorrelated part of the roughness has a smaller in-plane correlated length nu as compared to that of the correlation part of the roughness nc [9]. In the present work the interface structure of epitaxial Fe/Cr multilayers has been varied by irradiation with swift heavy ions and the resultant effect on the magneto transport properties has been studied. Effect of irradiation on the interface roughness as well as in-plane correlation length of both correlated and uncorrelated part of the
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
203
Z-axis
ξu
Reflectivity (arb. unit)
ξc
10
-1
10
-4
Experimental Fitted
Pristine 10
-7
-10
10
13
2
1x10 ions/cm -13
10
-16
Fig. 1. Schematics of the interface structure in multilayers. Z-axis is along the thickness of the multilayer. nc and nu are the in-plane correlation length of correlated and uncorrelated roughness, respectively.
roughness has been studied using X-ray reflectivity and X-ray diffuse scattering. 2. Experimental Fe/Cr multilayers having the structure: MgO(0 0 1) sub˚ )/[Fe (30 A ˚ )/Cr (12 A ˚ )]20 were prepared strate/Cr (100 A using electron beam evaporation in a UHV chamber. The base pressure in the chamber was better than ˚ Cr buffer layer has been depos5 · 109 mbar. The 100 A ited in order to provide a lattice matched template for the subsequent growth of the film [10]. Film was irradiated with 100 MeV Au ions using 15 UD pelletron of Nuclear Science Centre, New Delhi, with a fluence of 1 · 1013 ions/cm2. Interface structure of the multilayers was characterized using X-ray reflectivity (which gives information about the layer thicknesses and rms interface roughness) and X-ray diffuse scattering (which gives information about the in-plane correlation length), using ID32 beamline of ESRF, Grenoble, France. It may be noted that the electron density in Fe and Cr layers are very close to each other and therefore, in general, X-ray contrast between these two layers is very poor. In order to increase the contrast, measurements were done at energy of 7.111 keV (just below the absorption edge of Fe). Magnetoresistance at room temperature was measured using the standard four-probe technique in an external magnetic field in the plane of the film (CIP geometry). 3. Result and discussion Fig. 2 gives X-ray reflectivity data of the pristine and irradiated multilayer at 7.111 keV energy. Well-defined Bragg peaks up to second order corresponding to the multilayer periodicity are visible due to the large scattering contrast between Fe and Cr at this energy. The reflectivity patterns are fitted using Parratt’s formalism [11] with the bilayer thickness and the average rms interface roughnesses of Fe-on-Cr and Cr-on-Fe interfaces, taken as fitting vari-
10
0.05
0.10
0.15
0.20 0.25 -1 qz (Å)
0.30
0. 35
0. 40
Fig. 2. X-ray reflectivity of the pristine and irradiated (1 · 1013 ions/cm2) samples taken at the K edge of Iron.
ables. It is found that bilayer thicknesses are the same for both pristine and irradiated samples while interface roughness exhibits some variation. The results of fitting are given in Table 1. It may be noted that the roughness of Cr-on-Fe interface (rFe) is significantly more than that of the Fe-onCr interface (rCr) [12]. These results are in accordance with the conversion electron Mo¨ssbauer spectroscopy (CEMS) measurements, which suggest that Cr-on-Fe interface is more diffused than the Fe-on-Cr interface [13,14]. Table 1 also shows that the average interface roughness of both Cr-on-Fe and Fe-on-Cr interfaces increases upon irradiation (1 · 1013 ions/cm2). One may note that irradiation results in only a small increase in the interface roughness. It may be noted that the X-ray reflectivity measurements cannot differentiate between intermixing and interface roughness. Therefore it is possible that the observed increase in the roughness may be due the some intermixing at the interfaces upon irradiation. However in the existing theories in the literature on the effect of interface roughness on interlayer coupling/GMR no differentiation has been made between interface roughness or intermixing [8,15]. X-ray diffuse scattering measurements were done by keeping the scattering angle 2h fixed and varying the angle of incidence x from 0 to 2h. Measurements are reported for two different values of 2h: (1) Corresponding to the position of the first Bragg peak in the specular X-ray reflectivity and (2) at the adjacent minimum in the specular reflectivity. Fig. 3 gives the X-ray diffuse scattering results of both the pristine and irradiated samples. The data has been corrected for the variation in the illuminated volume of the sample with varying angle of incidence by multiplying with a factor sin(x)/sin(h), which has the effect of normalizing the data to constant sample volume [9]. The rocking curve taken at the first Bragg peak shows only a small variation with irradiation (Fig. 3(a)). In contrast to this, the rocking curve taken at the minimum adjacent to the Bragg peak exhibits considerable changes with
204
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
Table 1 Results of the XRR, diffuse scattering and magneto resistance measurements ˚] ˚] ˚ 2] rFe [A rCr [A vc (104) [A Fluence [ions/cm2]
˚ 2] vu (104) [A
GMR [%]
0 1 · 1013
10.1 25.7
11.62 ± 0.01 11.02 ± 0.01
12.0 ± 0.5 13.0 ± 0.5
6.0 ± 0.5 8.0 ± 0.5
15.6 17.8
rFe and rCr are the interface roughness of Cr-on-Fe and Fe-on-Cr interface, respectively, while vc and vu are the variance of diffuse scattering data taken at the first Bragg peak and the adjacent minimum, respectively.
Pristine
(a) 10
5
Irradiated
10 4
10 3
Intensity (arb.unit)
10 2
10 1 10 5
-1.5
-1.0
-0.5
0.0
0.5
(b)
1.0
1.5
Pristine Irradiated
10 4
10 3
10 2
10 1 -1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
-1
qx (Å)
tered intensity would mean a smaller in-plane correlation length. From Table 1, it may be noted that, while the variance vc of diffused scattering data taken at the first Bragg ˚ 2 to peak, shows a small increase (from 15.6 · 104 A 4 ˚ 2 17.8 · 10 A ) upon irradiation, variance vu of the data taken at the adjacent minimum shows a large change (from ˚ 2 to 25.7 · 104 A ˚ 2). Thus the diffused 10.1 · 104 A scattering measurements show that the irradiation results in a substantial decrease in the in-plane correlation length of the uncorrelated part of the roughness, while that of the correlated part remains almost unchanged. This is as per expectation, since modification produced at the interfaces due to irradiation is not expected to be correlated to one interface to the other. Fig. 4 gives that resistance of pristine and irradiated samples as a function of applied external magnetic field in the film plane. The value of magneto resistance is s obtained using the relation: R0RR 100, where R0 and Rs s are the resistance value at zero and saturating magnetic field, respectively and is included in Table 1. It may be noted that influence of 1 · 1013 ions/cm2 results in a decrease in GMR from 11.62% to 11.02%. Thus an increase ˚ , results in a in average interface roughness by 1.5 A decrease in GMR by 0.6%. Present results are consistence with the earlier studies in terms of the dependence of change in GMR on change in roughness where a change ˚ to 12 A ˚ resulted in a change in rms roughness from 6 A in GMR by 2% [2]. However it may be noted that in the earlier studies the interface roughness was achieved
Fig. 3. Diffuse scattering curves at first Bragg peak (a) and at the adjacent minimum (b). The lines are guides to the eye.
Iðqx Þ dqx
may be noted that the larger variance of the diffused scat-
1.00
Pristine 1E13 ions/cm2
0.98 Resistance (arb.unit)
the irradiation fluence (Fig. 3(b)). It may be noted that for 2h corresponding to the Bragg peak, the diffuse scattering has much stronger contribution from the correlated part of the roughness as compared to that from the uncorrelated part, while at 2h values corresponding to a minimum in the X-ray reflectivity, diffuse scattering has stronger contribution from the uncorrelated part of the roughness [9]. Therefore the diffuse scattering measurements show that the irradiation essentially results in variation in the uncorrelated part of the roughness. Perusal of Fig. 3(b) shows that the main qualitative change in the rocking curve is that as a result of irradiation, the intensity near qx = 0 is pushed out to the wings indicating that the in-plane correlation length is decreased. The statistical variance of the diffuse scatterR Iðqx Þq2x dqx ing curve defined as: R , is also given in Table 1. It
0.96 0.94 0.92 0.90 0.88 -800
-400
0
400
800
Magnetic field (kA/m) Fig. 4. In plan resistance verses magnetic field curves of pristine and irradiated Fe/Cr multilayers at room temperature.
A. Gupta, D. Kumar / Nucl. Instr. and Meth. in Phys. Res. B 244 (2006) 202–205
by simultaneous deposition of films on substrate of different surface roughness. Therefore the in-plane correlation length of the roughness was expected to be the same. On the other hand in the present case, the in-plane correlation length of uncorrelated part also got changed substantially. This suggests that the GMR is mainly affected by the change in rms roughness and is influenced only weakly by variation in the in-plane correlation length. In conclusion, the effect of swift heavy ion irradiation on Fe/Cr epitaxial multilayers has been studied. XRR results show that irradiation results in an increase in the average roughnesses of both interfaces rFe and rCr increases, while diffuse scattering measurements show that it causes a substantial decrease in the in-plane correlation length of uncorrelated part of the roughness, whereas in-plane correlation length of the correlated part changes only by a small amount. GMR measurements suggest that it is mainly affected by the change in root mean square roughness and is influenced only weakly by variation in the in-plane correlation length. Acknowledgement One of the authors Dileep Kumar is thankful to Council for scientific and Industrial research, New Delhi, India, for financial support in the form of senior research fellowship.
205
References [1] M. Kopcewicz, F. Stobiecki, J. Jagielski, B. Szymanski, M. Schmidt, J. Dubowik, J. Kalinowska, J. Appl. Phys. 93 (2003) 5514. [2] A. Gupta, A. Paul, S.M. Chaudhary, D.M. Phase, J. Phys. Soc. Jpn. 69 (2000) 2182. [3] A. Paul, J. Magn. Magn. Mater. 240 (2002) 497. [4] R. Schad, P. Belie¨n, G. Verbanck, C.D. Potter, H. Fischer, S. Lefebvre, M. Bessiere, V.V. Moshchalkov, Y. Bruynseraede, Phys. Rev. B 57 (1998) 13692. [5] M. Velez, I.K. Schuller, J. Magn. Magn. Mater. 184 (1998) 275. [6] R.Q. Hood, L.M. Falicov, D.R. Penn, Phys. Rev. B 49 (1994) 368. [7] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [8] R. Schad, P. Belie¨n, G. Verbanck, V.V. Moshchalkov, Y. Bruynseraede, H.E. Fischer, S. Lefebvre, B. Bessiere, Phys. Rev. B 59 (1999) 1242. [9] D.E. Savage, J. Kleiner, N. Schimke, Y.H. Phang, T. Jankowski, J. Jacobs, R. Kariotis, M.G. Lagally, J. Appl. Phys. 69 (1991) 1411. [10] E.E. Fullerton, M.J. Conover, J.E. Mattson, C.H. Sowers, S.D. Bader, Appl. Phys. Lett. 63 (1993) 1699. [11] L.G. Parratt, Phys. Rev. 95 (1954) 359. [12] A. Gupta, A. Paul, M. Gupta, C. Meneghini, U. Pietsch, K. Mibu, A. Maddalena, S.D. Toe, G. Principi, J. Magn. Magn. Mater. 272–276 (2004) 1219. [13] T. Shinjo, W. Keune, J. Magn. Magn. Mater. 200 (1999) 598. [14] V. Uzdin, W. Keune, M. Walterfang, J. Magn. Magn. Mater. 240 (2002) 504. [15] H. Ehrenreich, F. Spaepen, Solid State Phys. 56 (2001) 113.