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Magnetic modifications of thin CoFe films induced by Xe+-ion irradiation
Nuclear Instruments and Methods in Physics Research B 216 (2004) 350–354 www.elsevier.com/locate/nimb
Magnetic modifications of thin CoFe films induced by Xeþ-ion irradiation Ratnesh Gupta
*,1,
G.A. M€ uller, P. Schaaf, K. Zhang, K.P. Lieb
II. Physikalisches Institut and SFB 602, Universit€at G€ottingen, Bunsenstrasse 7-9, D-37073 G€ottingen, Germany
Abstract We have investigated magnetic field assisted implantations of 200 keV Xe-ions in 30–90 nm thick Fe50 Co50 alloy films on Si substrates at ion fluences of up to 1.3 · 1016 cm2 . Longitudinal magneto-optic kerr effect measurements at room temperature suggest that the coercivity of the films first decreases for increasing ion fluence, while the magnetic remanence develops as a superposition of fourfold and twofold uniaxial anisotropies. Higher Xe-ion fluences increase the density of pinning centers therefore causing slight increases of the coercivity and of the magnetostrictive anisotropy. Lattice expansion and ion-induced grain growth were measured parallel to MOKE by X-ray diffraction. 2003 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 75.30.Gw; 75.70.Ak; 75.70.)I Keywords: Ion implantation; Magnetic anisotropy; Thin film; Permendur; Magneto-optical kerr effect
1. Introduction Permendur (Fe50 Co50 ) is a commercially known alloy used for technological applications, in which soft magnetic materials with high permeability, high saturation magnetization and high Curie temperature are required [1]. Since its saturation magnetostriction coefficient is very large (kavg ¼ 6:5 106 polycrystalline films), considerable nonuniform, local magnetostrictive anisotropies may arise. When growing Fe50 Co50 on different substrates such as Ag, Cu, Au/MgO or
* Corresponding author. Tel.: +49-551-397-628; fax: +49551-394-433. E-mail address: [email protected] (R. Gupta). 1 On leave from Institute of Instrumentation, D.A. University, Khandwa Road, Indore 452017, India.
CoO, reduced coercitivities have been reported [2– 5]. TEM measurements on FeCo films deposited on CoO suggested that this reduction in coercivity is due to the decrease in grain size [4]. Similar conclusions have been drawn for FeCo films grown on Cu, showing for the remanence an inplane uniaxial anisotropy [5]. Soft magnetic properties of Fex Co1x alloy films of different compositions x have been studied after depositing them on various ferromagnetic alloys [6]. Ion-beam implantation has emerged as a promising tool for tailoring magnetic properties of thin magnetic films and multilayers [7–10]. Chappert et al. introduced the concept of helium ion irradiation to modify the magnetic properties of Co/Pt ultra-thin films [7]. The prerequisite condition for precise tuning in this system was to induce local short-range atomic displacements in the collision cascades. In the present work, we report on
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.060
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
changes of the structural and magnetic properties in Xe-ion irradiated Fe50 Co50 alloy films. This alloy has been chosen because the magnetic moment according to the Slater–Pauling curve is highest among all the 3d ferromagnetic alloys. Furthermore, as its magnetostriction constant is high, ioninduced defects leading to slight lattice dilatations may cause considerable magnetic texturing effects. Fex Co1x alloys are thermodynamically stable in the bcc structure for x ¼ 0:25–1.0 [11].
351
Magnetic hysteresis loops were measured at room temperature by MOKE with the linearly polarized beam of a He–Ne laser (k ¼ 632 nm) in the longitudinal configuration, which probes the in-plane magnetization. The samples were mounted in a MOKE goniometer with 1 precision and a maximum field of 1.5 kOe. The field direction was fixed and the long axis of the specimen was considered as reference line for the MOKE angular scans.
3. Results and discussion 2. Experimental details
100
0.8
80
0.7
60
0.6
20
40
60
80
total width (deg.)
The in-plane hysteresis curves for different thickness of permendur films measured after deposition show an increasing coercivity for increasing film thickness (see Fig. 1). The MOKE angular scans of the as-deposited films suggest a small in-plane anisotropy in the 30 nm thick specimen, but isotropy in other films. Fig. 1 depicts the coercivity in comparison with the total width of the (1 1 0) peak (in the XRD spectra) as a function of the film thickness. The coercivity is increasing with the film thickness up to about 60 nm and then reaches saturation. The corresponding variation of the total line width of the (1 1 0) peak with film thickness suggests that the increase in coercivity is due to an increase in crystalline grain size. No account was taken for any possible inhomogeneous lattice strains, which also would contribute to line broadening. Each grain boundary
Coercivity (Oe)
Permendur films with different thickness, typically 30–90 nm, were deposited by e-beam on Si(1 0 0) wafers. The material was produced by arc melting the proper amounts of both metals. The base pressure in the chamber was maintained at 1 · 108 mbar. 200 keV Xe-ions were implanted at room temperature and at fluences of (1– 10) · 1015 cm2 . This ion energy was chosen to adjust the projected ion range Rp of about 30 nm, according to TRIM95 [12] calculations, far from the metal/Si interface, in order to avoid ion-beam mixing effects. In the case of Xe-irradiated Ni films, we had observed that the maximum magnetic texturing effect occurred when the projected range was about half the film thickness [10]. The ion flux was maintained at about 1 lA/cm2 , in order to avoid beam heating. An XY beam sweeping system was used for homogeneous implantation over the full sample area of 7 · 10 mm2 . The irradiations were performed in the presence of an external magnetic field of 104 Oe, which was oriented in-plane along the short axis of the samples. In order to monitor the film thickness and sputtering and interface mixing effects, Rutherford backscattering spectroscopy (RBS) with 900 keV He2þ ions was performed, using two Si surface barrier detectors with about 12 keV energy resolution, positioned at scattering angles of ±165 relative to the beam axis. The ion implantations and subsequent RBS analyses of the samples were performed by means of the IONAS implanter [13]. Most samples were analyzed by X-ray diffraction in glancing angle (incidence angle ¼ 3) geometry.
100
Thickness (nm) Fig. 1. Thickness dependence of the coercivity (MOKE) and total peak width of the bcc (1 1 0) peak (XRD) of FeCo films on Si(1 0 0) at room temperature.
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
[110]
Asdep
Intensity (arb.unit)
1E15 1E16
42
44
46
2θ (deg.) Fig. 3. Glancing angle XRD scan of the (1 1 0) peak for asdeposited sample and after irradiations with 1 · 1015 and 1 · 1016 Xe-ions/cm2 .
Fig. 4 shows the coercivity of the hard axis obtained for the different Xe-ion fluences: it dramatically decreases at the low fluences up to 1 · 1015 cm2 and afterwards increases slightly. The coercivity and remanence are extrinsic magnetic properties, which are very sensitive to local structural properties of the films such as defects, grain size and strain. Thus, the fast decrease of the coercivity at the low fluences suggests that the ion
(d)
0.03
0.4
0.02
80
(c)
0.01
0.3
(b)
0.00
0
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400
Depth (1015 at./cm2)
600
60
0.2
(d) (c)
40
0.1
20
(b) (a)
400
600
800
Energy (keV) Fig. 2. RBS experimental and simulated spectra of FeCo films, (a) as-deposited and at Xe-ion fluences of (b) 1 · 1015 ions/cm2 , (c) 5 · 1015 ions/cm2 and (d) 10 · 1015 ions/cm2 . The inset shows the corresponding Xe depth profiles.
Coercivity (Oe)
Yield (arb. unit)
Xe Concentration
may act as local barrier, which enhances the coercivity of the relatively thicker films. Freeland et al. have suggested that the average grain size is an important parameter to determine the coercive field of thin films [14]. RBS spectra taken before and after implanting different ion fluences are presented in Fig. 2. Simulations performed with the RUMP code [15] indicate that the thickness of the FeCo films, being 73(1) nm after deposition, changed very little during the ion implantations, as consequence of surface sputtering. Furthermore, no effects of ionbeam mixing at the FeCo/Si interface were identified. The glancing angle XRD patterns shown in Fig. 3 confirm the polycrystalline bcc structure of the films. The lattice constant of the as-deposited film was deduced as 0.2842(2) nm, which value is slightly lower than that of Fe50 Co50 alloy bulk material, 0.2855(1) nm [16]. The Xe-ion fluence of 1 · 1015 cm2 results in an increase of the lattice constant 0.2860(2) nm and of the crystalline grain size 16 nm, as obtained by using the Scherrer equation. The value of lattice constant have been estimated on the basis of three main bcc peaks of FeCo. This trend continues for higher ion fluences: after implanting 1 · 1016 ions/cm2 , the lattice constant has increased to 0.2867(2) nm and the crystallite size to 18 nm.
∆ (Mr/MS)
352
0.0 0
4
8
12
0
Ion fluence (1015 ions/cm2) Fig. 4. Fluence dependent coercivity value of hard axis (right hand side) and total change in the remanance ratio (left hand side) as defined in the text.
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
implantation induces a stress relaxation in the films, correlated to an increase in the lattice constant value as evidenced by the XRD patterns. The energy transferred to the film during ion irradiation may affect the local atomic arrangement and improve the crystal structure. On the other hand, as the average magnetostriction constant of this alloy is quite high [1], a small change of the stress may lead to significant changes of the coercivity. At higher fluences, a different mechanism appears to be more important, the increasing density of pinning centers arising from the increasing number of Xe-ions stored in the film. To understand how Xe-ion irradiation affects the intrinsic magnetic behaviour, we measured with MOKE the angle-dependent hysteresis curves. Fig. 5 shows the angular modulation of the relative remanence Mr =Ms , Ms being the saturation magnetization, before and after ion irradiations at various fluences. These diagrams first demonstrate that the in-plane easy axis of the specimen is aligned parallel to the external magnetic field direction present during the implantations. For small ion fluences, one finds a superposition of a fourfold symmetric pattern, which is characteristic for the bcc FeCo structure and a twofold uniaxial anisotropy. The difference in the value of the rel-
90
90 1.0 135
45
(a)
0.9
135
0
0.9
(b)
0.8 180
0
0.9 225
1.0
225
315 1.0
270 90
1.0 135
45
0.8
315 270 90
1.0
(c)
0.6
135
45
ative remanence Mr =Ms between the most easy and hard axis, labelled DðMr =Ms Þ, is plotted in Fig. 4 as function of the Xe fluence. Fig. 4 clearly shows that the variations of the coercivity and of the quantity DðMr =Ms Þ are correlated. After implanting 1 · 1015 or 5 · 1015 Xe-ions/cm2 into the film, a pronounced fourfold symmetry is generated, leading to a rapid increase of DðMr =Ms Þ. Magnetic anisotropies depend fundamentally on the symmetry and the form of local interactions. The total contributions come from magneto-crystalline, magneto-elastic and isotropic anisotropy which contains demagnetising field and stresses in the film. The surface roughness is normally increasing by the ion fluences though it is not visible from the corresponding RBS spectra and it may also play a role to increase the demagnetising field factor. Thus, a decrease in DðMr =Ms Þ for the higher fluences of Xe-ions may probably be due to isotropic stress present due to the higher density of pinning centers. Thus, the strong dependence of the DðMr =Ms Þ on ion fluence suggest that it may happen due to an interplay between cubic anisotropy, uniaxial anisotropy and an induced stress component. In conclusion, we have demonstrated that magnetic field assisted ion beam implantation is an efficient technique to modify the extrinsic magnetic properties of FeCo alloy thin films and to engineer the grain size, magnetic anisotropy and coercivity. The magnetization direction is being aligned by a small external field during implantation. Xe-ion implantation at small fluences appears to reduce the internal stress produced during film deposition, while increasing ion fluences enhance the magnetic anisotropy. Similar results have been obtained for Xe-irradiated polycrystalline Ni films of similar thickness [10,17,18].
(d)
0.8 0.6
180
0
180
0.6
0.6
0.8
0.8 225
1.0
45
0.9
0.8 180
Mr / MS
1.0
353
315 270
0
Acknowledgements 225
1.0
315 270
Fig. 5. Polar diagram of remanence magnetization ratio for ion fluences (a) as-deposited, (b) 5 · 1014 ions/cm2 , (c) 1 · 1015 ions/ cm2 and (d) 5 · 1015 ions/cm2 . Note the offset of the Mr =MS scale.
It is a pleasure to thank D. Purschke and A. K€ onig for their help with the ion implantations, M. Kanbach is greatfully acknowledged for the preparation of alloy. This work has been funded by Deutsche Forschungsgemeinschaft.
354
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
References [1] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, Reading, MA, 1972, p. 529. [2] T.A. Lafford, M.R.J. Gibbs, R. Zuberek, C. Shearwood, J. Appl. Phys. 76 (1994) 6534. [3] T. Thomson, P.C. Riedi, C.L. Platt, A.E. Berkowitz, IEEE Trans. Magn. 34 (1998) 1045. [4] C.L. Platt, A.E. Berkowitz, D.J. Smith, M.R. McCartney, J. Appl. Phys. 88 (2000) 2058. [5] H.S. Jung, W.D. Doyle, J.E. Wittig, J.F. Al-Sahrab, J. Bentley, Appl. Phys. Lett. 81 (2002) 2415. [6] H. Katada, T. Shimatsu, I. Watanabe, H. Muraoka, Y. Nakamura, IEEE Trans. Magn. 38 (2002) 2225. [7] C. Chappert, H. Bernas, J. Ferre, V. Kottler, J.-P. Jamet, Y. Chen, E. Cambril, T. Devolder, F. Rousseaux, V. Mathet, H. Launois, Science 280 (1998) 1919. [8] G.A. M€ uller, R. Gupta, K.P. Lieb, P. Schaaf, Appl. Phys. Lett. 82 (2003) 73. [9] R. Gupta, K.P. Lieb, Y. Luo, G.A. M€ uller, P. Schaaf, K. Zhang, J. Magn. Magn. Mat., submitted for publication.
[10] K. Zhang, K.P. Lieb, G.A. M€ uller, P. Schaaf, M. Uhrmacher, M. M€ unzenberg, Phys. Rev. B, submitted for publication. [11] T. Nishizawa, K. Ishida, Bull. Alloy Phase Daigr. 5 (1984) 250. [12] J.F. Ziegeler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1985. [13] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke, K.P. Lieb, Nucl. Instr. and Meth. B 9 (1995) 234. [14] J.W. Freeland, K. Bussmann, Y.U. Idzerda, Appl. Phys. Lett. 76 (2000) 2603. [15] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [16] W.C. Ellis, E.S. Greiner, Trans. ASM 29 (1941) 415. [17] K.P. Lieb, K. Zhang, G.A. M€ uller, P. Schaaf, M. Uhrmacher, W. Felsch, M. M€ unzenberg, Acta Phys. Pol. A 100 (2001) 751. [18] K. Zhang, K.P. Lieb, P. Schaaf, M. Uhrmacher, W. Felsch, M. M€ unzenberg, Nucl. Instr. and Meth. B 161–163 (2000) 1020.
Magnetic modifications of thin CoFe films induced by Xeþ-ion irradiation Ratnesh Gupta
*,1,
G.A. M€ uller, P. Schaaf, K. Zhang, K.P. Lieb
II. Physikalisches Institut and SFB 602, Universit€at G€ottingen, Bunsenstrasse 7-9, D-37073 G€ottingen, Germany
Abstract We have investigated magnetic field assisted implantations of 200 keV Xe-ions in 30–90 nm thick Fe50 Co50 alloy films on Si substrates at ion fluences of up to 1.3 · 1016 cm2 . Longitudinal magneto-optic kerr effect measurements at room temperature suggest that the coercivity of the films first decreases for increasing ion fluence, while the magnetic remanence develops as a superposition of fourfold and twofold uniaxial anisotropies. Higher Xe-ion fluences increase the density of pinning centers therefore causing slight increases of the coercivity and of the magnetostrictive anisotropy. Lattice expansion and ion-induced grain growth were measured parallel to MOKE by X-ray diffraction. 2003 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 75.30.Gw; 75.70.Ak; 75.70.)I Keywords: Ion implantation; Magnetic anisotropy; Thin film; Permendur; Magneto-optical kerr effect
1. Introduction Permendur (Fe50 Co50 ) is a commercially known alloy used for technological applications, in which soft magnetic materials with high permeability, high saturation magnetization and high Curie temperature are required [1]. Since its saturation magnetostriction coefficient is very large (kavg ¼ 6:5 106 polycrystalline films), considerable nonuniform, local magnetostrictive anisotropies may arise. When growing Fe50 Co50 on different substrates such as Ag, Cu, Au/MgO or
* Corresponding author. Tel.: +49-551-397-628; fax: +49551-394-433. E-mail address: [email protected] (R. Gupta). 1 On leave from Institute of Instrumentation, D.A. University, Khandwa Road, Indore 452017, India.
CoO, reduced coercitivities have been reported [2– 5]. TEM measurements on FeCo films deposited on CoO suggested that this reduction in coercivity is due to the decrease in grain size [4]. Similar conclusions have been drawn for FeCo films grown on Cu, showing for the remanence an inplane uniaxial anisotropy [5]. Soft magnetic properties of Fex Co1x alloy films of different compositions x have been studied after depositing them on various ferromagnetic alloys [6]. Ion-beam implantation has emerged as a promising tool for tailoring magnetic properties of thin magnetic films and multilayers [7–10]. Chappert et al. introduced the concept of helium ion irradiation to modify the magnetic properties of Co/Pt ultra-thin films [7]. The prerequisite condition for precise tuning in this system was to induce local short-range atomic displacements in the collision cascades. In the present work, we report on
0168-583X/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2003.11.060
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
changes of the structural and magnetic properties in Xe-ion irradiated Fe50 Co50 alloy films. This alloy has been chosen because the magnetic moment according to the Slater–Pauling curve is highest among all the 3d ferromagnetic alloys. Furthermore, as its magnetostriction constant is high, ioninduced defects leading to slight lattice dilatations may cause considerable magnetic texturing effects. Fex Co1x alloys are thermodynamically stable in the bcc structure for x ¼ 0:25–1.0 [11].
351
Magnetic hysteresis loops were measured at room temperature by MOKE with the linearly polarized beam of a He–Ne laser (k ¼ 632 nm) in the longitudinal configuration, which probes the in-plane magnetization. The samples were mounted in a MOKE goniometer with 1 precision and a maximum field of 1.5 kOe. The field direction was fixed and the long axis of the specimen was considered as reference line for the MOKE angular scans.
3. Results and discussion 2. Experimental details
100
0.8
80
0.7
60
0.6
20
40
60
80
total width (deg.)
The in-plane hysteresis curves for different thickness of permendur films measured after deposition show an increasing coercivity for increasing film thickness (see Fig. 1). The MOKE angular scans of the as-deposited films suggest a small in-plane anisotropy in the 30 nm thick specimen, but isotropy in other films. Fig. 1 depicts the coercivity in comparison with the total width of the (1 1 0) peak (in the XRD spectra) as a function of the film thickness. The coercivity is increasing with the film thickness up to about 60 nm and then reaches saturation. The corresponding variation of the total line width of the (1 1 0) peak with film thickness suggests that the increase in coercivity is due to an increase in crystalline grain size. No account was taken for any possible inhomogeneous lattice strains, which also would contribute to line broadening. Each grain boundary
Coercivity (Oe)
Permendur films with different thickness, typically 30–90 nm, were deposited by e-beam on Si(1 0 0) wafers. The material was produced by arc melting the proper amounts of both metals. The base pressure in the chamber was maintained at 1 · 108 mbar. 200 keV Xe-ions were implanted at room temperature and at fluences of (1– 10) · 1015 cm2 . This ion energy was chosen to adjust the projected ion range Rp of about 30 nm, according to TRIM95 [12] calculations, far from the metal/Si interface, in order to avoid ion-beam mixing effects. In the case of Xe-irradiated Ni films, we had observed that the maximum magnetic texturing effect occurred when the projected range was about half the film thickness [10]. The ion flux was maintained at about 1 lA/cm2 , in order to avoid beam heating. An XY beam sweeping system was used for homogeneous implantation over the full sample area of 7 · 10 mm2 . The irradiations were performed in the presence of an external magnetic field of 104 Oe, which was oriented in-plane along the short axis of the samples. In order to monitor the film thickness and sputtering and interface mixing effects, Rutherford backscattering spectroscopy (RBS) with 900 keV He2þ ions was performed, using two Si surface barrier detectors with about 12 keV energy resolution, positioned at scattering angles of ±165 relative to the beam axis. The ion implantations and subsequent RBS analyses of the samples were performed by means of the IONAS implanter [13]. Most samples were analyzed by X-ray diffraction in glancing angle (incidence angle ¼ 3) geometry.
100
Thickness (nm) Fig. 1. Thickness dependence of the coercivity (MOKE) and total peak width of the bcc (1 1 0) peak (XRD) of FeCo films on Si(1 0 0) at room temperature.
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
[110]
Asdep
Intensity (arb.unit)
1E15 1E16
42
44
46
2θ (deg.) Fig. 3. Glancing angle XRD scan of the (1 1 0) peak for asdeposited sample and after irradiations with 1 · 1015 and 1 · 1016 Xe-ions/cm2 .
Fig. 4 shows the coercivity of the hard axis obtained for the different Xe-ion fluences: it dramatically decreases at the low fluences up to 1 · 1015 cm2 and afterwards increases slightly. The coercivity and remanence are extrinsic magnetic properties, which are very sensitive to local structural properties of the films such as defects, grain size and strain. Thus, the fast decrease of the coercivity at the low fluences suggests that the ion
(d)
0.03
0.4
0.02
80
(c)
0.01
0.3
(b)
0.00
0
200
400
Depth (1015 at./cm2)
600
60
0.2
(d) (c)
40
0.1
20
(b) (a)
400
600
800
Energy (keV) Fig. 2. RBS experimental and simulated spectra of FeCo films, (a) as-deposited and at Xe-ion fluences of (b) 1 · 1015 ions/cm2 , (c) 5 · 1015 ions/cm2 and (d) 10 · 1015 ions/cm2 . The inset shows the corresponding Xe depth profiles.
Coercivity (Oe)
Yield (arb. unit)
Xe Concentration
may act as local barrier, which enhances the coercivity of the relatively thicker films. Freeland et al. have suggested that the average grain size is an important parameter to determine the coercive field of thin films [14]. RBS spectra taken before and after implanting different ion fluences are presented in Fig. 2. Simulations performed with the RUMP code [15] indicate that the thickness of the FeCo films, being 73(1) nm after deposition, changed very little during the ion implantations, as consequence of surface sputtering. Furthermore, no effects of ionbeam mixing at the FeCo/Si interface were identified. The glancing angle XRD patterns shown in Fig. 3 confirm the polycrystalline bcc structure of the films. The lattice constant of the as-deposited film was deduced as 0.2842(2) nm, which value is slightly lower than that of Fe50 Co50 alloy bulk material, 0.2855(1) nm [16]. The Xe-ion fluence of 1 · 1015 cm2 results in an increase of the lattice constant 0.2860(2) nm and of the crystalline grain size 16 nm, as obtained by using the Scherrer equation. The value of lattice constant have been estimated on the basis of three main bcc peaks of FeCo. This trend continues for higher ion fluences: after implanting 1 · 1016 ions/cm2 , the lattice constant has increased to 0.2867(2) nm and the crystallite size to 18 nm.
∆ (Mr/MS)
352
0.0 0
4
8
12
0
Ion fluence (1015 ions/cm2) Fig. 4. Fluence dependent coercivity value of hard axis (right hand side) and total change in the remanance ratio (left hand side) as defined in the text.
R. Gupta et al. / Nucl. Instr. and Meth. in Phys. Res. B 216 (2004) 350–354
implantation induces a stress relaxation in the films, correlated to an increase in the lattice constant value as evidenced by the XRD patterns. The energy transferred to the film during ion irradiation may affect the local atomic arrangement and improve the crystal structure. On the other hand, as the average magnetostriction constant of this alloy is quite high [1], a small change of the stress may lead to significant changes of the coercivity. At higher fluences, a different mechanism appears to be more important, the increasing density of pinning centers arising from the increasing number of Xe-ions stored in the film. To understand how Xe-ion irradiation affects the intrinsic magnetic behaviour, we measured with MOKE the angle-dependent hysteresis curves. Fig. 5 shows the angular modulation of the relative remanence Mr =Ms , Ms being the saturation magnetization, before and after ion irradiations at various fluences. These diagrams first demonstrate that the in-plane easy axis of the specimen is aligned parallel to the external magnetic field direction present during the implantations. For small ion fluences, one finds a superposition of a fourfold symmetric pattern, which is characteristic for the bcc FeCo structure and a twofold uniaxial anisotropy. The difference in the value of the rel-
90
90 1.0 135
45
(a)
0.9
135
0
0.9
(b)
0.8 180
0
0.9 225
1.0
225
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270 90
1.0 135
45
0.8
315 270 90
1.0
(c)
0.6
135
45
ative remanence Mr =Ms between the most easy and hard axis, labelled DðMr =Ms Þ, is plotted in Fig. 4 as function of the Xe fluence. Fig. 4 clearly shows that the variations of the coercivity and of the quantity DðMr =Ms Þ are correlated. After implanting 1 · 1015 or 5 · 1015 Xe-ions/cm2 into the film, a pronounced fourfold symmetry is generated, leading to a rapid increase of DðMr =Ms Þ. Magnetic anisotropies depend fundamentally on the symmetry and the form of local interactions. The total contributions come from magneto-crystalline, magneto-elastic and isotropic anisotropy which contains demagnetising field and stresses in the film. The surface roughness is normally increasing by the ion fluences though it is not visible from the corresponding RBS spectra and it may also play a role to increase the demagnetising field factor. Thus, a decrease in DðMr =Ms Þ for the higher fluences of Xe-ions may probably be due to isotropic stress present due to the higher density of pinning centers. Thus, the strong dependence of the DðMr =Ms Þ on ion fluence suggest that it may happen due to an interplay between cubic anisotropy, uniaxial anisotropy and an induced stress component. In conclusion, we have demonstrated that magnetic field assisted ion beam implantation is an efficient technique to modify the extrinsic magnetic properties of FeCo alloy thin films and to engineer the grain size, magnetic anisotropy and coercivity. The magnetization direction is being aligned by a small external field during implantation. Xe-ion implantation at small fluences appears to reduce the internal stress produced during film deposition, while increasing ion fluences enhance the magnetic anisotropy. Similar results have been obtained for Xe-irradiated polycrystalline Ni films of similar thickness [10,17,18].
(d)
0.8 0.6
180
0
180
0.6
0.6
0.8
0.8 225
1.0
45
0.9
0.8 180
Mr / MS
1.0
353
315 270
0
Acknowledgements 225
1.0
315 270
Fig. 5. Polar diagram of remanence magnetization ratio for ion fluences (a) as-deposited, (b) 5 · 1014 ions/cm2 , (c) 1 · 1015 ions/ cm2 and (d) 5 · 1015 ions/cm2 . Note the offset of the Mr =MS scale.
It is a pleasure to thank D. Purschke and A. K€ onig for their help with the ion implantations, M. Kanbach is greatfully acknowledged for the preparation of alloy. This work has been funded by Deutsche Forschungsgemeinschaft.
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