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Stress-induced magnetic anisotropy in Xe-ion-irradiated Ni thin films
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 243 (2006) 51–57 www.elsevier.com/locate/nimb
Stress-induced magnetic anisotropy in Xe-ion-irradiated Ni thin films Kun Zhang
*
II. Physikalisches Institut and Sonderforschungsbereich 602, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany Received 24 February 2005; received in revised form 12 July 2005 Available online 12 September 2005
Abstract Samples consisting of 75 nm Ni films deposited on Si substrates were bent mechanically and irradiated with 200 keV Xe-ions at a dose of 4 · 1014 ions/cm2. Magneto-optical Kerr effect, Rutherford backscattering spectrometry and X-ray diffraction were used to investigate the changes in the magnetic and microstructural properties. Perfect uniaxial magnetic anisotropy was found in the Ni films after irradiation and removal of the samples from the target holder. The magnetic behavior is shown to be very sensitive to the external stress produced in the films. With increasing curvature of the bent samples (2 m1), the easy axis of the magnetic anisotropy rotated in the direction perpendicular to the bending axis, indicating a compressive stress in the films after irradiation and relaxation. 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 68.55.Ln; 75.30.Gw; 75.70.i Keywords: Magnetic anisotropy; Thin film; Ion implantation; Film stress
1. Introduction The macroscopic magnetic properties of ferromagnetic films are mainly dominated by their magnetic anisotropy. Thin films having an anisotropy along a particular direction can be applied as magnetic sensors or switches [1]. Hence, the formation and control of magnetic anisotropy in thin ferromagnetic films is a very interesting topic [2]. The magnetic properties of polycrystalline magnetic materials are sensitive to the microstructural changes induced by mechanical treatment and thermal history [3–8]. Moreover, several recent studies have shown that ion-beam implantation provides effective means for modifying the film properties, e.g. ion beam assisted deposition [9–12] and post-ion implantation [13–16]. The high local energy deposition of implanted heavy ions not only induces a number of dramatic changes in the crystal structure [17– *
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0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.241
20], but also leads to pronounced changes in the magnetization of ferromagnetic films [21–24]. However, the detailed physical mechanism is not well understood, yet. We have previously reported on magnetic anisotropies in thin Fe, Co, Ni and CoFe films observed at low Xeion fluences [14,21,22,25–28]. The present work focuses on investigations of the influence of external stress in thin Ni films during ion irradiation. The motivation was to correlate the magnetic properties with the changes of stress in the films, which are being induced during film deposition, ion-irradiation and/or external mechanical treatment. 2. Experiments Seventy five nanometer polycrystalline Ni-films were deposited in an electron-beam evaporation setup with a base pressure of 1 · 106 mbar at a deposition rate of 0.1 nm/s onto 0.5 mm thick naturally oxidized Si(1 0 0) substrates of 15 · 40 mm2. The substrates were covered by a Cu-mask, having in its centre a rectangular hole of
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
Fig. 1. Schematic illustration of the bending of a sample.
the long axis of the samples. The coercive force, Hc(u), and the relative remanence, Mr/Ms, with Mr being the remanence magnetization and Ms the saturation magnetization, were analyzed. X-ray diffraction (XRD) measurements were carried out to check changes in their crystal structure. Grazing angle measurements detected polycrystalline peaks for both as-deposited and irradiated samples. Measurements based on rocking curves also implied a polycrystalline structure. However, the h2h scan indicated that the Ni(1 1 1)texture dominates, as almost no other reflection was found. 3. Results
Normalized MOKE Signal
Fig. 2 shows the in-plane hysteresis loops measured along the easy and hard axes of the films. The two hysteresis loops of the as-deposited film are almost identical, and no magnetic anisotropy was detected. The coercive field of this sample was about 200 Oe. After irradiation, the samples were relaxed to reestablish their original flat shape, and then analyzed with MOKE. These films revealed magnetic anisotropies (see Fig. 2(b)–(d)). The coercive field was dramatically reduced, having values of 7–16 Oe for the film which was flat during ion-irradiation. The bent sample showed a bigger (smaller) coercive field in its easy (hard) axis than the one which was flat during irradiation. For the film with curvature of 1.8 m1, the coercive field was reduced close to zero along the hard axis, whereas it was 29 Oe in the easy axis direction. Figs. 3 and 4 illustrate the angular modulations of the coercive field and the relative remanence, respectively. The as-deposited film had an isotropic coercive field of 200 Oe 1.0 0.5 0.0 -0.5 -1.0 -1000 -500 0 500 1000 -300 -150 0 150 HMOKE (Oe) HMOKE (Oe)
(a) as-dep. Normalized MOKE Signal
7 · 10 mm2 to define the geometry of the deposited Ni films. After deposition, the samples were clamped onto a Cu target holder (see Fig. 1), by means of two steel bars, intersected by a steel wire. In this fashion the sample was bent, having its bending axis parallel to its short axis and its bending radius adjusted by the distance between the steel bars and the target holder. The bending radius of each sample was measured via a surface profilometer. By mounting the two steel bars parallel to the long axis of the film, it was possible to bend the sample, having the bending axis parallel to the long axis. The bent samples were then irradiated with 200 keV Xe+-ions at a fluence of 4 · 1014 ions/cm2 and a temperature of 100 K. After irradiation, the samples were removed from the target holder and the substrate was allowed to relax to resume its flat shape in which conditions the flat samples were then characterized. The as-deposited sample had a flat shape and no external stress. By bending the sample, a tensile stress r0 with its direction perpendicular to the bending axis was introduced into the film. During Xe-ion irradiation, this tensile stress was reduced, and then the stress in the film (rion) was smaller than r0. After the samples had been removed from the target holder, the Si-substrates resumed their flat shape and produced a compressive stress in the Ni films, having a direction perpendicular to the bending axis. The Xe-ion irradiations of the samples were performed at the Go¨ttingen heavy ion implanter IONAS [29]. With an electrostatic X–Y sweeping system (fx: 120 Hz, fy: 5 Hz) the ions were homogeneously distributed over the film. The Xe ion current was kept at about 0.3 lA in order to avoid heating of the sample during ion irradiation. The values of the projected ion energy and the ion distribution in Ni films were estimated with the SRIM 2003 code [30], and measured by RBS after ion implantation. The ion energy of 200 keV was chosen for the 75 nm Ni films to locate the ion distribution entirely within the film, so that there was no ion-irradiation induced interaction between the film and the substrate. The magnetic characterization concentrated on measuring changes in the coercive field and orientation of the in-plane anisotropy axis. In-plane hysteresis loops were measured at room temperature, using the magneto-optical Kerr effect (MOKE) with a 633 nm laser beam of 1.5 mm beam diameter and at various angles, u, with respect to
300
(b) 1/R=0
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easy axis
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(c) 1/R=0.3 m-1
300 -300 -150 0 150 HMOKE (Oe)
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(d) 1/R=1.8 m-1
Fig. 2. In-plane hysteresis loops for the as-deposited film (a), the irradiated flat film (b) and the bent, irradiated and then relaxed films (c, d) with the magnetic field applied along the easy and hard axes.
Hc (Oe)
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
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(d) 1/R=1.8 m
-1
Fig. 3. The coercive field of the as-deposited film and the 200 keV Xe+ irradiated 75 nm thick Ni/Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
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7–16 6–18
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5–35 1–54
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3.3 0.5
7–16 7–18 6–24 5–29 1–51
0.57–0.98 0.32–0.98 0.15–0.98 0.06–0.98 0.02–0.99
16.5 35.0 56.2 84.5 88.3
the anisotropy (i.e. the angle of maximum remanence) for all the samples. Briefly for all the cases, the bending films rotated their easy axes toward the direction of the stress, and this rotation increased with increasing external stress. At a curvature of 1.8 m1, the easy axis was oriented parallel to the direction of the stress. Since the magnetostriction constant for polycrystalline Ni is negative (k = 3.3 · 105), it is inferred that the stress in the film was compressive after the bent film has been irradiated and then relaxed to resume its original flat shape. The relative remanence increases (decreases) in the direction of the easy (hard) axis with an increasing curvature during irradiation. The amplitude of the angular modulation of the coercive field increased with a increase of the curvature. The changes in the crystal structure of these bent films were determined by XRD. Fig. 5 shows the h2h XRD
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(c) 1/R=0.9 m
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1/R (m1)
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(b) 1/R=0
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Table 1 The maxima and the minima values of the coercive field Hc and the relative remanence Mr/Ms, and the position of the anisotropy axis u0
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0
1000 2θ=44.558°
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(d) 1/R=1.8 m
FWHM=0.649°
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Fig. 4. The relative remanences of the as-deposited film and the 200 keV Xe+ irradiated 75 nm thick Ni/Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
1000
Counts
(see Fig. 3(a)) and an isotropic relative remanence of 0.53 (see Fig. 4(a)). After irradiation the films showed magnetic anisotropy. For the flat film, the maxima of their relative remanence (easy axis) and their coercive field pointed along the same direction, at an angle of 16.5 to the long axis of the film. This orientation is possibly due to ion-irradiation induced stress in the film. In addition to uniaxial anisotropy, the angular modulation of the remanence also showed two ‘‘smaller sharp peaks’’ near the direction of the hard axis in the flat film or the films with a very small curvature during irradiation (see Fig. 4(b)), but disappeared in the films with large curvature (see Fig. 4(d)). For the latter films, the maxima of the coercive field were located in the direction between the easy and the hard axis. Table 1 summarizes the extreme values of the coercive field Hc and the relative remanence Mr/Ms, and the symmetry angle u0 of
(a) as deposition
2000
(b) 1/R=0 2θ=44.502°
FWHM=0.412°
0 3000 2000 1000
-1
(c) 1/R=0.28 m 2θ=44.475°
FWHM=0.478°
0 3000 2000 1000
(d) 1/R=0.92 m 2θ=44.452°
-1
FWHM=0.492° Ni(200)
0
3000 2000
1000 2θ =44.434° 0 40
(e) 1/R=1.79 m
Ni(111)
-1
FWHM=0.521° 45
50
55
2θ (°) Fig. 5. XRD-spectra of the 200 keV Xe+ irradiated 75 nm thick Ni/ Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
stresses in such thin films: the stress produced during film deposition; the stress induced by ion irradiation; and the stress introduced by mechanical treatment. For the experiments on ion-beam induced magnetic properties, the use of stress-free films and stress-defined films is important. Some experimental results are discussed in this section.
patterns for these films with their bending axis parallel to the short axis during irradiation. For the as-deposited film, only Ni(1 1 1) texture was observed. After irradiation, a much smaller peak of Ni(2 0 0) texture was detected, too. For the flat film, the 2h-position of the Ni(1 1 1) peak moved slightly toward the smaller angle after irradiation. This shift indicates an expansion of the lattice in the normal direction, which was probably induced by the introduction of Xe-atoms in the Ni lattice. On the other hand, the irradiation yields a much sharper peak of Ni(1 1 1) reflection. Two possible reasons for this observation are the irradiation-induced grain growth and the decrease in the microstrain of the film. When the film was bent during irradiation and then relaxed after irradiation, the shift of the Ni(1 1 1) peak was bigger than that measured in the flat film. This extra shift reveals an additional expansion of the lattice constant in the direction normal to the film, which was apparently induced by a contraction of the lattice in the film plane. This means that, after the bent Ni-film had been irradiated and relaxed to resume its original flat shape, a compressive stress in the film plane was generated, which increased the FWHM of the Ni(1 1 1) peak. The shift of the 2h-position of the Ni(1 1 1) peak increased with rising 1/R, and so did its width.
4.1. Coupling of uniaxial and fourfold anisotropies The angular dependences of the relative remanence in the ion-irradiated Ni films have two small sharp peaks if there is little or no external stress during irradiation (see Fig. 4(b)). This phenomenon can be interpreted in terms of the coupling of a uniaxial anisotropy Ku and an effective fourfold anisotropy K eff 1 (see Fig. 6). The uniaxial anisotropy is induced by stress, including the irradiation-induced stress and the external stress, while the fourfold term is a result of magnetocrystalline anisotropy. The magnetic energy density can be described with the following function: 1 E ¼ K u sin2 u þ K eff sin2 2u. 4 1
ð1Þ
If Ku is smaller than K eff 1 , ‘‘two small peaks’’ of the remanence magnetization are expected. If the Ku is slightly bigger than K eff 1 , in a small region around the maximum value of the magnetic energy density the magnetization is situated in an eventual metastable non-equilibrium state, in which the magnetization is just perpendicular to the easy axis of the uniaxial anisotropy, and there is no force to turn the direction of the magnetization toward the easy axis. Therefore, in this situation, the film has a big component of magnetization in the applied field direction, even when the external magnetic field is removed. If the induced uniaxial anisotropy Ku is very big, the effective fourfold anisot-
4. Discussion and summary The experimental results obtained in the present work have revealed that stress can be an important factor in the ion-irradiation induced modification of the magnetic properties of thin Ni films. This finding indicates a way for future experiments to more completely understand ion-beam induced magnetic anisotropy. For this purpose, it is important to distinguish the different origins of the
Energy density
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Fig. 6. Illustration of the free energy density and the corresponding remanent magnetization for the film with the coupling of uniaxial and effective fourfold anisotropies.
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
ropy K eff 1 can be ignored. In this situation there is no metastable non-equilibrium state in the magnetic energy density so that no ‘‘small peaks’’ can be observed in the angular dependence of the remanence magnetization. For this reason, the ‘‘two small peaks’’ can be observed in our measurements only in flat Ni films or in bent films with a smaller curvature.
sidered to be located around the easy axis and around the hard axis, respectively. Around the hard axis, the film has a magnetic behavior similar to that of a single domain, and coherent rotation magnetization contributes to the coercive field. The in-plane hysteresis loops, and thus the coercive field, can be calculated with extreme magnetic energy conditions to obtain the equilibrium state [32]. One obtains
4.2. Domain nucleation versus rotation in stressed films Hc ¼ If the film remains flat during irradiation, the maximum of the coercive field occurs in the direction along the easy axis and the minimum occurs along the hard axis. However, for the bent films, the maximum of the coercive field is located in the position between easy and hard axis. The angular modulation also shows a different form. This difference indicates different magnetization processes in these films. To estimate the coercivity dependence on the azimuthal angle, a simple relation is used to fit the coercive field of a flat film [31]: H c ¼ H a cos2 ðu u0 Þ þ ½H 2cw H 2a cos2 ðu u0 Þ sin2 ðu u0 Þ1=2 ;
ð2Þ
where both the magnetization rotation and the domain wall motion are considered to contribute to the magnetization reversal, and the applied field H must overcome the component of the magnetic anisotropy field in the applied field direction, H a cosðu u0 Þ, and the domain wall pinning coercivity, Hcw. Fig. 7(a) shows the angular dependence of Hc for the film being flat during irradiation. For bent films, the hysteresis curves in all measured directions exhibit the discontinuity jump only at Hc, so that the magnetization process must be dominated either by domain nucleation or by coherent rotation. Therefore, the two different magnetization features in bent films are con-
20 (a) 1/R = 0 15 10
0 60
Ku sin 2ðu u0 Þ; Ms
Hc ¼
H c;e . cosðu u0 Þ
ð4Þ
Here Hc,e is the coercive field along the easy axis. The measured coercive field Hc for the film bent during irradiation is replotted in Fig. 7(b) with its fitting curve derived using Eqs. (3) and (4). 4.3. Separation of ion-induced and external stresses The anisotropy field Ha can be obtained by measuring the saturation field along the direction of the hard axis. Fig. 8 shows that the anisotropy field increases with increasing external stress. It should be noted that, at a given value of the curvature 1/R, the film whose bending axis is parallel to its short axis has a bigger anisotropy field than the film with its bending axis parallel to the long axis. This phenomenon is due to the ion-induced magnetic anisotropy, which occurs close to the long axis of the film. If a film was strongly bent during irradiation, the effective fourfold anisotropy can be ignored due to stronger
Experiment Fitting (b) 1/R = 1.8 m
ð3Þ
where Ku and Ms are the uniaxial anisotropy constant and the saturated magnetization, respectively. Around the easy axis, the displacement of domain walls occurs before the irreversible rotation magnetization. The domain wall expands as a result of the pressure caused by a magnetic field. If the external field is equal to the pinning field of the domains, the magnetization switches by 180. In this critical condition, the displacement is just half of the wavelength of the internal stress, and the coercivity obeys the following relation [32]:
bending axis || short axis
200
-1
Ha (Oe)
Hc (Oe)
5
55
40
100
20 bending axis || long a is
0 0
90
180 270 ϕ (deg.)
360
0 0.0
0.5
1.0
1.5
2.0
-1
1/R (m ) Fig. 7. Angular dependences of the measured coercive fields (square) for the films, which were flat (a) and bent (b) during irradiation, respectively. Also shown are the fitting curves (lines).
Fig. 8. The anisotropy field measured along the hard axis as a function of the bending curvature of the Ni films during 200 keV Xe-ion irradiation.
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
uniaxial anisotropy. The uniaxial anisotropy energy density E should consist of two parts: the ion-irradiation induced uniaxial anisotropy energy Kirr with the anisotropy axis u01 and the external stress-induced anisotropy energy Kr = 3kr/2 with its anisotropy axis perpendicular to the bending axis. Here, k and r are the magnetostriction constant and the stress, respectively. These two anisotropy energies act together to induce a new uniaxial anisotropy, with the anisotropy axis u0. For the film with its bending axis parallel to its short axis, the anisotropy energy density has the following form: 3 E ¼ K irr sin2 ðu u01 Þ krcos2 u ¼ K Su sin2 ðu u0 Þ; ð5Þ 2 while for the film with its bending axis parallel to its long axis, the anisotropy energy density is given by 3 E ¼ K irr sin2 ðu u01 Þ krcos2 ðu 90 Þ 2 ¼ K Lu sin2 ðu u0 Þ.
Acknowledgments The author would like to thank Professor K.P. Lieb, Dr. P. Schaaf, Dr. G.A. Mu¨ller and Dr. M. Uhrmacher for their collaboration and discussions during this work. This work was funded by Deutsche Forschungsgemeinschaft. References
ð6Þ
Here K uL=S is the anisotropy constant for the film with its bending axis parallel to its long/short axis. Solving (5) and (6), we reach 3 K L=S ¼ K irr ½1 2sin2 ðu01 u0 Þ krð2sin2 u0 1Þ; ð7Þ u 2 with ‘‘+’’ for the film having its bending axis parallel to its long axis and ‘‘’’ for the film having its bending axis parallel to its short axis. If r ! 1 (namely, 1/R ! 1), one gets a difference between K Su and K Lu for the same value of r(or R): DK ¼ K Su K Lu ¼ 2ðcos2 u01 sin2 u01 ÞK irr .
the direction perpendicular to the bending axis, indicating compressive stress in the films after irradiation and relaxation. Different angular modulations of the coercive field are discussed. The differential anisotropy fields between the film having its bending axis parallel to its long axis and the film having its bending axis parallel to its short axis have been interpreted by considering both the irradiationinduced anisotropy energy and the external-stress-induced anisotropy energy.
ð8Þ
Here DK shows the irradiation effect only (without the effect of external stress). With Ha = 2K/Ms, one obtains DHa = DK/Ms, which indicates that the film with its bending axis parallel to its short axis has a bigger anisotropy field than the film with its bending axis parallel to its long axis (see Fig. 5) if u01 is close to the long axis, namely u01 < 45. K Su and K Lu will have the same value for the same curvature 1/R, but only if u01 = 45. In our experiments, u01 has the value of 16.5. In summary, we have deposited pre-stressed Ni films on Si substrates, irradiated them with 200-keV Xe-ions at a fluence of 4 · 1014 ions/cm2, and then removed them from the target holder. An in-plane uniaxial compressive stress was induced in these films. The MOKE-measurements have shown that a stronger uniaxial magnetic anisotropy was induced in the Ni films, and their magnetic behavior was extremely sensitive to the external stress produced in the films. Besides uniaxial anisotropy, the angular modulation of the relative remanence showed two ‘‘smaller peaks’’ if the film was flat or bent with smaller curvature during irradiation. This phenomenon reveals a coupling of a uniaxial and a fourfold anisotropy. However at bigger curvatures, the film showed a perfect uniaxial magnetic anisotropy, and the easy axis of the magnetic anisotropy rotated to
[1] M. Hehn, S. Padovani, K. Ounadjela, J.P. Bucher, Phys. Rev. B 54 (1996) 3428. [2] K. Mandal et al., J. Magn. Magn. Mater. 220 (2000) 152. [3] K.N. Tu, W.K. Chu, J.W. Mayer, Thin Solid Films 25 (1981) 403. [4] H.J. Kim, B. Lee, S.K. Joo, J. Appl. Phys. 81 (1997) 3995. [5] K.R. Coffey, M.A. Parker, J.K. Howard, IEEE Trans. Magn. 31 (1995) 2737. [6] J.L. Mene´ndez, P. Caro, A. Cebollada, F. Briones, D. Garcı´a, M. Va´zquez, A. Hernando, J.A. Garcı´a, J. Magn. Magn. Mater. 206 (1999) 1. [7] D. Garcı´a, J.L. Mun˜oz, G. Kurlyandskaya, M. Va´zquez, M. Ali, M.R.J. Gibbs, J. Magn. Magn. Mater. 191 (1999) 339. [8] J. Wenda, L.J. Maksymowicz, J. Magn. Magn. Mater. 87 (1990) 268. [9] S. Yatsuya, M.B. Hintz, J. Appl. Phys. 75 (1994) 7084. [10] W.A. Lewis, M. Farle, B.M. Clemens, R.C. White, J. Appl. Phys. 75 (1994) 5644; M. Farle, H. Saffari, W.A. Lewis, E. Kay, S.B. Hagstrom, IEEE Trans. Magn. MAG-28 (1992) 2940. [11] S. Iwatsubo, T. Takahashi, M. Naoe, J. Appl. Phys. 83 (1998) 6667. [12] Y. Watanabe, N. Kitazawa, Y. Nakamura, J. Vac. Sci. Technol. A 18 (2000) 1567. [13] M. Neubauer, K.P. Lieb, M. Uhrmacher, P. Wodniecki, Europhys. Lett. 43 (1998) 177. [14] K.P. Lieb, K. Zhang, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, W. Felsch, M. Mu¨nzenberg, Acta Phys. Pol. A 100 (2001) 751. [15] J. Juraszek, A. Fnidiki, J. Appl. Phys. 89 (2001) 3151. [16] 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. [17] R.S. Averback, Nucl. Instr. and Meth. B 15 (1986) 675. [18] W. Bolse, Mat Sci. Rep. R12 (1994) 53. [19] K.P. Lieb, Contemp. Phys. 40 (1999) 1556; K.P. Lieb, W. Bolse, S. Dhar, S. Habe-nicht, M. Neubauer, F. Roccaforte, M. Uhrmacher, SPIG-19, Zlatibor (1998) 204. [20] H.I. Smith, J. Appl. Phys. 64 (1995) 2337. [21] M. Neubauer, N. Reinecke, M. Uhrmacher, K.P. Lieb, M. Mu¨nzenberg, W. Felsch, Nucl. Instr. and Meth. B 139 (1998) 332. [22] K. Zhang, K.P. Lieb, P. Schaaf, M. Uhrmacher, W. Felsch, M. Mu¨nzenberg, Nucl. Instr. and Meth. B 161–163 (2000) 1016. [23] D. Weller, J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Toney, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [24] T. Devolder, Phys. Rev. B 62 (2001) 5794. [25] G.A. Mu¨ller, K.P. Lieb, P. Schaaf, R. Gupta, Appl. Phys. Lett. 82 (2003) 73.
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57 [26] K. Zhang, R. Gupta, K.P. Lieb, Y. Luo, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, Europhys. Lett. 64 (2003) 668. [27] R. Gupta, G.A. Mu¨ller, P. Schaaf, K. Zhang, K.P. Lieb, Nucl. Instr. and Meth. B 216 (2004) 350. [28] K. Zhang, K.P. Lieb, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, M. Mu¨nzenberg, Eur. Phys. J. B 42 (2004) 193.
57
[29] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Pursche, K.P. Lieb, Nucl. Instr. and Meth. B 9 (1985) 234. [30] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1999. [31] M. Li, G.-C. Wang, H.-G. Min, J. Appl. Phys. 83 (1998) 5313. [32] S. Chikazumi, Physics of Ferromagnetism, Oxford, 1997.
Nuclear Instruments and Methods in Physics Research B 243 (2006) 51–57 www.elsevier.com/locate/nimb
Stress-induced magnetic anisotropy in Xe-ion-irradiated Ni thin films Kun Zhang
*
II. Physikalisches Institut and Sonderforschungsbereich 602, Universita¨t Go¨ttingen, Friedrich-Hund-Platz 1, D-37077 Go¨ttingen, Germany Received 24 February 2005; received in revised form 12 July 2005 Available online 12 September 2005
Abstract Samples consisting of 75 nm Ni films deposited on Si substrates were bent mechanically and irradiated with 200 keV Xe-ions at a dose of 4 · 1014 ions/cm2. Magneto-optical Kerr effect, Rutherford backscattering spectrometry and X-ray diffraction were used to investigate the changes in the magnetic and microstructural properties. Perfect uniaxial magnetic anisotropy was found in the Ni films after irradiation and removal of the samples from the target holder. The magnetic behavior is shown to be very sensitive to the external stress produced in the films. With increasing curvature of the bent samples (2 m1), the easy axis of the magnetic anisotropy rotated in the direction perpendicular to the bending axis, indicating a compressive stress in the films after irradiation and relaxation. 2005 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 68.55.Ln; 75.30.Gw; 75.70.i Keywords: Magnetic anisotropy; Thin film; Ion implantation; Film stress
1. Introduction The macroscopic magnetic properties of ferromagnetic films are mainly dominated by their magnetic anisotropy. Thin films having an anisotropy along a particular direction can be applied as magnetic sensors or switches [1]. Hence, the formation and control of magnetic anisotropy in thin ferromagnetic films is a very interesting topic [2]. The magnetic properties of polycrystalline magnetic materials are sensitive to the microstructural changes induced by mechanical treatment and thermal history [3–8]. Moreover, several recent studies have shown that ion-beam implantation provides effective means for modifying the film properties, e.g. ion beam assisted deposition [9–12] and post-ion implantation [13–16]. The high local energy deposition of implanted heavy ions not only induces a number of dramatic changes in the crystal structure [17– *
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0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.241
20], but also leads to pronounced changes in the magnetization of ferromagnetic films [21–24]. However, the detailed physical mechanism is not well understood, yet. We have previously reported on magnetic anisotropies in thin Fe, Co, Ni and CoFe films observed at low Xeion fluences [14,21,22,25–28]. The present work focuses on investigations of the influence of external stress in thin Ni films during ion irradiation. The motivation was to correlate the magnetic properties with the changes of stress in the films, which are being induced during film deposition, ion-irradiation and/or external mechanical treatment. 2. Experiments Seventy five nanometer polycrystalline Ni-films were deposited in an electron-beam evaporation setup with a base pressure of 1 · 106 mbar at a deposition rate of 0.1 nm/s onto 0.5 mm thick naturally oxidized Si(1 0 0) substrates of 15 · 40 mm2. The substrates were covered by a Cu-mask, having in its centre a rectangular hole of
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
Fig. 1. Schematic illustration of the bending of a sample.
the long axis of the samples. The coercive force, Hc(u), and the relative remanence, Mr/Ms, with Mr being the remanence magnetization and Ms the saturation magnetization, were analyzed. X-ray diffraction (XRD) measurements were carried out to check changes in their crystal structure. Grazing angle measurements detected polycrystalline peaks for both as-deposited and irradiated samples. Measurements based on rocking curves also implied a polycrystalline structure. However, the h2h scan indicated that the Ni(1 1 1)texture dominates, as almost no other reflection was found. 3. Results
Normalized MOKE Signal
Fig. 2 shows the in-plane hysteresis loops measured along the easy and hard axes of the films. The two hysteresis loops of the as-deposited film are almost identical, and no magnetic anisotropy was detected. The coercive field of this sample was about 200 Oe. After irradiation, the samples were relaxed to reestablish their original flat shape, and then analyzed with MOKE. These films revealed magnetic anisotropies (see Fig. 2(b)–(d)). The coercive field was dramatically reduced, having values of 7–16 Oe for the film which was flat during ion-irradiation. The bent sample showed a bigger (smaller) coercive field in its easy (hard) axis than the one which was flat during irradiation. For the film with curvature of 1.8 m1, the coercive field was reduced close to zero along the hard axis, whereas it was 29 Oe in the easy axis direction. Figs. 3 and 4 illustrate the angular modulations of the coercive field and the relative remanence, respectively. The as-deposited film had an isotropic coercive field of 200 Oe 1.0 0.5 0.0 -0.5 -1.0 -1000 -500 0 500 1000 -300 -150 0 150 HMOKE (Oe) HMOKE (Oe)
(a) as-dep. Normalized MOKE Signal
7 · 10 mm2 to define the geometry of the deposited Ni films. After deposition, the samples were clamped onto a Cu target holder (see Fig. 1), by means of two steel bars, intersected by a steel wire. In this fashion the sample was bent, having its bending axis parallel to its short axis and its bending radius adjusted by the distance between the steel bars and the target holder. The bending radius of each sample was measured via a surface profilometer. By mounting the two steel bars parallel to the long axis of the film, it was possible to bend the sample, having the bending axis parallel to the long axis. The bent samples were then irradiated with 200 keV Xe+-ions at a fluence of 4 · 1014 ions/cm2 and a temperature of 100 K. After irradiation, the samples were removed from the target holder and the substrate was allowed to relax to resume its flat shape in which conditions the flat samples were then characterized. The as-deposited sample had a flat shape and no external stress. By bending the sample, a tensile stress r0 with its direction perpendicular to the bending axis was introduced into the film. During Xe-ion irradiation, this tensile stress was reduced, and then the stress in the film (rion) was smaller than r0. After the samples had been removed from the target holder, the Si-substrates resumed their flat shape and produced a compressive stress in the Ni films, having a direction perpendicular to the bending axis. The Xe-ion irradiations of the samples were performed at the Go¨ttingen heavy ion implanter IONAS [29]. With an electrostatic X–Y sweeping system (fx: 120 Hz, fy: 5 Hz) the ions were homogeneously distributed over the film. The Xe ion current was kept at about 0.3 lA in order to avoid heating of the sample during ion irradiation. The values of the projected ion energy and the ion distribution in Ni films were estimated with the SRIM 2003 code [30], and measured by RBS after ion implantation. The ion energy of 200 keV was chosen for the 75 nm Ni films to locate the ion distribution entirely within the film, so that there was no ion-irradiation induced interaction between the film and the substrate. The magnetic characterization concentrated on measuring changes in the coercive field and orientation of the in-plane anisotropy axis. In-plane hysteresis loops were measured at room temperature, using the magneto-optical Kerr effect (MOKE) with a 633 nm laser beam of 1.5 mm beam diameter and at various angles, u, with respect to
300
(b) 1/R=0
1.0 0.5
easy axis
0.0
hard axis
-0.5 -1.0 -300 -150 0 150 HMOKE (Oe)
(c) 1/R=0.3 m-1
300 -300 -150 0 150 HMOKE (Oe)
300
(d) 1/R=1.8 m-1
Fig. 2. In-plane hysteresis loops for the as-deposited film (a), the irradiated flat film (b) and the bent, irradiated and then relaxed films (c, d) with the magnetic field applied along the easy and hard axes.
Hc (Oe)
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
200 150 100 0 180 100 210 200
Hc (Oe)
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(d) 1/R=1.8 m
-1
Fig. 3. The coercive field of the as-deposited film and the 200 keV Xe+ irradiated 75 nm thick Ni/Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
120
Mr/Ms
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Bending axis parallel to long axis
Hc (Oe)
Mr/Ms
u0 (deg.)
Hc (Oe)
Mr/Ms
u0 (deg.)
7–16 6–18
0.57–0.98 0.15–0.98
16.5 14.4
5–35 1–54
0.05–0.98 0.01–0.99
3.3 0.5
7–16 7–18 6–24 5–29 1–51
0.57–0.98 0.32–0.98 0.15–0.98 0.06–0.98 0.02–0.99
16.5 35.0 56.2 84.5 88.3
the anisotropy (i.e. the angle of maximum remanence) for all the samples. Briefly for all the cases, the bending films rotated their easy axes toward the direction of the stress, and this rotation increased with increasing external stress. At a curvature of 1.8 m1, the easy axis was oriented parallel to the direction of the stress. Since the magnetostriction constant for polycrystalline Ni is negative (k = 3.3 · 105), it is inferred that the stress in the film was compressive after the bent film has been irradiated and then relaxed to resume its original flat shape. The relative remanence increases (decreases) in the direction of the easy (hard) axis with an increasing curvature during irradiation. The amplitude of the angular modulation of the coercive field increased with a increase of the curvature. The changes in the crystal structure of these bent films were determined by XRD. Fig. 5 shows the h2h XRD
60 30
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3000
0.0 180 0.5
0 0.3 0.5 0.9 1.8
Bending axis parallel to short axis
330
210
60
300
(c) 1/R=0.9 m
30
1/R (m1)
300
(b) 1/R=0
60
Table 1 The maxima and the minima values of the coercive field Hc and the relative remanence Mr/Ms, and the position of the anisotropy axis u0
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(a) as-dep.
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53
0
1000 2θ=44.558°
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(d) 1/R=1.8 m
FWHM=0.649°
0 3000
-1
2000
Fig. 4. The relative remanences of the as-deposited film and the 200 keV Xe+ irradiated 75 nm thick Ni/Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
1000
Counts
(see Fig. 3(a)) and an isotropic relative remanence of 0.53 (see Fig. 4(a)). After irradiation the films showed magnetic anisotropy. For the flat film, the maxima of their relative remanence (easy axis) and their coercive field pointed along the same direction, at an angle of 16.5 to the long axis of the film. This orientation is possibly due to ion-irradiation induced stress in the film. In addition to uniaxial anisotropy, the angular modulation of the remanence also showed two ‘‘smaller sharp peaks’’ near the direction of the hard axis in the flat film or the films with a very small curvature during irradiation (see Fig. 4(b)), but disappeared in the films with large curvature (see Fig. 4(d)). For the latter films, the maxima of the coercive field were located in the direction between the easy and the hard axis. Table 1 summarizes the extreme values of the coercive field Hc and the relative remanence Mr/Ms, and the symmetry angle u0 of
(a) as deposition
2000
(b) 1/R=0 2θ=44.502°
FWHM=0.412°
0 3000 2000 1000
-1
(c) 1/R=0.28 m 2θ=44.475°
FWHM=0.478°
0 3000 2000 1000
(d) 1/R=0.92 m 2θ=44.452°
-1
FWHM=0.492° Ni(200)
0
3000 2000
1000 2θ =44.434° 0 40
(e) 1/R=1.79 m
Ni(111)
-1
FWHM=0.521° 45
50
55
2θ (°) Fig. 5. XRD-spectra of the 200 keV Xe+ irradiated 75 nm thick Ni/ Si(1 0 0) films with their bending axis parallel to their short axis and with different curvatures.
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
stresses in such thin films: the stress produced during film deposition; the stress induced by ion irradiation; and the stress introduced by mechanical treatment. For the experiments on ion-beam induced magnetic properties, the use of stress-free films and stress-defined films is important. Some experimental results are discussed in this section.
patterns for these films with their bending axis parallel to the short axis during irradiation. For the as-deposited film, only Ni(1 1 1) texture was observed. After irradiation, a much smaller peak of Ni(2 0 0) texture was detected, too. For the flat film, the 2h-position of the Ni(1 1 1) peak moved slightly toward the smaller angle after irradiation. This shift indicates an expansion of the lattice in the normal direction, which was probably induced by the introduction of Xe-atoms in the Ni lattice. On the other hand, the irradiation yields a much sharper peak of Ni(1 1 1) reflection. Two possible reasons for this observation are the irradiation-induced grain growth and the decrease in the microstrain of the film. When the film was bent during irradiation and then relaxed after irradiation, the shift of the Ni(1 1 1) peak was bigger than that measured in the flat film. This extra shift reveals an additional expansion of the lattice constant in the direction normal to the film, which was apparently induced by a contraction of the lattice in the film plane. This means that, after the bent Ni-film had been irradiated and relaxed to resume its original flat shape, a compressive stress in the film plane was generated, which increased the FWHM of the Ni(1 1 1) peak. The shift of the 2h-position of the Ni(1 1 1) peak increased with rising 1/R, and so did its width.
4.1. Coupling of uniaxial and fourfold anisotropies The angular dependences of the relative remanence in the ion-irradiated Ni films have two small sharp peaks if there is little or no external stress during irradiation (see Fig. 4(b)). This phenomenon can be interpreted in terms of the coupling of a uniaxial anisotropy Ku and an effective fourfold anisotropy K eff 1 (see Fig. 6). The uniaxial anisotropy is induced by stress, including the irradiation-induced stress and the external stress, while the fourfold term is a result of magnetocrystalline anisotropy. The magnetic energy density can be described with the following function: 1 E ¼ K u sin2 u þ K eff sin2 2u. 4 1
ð1Þ
If Ku is smaller than K eff 1 , ‘‘two small peaks’’ of the remanence magnetization are expected. If the Ku is slightly bigger than K eff 1 , in a small region around the maximum value of the magnetic energy density the magnetization is situated in an eventual metastable non-equilibrium state, in which the magnetization is just perpendicular to the easy axis of the uniaxial anisotropy, and there is no force to turn the direction of the magnetization toward the easy axis. Therefore, in this situation, the film has a big component of magnetization in the applied field direction, even when the external magnetic field is removed. If the induced uniaxial anisotropy Ku is very big, the effective fourfold anisot-
4. Discussion and summary The experimental results obtained in the present work have revealed that stress can be an important factor in the ion-irradiation induced modification of the magnetic properties of thin Ni films. This finding indicates a way for future experiments to more completely understand ion-beam induced magnetic anisotropy. For this purpose, it is important to distinguish the different origins of the
Energy density
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Remanence
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0
330 210 240
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Ku:K1=1:0.5
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K1 = 0 Ku ≠ 0
Fig. 6. Illustration of the free energy density and the corresponding remanent magnetization for the film with the coupling of uniaxial and effective fourfold anisotropies.
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
ropy K eff 1 can be ignored. In this situation there is no metastable non-equilibrium state in the magnetic energy density so that no ‘‘small peaks’’ can be observed in the angular dependence of the remanence magnetization. For this reason, the ‘‘two small peaks’’ can be observed in our measurements only in flat Ni films or in bent films with a smaller curvature.
sidered to be located around the easy axis and around the hard axis, respectively. Around the hard axis, the film has a magnetic behavior similar to that of a single domain, and coherent rotation magnetization contributes to the coercive field. The in-plane hysteresis loops, and thus the coercive field, can be calculated with extreme magnetic energy conditions to obtain the equilibrium state [32]. One obtains
4.2. Domain nucleation versus rotation in stressed films Hc ¼ If the film remains flat during irradiation, the maximum of the coercive field occurs in the direction along the easy axis and the minimum occurs along the hard axis. However, for the bent films, the maximum of the coercive field is located in the position between easy and hard axis. The angular modulation also shows a different form. This difference indicates different magnetization processes in these films. To estimate the coercivity dependence on the azimuthal angle, a simple relation is used to fit the coercive field of a flat film [31]: H c ¼ H a cos2 ðu u0 Þ þ ½H 2cw H 2a cos2 ðu u0 Þ sin2 ðu u0 Þ1=2 ;
ð2Þ
where both the magnetization rotation and the domain wall motion are considered to contribute to the magnetization reversal, and the applied field H must overcome the component of the magnetic anisotropy field in the applied field direction, H a cosðu u0 Þ, and the domain wall pinning coercivity, Hcw. Fig. 7(a) shows the angular dependence of Hc for the film being flat during irradiation. For bent films, the hysteresis curves in all measured directions exhibit the discontinuity jump only at Hc, so that the magnetization process must be dominated either by domain nucleation or by coherent rotation. Therefore, the two different magnetization features in bent films are con-
20 (a) 1/R = 0 15 10
0 60
Ku sin 2ðu u0 Þ; Ms
Hc ¼
H c;e . cosðu u0 Þ
ð4Þ
Here Hc,e is the coercive field along the easy axis. The measured coercive field Hc for the film bent during irradiation is replotted in Fig. 7(b) with its fitting curve derived using Eqs. (3) and (4). 4.3. Separation of ion-induced and external stresses The anisotropy field Ha can be obtained by measuring the saturation field along the direction of the hard axis. Fig. 8 shows that the anisotropy field increases with increasing external stress. It should be noted that, at a given value of the curvature 1/R, the film whose bending axis is parallel to its short axis has a bigger anisotropy field than the film with its bending axis parallel to the long axis. This phenomenon is due to the ion-induced magnetic anisotropy, which occurs close to the long axis of the film. If a film was strongly bent during irradiation, the effective fourfold anisotropy can be ignored due to stronger
Experiment Fitting (b) 1/R = 1.8 m
ð3Þ
where Ku and Ms are the uniaxial anisotropy constant and the saturated magnetization, respectively. Around the easy axis, the displacement of domain walls occurs before the irreversible rotation magnetization. The domain wall expands as a result of the pressure caused by a magnetic field. If the external field is equal to the pinning field of the domains, the magnetization switches by 180. In this critical condition, the displacement is just half of the wavelength of the internal stress, and the coercivity obeys the following relation [32]:
bending axis || short axis
200
-1
Ha (Oe)
Hc (Oe)
5
55
40
100
20 bending axis || long a is
0 0
90
180 270 ϕ (deg.)
360
0 0.0
0.5
1.0
1.5
2.0
-1
1/R (m ) Fig. 7. Angular dependences of the measured coercive fields (square) for the films, which were flat (a) and bent (b) during irradiation, respectively. Also shown are the fitting curves (lines).
Fig. 8. The anisotropy field measured along the hard axis as a function of the bending curvature of the Ni films during 200 keV Xe-ion irradiation.
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K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57
uniaxial anisotropy. The uniaxial anisotropy energy density E should consist of two parts: the ion-irradiation induced uniaxial anisotropy energy Kirr with the anisotropy axis u01 and the external stress-induced anisotropy energy Kr = 3kr/2 with its anisotropy axis perpendicular to the bending axis. Here, k and r are the magnetostriction constant and the stress, respectively. These two anisotropy energies act together to induce a new uniaxial anisotropy, with the anisotropy axis u0. For the film with its bending axis parallel to its short axis, the anisotropy energy density has the following form: 3 E ¼ K irr sin2 ðu u01 Þ krcos2 u ¼ K Su sin2 ðu u0 Þ; ð5Þ 2 while for the film with its bending axis parallel to its long axis, the anisotropy energy density is given by 3 E ¼ K irr sin2 ðu u01 Þ krcos2 ðu 90 Þ 2 ¼ K Lu sin2 ðu u0 Þ.
Acknowledgments The author would like to thank Professor K.P. Lieb, Dr. P. Schaaf, Dr. G.A. Mu¨ller and Dr. M. Uhrmacher for their collaboration and discussions during this work. This work was funded by Deutsche Forschungsgemeinschaft. References
ð6Þ
Here K uL=S is the anisotropy constant for the film with its bending axis parallel to its long/short axis. Solving (5) and (6), we reach 3 K L=S ¼ K irr ½1 2sin2 ðu01 u0 Þ krð2sin2 u0 1Þ; ð7Þ u 2 with ‘‘+’’ for the film having its bending axis parallel to its long axis and ‘‘’’ for the film having its bending axis parallel to its short axis. If r ! 1 (namely, 1/R ! 1), one gets a difference between K Su and K Lu for the same value of r(or R): DK ¼ K Su K Lu ¼ 2ðcos2 u01 sin2 u01 ÞK irr .
the direction perpendicular to the bending axis, indicating compressive stress in the films after irradiation and relaxation. Different angular modulations of the coercive field are discussed. The differential anisotropy fields between the film having its bending axis parallel to its long axis and the film having its bending axis parallel to its short axis have been interpreted by considering both the irradiationinduced anisotropy energy and the external-stress-induced anisotropy energy.
ð8Þ
Here DK shows the irradiation effect only (without the effect of external stress). With Ha = 2K/Ms, one obtains DHa = DK/Ms, which indicates that the film with its bending axis parallel to its short axis has a bigger anisotropy field than the film with its bending axis parallel to its long axis (see Fig. 5) if u01 is close to the long axis, namely u01 < 45. K Su and K Lu will have the same value for the same curvature 1/R, but only if u01 = 45. In our experiments, u01 has the value of 16.5. In summary, we have deposited pre-stressed Ni films on Si substrates, irradiated them with 200-keV Xe-ions at a fluence of 4 · 1014 ions/cm2, and then removed them from the target holder. An in-plane uniaxial compressive stress was induced in these films. The MOKE-measurements have shown that a stronger uniaxial magnetic anisotropy was induced in the Ni films, and their magnetic behavior was extremely sensitive to the external stress produced in the films. Besides uniaxial anisotropy, the angular modulation of the relative remanence showed two ‘‘smaller peaks’’ if the film was flat or bent with smaller curvature during irradiation. This phenomenon reveals a coupling of a uniaxial and a fourfold anisotropy. However at bigger curvatures, the film showed a perfect uniaxial magnetic anisotropy, and the easy axis of the magnetic anisotropy rotated to
[1] M. Hehn, S. Padovani, K. Ounadjela, J.P. Bucher, Phys. Rev. B 54 (1996) 3428. [2] K. Mandal et al., J. Magn. Magn. Mater. 220 (2000) 152. [3] K.N. Tu, W.K. Chu, J.W. Mayer, Thin Solid Films 25 (1981) 403. [4] H.J. Kim, B. Lee, S.K. Joo, J. Appl. Phys. 81 (1997) 3995. [5] K.R. Coffey, M.A. Parker, J.K. Howard, IEEE Trans. Magn. 31 (1995) 2737. [6] J.L. Mene´ndez, P. Caro, A. Cebollada, F. Briones, D. Garcı´a, M. Va´zquez, A. Hernando, J.A. Garcı´a, J. Magn. Magn. Mater. 206 (1999) 1. [7] D. Garcı´a, J.L. Mun˜oz, G. Kurlyandskaya, M. Va´zquez, M. Ali, M.R.J. Gibbs, J. Magn. Magn. Mater. 191 (1999) 339. [8] J. Wenda, L.J. Maksymowicz, J. Magn. Magn. Mater. 87 (1990) 268. [9] S. Yatsuya, M.B. Hintz, J. Appl. Phys. 75 (1994) 7084. [10] W.A. Lewis, M. Farle, B.M. Clemens, R.C. White, J. Appl. Phys. 75 (1994) 5644; M. Farle, H. Saffari, W.A. Lewis, E. Kay, S.B. Hagstrom, IEEE Trans. Magn. MAG-28 (1992) 2940. [11] S. Iwatsubo, T. Takahashi, M. Naoe, J. Appl. Phys. 83 (1998) 6667. [12] Y. Watanabe, N. Kitazawa, Y. Nakamura, J. Vac. Sci. Technol. A 18 (2000) 1567. [13] M. Neubauer, K.P. Lieb, M. Uhrmacher, P. Wodniecki, Europhys. Lett. 43 (1998) 177. [14] K.P. Lieb, K. Zhang, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, W. Felsch, M. Mu¨nzenberg, Acta Phys. Pol. A 100 (2001) 751. [15] J. Juraszek, A. Fnidiki, J. Appl. Phys. 89 (2001) 3151. [16] 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. [17] R.S. Averback, Nucl. Instr. and Meth. B 15 (1986) 675. [18] W. Bolse, Mat Sci. Rep. R12 (1994) 53. [19] K.P. Lieb, Contemp. Phys. 40 (1999) 1556; K.P. Lieb, W. Bolse, S. Dhar, S. Habe-nicht, M. Neubauer, F. Roccaforte, M. Uhrmacher, SPIG-19, Zlatibor (1998) 204. [20] H.I. Smith, J. Appl. Phys. 64 (1995) 2337. [21] M. Neubauer, N. Reinecke, M. Uhrmacher, K.P. Lieb, M. Mu¨nzenberg, W. Felsch, Nucl. Instr. and Meth. B 139 (1998) 332. [22] K. Zhang, K.P. Lieb, P. Schaaf, M. Uhrmacher, W. Felsch, M. Mu¨nzenberg, Nucl. Instr. and Meth. B 161–163 (2000) 1016. [23] D. Weller, J.E.E. Baglin, A.J. Kellock, K.A. Hannibal, M.F. Toney, G. Kusinski, S. Lang, L. Folks, M.E. Best, B.D. Terris, J. Appl. Phys. 87 (2000) 5768. [24] T. Devolder, Phys. Rev. B 62 (2001) 5794. [25] G.A. Mu¨ller, K.P. Lieb, P. Schaaf, R. Gupta, Appl. Phys. Lett. 82 (2003) 73.
K. Zhang / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 51–57 [26] K. Zhang, R. Gupta, K.P. Lieb, Y. Luo, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, Europhys. Lett. 64 (2003) 668. [27] R. Gupta, G.A. Mu¨ller, P. Schaaf, K. Zhang, K.P. Lieb, Nucl. Instr. and Meth. B 216 (2004) 350. [28] K. Zhang, K.P. Lieb, G.A. Mu¨ller, P. Schaaf, M. Uhrmacher, M. Mu¨nzenberg, Eur. Phys. J. B 42 (2004) 193.
57
[29] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Pursche, K.P. Lieb, Nucl. Instr. and Meth. B 9 (1985) 234. [30] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1999. [31] M. Li, G.-C. Wang, H.-G. Min, J. Appl. Phys. 83 (1998) 5313. [32] S. Chikazumi, Physics of Ferromagnetism, Oxford, 1997.