- Email: [email protected]
Ion-beam induced magnetic anisotropies in iron films
Nuclear Instruments and Methods in Physics Research B 139 (1998) 332±337
Ion-beam induced magnetic anisotropies in iron ®lms M. Neubauer a, N. Reinecke a, M. Uhrmacher a,*, K.P. Lieb a, M. M unzenberg b, b W. Felsch a b
II. Physikalisches Institut, Universit at G ottingen and Sonderforschungsbereich 345, D-37073 G ottingen, Germany I. Physikalisches Institut, Universit at G ottingen and Sonderforschungsbereich 345, D-37073 G ottingen, Germany
Abstract 100±300 nm thin Fe layers evaporated onto crystalline and amorphous Si or SiO2 substrates were irradiated, at 77 K, with 1014 ±1016 Xe -ions/cm2 at 450 keV beam energy. The magnetizations in the ®lms were measured by means of Perturbed Angular Correlation (PAC) spectroscopy with implanted 111 In tracer ions, or the Magneto-Optic Kerr Eect (MOKE). Upon ion implantation, dramatic changes of the magnetic anisotropy were observed which are attributed to ion-beam enhanced lateral grain growth. Very little in¯uence of the deposition parameters (type and cristallinity of substrate, evaporation rate) on the anisotropic magnetization was found. Ó 1998 Elsevier Science B.V. PACS: 75.70.Ak; 81.40.Wx; 76.80.+y
1. Introduction Ion irradiations of thin metallic ®lms induce a number of interesting and technologically important modi®cations, many of which are not at all understood theoretically. The high local energy deposition of implanted heavy ions not only leads to ballistic eects like sputtering and ballistic interface mixing in multilayers, but can induce changes of the microstructure of the ®lms. Many of these phenomena seem to be related to the formation of thermal spikes which are expected to occur if
* Corresponding author. Address: M. Uhrmacher, Universitat G ottingen, II. Physikalisches Institut, Bunsenstrasse 7-9, 37073 G ottingen, Germany. Tel.: 0551-397613; fax: 0551394493; e-mail: [email protected].
the average element number of the matrix and the projectile exceeds Z 20 [1±3]. In our recent studies of Ag/Fe bilayers irradiated with Ar and Xe ions, we have encountered ion-beam enhanced grain growth and texturing in the Ag-top layer as well as demixing at the Ag/Fe interface, as proven by Rutherford Backscattering Spectrometry (RBS), RBS-channeling, Scanning Tunneling Microscopy (STM) and hyper®ne interaction methods [4±8]. It appears that in this thermodynamically non-miscible system, as in many other miscible metallic multilayer systems [9±12], (local) thermal spikes are a very ecient driving mechanism for the atomic transport processes which happen at low substrate temperatures. The present work addresses to dramatic changes of the magnetic structure of thin Fe ®lms irradiated with heavy ions. We have observed,
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 6 8 - 3
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
333
via Perturbed Angular Correlation (PAC) spectroscopy with radioactive 111 In tracer ions, changes of the magnetic anisotropy within the Fe-®lms, upon irradiations with Xe ions. In addition to the hyper®ne analysis, we have also used the MagnetoOptic Kerr Eect (MOKE) to measure the magnetization curves along dierent directions of the Xeirradiated Fe ®lms and to study their dependence on the ion ¯uence and the procedures the ®lms were prepared (type, cristallinity and temperature of the substrate, evaporation rate of iron). 2. Experimental details The 100±300 nm thick Fe layers of 4N quality were deposited via electron evaporation onto Si or SiO2 substrates, at a rate of 0.1±0.3 nm/s as measured with a quartz crystal. The thickness of each layer was checked by RBS at 900 keV a-particle energy. We used cristalline Si (1 0 0), Si (1 1 0) and a-SiO2 substrates as well as amorphous SiO2 substrates obtained by heating Si wafers for 24 h at 900°C in air. The ®lms were prepared in two evaporation apparatus: in a high vacuum (HV) chamber at the base pressure of 10ÿ5 Pa, the uncooled substrate was at a distance of 26 cm from the electron gun. Alternatively, we also used an UHV chamber at the base pressure of 10ÿ9 Pa in which the water-cooled substrate was 15 cm away from the electron gun. The 111 In tracer implantations (about 8 ´ 1012 /cm2 of 111 In ions implanted at 280 keV onto a 4-mm-diameter beam spot) and Xe-ion irradiations (1014 ±1016 /cm2 at 450 keV) were performed with the G ottingen ion implanter IONAS [13], at a pressure of 5 ´ 10ÿ4 Pa in the irradiation chamber. During the Xe-implantations, the samples were cooled to 77 K and the ions covered a 10 ´ 10 mm2 beam spot on the target, as achieved by means of an electric x± y sweeping system. The mean range of the implanted Xe-ions (111 In ions) in Fe, as predicted by the TRIM code [14], is about 70 nm (50 nm) and hence, is less than the Fe-®lm thickness. Ion-induced processes at the Fe/substrate interface therefore should not play a role. The PAC spectra were taken at room temperature, by means of a set-up of four BaF2 detectors
Fig. 1. PAC perturbation functions R(t) and Fourier transforms A(x) of a Ag/Fe (50/130 nm) Fe layer doped with 111 In atoms at 280 keV implantation energy (a) and irradiated with 6 ´ 1015 Xe-ions/cm2 at 450 keV (b). The change of the magnetic anisotropy can be seen from the ratio of the two components x0L and 2x0L in the Fourier spectra at the Larmor frequency x0L 0:56 GHz.
in 90° geometry, having a time resolution of 1.5 ns which are coupled to a fast-fast coincidence circuit [15]. The samples were oriented in a plane which was perpendicular to the detector plane and intersecting it at 45° to the detector directions (see inset of Fig. 1). A hyper®ne ®eld pointing parallel to the detector plane (k) shows an oscillation in the PAC perturbation function at the Larmor frequency xL , while a hyper®ne ®eld pointing perpendicular to the detector-plane along the 45° direction (^) induces a precession at twice the Larmor frequency, 2xL . In this way, an anisotropic hyper®ne ®eld can be clearly detected, since for a random orientation of the magnetization both frequencies have equal intensities in the Fourier spectrum. The MOKE apparatus used in the present analysis to measure the macroscopic magnetization in the 100 nm top layer of the sample has been described in [16]. A polarized laser beam at 532 nm wavelength was employed. These samples had, of course, not been doped with the 111 In activity. 3. Results of the PAC experiments The primary information on ion-induced changes of the magnetic anisotropy in 200 nm Fe foils
334
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
came from PAC analyses. Fig. 1 shows PAC perturbation spectra R(t) and their Fourier transforms A(x) obtained directly after ®lm deposition (a) and after an irradiation with 6 ´ 1015 Xe-ions/cm2 at 77 K (b). In this case we used an Ag/Fe bilayer sample having a 50 nm thick Ag top layer covering a 150 nm Fe layer. In previous experiments, we had shown that the ion-induced intermixing eect at the bilayer interface is counterbalanced by thermal-spike phase segregation leaving a ¯at interface and a very small mixing rate. Both Fourier spectra exhibit components having the Larmor frequency x0L 0.56 GHz at room temperature and twice this value, typical for 111 In atoms sitting on substitutional, defect-free
lattice sites [17,18]. A small fraction of tracer atoms having trapped radiation defects can be identi®ed on the basis of their smaller Larmor frequency xdL 0.53 GHz [4,5]. This component produces tails at the low-frequency edges in the Fourier spectra. Clearly, the orientation of the magnetization has changed as a consequence of the Xe-ion irradiation, as can be distinguished from the ratios of the components with x0L and 2x0L before and after Xe implantation.This ratio has decreased from 1.07(4) which value is typical for a random distribution of the magnetization within the ®lm to 0.13(4), i.e. the Xe implantation has induced a magnetization within the detector plane.
Fig. 2. Hysteresis curves of a Fe ®lm deposited at a rate of 0.3 nm/s onto Si (1 0 0) and irradiated with 4 ´ 1014 , 4 ´ 1015 and 4 ´ 10 16 Xe-ions/cm2 , at 77 K. The orientation of the polarizing ®eld is given by the angle h 0°, 90° and 45° relative to an axis along the boundary of the ®lm.
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
335
4. Results of the MOKE analysis In order to check the in¯uence of the ®lm deposition parameters on the ion-induced magnetic anisotropy, magnetization measurements by means of MOKE were carried out for as-deposited and ionirradiated Fe ®lms. In particular, we used layers evaporated onto the dierent Si and SiO2 substrates mentioned before, at dierent deposition rates (0.1±0.3 nm/s) and for dierent pressures in the evaporation chamber. Fig. 2 illustrates hysteresis curves taken for a 130 nm thick sample deposited in the HV chamber at a rate of 0.3 nm/s onto a Si (1 0 0) substrate and irradiated with 4 ´ 1014 , 4 ´ 1015 and 4 ´ 1016 Xe-ions/cm2 . The quantities to be discussed later are the coercive ®eld Hc and the ®eld strength HA necessary to reach 90% of the saturation magnetization Ms , as indicated in Fig. 2. The polarizing ®eld H pointed along the angles h 0°, 90° and 45° relative to an axis of the specimen's surface. For the as-deposited ®lm, one notes a nearly isotropic distribution of the coercive ®eld, Hc 51(4) Oe, and relative remanence, Mr /Ms 0.79 within the sample plane. At a Xe¯uence of 4 ´ 1014 ions/cm2 , there appears an anisotropy of the remanent magnetization along the 0° axis, Mr (0°)/Ms (0°) 0.93 versus Mr (90°)/ Ms (90°) 0.47, and of the ®eld strength, HA (0°) 1(3) Oe versus HA (90°) 13(3) Oe. This magnetic anisotropy does not change much during further irradiations and reaches Mr (0°)/Ms (0°) 0.97 versus Mr (90°)/Ms (90°) 0.36 at the highest ¯uence of U 1016 ions/cm2 . Similar results were obtained for Fe ®lms on Si (1 1 0) and amorphous and crystalline SiO2 substrates as well as for samples evaporated at a rate of 0.1 or 0.2 nm/s. The measured angular dependence of the remanence Mr /Ms is shown in Fig. 3. This Fe ®lm had been deposited at a rate of 0.1 nm/s onto Si (1 0 0), under HV conditions, and bombarded with only 1014 Xe-ions/cm2 . The remanence follows a periodic function, its minimal value being Mr /Ms 0.6. On the top of Fig. 3, the á1 1 0ñ orientation of the Fe grains and the easy (1 0 0) magnetization axes along the directions h 155(9)° and 335(9)° are indicated as derived from the angular dependence of Mr /Ms . We conclude that the polycristalline Fe ®lm which originally had
Fig. 3. (a) Orientation of the Fe crystallites in a 130 nm Fe foil evaporated onto Si (1 0 0) at a rate of 0.1 nm/s and irradiated with 4 ´ 1014 Xe-ions/cm2 , as deduced from the measured angular dependence of the relative remanence Mr /Ms plotted in (b).
no preferred magnetization has gained, under the ion bombardment, a magnetic texture. In that respect, the MOKE analysis strongly supports the ®nding of the PAC experiment. Somewhat dierent MOKE results were found for 185 nm Fe ®lms deposited at a rate of 0.3 nm/s under UHV conditions on water-cooled Si (1 0 0) substrates as shown in Fig. 4: Up to a Xe ¯uence of 1015 /cm2 , no anisotropic magnetization appeared, but the isotropic remanence rose from Mr /Ms 0.63 to 0.82. Only after further irradiation with a total of 1016 Xe-ions/cm2 did the remanence at 90° further rise to Mr /Ms 0.92, while it decreased to 0.73 in the 0° direction. In this sample, the anisotropic magnetization therefore sets in at higher ion ¯uences than in the samples described before. Although we so far do not have a full interpretation of this dierence, we note that the main dierence between these and the former samples is pobably caused by a dierent grain size in the as-deposited state [19]. Arguing from our previous Xe-implantation experiment of thin Ag ®lms [7,8], we infer that the ion bombardment induces a grain growth as a consequences of thermal spikes.
336
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
Fig. 4. Same as Fig. 2, but for a ®lm deposited under UHV conditions onto a water-cooled Si (1 0 0) substrate.
Fig. 5 illustrates the measured dependence of the coercive ®eld Hc as of the ion ¯uence U, for four ®lms deposited under HV and UHV conditions. Assuming that the ion irradiation leads to grain growth in the ®lm plane, as also inferred for the Fe ®lms from the MOKE data shown in Fig. 3, one expects a relationship Hc 3cB = DJs / cB = Js U1=n ; where Js l0 Ms denotes the saturation polarization, cB the energy of the Bloch domain, and D the grain size [19]. The curves shown in Fig. 5 exhibit exponents of n 5 for the samples deposited onto the uncooled substrates in the HV chamber, and n 1.6 for the samples deposited under UHV conditions onto the water-cooled substrates. X-Ray Diraction of the latter Fe-®lms performed
with the Cu-Ka line showed that the line widths of the [1 1 0] re¯exes are consistent with a grain size of 33(5) nm, in the vertical direction, before and after ion irradiation. As to the change of the lateral grain size due to ion bombardment, a STM analysis in air was not successful, due to the oxide surface layer. For that reason, further crystallographic investigations of the microstructure and texture of the Fe ®lms, via XRD and STM, are necessary to check the structural properties and their correlation with the magnetization. 5. Summary Irradiations of 100±300 nm thick Fe ®lms by energetic Xe ions at 77 K have led to pronounced
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
337
Acknowledgements The authors are grateful to Dr. L. Ziegeler for help in preparing the 111 In tracer implantations, and to D. Purschke who cooperated in the RBS analyses and Xe implantations. This work was supported by Deutsche Forschungsgemeinschaft. References
Fig. 5. Fluence dependence of the coercive ®eld Hc for samples deposited under HV (open symbols) and UHV (closed symbols) conditions at the deposition rates given. For further details, see text.
changes of the magnetic anisotropy within the ®lm plane, as veri®ed via PAC and MOKE analyses. The latter experiments have demonstrated that, at a Xe-ion ¯uence as low as 1014 /cm2 , the coercive ®eld Hc and magnetic remanence Mr /Ms get anisotropic within the foil plane. The anisotropy increases with the ion ¯uence U according to a Uÿ1=n dependence, where the exponent n was found to range between n 1.5 and 5.5. The exponent n appears to depend on the grain size within the as-deposited ®lms. The MOKE experiments have also shown that neither the nature and crystallinity of the substrate nor the deposition rate during electron evaporation do appreciably in¯uence the magnetic anisotropy. We tentatively correlate the ion-induced magnetic texturing of the Fe-foils with changes of the lateral grain size and orientation as previously established for Xe-irradiated Ag ®lms [7,8]. Experiments are in progress which address to the dependence of the magnetic anisotropy on the ®lm thickness and ion species and to detailed studies of the microstructure before and after ion irradiation.
[1] W.L. Johnson et al., Nucl. Instr. and Meth. B 7/8 (1985) 657; R.S. Averback, Nucl. Instr. and Meth. B 15 (1986) 675. [2] Y.T. Cheng, Mat. Sci. Rep. 5 (1990) 45. [3] W. Bolse, Mat. Sci. Rep. R 12 (1994) 53. [4] M. Neubauer, K.P. Lieb, P. Schaaf, M. Uhrmacher, Phys. Rev. B 53 (1996) 10237. [5] M. Neubauer, K.P. Lieb, P. Schaaf, M. Uhrmacher, Thin Solid Films 275 (1996) 69; Europhys. Lett., submitted. [6] H.U. Krebs, Y. Luo, M. St ormer, A. Crespo, P. Schaaf, W. Bolse, Appl. Phys. A 61 (1995) 591. [7] A. Crespo-Sosa, P. Schaaf, W. Bolse, K.P. Lieb, M. Gimbel, U. Geyer, C. Tosello, Phys. Rev. B 53 (1996) 14795. [8] A. Crespo-Sosa, W. Bolse, U. Geyer, M. Gimbel, K.P. Lieb, in: J.L. Duggau, I.L. Morgan (Eds.), Application of Accelerators in Research and Industry, AIP Press, New York, 1997, p. 945. [9] T. Weber, K.P. Lieb, J. Appl. Phys. 73 (1993) 3499. [10] F. Shi, T. Weber, W. Bolse, K.P. Lieb, Appl. Phys. A 57 (1993) 343. [11] F. Shi, W. Bolse, K.P. Lieb, J. Appl. Phys. 78 (1995) 2303. [12] K.P. Lieb, W. Bolse, M. Uhrmacher, Nucl. Instr. and Meth. B 89 (1995) 277. [13] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke, K.P. Lieb, Nucl. Instr. and Meth. B 9 (1985) 234. [14] J.P. Biersack, Program TRIM, private communication. [15] A. Bartos, K. Schemmerling, Th. Wenzel, M. Uhramacher, Nucl. Instr. and Meth. A 330 (1993) 132. [16] M. M unzenberg, Diploma Thesis (unpubl.), G ottingen, 1997. [17] R.S. Raghavan, P. Raghavan, Nucl. Instr. and Meth. 92 (1971) 437. [18] B. Lindgren, Y.K. Vijay, Hyper®ne Interactions 9 (1981) 379. [19] E. Kneller, Ferromagnetismus, Springer, Berlin, 1962.
Ion-beam induced magnetic anisotropies in iron ®lms M. Neubauer a, N. Reinecke a, M. Uhrmacher a,*, K.P. Lieb a, M. M unzenberg b, b W. Felsch a b
II. Physikalisches Institut, Universit at G ottingen and Sonderforschungsbereich 345, D-37073 G ottingen, Germany I. Physikalisches Institut, Universit at G ottingen and Sonderforschungsbereich 345, D-37073 G ottingen, Germany
Abstract 100±300 nm thin Fe layers evaporated onto crystalline and amorphous Si or SiO2 substrates were irradiated, at 77 K, with 1014 ±1016 Xe -ions/cm2 at 450 keV beam energy. The magnetizations in the ®lms were measured by means of Perturbed Angular Correlation (PAC) spectroscopy with implanted 111 In tracer ions, or the Magneto-Optic Kerr Eect (MOKE). Upon ion implantation, dramatic changes of the magnetic anisotropy were observed which are attributed to ion-beam enhanced lateral grain growth. Very little in¯uence of the deposition parameters (type and cristallinity of substrate, evaporation rate) on the anisotropic magnetization was found. Ó 1998 Elsevier Science B.V. PACS: 75.70.Ak; 81.40.Wx; 76.80.+y
1. Introduction Ion irradiations of thin metallic ®lms induce a number of interesting and technologically important modi®cations, many of which are not at all understood theoretically. The high local energy deposition of implanted heavy ions not only leads to ballistic eects like sputtering and ballistic interface mixing in multilayers, but can induce changes of the microstructure of the ®lms. Many of these phenomena seem to be related to the formation of thermal spikes which are expected to occur if
* Corresponding author. Address: M. Uhrmacher, Universitat G ottingen, II. Physikalisches Institut, Bunsenstrasse 7-9, 37073 G ottingen, Germany. Tel.: 0551-397613; fax: 0551394493; e-mail: [email protected].
the average element number of the matrix and the projectile exceeds Z 20 [1±3]. In our recent studies of Ag/Fe bilayers irradiated with Ar and Xe ions, we have encountered ion-beam enhanced grain growth and texturing in the Ag-top layer as well as demixing at the Ag/Fe interface, as proven by Rutherford Backscattering Spectrometry (RBS), RBS-channeling, Scanning Tunneling Microscopy (STM) and hyper®ne interaction methods [4±8]. It appears that in this thermodynamically non-miscible system, as in many other miscible metallic multilayer systems [9±12], (local) thermal spikes are a very ecient driving mechanism for the atomic transport processes which happen at low substrate temperatures. The present work addresses to dramatic changes of the magnetic structure of thin Fe ®lms irradiated with heavy ions. We have observed,
0168-583X/98/$19.00 Ó 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 8 - 5 8 3 X ( 9 7 ) 0 0 9 6 8 - 3
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
333
via Perturbed Angular Correlation (PAC) spectroscopy with radioactive 111 In tracer ions, changes of the magnetic anisotropy within the Fe-®lms, upon irradiations with Xe ions. In addition to the hyper®ne analysis, we have also used the MagnetoOptic Kerr Eect (MOKE) to measure the magnetization curves along dierent directions of the Xeirradiated Fe ®lms and to study their dependence on the ion ¯uence and the procedures the ®lms were prepared (type, cristallinity and temperature of the substrate, evaporation rate of iron). 2. Experimental details The 100±300 nm thick Fe layers of 4N quality were deposited via electron evaporation onto Si or SiO2 substrates, at a rate of 0.1±0.3 nm/s as measured with a quartz crystal. The thickness of each layer was checked by RBS at 900 keV a-particle energy. We used cristalline Si (1 0 0), Si (1 1 0) and a-SiO2 substrates as well as amorphous SiO2 substrates obtained by heating Si wafers for 24 h at 900°C in air. The ®lms were prepared in two evaporation apparatus: in a high vacuum (HV) chamber at the base pressure of 10ÿ5 Pa, the uncooled substrate was at a distance of 26 cm from the electron gun. Alternatively, we also used an UHV chamber at the base pressure of 10ÿ9 Pa in which the water-cooled substrate was 15 cm away from the electron gun. The 111 In tracer implantations (about 8 ´ 1012 /cm2 of 111 In ions implanted at 280 keV onto a 4-mm-diameter beam spot) and Xe-ion irradiations (1014 ±1016 /cm2 at 450 keV) were performed with the G ottingen ion implanter IONAS [13], at a pressure of 5 ´ 10ÿ4 Pa in the irradiation chamber. During the Xe-implantations, the samples were cooled to 77 K and the ions covered a 10 ´ 10 mm2 beam spot on the target, as achieved by means of an electric x± y sweeping system. The mean range of the implanted Xe-ions (111 In ions) in Fe, as predicted by the TRIM code [14], is about 70 nm (50 nm) and hence, is less than the Fe-®lm thickness. Ion-induced processes at the Fe/substrate interface therefore should not play a role. The PAC spectra were taken at room temperature, by means of a set-up of four BaF2 detectors
Fig. 1. PAC perturbation functions R(t) and Fourier transforms A(x) of a Ag/Fe (50/130 nm) Fe layer doped with 111 In atoms at 280 keV implantation energy (a) and irradiated with 6 ´ 1015 Xe-ions/cm2 at 450 keV (b). The change of the magnetic anisotropy can be seen from the ratio of the two components x0L and 2x0L in the Fourier spectra at the Larmor frequency x0L 0:56 GHz.
in 90° geometry, having a time resolution of 1.5 ns which are coupled to a fast-fast coincidence circuit [15]. The samples were oriented in a plane which was perpendicular to the detector plane and intersecting it at 45° to the detector directions (see inset of Fig. 1). A hyper®ne ®eld pointing parallel to the detector plane (k) shows an oscillation in the PAC perturbation function at the Larmor frequency xL , while a hyper®ne ®eld pointing perpendicular to the detector-plane along the 45° direction (^) induces a precession at twice the Larmor frequency, 2xL . In this way, an anisotropic hyper®ne ®eld can be clearly detected, since for a random orientation of the magnetization both frequencies have equal intensities in the Fourier spectrum. The MOKE apparatus used in the present analysis to measure the macroscopic magnetization in the 100 nm top layer of the sample has been described in [16]. A polarized laser beam at 532 nm wavelength was employed. These samples had, of course, not been doped with the 111 In activity. 3. Results of the PAC experiments The primary information on ion-induced changes of the magnetic anisotropy in 200 nm Fe foils
334
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
came from PAC analyses. Fig. 1 shows PAC perturbation spectra R(t) and their Fourier transforms A(x) obtained directly after ®lm deposition (a) and after an irradiation with 6 ´ 1015 Xe-ions/cm2 at 77 K (b). In this case we used an Ag/Fe bilayer sample having a 50 nm thick Ag top layer covering a 150 nm Fe layer. In previous experiments, we had shown that the ion-induced intermixing eect at the bilayer interface is counterbalanced by thermal-spike phase segregation leaving a ¯at interface and a very small mixing rate. Both Fourier spectra exhibit components having the Larmor frequency x0L 0.56 GHz at room temperature and twice this value, typical for 111 In atoms sitting on substitutional, defect-free
lattice sites [17,18]. A small fraction of tracer atoms having trapped radiation defects can be identi®ed on the basis of their smaller Larmor frequency xdL 0.53 GHz [4,5]. This component produces tails at the low-frequency edges in the Fourier spectra. Clearly, the orientation of the magnetization has changed as a consequence of the Xe-ion irradiation, as can be distinguished from the ratios of the components with x0L and 2x0L before and after Xe implantation.This ratio has decreased from 1.07(4) which value is typical for a random distribution of the magnetization within the ®lm to 0.13(4), i.e. the Xe implantation has induced a magnetization within the detector plane.
Fig. 2. Hysteresis curves of a Fe ®lm deposited at a rate of 0.3 nm/s onto Si (1 0 0) and irradiated with 4 ´ 1014 , 4 ´ 1015 and 4 ´ 10 16 Xe-ions/cm2 , at 77 K. The orientation of the polarizing ®eld is given by the angle h 0°, 90° and 45° relative to an axis along the boundary of the ®lm.
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
335
4. Results of the MOKE analysis In order to check the in¯uence of the ®lm deposition parameters on the ion-induced magnetic anisotropy, magnetization measurements by means of MOKE were carried out for as-deposited and ionirradiated Fe ®lms. In particular, we used layers evaporated onto the dierent Si and SiO2 substrates mentioned before, at dierent deposition rates (0.1±0.3 nm/s) and for dierent pressures in the evaporation chamber. Fig. 2 illustrates hysteresis curves taken for a 130 nm thick sample deposited in the HV chamber at a rate of 0.3 nm/s onto a Si (1 0 0) substrate and irradiated with 4 ´ 1014 , 4 ´ 1015 and 4 ´ 1016 Xe-ions/cm2 . The quantities to be discussed later are the coercive ®eld Hc and the ®eld strength HA necessary to reach 90% of the saturation magnetization Ms , as indicated in Fig. 2. The polarizing ®eld H pointed along the angles h 0°, 90° and 45° relative to an axis of the specimen's surface. For the as-deposited ®lm, one notes a nearly isotropic distribution of the coercive ®eld, Hc 51(4) Oe, and relative remanence, Mr /Ms 0.79 within the sample plane. At a Xe¯uence of 4 ´ 1014 ions/cm2 , there appears an anisotropy of the remanent magnetization along the 0° axis, Mr (0°)/Ms (0°) 0.93 versus Mr (90°)/ Ms (90°) 0.47, and of the ®eld strength, HA (0°) 1(3) Oe versus HA (90°) 13(3) Oe. This magnetic anisotropy does not change much during further irradiations and reaches Mr (0°)/Ms (0°) 0.97 versus Mr (90°)/Ms (90°) 0.36 at the highest ¯uence of U 1016 ions/cm2 . Similar results were obtained for Fe ®lms on Si (1 1 0) and amorphous and crystalline SiO2 substrates as well as for samples evaporated at a rate of 0.1 or 0.2 nm/s. The measured angular dependence of the remanence Mr /Ms is shown in Fig. 3. This Fe ®lm had been deposited at a rate of 0.1 nm/s onto Si (1 0 0), under HV conditions, and bombarded with only 1014 Xe-ions/cm2 . The remanence follows a periodic function, its minimal value being Mr /Ms 0.6. On the top of Fig. 3, the á1 1 0ñ orientation of the Fe grains and the easy (1 0 0) magnetization axes along the directions h 155(9)° and 335(9)° are indicated as derived from the angular dependence of Mr /Ms . We conclude that the polycristalline Fe ®lm which originally had
Fig. 3. (a) Orientation of the Fe crystallites in a 130 nm Fe foil evaporated onto Si (1 0 0) at a rate of 0.1 nm/s and irradiated with 4 ´ 1014 Xe-ions/cm2 , as deduced from the measured angular dependence of the relative remanence Mr /Ms plotted in (b).
no preferred magnetization has gained, under the ion bombardment, a magnetic texture. In that respect, the MOKE analysis strongly supports the ®nding of the PAC experiment. Somewhat dierent MOKE results were found for 185 nm Fe ®lms deposited at a rate of 0.3 nm/s under UHV conditions on water-cooled Si (1 0 0) substrates as shown in Fig. 4: Up to a Xe ¯uence of 1015 /cm2 , no anisotropic magnetization appeared, but the isotropic remanence rose from Mr /Ms 0.63 to 0.82. Only after further irradiation with a total of 1016 Xe-ions/cm2 did the remanence at 90° further rise to Mr /Ms 0.92, while it decreased to 0.73 in the 0° direction. In this sample, the anisotropic magnetization therefore sets in at higher ion ¯uences than in the samples described before. Although we so far do not have a full interpretation of this dierence, we note that the main dierence between these and the former samples is pobably caused by a dierent grain size in the as-deposited state [19]. Arguing from our previous Xe-implantation experiment of thin Ag ®lms [7,8], we infer that the ion bombardment induces a grain growth as a consequences of thermal spikes.
336
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
Fig. 4. Same as Fig. 2, but for a ®lm deposited under UHV conditions onto a water-cooled Si (1 0 0) substrate.
Fig. 5 illustrates the measured dependence of the coercive ®eld Hc as of the ion ¯uence U, for four ®lms deposited under HV and UHV conditions. Assuming that the ion irradiation leads to grain growth in the ®lm plane, as also inferred for the Fe ®lms from the MOKE data shown in Fig. 3, one expects a relationship Hc 3cB = DJs / cB = Js U1=n ; where Js l0 Ms denotes the saturation polarization, cB the energy of the Bloch domain, and D the grain size [19]. The curves shown in Fig. 5 exhibit exponents of n 5 for the samples deposited onto the uncooled substrates in the HV chamber, and n 1.6 for the samples deposited under UHV conditions onto the water-cooled substrates. X-Ray Diraction of the latter Fe-®lms performed
with the Cu-Ka line showed that the line widths of the [1 1 0] re¯exes are consistent with a grain size of 33(5) nm, in the vertical direction, before and after ion irradiation. As to the change of the lateral grain size due to ion bombardment, a STM analysis in air was not successful, due to the oxide surface layer. For that reason, further crystallographic investigations of the microstructure and texture of the Fe ®lms, via XRD and STM, are necessary to check the structural properties and their correlation with the magnetization. 5. Summary Irradiations of 100±300 nm thick Fe ®lms by energetic Xe ions at 77 K have led to pronounced
M. Neubauer et al. / Nucl. Instr. and Meth. in Phys. Res. B 139 (1998) 332±337
337
Acknowledgements The authors are grateful to Dr. L. Ziegeler for help in preparing the 111 In tracer implantations, and to D. Purschke who cooperated in the RBS analyses and Xe implantations. This work was supported by Deutsche Forschungsgemeinschaft. References
Fig. 5. Fluence dependence of the coercive ®eld Hc for samples deposited under HV (open symbols) and UHV (closed symbols) conditions at the deposition rates given. For further details, see text.
changes of the magnetic anisotropy within the ®lm plane, as veri®ed via PAC and MOKE analyses. The latter experiments have demonstrated that, at a Xe-ion ¯uence as low as 1014 /cm2 , the coercive ®eld Hc and magnetic remanence Mr /Ms get anisotropic within the foil plane. The anisotropy increases with the ion ¯uence U according to a Uÿ1=n dependence, where the exponent n was found to range between n 1.5 and 5.5. The exponent n appears to depend on the grain size within the as-deposited ®lms. The MOKE experiments have also shown that neither the nature and crystallinity of the substrate nor the deposition rate during electron evaporation do appreciably in¯uence the magnetic anisotropy. We tentatively correlate the ion-induced magnetic texturing of the Fe-foils with changes of the lateral grain size and orientation as previously established for Xe-irradiated Ag ®lms [7,8]. Experiments are in progress which address to the dependence of the magnetic anisotropy on the ®lm thickness and ion species and to detailed studies of the microstructure before and after ion irradiation.
[1] W.L. Johnson et al., Nucl. Instr. and Meth. B 7/8 (1985) 657; R.S. Averback, Nucl. Instr. and Meth. B 15 (1986) 675. [2] Y.T. Cheng, Mat. Sci. Rep. 5 (1990) 45. [3] W. Bolse, Mat. Sci. Rep. R 12 (1994) 53. [4] M. Neubauer, K.P. Lieb, P. Schaaf, M. Uhrmacher, Phys. Rev. B 53 (1996) 10237. [5] M. Neubauer, K.P. Lieb, P. Schaaf, M. Uhrmacher, Thin Solid Films 275 (1996) 69; Europhys. Lett., submitted. [6] H.U. Krebs, Y. Luo, M. St ormer, A. Crespo, P. Schaaf, W. Bolse, Appl. Phys. A 61 (1995) 591. [7] A. Crespo-Sosa, P. Schaaf, W. Bolse, K.P. Lieb, M. Gimbel, U. Geyer, C. Tosello, Phys. Rev. B 53 (1996) 14795. [8] A. Crespo-Sosa, W. Bolse, U. Geyer, M. Gimbel, K.P. Lieb, in: J.L. Duggau, I.L. Morgan (Eds.), Application of Accelerators in Research and Industry, AIP Press, New York, 1997, p. 945. [9] T. Weber, K.P. Lieb, J. Appl. Phys. 73 (1993) 3499. [10] F. Shi, T. Weber, W. Bolse, K.P. Lieb, Appl. Phys. A 57 (1993) 343. [11] F. Shi, W. Bolse, K.P. Lieb, J. Appl. Phys. 78 (1995) 2303. [12] K.P. Lieb, W. Bolse, M. Uhrmacher, Nucl. Instr. and Meth. B 89 (1995) 277. [13] M. Uhrmacher, K. Pampus, F.J. Bergmeister, D. Purschke, K.P. Lieb, Nucl. Instr. and Meth. B 9 (1985) 234. [14] J.P. Biersack, Program TRIM, private communication. [15] A. Bartos, K. Schemmerling, Th. Wenzel, M. Uhramacher, Nucl. Instr. and Meth. A 330 (1993) 132. [16] M. M unzenberg, Diploma Thesis (unpubl.), G ottingen, 1997. [17] R.S. Raghavan, P. Raghavan, Nucl. Instr. and Meth. 92 (1971) 437. [18] B. Lindgren, Y.K. Vijay, Hyper®ne Interactions 9 (1981) 379. [19] E. Kneller, Ferromagnetismus, Springer, Berlin, 1962.