- Email: [email protected]
Effect of ion beam irradiation on studies of surface magnetism via capture of spin-polarized electrons from a thin Co film on Cu(0 0 1)
Nuclear Instruments and Methods in Physics Research B 232 (2005) 16–21 www.elsevier.com/locate/nimb
Effect of ion beam irradiation on studies of surface magnetism via capture of spin-polarized electrons from a thin Co film on Cu(0 0 1) T. Bernhard, H. Winter
*
Institut fu¨r Physik der Humboldt – Universita¨t zu Berlin, Newtonstr. 15, D-12489 Berlin-Adlershof, Germany Available online 27 April 2005
Abstract The magnetism of ultrathin Co films deposited on a Cu(0 0 1) substrate is studied via capture of spin-polarized electrons into the He I 3p 3P term after grazing scattering of 25 keV He+ ions. For films of 5 ML we explore the effect of ion beam irradiation on the data and investigate the removal of film atoms by incident ions via Auger electron spectroscopy. From our work we deduce sputtering yields for bombardment of the film surface with He+ and Ar+ ions and determine the regime of ion fluences where the method of electron capture is applicable for studies on surface magnetism of ultrathin films. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Studies on magnetic properties of surfaces have obtained increased attention in the past decade, since various applications are affected in a decisive manner by processes at the vacuum solid interface [1]. This statement holds, in particular, for ultrathin films, where a variety of fascinating new effects are observed [2]. As example, we mention the giant magneto resistance (GMR) effect [3,4] or manipulation of magnetic behaviour via *
Corresponding author. Tel.: +49 30 2093 7891; fax: +49 30 2093 7899. E-mail address: [email protected] (H. Winter).
ion beam irradiation [5,6]. In addition to the variety of ‘‘established’’ techniques to investigate magnetism of ultrathin films – as magneto optical Kerr effect (MOKE), scanning electron microscopy with polarization analysis (SEMPA), spinpolarized low energy electron diffraction (SPLEED), the capture of spin-polarized electrons into atomic terms after scattering of fast ions from the surface of ultrathin films has been employed for studies of magnetic properties [7]. Here we will concentrate on a specific version of the electron capture method, i.e. the detection of spin-polarized electrons bound in excited atomic terms via measurement of the polarization of the fluorescence light [8].
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.018
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
This method was applied for studies on magnetism for a variety of different clean metal surfaces [8–10] and in recent years also for ultrathin films on magnetic [11] and non-magnetic substrates [12,13]. Capture of electrons into excited atomic terms as a spectroscopic method has a high sensitivity to the topmost surface layer, since, in a simple picture, excited atomic terms cannot survive within or close to a solid. This is manifested by high electron transition rates for those terms close to the topmost surface layer which result in high probabilities for electron loss. Only for sufficiently low transition rates, i.e. at some distance of the atom core from the surface plane, excited electronic terms can survive and probe electronic features in front of the surface only. Recently, Pfandzelter and Potthoff [14] have presented a demonstration of this property by comparing layer resolved mean-field calculations with data obtained with different methods for studies on surface magnetism as function of temperature. The authors showed that capture of electrons into excited atomic terms has indeed the expected high sensitivity to the region in front of the surface plane. Irrespective of the attractive properties of the method, basic mechanisms are still matter of debate and not cleared up so far. This holds, in particular, for the capture process which has to be understood microscopically in terms of electron capture from a system with ‘‘complicated’’ electronic structure. In the last two decades considerable success can be stated in the description of electron capture mechanisms for scattering from surfaces of free-electron metals [15,16] as well as noble metals with projected band gaps [17,18]. However, for magnetic surfaces described by a spin resolved electronic band structure calculations of capture processes are far from being complete. This holds especially for the incorporation of dynamic effects into the model, since established concepts for charge transfer from simple metal surfaces approximated by a free-electron gas (Galilei transformation of plane waves, concept of shifted Fermi sphere [15]) are not applicable here. A further problem in using the electron capture method is the effect of the bombardment of the
17
film surface with ions. Ions are also used for the preparation of the target surface where for a sufficiently high dose (fluence) of scattered ions the surface is modified via ‘‘sputtering’’ in a controlled manner, i.e. removal of defect structures and adsorbed atoms from the surface plane. Since the capture of spin-polarized electrons is detected via fluorescence light with a small solid angle, considerable ion currents of several tens of nA are needed, in order to achieve a sufficiently high signal-to-noise ratio in the measurements. Therefore it is crucial to support the so far only vaguely proven statements of a small or even negligible influence of the ion beam irradiation on the film surface. In the work presented here we have studied the magnetic properties of ultrathin Co films grown on a Cu(0 0 1) surface as function of the fluence of incident He+ ions. By means of Auger electron spectroscopy we monitor the removal of atoms from the film as function of ion fluence. From our studies we derive limits for the application of the electron capture method concerning investigations on the long range magnetic order of ultrathin films.
2. Experiment and results In our experiments we have scattered 25 keV He atoms as well as He+ and Ar+ ions under a grazing angle of incidence U = 1.6° from the surface of a thin Co film expitaxially grown on a Cu(0 0 1) surface. Growth of the film was inspected by the intensity of specularly reflected He atoms showing pronounced oscillations with Co coverage, a signature for a near-perfect layer-by-layer growth [19]. The atoms are detected with a channeltron detector positioned on a precision manipulator for recording polar angular distributions of scattered particles. The target was kept at a base pressure in the upper 1011 mbar domain and magnetized normal to the scattering plane by means of a pair of external Helmholtz coils. In order to avoid angular deflections of incident projectiles by the external magnetic field during measurements of hysteresis loops, neutral He beams were used. They were produced via electron capture in a He gas target in the beamline of our setup. Since for
18
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
He atoms with an energy of 25 keV a charge equilibrium via electron capture and loss is reached close to the surface, no difference is found compared to data obtained for incident ions. The polarized fluorescence light emitted in the He I 2s 3S–3p 3P, k = 389 nm transition is detected normal to the scattering plane by means of a quarter wave plate, a linear polarizer, and a EMI-6265 photomultiplier. With specific angles of rotation of the quarter wave plate relative to the linear polarizer the two intensity components with left and right handed circular polarization are obtained. Sputtering effects on the films are observed via Auger electron spectroscopy of the Cu LMM line at an electron energy of about 650 eV where the production of inner shell holes is achieved with a beam of 4 keV electrons from an electron gun directed under large angle impact onto the target. Emitted electrons are detected using a CLAM2-spectrometer (VG Instruments). In Fig. 1 we show hysteresis loops obtained for scattering of He atoms from the surface of a 5 ML Co film deposited on Cu(0 0 1) at a temperature of T = 140 K (left panel) and T = 410 K. In the measurements the target is kept at a temperature of T = 140 K (clearly below the Curie temperature for this system [13]), and we recorded the degree of circularly polarized light given by the Stokes
0.0
0.0
140K
410K
-0.1
-0.1
0.25
-0.2
-0.2
0.20
-0.3
-0.3
0.15
-0.4
-0.4
0.10
-0.5
-0.5
0.05
-10
-5
0
5
10 -10
-5
0
5
10
magnetic field (mT) Fig. 1. Stokes parameter S/I for the He I 2s 3S–3p 3P, k = 389 nm transition as function of external magnetic field for scattering of 25 keV He atoms under Uin = 1.6° from a 5 ML Co film grown on Cu(0 0 1) at 140 K (left panel) and 410 K (right panel). Measurements are performed at 140 K.
Ps
S/I
parameter S/I = (I(r) I(r+))/(I(r) + I(r+)) where I(r ) and I(r+) denote the intensity of light with negative and positive helicity, respectively. S/I is closely related to the polarizations PS and PL of an atomic ensemble with electronic spin S and orbital angular momentum L. In passing we note that transitions in the triplet systems are investigated here (S = 1) so that after capture of one polarized electron coupling of spins for two electrons in the excited He atom takes place. Details on the anisotropy transfer in the coupling are not understood so far; in particular, the substantially higher PS for one electron capture observed in triplet in comparison to doublet systems is an open question [9]. Nevertheless, there are good arguments that for constant scattering conditions (esp. projectile velocity) the measured PS can be related to the electronic spin polarization of a long range magnetic order at the surface. In the data shown in Fig. 1 we reveal a clearly higher variation of S/I and higher coercive fields for growth at the lower temperature (140 K). In a recent paper we have ascribed this observation to the effect of intermixing at growth at the elevated temperature which results for the specific system to about 2 ML Cu atoms floating on top of the Co film [13]. In our studies on ion irradiation effects presented here we find support for this interpretation. In Fig. 2 we have plotted the spin
0.00
130K
410K
sputtering
growth
0
1
2
3
4
5
coverage (ML)
Fig. 2. Electron spin polarization PS as function of Co coverage on Cu(0 0 1) for Co films grown at 130 K and 410 K. Both films were bombarded with 25 keV He+ ions. Co coverage is deduced from Auger electron spectra.
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
polarization of captured electron PS as function of film coverage for growth and removal (via sputtering by He+ ions) of the film. PS is deduced from S/I for reversed settings of the target magnetization and concepts of coupling of quantum mechanical angular momenta [9]. During sputtering the data for 410 K growth increase and reach after a reduction of the film coverage from 5 ML to about 3 ML the data for the film grown at 130 K, where no intermixing processes take place. This result is consistent with our finding of a concentration of 2 ML Cu atoms in the top layers of Co film as derived in independent work with Auger electron spectroscopy [20]. The data demonstrates clearly the effect of impurity atoms (Cu) on the surface magnetism of the Co film. The reduction of the coverage of the film via sputtering with ions is monitored via Auger electron spectroscopy. In Fig. 3 we show electron spectra for the Co LMM transition at about 650 eV as function of fluence F of He+ ions impinging at an angle of Uin = 1.6° on the surface of a Co film grown at 140 K. F is given here in terms of number of atoms equivalent to ML of the film. With increasing fluence the intensity of the Auger line is gradually reduced which indicates the removal of Co atoms from the film surface (escape depth of 650 eV electrons is about 6 ML). From electron spectra as displayed in Fig. 3 we derive the amount
19
of Co atoms removed via sputtering from the film. A plot of the results as function of ion fluence is shown in Fig. 4 for irradiation of the film surface with He+ (full circles) and Ar+ (full triangles) ions. The solid curves in the figure are best fits to an arbitrarily chosen power expansion which reveals for the He+ data slight variations in slope. Sputtering with Ar+ ions is much more efficient than with He+ atoms so that only two data points fall into the regime shown in Fig. 4. Since the polarization measurements are performed with He+ ions and He atoms, we will concentrate here on the results for those projectiles and note for the heavier projectile only that our observation is in qualitative accord with the expected behaviour. From the slope of the curve shown in Fig. 4 we obtain the changes of removed Co atoms for a given ion fluence. This allows us to derive zsputter yields as function of fluence. From the plot of sputtering yields for Co atoms YCo in Fig. 5 for grazing sputtering of the Co film with 25 keV He+ ions one reveals an interesting behaviour of YCo as function of fluence. In the beginning of the irradiation of the surface, sputter yields are small, reach a maximum of YCo 0.1, and decrease slightly for higher fluences. The sputter yields for 25 keV Ar+ ions were not investigated in detail here and will serve only as a
Ar +
F= 0ML removed Co (ML)
Co-LMM-Auger-intensity (norm.)
2.0
1.04 11 1.02
28 40
1.00
1.5
He + 1.0
0.5
25 keV He +, Ar+ - Co/Cu(001)
Cu 0.0
620
630
640
650
660
670
0
5
10
15
20
25
fluence F (ML)
electron-energy (eV) Fig. 3. Intensity of Co Auger signal as function of electron energy for bombardment of 5 ML Co film grown on Cu(0 0 1) for increasing fluence of 25 keV He+ ions impinging on surface under Uin = 1.6°.
Fig. 4. Removal of Co atoms from 5 ML Co film grown on Cu(0 0 1) at room temperature as function of fluence of 25 keV He+ ions (full circles) and Ar+ ions (full triangles) impinging on surface under Uin = 1.6°. Solid curves: fits with polynomial function.
20
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21 0.8
sputtering yield YCo
25 keV He+, Ar + - Co/Cu(001) 0.6
Ar+
0.4
0.2
0.0
He+
0
5
10
15
20
25
fluence F (ML) Fig. 5. Sputtering yield for Co atoms YCo as function of ion fluence for bombardment of 5 ML Co film grown on Cu(0 0 1) at room temperature by 25 keV He+ ions (solid curve) and Ar+ ions (dashed curve) impinging on surface under Uin = 1.6°.
demonstration of the expected more efficient removal of target material caused by the larger energy transfer to surface atoms in elastic collisions owing to the considerably larger mass. We also investigated angular distributions for scattered projectiles and found that the intensity is reduced over the range of fluence displayed in Fig. 4 by about 40%, combined with a slight broadening of the widths of the corresponding angular distributions. From this observation we conclude that the surface of the film gets gradually rougher with ion irradiation. Signatures for a type of ‘‘inverse growth’’ manifested by oscillations in the intensity [21] could not be observed in our work. 3. Discussion From our work important conclusions concerning the use of fast ions for studies on surface magnetism via capture of polarized electrons into excited atomic terms can be drawn. In order to estimate the regime of applicability of the method, we mention that an ion fluence equivalent to typically 1–2 ML is needed for recording hysteresis loops as shown in Fig. 1. Such fluences are reached in measurements with currents of some 10 nA after about 20 min. From the sputtering yields given in Fig. 5 we estimate for the initial bombardment a mass removal of some percent of a ML. As a con-
sequence several measurements of complete hysteresis loops can be performed before the modification of the film surface affects the studies in a substantial manner. Concerning the sputtering process, there is a long standing experience that this process is clearly different under grazing than for large angle impact. This feature has been routinely used to prepare well defined and flat crystal surfaces [16] and for the smoothening during film growth [22]. A striking observation of our study is the dependence of the sputtering yield on the ion fluence. A similar effect was observed recently for bombardment of Pt(1 1 1) with 5 keV Ar+ ions under grazing incidence and explained by drastically enhanced sputtering yields for collisions with steps Ystep compared to those for scattering from (flat) terraces Yterr [23]. Analysis of experimental data with molecular dynamics calculations imply that these yields differ by more than one order of magnitude, i.e. under grazing sputtering mass removal almost completely proceeds at steps (and other surface defects). By a simple geometrical model it was shown that sputtering depends on the widths of terraces L and step heights h. Only for terraces smaller than a critical width Lc ¼ 2h= tan Uin direct encounter of all projectiles hitting a terrace with step atoms will occur, otherwise the ions are reflected from the terrace with negligible sputtering effects. This model might explain our experimental observation that for the initial stage of bombardment of the surface the sputtering yield increases with fluence. In this regime, ion impact will enhance the step density at the surface and the sputtering yield will increase, until the critical width for terraces and saturation of the sputter yields is reached. In our experiments we have observed a decrease of YCo for higher fluences (cf. Fig. 5). This decrease takes place at fluences above about 8 ML where about half of the first ML is removed from the film. For this range of fluences we conclude from the angular distributions for scattered projectiles that the concentration of defects of the film surface is gradually enhanced. A description of the sputtering process under incorporation of a modelling of the morphology of the film with a realistic defect structure is beyond the scope of the present work.
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
Acknowledgments This work is supported by the Deutsche Forschungsgemeinschaft under contract Wi 1336. We thank R.A. Noack, K. Maass, M. Baron, and Dr. M. Gruyters for their assistance in preparation and running of the experiments and for helpful discussions. References [1] U. Gradmann, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 7/1, Elsevier, Amsterdam, 1993, p. 1. [2] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Springer, Berlin, 1994. [3] P. Gru¨nberg, R. Schreiber, Y. Pang, M.B. Brodsky, H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [4] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Crenzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [5] S.O. Demokritov, C. Bayer, S. Poppe, M. Rickart, J. Fassbender, B. Hillebrands, D.I. Kholin, N.M. Kreines, O.M. Liedke, Phys. Rev. Lett. 90 (2003) 097201. [6] R. Moroni, D. Sekiba, F. Buatier de Mongeot, G. Gonella, C. Boragno, L. Mattera, U. Valbusa, Phys. Rev. Lett. 91 (2003) 167207. [7] F.B. Duning, C. Rau, G.K. Walters, Comments Solid State Phys. 12 (1985) 17.
21
[8] H. Winter, H. Hagedorn, R. Zimny, H. Nienhaus, J. Kirschner, Phys. Rev. Lett. 62 (1989) 296. [9] H. Winter, Z. Physik D 23 (1992) 41. [10] J. Manske, M. Dirska, G. Lubinski, M. Schlehberger, A. Na¨rmann, R. Hoekstra, J. Magn. Magn. Mater. 168 (1997) 249. [11] T. Igel, R. Pfandzelter, H. Winter, Appl. Surf. Sci. 142 (1999) 532. [12] A.R. Bachmann, S. Speller, J. Manske, M. Schleberger, A. Na¨rmann, W. Heiland, in: J.L. Moran-Lopez (Ed.), The Physics of Low Dimensional Systems, Kluwer Academic, Plenum Publishers, New York, 2001, p. 265. [13] M. Gruyters, T. Bernhard, H. Winter, J. Magn. Magn. Mater. 292 (2005) 192. [14] R. Pfandzelter, M. Potthoff, Phys. Rev. B 64 (2001) 14045. [15] J. Los, J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. [16] H. Winter, Phys. Rep. 367 (2002) 387. [17] A.G. Borisov, A.K. Kazansky, J.P. Gauyacq, Phys. Rev. Lett. 80 (1998) 1996. [18] T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, A.K. Kazansky, Phys. Rev. Lett. 84 (2000) 2517. [19] R. Pfandzelter, Phys. Rev. B 57 (1998) 15496. [20] T. Bernhard, M. Gruyters, H. Winter, Surf. Sci. 575 (2005) 154. [21] M.V. Ramana Murty, T. Curcic, A. Judy, B.H. Cooper, A.R. Woll, J.D. Brock, S. Kycia, R.L. Headrick, Phys. Rev. Lett. 80 (1998) 4713. [22] K.C. Ruthe, S.A. Barnett, Surf. Sci. Lett. 538 (2003) L460. [23] H. Hansen, C. Polop, T. Michely, A. Friedrich, H.M. Urbassek, Phys. Rev. Lett. 92 (2004) 246106.
Effect of ion beam irradiation on studies of surface magnetism via capture of spin-polarized electrons from a thin Co film on Cu(0 0 1) T. Bernhard, H. Winter
*
Institut fu¨r Physik der Humboldt – Universita¨t zu Berlin, Newtonstr. 15, D-12489 Berlin-Adlershof, Germany Available online 27 April 2005
Abstract The magnetism of ultrathin Co films deposited on a Cu(0 0 1) substrate is studied via capture of spin-polarized electrons into the He I 3p 3P term after grazing scattering of 25 keV He+ ions. For films of 5 ML we explore the effect of ion beam irradiation on the data and investigate the removal of film atoms by incident ions via Auger electron spectroscopy. From our work we deduce sputtering yields for bombardment of the film surface with He+ and Ar+ ions and determine the regime of ion fluences where the method of electron capture is applicable for studies on surface magnetism of ultrathin films. Ó 2005 Elsevier B.V. All rights reserved.
1. Introduction Studies on magnetic properties of surfaces have obtained increased attention in the past decade, since various applications are affected in a decisive manner by processes at the vacuum solid interface [1]. This statement holds, in particular, for ultrathin films, where a variety of fascinating new effects are observed [2]. As example, we mention the giant magneto resistance (GMR) effect [3,4] or manipulation of magnetic behaviour via *
Corresponding author. Tel.: +49 30 2093 7891; fax: +49 30 2093 7899. E-mail address: [email protected] (H. Winter).
ion beam irradiation [5,6]. In addition to the variety of ‘‘established’’ techniques to investigate magnetism of ultrathin films – as magneto optical Kerr effect (MOKE), scanning electron microscopy with polarization analysis (SEMPA), spinpolarized low energy electron diffraction (SPLEED), the capture of spin-polarized electrons into atomic terms after scattering of fast ions from the surface of ultrathin films has been employed for studies of magnetic properties [7]. Here we will concentrate on a specific version of the electron capture method, i.e. the detection of spin-polarized electrons bound in excited atomic terms via measurement of the polarization of the fluorescence light [8].
0168-583X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.03.018
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
This method was applied for studies on magnetism for a variety of different clean metal surfaces [8–10] and in recent years also for ultrathin films on magnetic [11] and non-magnetic substrates [12,13]. Capture of electrons into excited atomic terms as a spectroscopic method has a high sensitivity to the topmost surface layer, since, in a simple picture, excited atomic terms cannot survive within or close to a solid. This is manifested by high electron transition rates for those terms close to the topmost surface layer which result in high probabilities for electron loss. Only for sufficiently low transition rates, i.e. at some distance of the atom core from the surface plane, excited electronic terms can survive and probe electronic features in front of the surface only. Recently, Pfandzelter and Potthoff [14] have presented a demonstration of this property by comparing layer resolved mean-field calculations with data obtained with different methods for studies on surface magnetism as function of temperature. The authors showed that capture of electrons into excited atomic terms has indeed the expected high sensitivity to the region in front of the surface plane. Irrespective of the attractive properties of the method, basic mechanisms are still matter of debate and not cleared up so far. This holds, in particular, for the capture process which has to be understood microscopically in terms of electron capture from a system with ‘‘complicated’’ electronic structure. In the last two decades considerable success can be stated in the description of electron capture mechanisms for scattering from surfaces of free-electron metals [15,16] as well as noble metals with projected band gaps [17,18]. However, for magnetic surfaces described by a spin resolved electronic band structure calculations of capture processes are far from being complete. This holds especially for the incorporation of dynamic effects into the model, since established concepts for charge transfer from simple metal surfaces approximated by a free-electron gas (Galilei transformation of plane waves, concept of shifted Fermi sphere [15]) are not applicable here. A further problem in using the electron capture method is the effect of the bombardment of the
17
film surface with ions. Ions are also used for the preparation of the target surface where for a sufficiently high dose (fluence) of scattered ions the surface is modified via ‘‘sputtering’’ in a controlled manner, i.e. removal of defect structures and adsorbed atoms from the surface plane. Since the capture of spin-polarized electrons is detected via fluorescence light with a small solid angle, considerable ion currents of several tens of nA are needed, in order to achieve a sufficiently high signal-to-noise ratio in the measurements. Therefore it is crucial to support the so far only vaguely proven statements of a small or even negligible influence of the ion beam irradiation on the film surface. In the work presented here we have studied the magnetic properties of ultrathin Co films grown on a Cu(0 0 1) surface as function of the fluence of incident He+ ions. By means of Auger electron spectroscopy we monitor the removal of atoms from the film as function of ion fluence. From our studies we derive limits for the application of the electron capture method concerning investigations on the long range magnetic order of ultrathin films.
2. Experiment and results In our experiments we have scattered 25 keV He atoms as well as He+ and Ar+ ions under a grazing angle of incidence U = 1.6° from the surface of a thin Co film expitaxially grown on a Cu(0 0 1) surface. Growth of the film was inspected by the intensity of specularly reflected He atoms showing pronounced oscillations with Co coverage, a signature for a near-perfect layer-by-layer growth [19]. The atoms are detected with a channeltron detector positioned on a precision manipulator for recording polar angular distributions of scattered particles. The target was kept at a base pressure in the upper 1011 mbar domain and magnetized normal to the scattering plane by means of a pair of external Helmholtz coils. In order to avoid angular deflections of incident projectiles by the external magnetic field during measurements of hysteresis loops, neutral He beams were used. They were produced via electron capture in a He gas target in the beamline of our setup. Since for
18
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
He atoms with an energy of 25 keV a charge equilibrium via electron capture and loss is reached close to the surface, no difference is found compared to data obtained for incident ions. The polarized fluorescence light emitted in the He I 2s 3S–3p 3P, k = 389 nm transition is detected normal to the scattering plane by means of a quarter wave plate, a linear polarizer, and a EMI-6265 photomultiplier. With specific angles of rotation of the quarter wave plate relative to the linear polarizer the two intensity components with left and right handed circular polarization are obtained. Sputtering effects on the films are observed via Auger electron spectroscopy of the Cu LMM line at an electron energy of about 650 eV where the production of inner shell holes is achieved with a beam of 4 keV electrons from an electron gun directed under large angle impact onto the target. Emitted electrons are detected using a CLAM2-spectrometer (VG Instruments). In Fig. 1 we show hysteresis loops obtained for scattering of He atoms from the surface of a 5 ML Co film deposited on Cu(0 0 1) at a temperature of T = 140 K (left panel) and T = 410 K. In the measurements the target is kept at a temperature of T = 140 K (clearly below the Curie temperature for this system [13]), and we recorded the degree of circularly polarized light given by the Stokes
0.0
0.0
140K
410K
-0.1
-0.1
0.25
-0.2
-0.2
0.20
-0.3
-0.3
0.15
-0.4
-0.4
0.10
-0.5
-0.5
0.05
-10
-5
0
5
10 -10
-5
0
5
10
magnetic field (mT) Fig. 1. Stokes parameter S/I for the He I 2s 3S–3p 3P, k = 389 nm transition as function of external magnetic field for scattering of 25 keV He atoms under Uin = 1.6° from a 5 ML Co film grown on Cu(0 0 1) at 140 K (left panel) and 410 K (right panel). Measurements are performed at 140 K.
Ps
S/I
parameter S/I = (I(r) I(r+))/(I(r) + I(r+)) where I(r ) and I(r+) denote the intensity of light with negative and positive helicity, respectively. S/I is closely related to the polarizations PS and PL of an atomic ensemble with electronic spin S and orbital angular momentum L. In passing we note that transitions in the triplet systems are investigated here (S = 1) so that after capture of one polarized electron coupling of spins for two electrons in the excited He atom takes place. Details on the anisotropy transfer in the coupling are not understood so far; in particular, the substantially higher PS for one electron capture observed in triplet in comparison to doublet systems is an open question [9]. Nevertheless, there are good arguments that for constant scattering conditions (esp. projectile velocity) the measured PS can be related to the electronic spin polarization of a long range magnetic order at the surface. In the data shown in Fig. 1 we reveal a clearly higher variation of S/I and higher coercive fields for growth at the lower temperature (140 K). In a recent paper we have ascribed this observation to the effect of intermixing at growth at the elevated temperature which results for the specific system to about 2 ML Cu atoms floating on top of the Co film [13]. In our studies on ion irradiation effects presented here we find support for this interpretation. In Fig. 2 we have plotted the spin
0.00
130K
410K
sputtering
growth
0
1
2
3
4
5
coverage (ML)
Fig. 2. Electron spin polarization PS as function of Co coverage on Cu(0 0 1) for Co films grown at 130 K and 410 K. Both films were bombarded with 25 keV He+ ions. Co coverage is deduced from Auger electron spectra.
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
polarization of captured electron PS as function of film coverage for growth and removal (via sputtering by He+ ions) of the film. PS is deduced from S/I for reversed settings of the target magnetization and concepts of coupling of quantum mechanical angular momenta [9]. During sputtering the data for 410 K growth increase and reach after a reduction of the film coverage from 5 ML to about 3 ML the data for the film grown at 130 K, where no intermixing processes take place. This result is consistent with our finding of a concentration of 2 ML Cu atoms in the top layers of Co film as derived in independent work with Auger electron spectroscopy [20]. The data demonstrates clearly the effect of impurity atoms (Cu) on the surface magnetism of the Co film. The reduction of the coverage of the film via sputtering with ions is monitored via Auger electron spectroscopy. In Fig. 3 we show electron spectra for the Co LMM transition at about 650 eV as function of fluence F of He+ ions impinging at an angle of Uin = 1.6° on the surface of a Co film grown at 140 K. F is given here in terms of number of atoms equivalent to ML of the film. With increasing fluence the intensity of the Auger line is gradually reduced which indicates the removal of Co atoms from the film surface (escape depth of 650 eV electrons is about 6 ML). From electron spectra as displayed in Fig. 3 we derive the amount
19
of Co atoms removed via sputtering from the film. A plot of the results as function of ion fluence is shown in Fig. 4 for irradiation of the film surface with He+ (full circles) and Ar+ (full triangles) ions. The solid curves in the figure are best fits to an arbitrarily chosen power expansion which reveals for the He+ data slight variations in slope. Sputtering with Ar+ ions is much more efficient than with He+ atoms so that only two data points fall into the regime shown in Fig. 4. Since the polarization measurements are performed with He+ ions and He atoms, we will concentrate here on the results for those projectiles and note for the heavier projectile only that our observation is in qualitative accord with the expected behaviour. From the slope of the curve shown in Fig. 4 we obtain the changes of removed Co atoms for a given ion fluence. This allows us to derive zsputter yields as function of fluence. From the plot of sputtering yields for Co atoms YCo in Fig. 5 for grazing sputtering of the Co film with 25 keV He+ ions one reveals an interesting behaviour of YCo as function of fluence. In the beginning of the irradiation of the surface, sputter yields are small, reach a maximum of YCo 0.1, and decrease slightly for higher fluences. The sputter yields for 25 keV Ar+ ions were not investigated in detail here and will serve only as a
Ar +
F= 0ML removed Co (ML)
Co-LMM-Auger-intensity (norm.)
2.0
1.04 11 1.02
28 40
1.00
1.5
He + 1.0
0.5
25 keV He +, Ar+ - Co/Cu(001)
Cu 0.0
620
630
640
650
660
670
0
5
10
15
20
25
fluence F (ML)
electron-energy (eV) Fig. 3. Intensity of Co Auger signal as function of electron energy for bombardment of 5 ML Co film grown on Cu(0 0 1) for increasing fluence of 25 keV He+ ions impinging on surface under Uin = 1.6°.
Fig. 4. Removal of Co atoms from 5 ML Co film grown on Cu(0 0 1) at room temperature as function of fluence of 25 keV He+ ions (full circles) and Ar+ ions (full triangles) impinging on surface under Uin = 1.6°. Solid curves: fits with polynomial function.
20
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21 0.8
sputtering yield YCo
25 keV He+, Ar + - Co/Cu(001) 0.6
Ar+
0.4
0.2
0.0
He+
0
5
10
15
20
25
fluence F (ML) Fig. 5. Sputtering yield for Co atoms YCo as function of ion fluence for bombardment of 5 ML Co film grown on Cu(0 0 1) at room temperature by 25 keV He+ ions (solid curve) and Ar+ ions (dashed curve) impinging on surface under Uin = 1.6°.
demonstration of the expected more efficient removal of target material caused by the larger energy transfer to surface atoms in elastic collisions owing to the considerably larger mass. We also investigated angular distributions for scattered projectiles and found that the intensity is reduced over the range of fluence displayed in Fig. 4 by about 40%, combined with a slight broadening of the widths of the corresponding angular distributions. From this observation we conclude that the surface of the film gets gradually rougher with ion irradiation. Signatures for a type of ‘‘inverse growth’’ manifested by oscillations in the intensity [21] could not be observed in our work. 3. Discussion From our work important conclusions concerning the use of fast ions for studies on surface magnetism via capture of polarized electrons into excited atomic terms can be drawn. In order to estimate the regime of applicability of the method, we mention that an ion fluence equivalent to typically 1–2 ML is needed for recording hysteresis loops as shown in Fig. 1. Such fluences are reached in measurements with currents of some 10 nA after about 20 min. From the sputtering yields given in Fig. 5 we estimate for the initial bombardment a mass removal of some percent of a ML. As a con-
sequence several measurements of complete hysteresis loops can be performed before the modification of the film surface affects the studies in a substantial manner. Concerning the sputtering process, there is a long standing experience that this process is clearly different under grazing than for large angle impact. This feature has been routinely used to prepare well defined and flat crystal surfaces [16] and for the smoothening during film growth [22]. A striking observation of our study is the dependence of the sputtering yield on the ion fluence. A similar effect was observed recently for bombardment of Pt(1 1 1) with 5 keV Ar+ ions under grazing incidence and explained by drastically enhanced sputtering yields for collisions with steps Ystep compared to those for scattering from (flat) terraces Yterr [23]. Analysis of experimental data with molecular dynamics calculations imply that these yields differ by more than one order of magnitude, i.e. under grazing sputtering mass removal almost completely proceeds at steps (and other surface defects). By a simple geometrical model it was shown that sputtering depends on the widths of terraces L and step heights h. Only for terraces smaller than a critical width Lc ¼ 2h= tan Uin direct encounter of all projectiles hitting a terrace with step atoms will occur, otherwise the ions are reflected from the terrace with negligible sputtering effects. This model might explain our experimental observation that for the initial stage of bombardment of the surface the sputtering yield increases with fluence. In this regime, ion impact will enhance the step density at the surface and the sputtering yield will increase, until the critical width for terraces and saturation of the sputter yields is reached. In our experiments we have observed a decrease of YCo for higher fluences (cf. Fig. 5). This decrease takes place at fluences above about 8 ML where about half of the first ML is removed from the film. For this range of fluences we conclude from the angular distributions for scattered projectiles that the concentration of defects of the film surface is gradually enhanced. A description of the sputtering process under incorporation of a modelling of the morphology of the film with a realistic defect structure is beyond the scope of the present work.
T. Bernhard, H. Winter / Nucl. Instr. and Meth. in Phys. Res. B 232 (2005) 16–21
Acknowledgments This work is supported by the Deutsche Forschungsgemeinschaft under contract Wi 1336. We thank R.A. Noack, K. Maass, M. Baron, and Dr. M. Gruyters for their assistance in preparation and running of the experiments and for helpful discussions. References [1] U. Gradmann, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 7/1, Elsevier, Amsterdam, 1993, p. 1. [2] J.A.C. Bland, B. Heinrich (Eds.), Ultrathin Magnetic Structures, Springer, Berlin, 1994. [3] P. Gru¨nberg, R. Schreiber, Y. Pang, M.B. Brodsky, H. Sowers, Phys. Rev. Lett. 57 (1986) 2442. [4] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Eitenne, G. Crenzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472. [5] S.O. Demokritov, C. Bayer, S. Poppe, M. Rickart, J. Fassbender, B. Hillebrands, D.I. Kholin, N.M. Kreines, O.M. Liedke, Phys. Rev. Lett. 90 (2003) 097201. [6] R. Moroni, D. Sekiba, F. Buatier de Mongeot, G. Gonella, C. Boragno, L. Mattera, U. Valbusa, Phys. Rev. Lett. 91 (2003) 167207. [7] F.B. Duning, C. Rau, G.K. Walters, Comments Solid State Phys. 12 (1985) 17.
21
[8] H. Winter, H. Hagedorn, R. Zimny, H. Nienhaus, J. Kirschner, Phys. Rev. Lett. 62 (1989) 296. [9] H. Winter, Z. Physik D 23 (1992) 41. [10] J. Manske, M. Dirska, G. Lubinski, M. Schlehberger, A. Na¨rmann, R. Hoekstra, J. Magn. Magn. Mater. 168 (1997) 249. [11] T. Igel, R. Pfandzelter, H. Winter, Appl. Surf. Sci. 142 (1999) 532. [12] A.R. Bachmann, S. Speller, J. Manske, M. Schleberger, A. Na¨rmann, W. Heiland, in: J.L. Moran-Lopez (Ed.), The Physics of Low Dimensional Systems, Kluwer Academic, Plenum Publishers, New York, 2001, p. 265. [13] M. Gruyters, T. Bernhard, H. Winter, J. Magn. Magn. Mater. 292 (2005) 192. [14] R. Pfandzelter, M. Potthoff, Phys. Rev. B 64 (2001) 14045. [15] J. Los, J.J.C. Geerlings, Phys. Rep. 190 (1990) 133. [16] H. Winter, Phys. Rep. 367 (2002) 387. [17] A.G. Borisov, A.K. Kazansky, J.P. Gauyacq, Phys. Rev. Lett. 80 (1998) 1996. [18] T. Hecht, H. Winter, A.G. Borisov, J.P. Gauyacq, A.K. Kazansky, Phys. Rev. Lett. 84 (2000) 2517. [19] R. Pfandzelter, Phys. Rev. B 57 (1998) 15496. [20] T. Bernhard, M. Gruyters, H. Winter, Surf. Sci. 575 (2005) 154. [21] M.V. Ramana Murty, T. Curcic, A. Judy, B.H. Cooper, A.R. Woll, J.D. Brock, S. Kycia, R.L. Headrick, Phys. Rev. Lett. 80 (1998) 4713. [22] K.C. Ruthe, S.A. Barnett, Surf. Sci. Lett. 538 (2003) L460. [23] H. Hansen, C. Polop, T. Michely, A. Friedrich, H.M. Urbassek, Phys. Rev. Lett. 92 (2004) 246106.