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Magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films measured by a UHV cantilever beam technique
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s u r f a c e science ELSEVIER
Surface Science 331-333 (1995) 1398-1403
Magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films measured by a UHV cantilever beam technique R. Koch *, M. Weber, E. Henze, K.H. Rieder Freie Universitiit Berlin, Institut Jfir Experimentalphysik, Arnimallee 14, D-14195 Berlin, Germany Received 7 September 1994; accepted for publication 2 December 1994
Abstract
The magnetization, magnetostriction and intrinsic stress of various polycrystalline Fe films have been investigated with an extended version of a UHV cantilever beam technique. Due to the high sensitivity of this technique absolute values of the respective properties can be determined of films approaching even monolayer thickness. The saturation magnetization of Fe/glass is equal to the respective bulk value down to film thicknesses of 3 nm, where the films percolate; on mica and oxidized Si a decrease by about 30% is observed at mean film thicknesses lower than 10 nm, probably due to intermixing effects at the interface. On all three substrates a strong dependence of the saturation magnetostriction on film thickness is observed at mean film thicknesses lower than 10-20 nm. Keywords: Iron; Magnetic films; Magnetic measurements; Magnetic phenomena; Polycrystalline thin films
I. Introduction
Ultrathin magnetic films of thicknesses up to a few monolayers have become subject of intense investigation in the last years [1]. In the course of these investigations it was discovered that the magnetic film properties frequently alter in the thickness range of a few monolayers, i.e. when the film dimensionality is reduced from 3D to 2D; for example, the Curie temperature decreases [2], perpendicular magnetic anisotropies are stabilized [3], magnetic moments are
* Corresponding author.
found to be enhanced [4], etc. The experimental techniques employed so far to study the magnetic properties of thin films can be divided into two groups: (i) bulk techniques, which have been substantially improved with respect to their sensitivity, such as torque magnetometers [5], Mfssbauer spectroscopy [6], FMR (ferromagnetic resonance) [7] or SMOKE (surface magneto-optic Kerr effect) [8], and (ii) a variety of spin-polarized (SP) versions of standard surface science techniques, such as SP-photoelectron spectroscopy [9], SP-AES (Auger electron spectroscopy) [10], SP-HREELS (high resolution electron energy loss spectroscopy) [11], SP-LEEM (low energy electron microscopy) [12], SP-SEM
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 0 6 8 - 2
1399
R. Koch et al. /Surface Science 331-333 (1995) 1398-1403
(scanning electron microscopy) [13,14], etc. The vast majority of techniques, however, does not determine absolute values of the film magnetization M and therefore is not suited to verify theoretical predictions of, e.g., thickness dependent magnetic moments [15]. Meanwhile there is also a strong demand for measurements of the magnetostrictive film properties [1], i.e. the change of their equilibrium dimensions upon the variation of magnetization. In the case of thin films magnetostrictive effects usually give rise to magnetostrictive stress, because adaption to the new film dimensions is inhibited by adhesion to the substrate. Since magnetostrictive stress together with growth inherent intrinsic stress contributes to the magnetoelastic energy, which is one of the components to the total magnetic energy, it may have impact on the magnetic anisotropy of the films [16,17]. In two recent investigations a considerable increase of the magnitude of the magnetostriction coefficients As of NiFe alloys was reported for film thicknesses below 10 nm [18] and 2 nm, respectively [191. Recently we have demonstrated [20] that with a modified version of the cantilever beam technique it is possible to determine quantitatively and in situ in UttV (i) the magnetization, (ii) the magnetostriction and (iii) the intrinsic stress of thin films. The high sensitivity of the method enables measurements even at film thicknesses approaching the monolayer. In addition, important information on microstmcture and growth mode of the films can be deduced from the thickness dependence of the film stress, as has been established in various studies of polycrystalline [21] and epitaxial thin films [22]. This is of special importance for magnetic thin films, because many magnetic properties such as the magnetic anisotropy, the coercive field or the magnetostriction are strongly influenced by morphological parameters. In the following we present further results measured with the new technique. After a brief description of the extended cantilever beam technique and giving the experimental details, we discuss the respective magnetic properties of polycrystalline Fe films, which were UHV-deposited onto glass, mica and oxidized Si substrates. As found in the preliminary investigation [20] there is a considerable dependence of the magnetic propertie s on film thickness, in particular of the magnetostriction.
2. Cantilever beam technique Fig. la depicts a schematic illustration of the cantilever beam device which is mounted in the center of three orthogonal pairs of Helmholtz-like coils in order to apply homogeneous magnetic fields in any direction of space. The cantilever beam substrate (S) is clamped on the left side, its right side is ready to move, when the force distribution in the thin films, that are deposited from below, changes. This happens, when the films generate intrinsic stress due to growth inherent processes or as a result of magnetostrictive stress, when the film magnetization is altered. In order to determine the film magnetization, use is made of the fact that magnetic dipoles m experience a torque Tm = m × B in homogeneous magnetic fields B, the y-component Ty of which gives rise to a respective bending of the substrate. Due to the magnetic anisotropy of most magnetic thin films two simple situations frequently can be arranged, where the substrate deflection directly reflects the total film magnetization: (i) in the very common case of a strong in-plane anisotropy (due to the demagnetizing field), by magnetizing the films in the x-direction (i.e. along the length of the cantilever beam) and applying a small vertical field B z (Fig. lb) and (ii) in the case of an out-of-plane anis0tropy
b) Bz -1
;Mx c)
Fig. 1. (a) Schematic illustration of the cantilever beam device mounted in the center of three orthogonal pairs of Helmholtz-like coils for measurements of the magnetization, magnetostriction and intrinsic stress of magnetic thin films. The displacement of the substrate's (S) free end is detected with high sensitivity by a differential capacitance technique (C). (b, c) Schematic illustration of the measurement of magnetization M of films exhibiting strong (b) in-plane and (c) perpendicular anisotropy.
1400
R. Koch et a l . / Surface Science 331-333 (1995) 1398-1403
by magnetizing the films in the z-direction and applying a magnetic field in the x-direction (Fig. lc). The decisive point of the method is the accurate detection of the displacement of the free end of the cantilever beam substrate, from which quantitative values of the involved forces and torques can be derived. The appropriate equations are given in Ref. [20]. The substrate deflection of our apparatus is determined by a highly sensitive capacitance technique (C) combined with lock-in-assisted signal detection (for details see Refs. [23,24]). The smallest detectable deflection is approximately 1 nm, which corresponds to minimum detectable torques and forces of about 5 X 10 -a° and 0.003 N / m , respectively, for 0.1 mm thick substrates. Therefore with our apparatus the misfit stress of submonolayer films is easily resolved [25] (1% of misfit yields film forces of about 0.2 N / m ) , it also allows to determine the magnetization of films with thicknesses of 1 - 2 monolayers. For the magnetostrictive measurements the limit of resolution is reached at film thicknesses of 1 - 2 nm depending on the magnitude of the magnetostrictive constant ( 1 0 - 4 - 1 0 - 6). Moreover, it is possible to improve the sensitivity of the cantilever beam device further by performing the magnetic measurements in a dynamical mode. Upon using magnetic fields that rotate within the film plane for the magnetostrictive measurements and oscillating perpendicular fields for the magnetization measurements the cantilever beam substrate starts to vibrate. The amplitude of vibration depends on the frequency of the driving field and is the largest, when the substrate vibrates in resonance. Because of the amplification effect the substrate deflection no longer corresponds to absolute quantities; it can be calibrated, however, by depositing a thicker film, where both static and dynamic measurements are possible.
3. Experimental The experiments were performed in a UHV system with a base pressure better than 1 X 10 - l ° mbar (see Ref. [24]). The Fe films were electron beam evaporated from Knudsen-type tungsten crucibles onto 30 X 5 X (0.10-0.15) mm 3 cantilever beam substrates which were thermally equilibrated at 300
K. The mean film thickness was determined with a quartz crystal microbalance calibrated by a second quartz in the substrate position. The deposition rate was 0.005 n m / s and the residual gas pressure during deposition was better than 2 x 10 - 9 mbar. All substrates have been carefully outgassed at temperatures of 600-750 K prior to the film deposition. The intrinsic film stress was determined continuously during the film deposition, the magnetic measurements were started about 15 min later. All hysteresis loops were obtained by varying the magnetic field in the x-direction (compare Fig. 1) and measuring the substrate displacement during short pulses of a vertical field B z of 1700-3400 A / m . The remnant magnetostrictive constant A was determined from the substrate deflection after saturating the Fe films in the y-direction and subsequently applying a respective field in the x-direction. To calculate quantitative values for the mechanical and magnetic properties the cantilever beam device was calibrated by applying a known weight, by means of which a defined substrate deflection is produced.
4. Results and discussion In this section we discuss the results of polycrystalline Fe films, which were UHV-deposited at various film thicknesses onto glass, mica and oxidized Si substrates. The intrinsic stress of the films has been studied previously [26,20]. In each case it is tensile, nearly independent of film thickness and exhibits values of ( 1 - 2 ) × 10 9 N / m 2. From the thickness dependence of the film stress it can be concluded (for a detailed discussion see Refs. [21,22]) that all Fe films have grown by columnar grain growth, a modification of Volmer-Weber or island growth for metals with low mobility. It is dominated by the tensile stress of non-equilibrium small-angle grainboundaries [27], which form the major defects of columnarly grained films. The first appearance of tensile stress is indicative of the beginning coalescence of individual islands and, due to the low mobility of Fe at 300 K, lies close to percolation ( ~ 2 - 3 nm) as confirmed by simultaneous electrical conductance measurements. From the film thickness at percolation an average grain size of about 6 nm can be estimated (see Ref. [24]) in accordance with a
R. Koch et al. / Surface Science 331-333 (1995) 1398-1403
1401
Fig. 2 shows the magnetic hysteresis loops of various Fe films deposited onto the three different substrates (glass, mica and oxidized Si). The Fe
scanning tunnelling microscopy investigation of analogous Fe film deposited onto mica substrates [281.
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Fig. 2. Hysteresis loops of polycrystalline Fe films UHV-deposited onto glass, mica and oxidized Si substrates. For comparison, the saturation magnetization of bulk iron is 1.76 MA/m.
1402
R. Koch et al. // Surface Science 331-333 (1995) 1398-1403
films exhibit mean thicknesses ranging from 100 nm down to 3 nm, at which the films are already electrically conducting (percolation). At mean thicknesses smaller than about 3 nm the Fe films exhibit (super-) paramagnetic behaviour, which is typical of thin films in the island stage of film growth. Notice that all hysteresis loops are well resolved, From the signal to noise ratio of the 3 nm films ( ~ 20 monolayers) a sensitivity down to film thicknesses of 1-2 monolayers can be extrapolated. The experimental saturation magnetization M s of the Fe films on glass in the entire thickness range is 1.78 _ 0.08 M A / m , in good agreement with the respective bulk value of 1.76 M A / m . M s remains unchanged, when the films are exposed to an atmosphere of nitrogen after they have been coated by a thin layer of Ag (Fig. 2, Fe/glass: bottom left). Also on the other two substrates at mean film thicknesses higher than about 10 nm, within experimental error, M s equals the bulk value. At film thicknesses smaller than 10 nm, however, a decrease of M s by about 30% is observed, probably due to intermixing effects at the interface by means of which part of the Fe atoms have become magnetically ineffective. On all substrates the coercive field of the films in the channel stage (3-5 nm) is significantly smaller than that of the continuous films. Obviously the grain boundaries, which are formed when thin films become continuous, act as barriers for the movement of domain walls. It is interesting to note that the coercive field is also increased, when the channels of the Fe films are
Fe/Glass
filled with a non-magnetic material such as Ag (Fig. 2, Fe/glass: bottom left) Contrary to the magnetization the magnetostrictive properties of the Fe films on the three substrates (Fig. 3) exhibit a distinct dependence on film thickness. Between 100 and 20 nm the saturation magnetostriction As of the Fe/glass films is - 4 . 6 X 10 -6, which within experimental error is equal to the bulk value of polycrystalline Fe ( - 4.4 X 10 -6 ). At mean film thicknesses lower than 10 nm significant departures from the bulk value are observed: the magnitude of As decreases to - 1.7 X 10 -6 at 8 nm; at 3 nm even its sign has changed (A s = + 1.5 X 10-6). An analogous behaviour is found on mica and oxidized Si, As of the thicker films, however, remains smaller than the respective bulk value, probably reflecting the intermixing effects at the interface as indicated by the magnetization measurements. By now, however, we have a very limited understanding of thin film magnetostriction at low film thicknesses, particularly due to the fact that only a small number of experimental data is available. It is well known that the magnetostrictive constants usually are anisotropic and therefore strongly dependent on the actual film structure. Consequently a detailed crystallographic, morphological and compositional analysis of the films in the respective thickness range is required, which in the case of our (presumably) polycrystalline films, among other morphological and chemical features, could disclose a possible texture or change of texture of the films. Surface contribu-
Fe/Mica(O01)
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Fig. 3. Remnant magnetostrictive properties of polycrystalline Fe films UHV-depositedonto glass, mica and oxidized Si substrates. For comparison, the saturation magnetostrictionof bulk iron is -4.4 X 10 -6.
R. Koch et al. / Surface Science 331-333 (1995) 1398-1403
tions - as proposed recently by Song et al. for 2 nm thick NiFe films [19] - may account for differences in the monolayer range but, in our opinion, does not explain the dramatic decrease of the value of As at film thicknesses of about 10-20 nm.
5. C o n c l u s i o n s
We have investigated the magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films UHV-deposited onto glass, mica and oxidized Si substrates. We employed a highly sensitive UHV cantilever beam technique for this study, which provides absolute values for the three properties. On the glass substrates the saturation magnetization equals the bulk value of polycrystalline Fe down to film thicknesses of 3 nm, where the films percolate; on mica and oxidized Si a decrease by about 30% is observed at mean film thicknesses lower than 10 nm, probably due to intermixing effects at the interface. On all three substrates a dramatic deviation of the saturation magnetostriction from the respective bulk value was observed at mean film thicknesses lower than 10-20 nm indicating that the magnetostriction is a parameter even more sensitive to growth characteristics than the film magnetization.
Acknowledgements
The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschafl (Sfb 290).
References [1] L.M. Falicov, D.T. Pierce, S.D. Bader, R. Gronsky, ICB. Hathaway, H.J. Hopster, D.N. Lambeth, S.S.P. Parkin, G.
1403
Prinz, M. Salamon, I.K. Schuller and R.H. Victoria, J. Mater. Res. 5 (1990) 1299. [2] C.M. Schneider, P. Bressler, P. Schuster, J. Kirschner, J.J. de Miguel and R. Miranda, Phys. Rev. Lett. 64 (1990) 1059. [3] R. Allenspach, J. Magn. Magn. Mater. 129 (1994) 160. [4] C. Turtur and G. Bayreuther, Phys. Rev. Lett. 72 (1994) 1557. [5] J. Hur and S.-C. Shin, Appl. Phys. Lett. 62 (1993) 2140. [6] M. Przybylski, J. Korecld and U. Gradmann, Appl. Phys. A 52 (1991) 33. [7] F. Gerhardter, Y. Li and K. Baberschke, Phys. Rev. B 47 (1993) 11204. [8] S.D. Bader, J. Magn. Magn. Mater. 100 (1991) 440. [9] M. Campagna, D.T. Pierce, F. Meier, K. Sattler and H.C. Siegmann, Adv. Electron. Electron Phys. 41 (1976) 113. [10] M. Taborelli, R. AUenspach, G. Boffa and M. Landolt, Phys. Rev. Lett. 56 (1986) 2869. [11] D.L. Abraham and H. Hopster, Phys. Rev. Lett. 62 (1989) 1157. [12] K. Grzelakowski and E. Bauer, to be published. [13] H.P. Oepen and J. Kirschner, Phys. Rev. Lett. 62 (1989) 819. [14] R. Allenspach, M. Stampanoni and A. Bischof, Phys. Rev. Lett. 65 (1990) 3344. [15] C. Li and A.J. Freeman, Phys. Rev. B 43 (1991) 780. [16] F.J.A. den Broeder, W. Hoving and P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [17] R.H. Victoria and J.M. MacLaren, Phys. Rev. B 47 (1993) 11583. [18] R.M. Valletta, G. Guthmiller and G. Gorman, J. Vac. Sci. Technol. A 9 (1991) 2093. [19] O. Song, C.A. Ballentine and R.C. O'Handley, Appl. Phys. Lett. 64 (1994) 2593. [20] M. Weber, R. Koch and K.H. Rieder, Phys. Rev. Lett. 73 (1994) 1166. [21] R. Abermann, Vacuum 41 (1990) 1279. [22] R. Koch, D. Winau and K.H. Rieder, Phys. Scr. T 49 1059 (1993) 1059. [23] R. Koch, H. Leonhard, G. Thurner and R. Abermann, Rev. Sci. Instr. 61 (1990) 3859. [24] D. Winan, R. Koch, A. Fiihrmann and K.H. Rieder, J. Appl. Phys. 70 (1991) 3081. [25] R. Koch, D. Winau, K. Thiirmer, M. Weber and K.H. Rieder, Europhys. Lett. 21 (1993) 213. [26] D. Winau, R. Koch und K.H. Rieder, Appl. Phys. Lett. 59 (1991) 1072. [27] R.W. Hoffman, in: Physics of Thin Films, Vol. 3, Eds. G. Hass and R.E. Thun (Academic Press, New York, 1966)p. 211. [28] V. Hoffmann, Diploma Thesis, Freie Universifiit Berlin, 1992.
s u r f a c e science ELSEVIER
Surface Science 331-333 (1995) 1398-1403
Magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films measured by a UHV cantilever beam technique R. Koch *, M. Weber, E. Henze, K.H. Rieder Freie Universitiit Berlin, Institut Jfir Experimentalphysik, Arnimallee 14, D-14195 Berlin, Germany Received 7 September 1994; accepted for publication 2 December 1994
Abstract
The magnetization, magnetostriction and intrinsic stress of various polycrystalline Fe films have been investigated with an extended version of a UHV cantilever beam technique. Due to the high sensitivity of this technique absolute values of the respective properties can be determined of films approaching even monolayer thickness. The saturation magnetization of Fe/glass is equal to the respective bulk value down to film thicknesses of 3 nm, where the films percolate; on mica and oxidized Si a decrease by about 30% is observed at mean film thicknesses lower than 10 nm, probably due to intermixing effects at the interface. On all three substrates a strong dependence of the saturation magnetostriction on film thickness is observed at mean film thicknesses lower than 10-20 nm. Keywords: Iron; Magnetic films; Magnetic measurements; Magnetic phenomena; Polycrystalline thin films
I. Introduction
Ultrathin magnetic films of thicknesses up to a few monolayers have become subject of intense investigation in the last years [1]. In the course of these investigations it was discovered that the magnetic film properties frequently alter in the thickness range of a few monolayers, i.e. when the film dimensionality is reduced from 3D to 2D; for example, the Curie temperature decreases [2], perpendicular magnetic anisotropies are stabilized [3], magnetic moments are
* Corresponding author.
found to be enhanced [4], etc. The experimental techniques employed so far to study the magnetic properties of thin films can be divided into two groups: (i) bulk techniques, which have been substantially improved with respect to their sensitivity, such as torque magnetometers [5], Mfssbauer spectroscopy [6], FMR (ferromagnetic resonance) [7] or SMOKE (surface magneto-optic Kerr effect) [8], and (ii) a variety of spin-polarized (SP) versions of standard surface science techniques, such as SP-photoelectron spectroscopy [9], SP-AES (Auger electron spectroscopy) [10], SP-HREELS (high resolution electron energy loss spectroscopy) [11], SP-LEEM (low energy electron microscopy) [12], SP-SEM
0039-6028/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved
SSDI 0 0 3 9 - 6 0 2 8 ( 9 5 ) 0 0 0 6 8 - 2
1399
R. Koch et al. /Surface Science 331-333 (1995) 1398-1403
(scanning electron microscopy) [13,14], etc. The vast majority of techniques, however, does not determine absolute values of the film magnetization M and therefore is not suited to verify theoretical predictions of, e.g., thickness dependent magnetic moments [15]. Meanwhile there is also a strong demand for measurements of the magnetostrictive film properties [1], i.e. the change of their equilibrium dimensions upon the variation of magnetization. In the case of thin films magnetostrictive effects usually give rise to magnetostrictive stress, because adaption to the new film dimensions is inhibited by adhesion to the substrate. Since magnetostrictive stress together with growth inherent intrinsic stress contributes to the magnetoelastic energy, which is one of the components to the total magnetic energy, it may have impact on the magnetic anisotropy of the films [16,17]. In two recent investigations a considerable increase of the magnitude of the magnetostriction coefficients As of NiFe alloys was reported for film thicknesses below 10 nm [18] and 2 nm, respectively [191. Recently we have demonstrated [20] that with a modified version of the cantilever beam technique it is possible to determine quantitatively and in situ in UttV (i) the magnetization, (ii) the magnetostriction and (iii) the intrinsic stress of thin films. The high sensitivity of the method enables measurements even at film thicknesses approaching the monolayer. In addition, important information on microstmcture and growth mode of the films can be deduced from the thickness dependence of the film stress, as has been established in various studies of polycrystalline [21] and epitaxial thin films [22]. This is of special importance for magnetic thin films, because many magnetic properties such as the magnetic anisotropy, the coercive field or the magnetostriction are strongly influenced by morphological parameters. In the following we present further results measured with the new technique. After a brief description of the extended cantilever beam technique and giving the experimental details, we discuss the respective magnetic properties of polycrystalline Fe films, which were UHV-deposited onto glass, mica and oxidized Si substrates. As found in the preliminary investigation [20] there is a considerable dependence of the magnetic propertie s on film thickness, in particular of the magnetostriction.
2. Cantilever beam technique Fig. la depicts a schematic illustration of the cantilever beam device which is mounted in the center of three orthogonal pairs of Helmholtz-like coils in order to apply homogeneous magnetic fields in any direction of space. The cantilever beam substrate (S) is clamped on the left side, its right side is ready to move, when the force distribution in the thin films, that are deposited from below, changes. This happens, when the films generate intrinsic stress due to growth inherent processes or as a result of magnetostrictive stress, when the film magnetization is altered. In order to determine the film magnetization, use is made of the fact that magnetic dipoles m experience a torque Tm = m × B in homogeneous magnetic fields B, the y-component Ty of which gives rise to a respective bending of the substrate. Due to the magnetic anisotropy of most magnetic thin films two simple situations frequently can be arranged, where the substrate deflection directly reflects the total film magnetization: (i) in the very common case of a strong in-plane anisotropy (due to the demagnetizing field), by magnetizing the films in the x-direction (i.e. along the length of the cantilever beam) and applying a small vertical field B z (Fig. lb) and (ii) in the case of an out-of-plane anis0tropy
b) Bz -1
;Mx c)
Fig. 1. (a) Schematic illustration of the cantilever beam device mounted in the center of three orthogonal pairs of Helmholtz-like coils for measurements of the magnetization, magnetostriction and intrinsic stress of magnetic thin films. The displacement of the substrate's (S) free end is detected with high sensitivity by a differential capacitance technique (C). (b, c) Schematic illustration of the measurement of magnetization M of films exhibiting strong (b) in-plane and (c) perpendicular anisotropy.
1400
R. Koch et a l . / Surface Science 331-333 (1995) 1398-1403
by magnetizing the films in the z-direction and applying a magnetic field in the x-direction (Fig. lc). The decisive point of the method is the accurate detection of the displacement of the free end of the cantilever beam substrate, from which quantitative values of the involved forces and torques can be derived. The appropriate equations are given in Ref. [20]. The substrate deflection of our apparatus is determined by a highly sensitive capacitance technique (C) combined with lock-in-assisted signal detection (for details see Refs. [23,24]). The smallest detectable deflection is approximately 1 nm, which corresponds to minimum detectable torques and forces of about 5 X 10 -a° and 0.003 N / m , respectively, for 0.1 mm thick substrates. Therefore with our apparatus the misfit stress of submonolayer films is easily resolved [25] (1% of misfit yields film forces of about 0.2 N / m ) , it also allows to determine the magnetization of films with thicknesses of 1 - 2 monolayers. For the magnetostrictive measurements the limit of resolution is reached at film thicknesses of 1 - 2 nm depending on the magnitude of the magnetostrictive constant ( 1 0 - 4 - 1 0 - 6). Moreover, it is possible to improve the sensitivity of the cantilever beam device further by performing the magnetic measurements in a dynamical mode. Upon using magnetic fields that rotate within the film plane for the magnetostrictive measurements and oscillating perpendicular fields for the magnetization measurements the cantilever beam substrate starts to vibrate. The amplitude of vibration depends on the frequency of the driving field and is the largest, when the substrate vibrates in resonance. Because of the amplification effect the substrate deflection no longer corresponds to absolute quantities; it can be calibrated, however, by depositing a thicker film, where both static and dynamic measurements are possible.
3. Experimental The experiments were performed in a UHV system with a base pressure better than 1 X 10 - l ° mbar (see Ref. [24]). The Fe films were electron beam evaporated from Knudsen-type tungsten crucibles onto 30 X 5 X (0.10-0.15) mm 3 cantilever beam substrates which were thermally equilibrated at 300
K. The mean film thickness was determined with a quartz crystal microbalance calibrated by a second quartz in the substrate position. The deposition rate was 0.005 n m / s and the residual gas pressure during deposition was better than 2 x 10 - 9 mbar. All substrates have been carefully outgassed at temperatures of 600-750 K prior to the film deposition. The intrinsic film stress was determined continuously during the film deposition, the magnetic measurements were started about 15 min later. All hysteresis loops were obtained by varying the magnetic field in the x-direction (compare Fig. 1) and measuring the substrate displacement during short pulses of a vertical field B z of 1700-3400 A / m . The remnant magnetostrictive constant A was determined from the substrate deflection after saturating the Fe films in the y-direction and subsequently applying a respective field in the x-direction. To calculate quantitative values for the mechanical and magnetic properties the cantilever beam device was calibrated by applying a known weight, by means of which a defined substrate deflection is produced.
4. Results and discussion In this section we discuss the results of polycrystalline Fe films, which were UHV-deposited at various film thicknesses onto glass, mica and oxidized Si substrates. The intrinsic stress of the films has been studied previously [26,20]. In each case it is tensile, nearly independent of film thickness and exhibits values of ( 1 - 2 ) × 10 9 N / m 2. From the thickness dependence of the film stress it can be concluded (for a detailed discussion see Refs. [21,22]) that all Fe films have grown by columnar grain growth, a modification of Volmer-Weber or island growth for metals with low mobility. It is dominated by the tensile stress of non-equilibrium small-angle grainboundaries [27], which form the major defects of columnarly grained films. The first appearance of tensile stress is indicative of the beginning coalescence of individual islands and, due to the low mobility of Fe at 300 K, lies close to percolation ( ~ 2 - 3 nm) as confirmed by simultaneous electrical conductance measurements. From the film thickness at percolation an average grain size of about 6 nm can be estimated (see Ref. [24]) in accordance with a
R. Koch et al. / Surface Science 331-333 (1995) 1398-1403
1401
Fig. 2 shows the magnetic hysteresis loops of various Fe films deposited onto the three different substrates (glass, mica and oxidized Si). The Fe
scanning tunnelling microscopy investigation of analogous Fe film deposited onto mica substrates [281.
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4
6
[kA/m]
Fig. 2. Hysteresis loops of polycrystalline Fe films UHV-deposited onto glass, mica and oxidized Si substrates. For comparison, the saturation magnetization of bulk iron is 1.76 MA/m.
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R. Koch et al. // Surface Science 331-333 (1995) 1398-1403
films exhibit mean thicknesses ranging from 100 nm down to 3 nm, at which the films are already electrically conducting (percolation). At mean thicknesses smaller than about 3 nm the Fe films exhibit (super-) paramagnetic behaviour, which is typical of thin films in the island stage of film growth. Notice that all hysteresis loops are well resolved, From the signal to noise ratio of the 3 nm films ( ~ 20 monolayers) a sensitivity down to film thicknesses of 1-2 monolayers can be extrapolated. The experimental saturation magnetization M s of the Fe films on glass in the entire thickness range is 1.78 _ 0.08 M A / m , in good agreement with the respective bulk value of 1.76 M A / m . M s remains unchanged, when the films are exposed to an atmosphere of nitrogen after they have been coated by a thin layer of Ag (Fig. 2, Fe/glass: bottom left). Also on the other two substrates at mean film thicknesses higher than about 10 nm, within experimental error, M s equals the bulk value. At film thicknesses smaller than 10 nm, however, a decrease of M s by about 30% is observed, probably due to intermixing effects at the interface by means of which part of the Fe atoms have become magnetically ineffective. On all substrates the coercive field of the films in the channel stage (3-5 nm) is significantly smaller than that of the continuous films. Obviously the grain boundaries, which are formed when thin films become continuous, act as barriers for the movement of domain walls. It is interesting to note that the coercive field is also increased, when the channels of the Fe films are
Fe/Glass
filled with a non-magnetic material such as Ag (Fig. 2, Fe/glass: bottom left) Contrary to the magnetization the magnetostrictive properties of the Fe films on the three substrates (Fig. 3) exhibit a distinct dependence on film thickness. Between 100 and 20 nm the saturation magnetostriction As of the Fe/glass films is - 4 . 6 X 10 -6, which within experimental error is equal to the bulk value of polycrystalline Fe ( - 4.4 X 10 -6 ). At mean film thicknesses lower than 10 nm significant departures from the bulk value are observed: the magnitude of As decreases to - 1.7 X 10 -6 at 8 nm; at 3 nm even its sign has changed (A s = + 1.5 X 10-6). An analogous behaviour is found on mica and oxidized Si, As of the thicker films, however, remains smaller than the respective bulk value, probably reflecting the intermixing effects at the interface as indicated by the magnetization measurements. By now, however, we have a very limited understanding of thin film magnetostriction at low film thicknesses, particularly due to the fact that only a small number of experimental data is available. It is well known that the magnetostrictive constants usually are anisotropic and therefore strongly dependent on the actual film structure. Consequently a detailed crystallographic, morphological and compositional analysis of the films in the respective thickness range is required, which in the case of our (presumably) polycrystalline films, among other morphological and chemical features, could disclose a possible texture or change of texture of the films. Surface contribu-
Fe/Mica(O01)
Fe/SiOx
~'--6 .....
x z
i. . . . . .
i
-4
~-2 I,-
.1 Z
0
2 -6
-4
-2
0
2
4
-4
-2
0
MAGNETIC FIELD
2
4
-4
-2
0
2
4
6
[kNm I
Fig. 3. Remnant magnetostrictive properties of polycrystalline Fe films UHV-depositedonto glass, mica and oxidized Si substrates. For comparison, the saturation magnetostrictionof bulk iron is -4.4 X 10 -6.
R. Koch et al. / Surface Science 331-333 (1995) 1398-1403
tions - as proposed recently by Song et al. for 2 nm thick NiFe films [19] - may account for differences in the monolayer range but, in our opinion, does not explain the dramatic decrease of the value of As at film thicknesses of about 10-20 nm.
5. C o n c l u s i o n s
We have investigated the magnetization, magnetostriction and intrinsic stress of polycrystalline Fe films UHV-deposited onto glass, mica and oxidized Si substrates. We employed a highly sensitive UHV cantilever beam technique for this study, which provides absolute values for the three properties. On the glass substrates the saturation magnetization equals the bulk value of polycrystalline Fe down to film thicknesses of 3 nm, where the films percolate; on mica and oxidized Si a decrease by about 30% is observed at mean film thicknesses lower than 10 nm, probably due to intermixing effects at the interface. On all three substrates a dramatic deviation of the saturation magnetostriction from the respective bulk value was observed at mean film thicknesses lower than 10-20 nm indicating that the magnetostriction is a parameter even more sensitive to growth characteristics than the film magnetization.
Acknowledgements
The authors gratefully acknowledge financial support by the Deutsche Forschungsgemeinschafl (Sfb 290).
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