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Cu(110 ) thin films
Surface Science 507–510 (2002) 324–329 www.elsevier.com/locate/susc
Structure and magnetism of Fe/Cu(1 1 0) thin films S. Tacchi a,*, F. Bruno b, G. Carlotti a, D. Cvetko b,c,d, L. Floreano b, G. Gubbiotti e, M. Madami a, A. Morgante b,f, A. Verdini b a
INFM and Dipartimento di Fisica dell’ Universit a di Perugia, Via Pascoli, I-06123 Perugia, Italy b Laboratorio TASC-INFM, 34012 Basovizza, Trieste, Italy c J. Stefan Institute, Department of Physics, University of Ljubljana, Ljubljana, Slovenia d Sincrotrone Trieste, 34012 Basovizza, Trieste, Italy e INFM, Unit a di Perugia, Via Pascoli, I-06123, Perugia, Italy f Dipartimento di Fisica dell’ Universita di Trieste, Via Valerio 2, I-34100, Trieste, Italy
Abstract We report on the structural and magnetic properties of thin Fe films grown on the Cu(1 1 0) surface. In-plane grazing-incidence X-ray diffraction has been used to measure the lateral lattice spacing of the Fe films. Complementary information about the structure of the topmost layers has been obtained by means of Auger and photoelectron diffraction. The Fe film grows pseudomorphic with the substrate up to a thickness of about 0.8 nm. The diffraction feature of a new phase is observed at 1.6 nm, with a corresponding interplanar distance close to the bulk body centered cubic (bcc) Fe one, which is eventually recovered at higher thickness (6.4 nm). From comparison between X-ray diffraction and photoelectron diffraction, it is suggested that the bcc-like Fe grows on the (1 0 0) surface with its [1 1 0] axis oriented along the [0 0 1] substrate direction. The photoelectron diffraction data also indicate a strong faceting in the [1 1 0] substrate direction. This morphology is believed to contribute to the in-plane uniaxial magnetic anisotropy observed by Kerr effect and Brillouin light scattering from spin waves. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Iron; Magnetic films; Metal–metal magnetic heterostructures; Angle resolved photoemission; X-ray scattering, diffraction, and reflection; Light scattering
1. Introduction In recent years a great effort has been devoted to understand the correlation between the magnetic and structural properties of ultrathin films (two-dimensional systems). In particular, iron films grown on copper single crystals have attracted considerable attention, since face centered cubic (fcc) iron can be formed by epitaxial deposition
*
Corresponding author. Fax: +390754466. E-mail address: silvia.tacchi@fisica.unipg.it (S. Tacchi).
on a suitable Cu substrate, thus displaying magnetic properties different from the Fe bulk ones [1]. Most studies were concerned with Fe growth on Cu(1 0 0) single crystals, where a spin reorientation transition has been found to be associated to an fcc to body centered cubic (bcc) martensitic phase transition [2]. The structural and magnetic properties of Fe films grown on Cu(1 1 0) are considerably less studied. Previous He atom scattering and photoelectron diffraction (PED) experiments showed a Stranski–Krastanov growth characterised by an initial pseudomorphic phase [3–5]. Low energy electron diffraction (LEED)
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 6 5 - 7
S. Tacchi et al. / Surface Science 507–510 (2002) 324–329
studies have revealed the formation of (1 1 1) facets in the [1 1 0] surface direction already after the deposition of a few monolayers (ML) [6]. The occurrence of Cu segregation has been also considered. Early experiments by CO titration revealed the presence of a consistent Cu surface concentration (up to 20%) up to 5 ML thickness [5]. More recently, a very accurate PED study concluded Cu surface segregation to be negligible in the same thickness range [4]. In fact, both techniques show their limits when dealing with this issue, due to both the strong influence of steps and defects (which are present with a high density on these Fe films) on the CO titration method, and the difficulty in discriminating between Fe and Cu atoms (which have similar scattering factor) when making multiple scattering simulations of the PED measurements. In general, these studies were limited to films with thickness below about 1.5 nm, i.e. consisting of a few MLs. Recently, at Perugia University, the magnetic properties of iron films with thickness in the range between 0.2 and 10 nm grown on Cu(1 1 0) single crystal, have been investigated in-situ by means of Brillouin light scattering (BLS) and surface magneto-optical Kerr effect (SMOKE) [7,8], while their structural quality has been examined by LEED. For Fe film thickness up to 0.6 nm the LEED patterns are similar to those of the clean Cu substrate, while for thicker films the diffraction patterns rapidly vanish, indicating the formation of a disordered film. The question arises, however, of a deeper understanding of the complex atomic arrangements in films as thick as several nanometers. In fact, as shown in Fig. 1 for a couple of specimens evaporated and studied in situ, these films exhibit a strong uniaxial anisotropy, with magnetic ‘‘easy’’ and ‘‘hard’’ directions along the [0 0 1] and [1 1 0] substrate directions, respectively. The presence of such an anisotropy is put in evidence by the different shape of the hysteresis loops measured by SMOKE (inset of Fig. 1), as well as by the characteristic nonmonotonic dependence of the spin wave frequency as a function of the external magnetic field applied along the hard direction [9]. In particular, an external field of about 700–800 Oe is required to reach the saturation of the hysteresis loop along the substrate [1 1 0] direction. This is reflected in
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Fig. 1. Spin wave frequencies, measured by BLS for two different Fe films, with thickness 1 and 4 nm. The points are the measured values, while the lines are the best fit curves, obtained by the model described in Ref. [7]. The external magnetic field H is applied along the magnetic hard axis ([1 1 0] substrate direction), while the angle of incidence of light is kept fixed at 45°. The bottom insert shows the spin wave frequencies for an external field applied along the perpendicular [0 0 1] substrate direction, i.e. along the magnetic easy axis. In the upper inset, the hysteresis cycles corresponding to the above two directions are shown for the 4 nm thick specimen.
the marked minimum in the evolution of the spin wave frequency shown in Fig. 1. Remarkably, this uniaxial anisotropy with the easy magnetisation axis along the substrate [0 0 1] direction, persisted even in iron films as thick as 10 nm, where the LEED patterns indicated a disordered surface, at least on a long-range scale [7]. The aim of this work is to examine the structural properties of Fe films grown on Cu(1 1 0) up to film thickness as large as several nanometers and to reveal the correlation with the above mentioned magnetic properties. We present preliminary results obtained at the ALOISA Beamline of the Elettra Synchrotron (Trieste, Italy) [10], by exploiting the grazing-incidence X-ray diffraction (GIXRD) and the photo (or Auger) Electron Diffraction (ED) surface investigation techniques. The former technique has been used to gather the information about the in-plane structural
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parameters, while the angular positions of the ED forward focusing peaks provided chemically selected, short range information on the structure of the film [11]. Moreover, the two techniques give complementary information about the film structure since ED is sensitive to the uppermost film layers (like LEED), while GIXRD probes the whole film structure together with the substrate one.
2. Experimental The ALOISA analysis chamber (base pressure in the 1011 mbar range) and its detection system for both ED and GIXRD is described in details elsewhere [10]. For the surface preparation, the sample is translated in the preparation chamber (base pressure of 1–2 1010 mbar) equipped with facilities for Arþ sputtering, evaporation cells and a in situ RHEED monitoring system. Before each Fe deposition, the Cu(1 1 0) single crystal substrate was cleaned by Arþ sputtering at 1.0 keV and subsequent flash to about 700 K. This resulted in a well ordered surface displaying a sharp (1 1) RHEED pattern. The photoemission surveys at grazing incidence were used periodically to check the absence of contamination due to impurities such as carbon or oxygen. Four iron films of different thickness were grown at room temperature by e-beam evaporation from an Omicron cell at a rate of about 0.1 nm/min. The deposition flux was calibrated with a quartz microbalance, while the film thickness was monitored during the evaporation by X-ray reflectivity (XRR) measurements. In a sequence of experiments, Fe evaporation was stopped at different points of the reflectivity deposition curve, thus ensuring a reliable estimate of the coverage. The ED polar scans were measured by rotating the electron analyser in the plane defined by the surface normal and the beam axis, while keeping fixed the grazing angle, the surface azimuthal orientation relative to the scattering plane and the photon polarisation orientation. We considered emission along the two main symmetry directions of the substrate, [0 0 1] and [1 1 0]. The photon energy was set to about 950 eV, in order to look at several photoelectron and
Auger peaks of both Fe and Cu. The emission intensities have been measured at the maximum of the photoemission line and at suitable chosen energies aside the peak, for a proper subtraction of the background of secondary electrons. In order to compare the ED patterns taken at different thickness, the diffractive contribute to the polar scans has been singled out from the experimental data by evaluating the anisotropy vðhÞ function defined as: vðhÞ ¼ ½IðhÞ ISOðhÞ=ISOðhÞ, where h is the polar angle, IðhÞ is the measured intensity and ISOðhÞ is the smooth non-diffractive isotropic background which can be expressed in an analytical form [12]. The v-functions have been extracted for each scan by subtracting the corresponding ISOðhÞ, as obtained by a fit to the experimental intensities. 3. Results and discussion We have taken GIXRD radial scans in the Cu reciprocal lattice across the (0 0 2) in-plane substrate reflection. They are shown in Fig. 2 as a
Fig. 2. In-plane radial scans in the Cu reciprocal lattice, as a function of the momentum transfer along the [0 0 1] substrate direction, obtained at fixed scattering geometry by varying the photon energy. The data are shown as a function of the reciprocal lattice unit of the fcc Cu(1 1 0) substrate for four thickness Fe films (0.8, 1.6, 3.2 and 6.4 nm). The full and dashed vertical lines indicate the position of the substrate (0 0 2) peak and that of the Fe bulk (2 2 0) peak, respectively.
S. Tacchi et al. / Surface Science 507–510 (2002) 324–329
function of the parallel momentum transfer along the substrate [0 0 1] direction for four different thickness Fe films. It can be seen that only the (0 0 2) diffraction peak, corresponding to the Cu lattice parameter a ¼ 0:3615 nm, is present at low coverage (0.8 nm). This is consistent with the pseudomorphic fcc Fe growth previously observed by LEED [3,7]. At Fe thickness larger than 0.8 nm, an additional peak is observed, whose corresponding parallel wave vector decreases as the thickness is increased. We attribute the appearance of the extra peak to the formation of a new Fe phase. The corresponding values of the lateral lattice spacing are reported in Table 1, together with an estimate of the domain size, as obtained from the diffraction peak full width at half maximum. It should be noted that, by increasing the film thickness, the in-plane lattice spacing of the new Fe phase approaches the value of 0.4053 nm, which corresponds to the diagonal [1 1 0] of the square bcc(1 0 0) lattice cell of bulk Fe. These findings suggest that, at a thickness between 0.8 and 1.6 nm, the Fe film undergoes a structural transition from a pseudomorphic fcc phase to a bcc-like one, with the Fe bcc [1 1 0] direction oriented along the [0 0 1] substrate direction. In order to discriminate between the possible growth directions of the bcc-like phase, we also measured radial scans along the substrate [1 1 0] direction, but we did not detect any extra diffraction peak around the (2 2 0) in-plane reflection of the substrate. Taking into account the commonly accepted high level of film roughness, the absence of this extra-peak might be attributed to the absence of long range order correlation for the Fe films along the [1 1 0] substrate direction, even if a non-tetragonal lattice distortion of the new bcclike phase cannot be excluded.
Table 1 Experimental values of the lateral lattice spacing and estimate of the domain size obtained by GIXRD on Fe films of different thickness along the substrate [0 0 1] direction Film thickness (nm) Lattice spacing (nm) Domain size (nm)
1.6 0.3977 5.0
3.2 0.4005 5.5
The error for the lattice spacing is 0.0005 nm.
6.4 0.4028 6.0
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Fig. 3. Experimental polar ED v-functions of the Fe LMM Auger (K.E. 642 eV) taken for the four Fe thickness films (same markers of Fig. 2). The polar angle is measured with respect to the surface normal. The vertical lines denote the angular position of the main forward scattering peaks characteristic of an fcc(1 1 0) surface. Upper panel: polar scan taken along the substrate [0 0 1] direction. Lower panel: polar scan taken along the [1 1 0] substrate direction.
A further insight into the structure of the Fe films was obtained from the analysis of the ED scans taken along the two main symmetry directions [0 0 1] and [1 1 0] of the substrate. The Fe LMM and Cu LVV Auger v-functions are reported in Figs. 3 and 4, respectively, for different values of the nominal film thickness. The Fe ED scans relative to the 0.8 nm thick Fe film exhibit an anisotropy resembling that expected for an fcc(1 1 0) surface, in fact, the main intensity maxima are observed along the surface normal and at polar angles of about 55° and 45° for the [0 0 1] and [1 1 0] scans, respectively, where the forward scattered electrons are focussed by the high density atom rows. In addition, the Fe ED
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Fig. 4. Experimental polar ED v-functions of the Cu LVV Auger (K.E. 913 eV) taken for the four Fe thickness films (same markers of Fig. 2). The polar angle is measured with respect to the surface normal. The vertical lines denote the angular position of the main forward scattering peaks characteristic of the fcc(1 1 0) surface. Upper panel: polar scan taken along the substrate [0 0 1] direction. Lower panel: polar scan taken along the [1 1 0] substrate direction.
scans at 0.8 nm are very similar to the corresponding Cu ED scans (Fig. 4), thus confirming previous observations of a pseudomorphic film in this thickness range. For higher coverage, the Fe signal modulation changes dramatically. By visual inspection of these high coverage polar scans, it is evident the disappearance of the ED peak at 45° along the [1 1 0] substrate direction, and the appearance of a strong and asymmetric ED peak between 25° and 30° along the [0 0 1] substrate direction. While multiple scattering calculations are required for a quantitative structure determination, we can understand the symmetry of the growing surface by comparing the experimentally observed polar directions of forward scattering
with the orientation of the high density atom rows for a few model surfaces, such as (1 0 0), (1 1 0) and (1 1 1). Simple geometrical considerations show that only a bcc(1 0 0) surface along its [1 1 0] direction presents high density atom rows, which are compatible with the anisotropy peaks displayed by the Fe ED scans taken along the [0 0 1] substrate direction (Fig. 3, upper panel). In particular, the broad ED peak with the maximum at about 27° and a marked shoulder at about 37° and the peak at 55° would correspond to the [1 1 3], [1 1 2] and [1 1 1] directions (in the bcc(1 0 0) reference system), which yield angles of 25.24°, 35.26° and 54.74°, respectively. For such a bcc(1 0 0)-oriented Fe film (with the [1 1 0] bcc axis oriented along the substrate [0 0 1] direction), the polar scans along the [1 1 0] and the [0 0 1] substrate direction should be equivalent, thanks to the fourfold cubic symmetry. On the contrary, in the polar scan along the [1 1 0] substrate direction (lower panel of Fig. 3), marked minima are visible not only at 30°, but also along the surface normal. This is a clear fingerprint of a strong faceting in the [1 1 0] substrate direction. In this case the main forward scattering scattering peak is no longer oriented along the surface normal, but along the normal to the facet, thus modifying the whole polar scan. As a result, the Fe ED anisotropy along the [1 1 0] substrate direction does not allow a simple visual analysis and multiple scattering calculations are needed. By combining the Fe ED informations with those of GIXRD, we envisage a surface morphology characterised by Fe islands elongated about 5 nm in the [0 0 1] substrate direction with extended faceting in the [1 1 0] substrate direction. This rough morphology would also explain the ED Cu scans taken at high Fe coverage, which are shown in Fig. 4. In fact, the Cu Auger v-function obtained after the deposition of 6.4 nm of Fe is still displaying a strong anisotropy along the [1 1 0] substrate direction, indicating that the Cu signal is originated by atoms close to the surface. Moreover, the anisotropy along the [1 1 0] substrate direction is very similar to that obtained for the thinner films, this observation exclude the Cu signal to be originated by segregated atoms (which would retain the bcc-like Fe structure) and
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requires the layers close to the interface to be partly uncovered. The reduced lateral connectivity of the Fe film might be the origin of the magnetic anisotropy, which shows the easy magnetisation axis to be parallel to the [0 0 1] substrate direction. At present, the possible contribute to the magnetic anisotropy by a non-tetragonal distortion of the Fe lattice cell cannot be excluded, and more extended GIXRD studies are needed to settle this structural issue. In conclusion, the combined use of GIXRD and ED indicates that fcc-Fe grows pseudomorphically on Cu(1 1 0) up to 0.8 nm films thickness. For higher coverage a new phase sets in, which displays a distorted bcc(1 0 0) surface symmetry with its [1 1 0] axis oriented along the [0 0 1] substrate direction. This new phase gradually evolves towards the Fe bulk bcc structure as the film thickness is increased. We suggest that the appearance of the new phase is also accompanied by morphological modifications of the growing surface, with the formation of facets in the [1 1 0] substrate direction. This morphology may be responsible for the strong in-plane uniaxial magnetic anisotropy observed in spin waves studies. A quantitative analysis of the ED scans, taking into account the lateral lattice spacing as given by GIXRD, is currently in progress and should allow to disentangle the interplay between the structure and morphological properties of the Fe film.
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Acknowledgements Financial support by INFM, as well as by MURST-COFIN-2000 is kindly acknowledged. References [1] P. Marcus, F. Jona, Surf. Rev. Lett. 1 (1994) 15; D. Li et al., Phys. Rev. Lett. 72 (1994) 3112; R. Allenspach, A. Bischof, Phys. Rev. Lett. 69 (1992) 3385. [2] S. M€ uller et al., Phys. Rev. Lett 74 (1995) 765; K. Kalki et al., Phys. Rev B 48 (1993) R18344; A. Biedermann, M. Schmid, P. Varga, Phys. Rev. Lett. 86 (2001) 464. [3] C. Pflitsch, R. David, L.K. Verheij, R. Franchy, Europhys. Lett. 53 (2001) 388. [4] A. Theobald, S. Bao, V. Fernandez, K.M. Scindler, O. Schaff, V. Fritzsche, A.M. Bradshaw, N. Booth, D.P. Woodruff, Surf. Sci. 385 (1997) 107. [5] M.T. Kief, W.F. Egelhoff, Phys. Rev. B 47 (1993) 10785. [6] D. Tian, F. Jona, P.M. Marcus, Phys. Rev. B 45 (1992) 11216. [7] L. Albini, G. Carlotti, G. Gubbiotti, M. Madami, S. Tacchi, J. Appl. Phys. 89 (2001) 7383. [8] G. Gubbiotti, G. Carlotti, M. Madami, S. Tacchi, L. Verdini, Phys. Stat. Sol. (a) 189 (2002) 403. [9] G. Carlotti, G. Gubbiotti, Rivista del Nuovo Cimento 22 (1999) 41, and references therein. [10] L. Floreano et al., Rev. Sci. Inst. 70 (1999) 3855, An updated presentation of the beamline can be found at. [11] F. Bruno et al., cond-mat/0103458, submitted; F. Bruno et al., Appl. Surf. Sci. 162, 163 (2000) 340–345. [12] F. Bruno, Ph.D. Thesis, Methods for growth experiments at ALOISA, November 2000, University of Trieste.
Structure and magnetism of Fe/Cu(1 1 0) thin films S. Tacchi a,*, F. Bruno b, G. Carlotti a, D. Cvetko b,c,d, L. Floreano b, G. Gubbiotti e, M. Madami a, A. Morgante b,f, A. Verdini b a
INFM and Dipartimento di Fisica dell’ Universit a di Perugia, Via Pascoli, I-06123 Perugia, Italy b Laboratorio TASC-INFM, 34012 Basovizza, Trieste, Italy c J. Stefan Institute, Department of Physics, University of Ljubljana, Ljubljana, Slovenia d Sincrotrone Trieste, 34012 Basovizza, Trieste, Italy e INFM, Unit a di Perugia, Via Pascoli, I-06123, Perugia, Italy f Dipartimento di Fisica dell’ Universita di Trieste, Via Valerio 2, I-34100, Trieste, Italy
Abstract We report on the structural and magnetic properties of thin Fe films grown on the Cu(1 1 0) surface. In-plane grazing-incidence X-ray diffraction has been used to measure the lateral lattice spacing of the Fe films. Complementary information about the structure of the topmost layers has been obtained by means of Auger and photoelectron diffraction. The Fe film grows pseudomorphic with the substrate up to a thickness of about 0.8 nm. The diffraction feature of a new phase is observed at 1.6 nm, with a corresponding interplanar distance close to the bulk body centered cubic (bcc) Fe one, which is eventually recovered at higher thickness (6.4 nm). From comparison between X-ray diffraction and photoelectron diffraction, it is suggested that the bcc-like Fe grows on the (1 0 0) surface with its [1 1 0] axis oriented along the [0 0 1] substrate direction. The photoelectron diffraction data also indicate a strong faceting in the [1 1 0] substrate direction. This morphology is believed to contribute to the in-plane uniaxial magnetic anisotropy observed by Kerr effect and Brillouin light scattering from spin waves. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Iron; Magnetic films; Metal–metal magnetic heterostructures; Angle resolved photoemission; X-ray scattering, diffraction, and reflection; Light scattering
1. Introduction In recent years a great effort has been devoted to understand the correlation between the magnetic and structural properties of ultrathin films (two-dimensional systems). In particular, iron films grown on copper single crystals have attracted considerable attention, since face centered cubic (fcc) iron can be formed by epitaxial deposition
*
Corresponding author. Fax: +390754466. E-mail address: silvia.tacchi@fisica.unipg.it (S. Tacchi).
on a suitable Cu substrate, thus displaying magnetic properties different from the Fe bulk ones [1]. Most studies were concerned with Fe growth on Cu(1 0 0) single crystals, where a spin reorientation transition has been found to be associated to an fcc to body centered cubic (bcc) martensitic phase transition [2]. The structural and magnetic properties of Fe films grown on Cu(1 1 0) are considerably less studied. Previous He atom scattering and photoelectron diffraction (PED) experiments showed a Stranski–Krastanov growth characterised by an initial pseudomorphic phase [3–5]. Low energy electron diffraction (LEED)
0039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 6 5 - 7
S. Tacchi et al. / Surface Science 507–510 (2002) 324–329
studies have revealed the formation of (1 1 1) facets in the [1 1 0] surface direction already after the deposition of a few monolayers (ML) [6]. The occurrence of Cu segregation has been also considered. Early experiments by CO titration revealed the presence of a consistent Cu surface concentration (up to 20%) up to 5 ML thickness [5]. More recently, a very accurate PED study concluded Cu surface segregation to be negligible in the same thickness range [4]. In fact, both techniques show their limits when dealing with this issue, due to both the strong influence of steps and defects (which are present with a high density on these Fe films) on the CO titration method, and the difficulty in discriminating between Fe and Cu atoms (which have similar scattering factor) when making multiple scattering simulations of the PED measurements. In general, these studies were limited to films with thickness below about 1.5 nm, i.e. consisting of a few MLs. Recently, at Perugia University, the magnetic properties of iron films with thickness in the range between 0.2 and 10 nm grown on Cu(1 1 0) single crystal, have been investigated in-situ by means of Brillouin light scattering (BLS) and surface magneto-optical Kerr effect (SMOKE) [7,8], while their structural quality has been examined by LEED. For Fe film thickness up to 0.6 nm the LEED patterns are similar to those of the clean Cu substrate, while for thicker films the diffraction patterns rapidly vanish, indicating the formation of a disordered film. The question arises, however, of a deeper understanding of the complex atomic arrangements in films as thick as several nanometers. In fact, as shown in Fig. 1 for a couple of specimens evaporated and studied in situ, these films exhibit a strong uniaxial anisotropy, with magnetic ‘‘easy’’ and ‘‘hard’’ directions along the [0 0 1] and [1 1 0] substrate directions, respectively. The presence of such an anisotropy is put in evidence by the different shape of the hysteresis loops measured by SMOKE (inset of Fig. 1), as well as by the characteristic nonmonotonic dependence of the spin wave frequency as a function of the external magnetic field applied along the hard direction [9]. In particular, an external field of about 700–800 Oe is required to reach the saturation of the hysteresis loop along the substrate [1 1 0] direction. This is reflected in
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Fig. 1. Spin wave frequencies, measured by BLS for two different Fe films, with thickness 1 and 4 nm. The points are the measured values, while the lines are the best fit curves, obtained by the model described in Ref. [7]. The external magnetic field H is applied along the magnetic hard axis ([1 1 0] substrate direction), while the angle of incidence of light is kept fixed at 45°. The bottom insert shows the spin wave frequencies for an external field applied along the perpendicular [0 0 1] substrate direction, i.e. along the magnetic easy axis. In the upper inset, the hysteresis cycles corresponding to the above two directions are shown for the 4 nm thick specimen.
the marked minimum in the evolution of the spin wave frequency shown in Fig. 1. Remarkably, this uniaxial anisotropy with the easy magnetisation axis along the substrate [0 0 1] direction, persisted even in iron films as thick as 10 nm, where the LEED patterns indicated a disordered surface, at least on a long-range scale [7]. The aim of this work is to examine the structural properties of Fe films grown on Cu(1 1 0) up to film thickness as large as several nanometers and to reveal the correlation with the above mentioned magnetic properties. We present preliminary results obtained at the ALOISA Beamline of the Elettra Synchrotron (Trieste, Italy) [10], by exploiting the grazing-incidence X-ray diffraction (GIXRD) and the photo (or Auger) Electron Diffraction (ED) surface investigation techniques. The former technique has been used to gather the information about the in-plane structural
326
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parameters, while the angular positions of the ED forward focusing peaks provided chemically selected, short range information on the structure of the film [11]. Moreover, the two techniques give complementary information about the film structure since ED is sensitive to the uppermost film layers (like LEED), while GIXRD probes the whole film structure together with the substrate one.
2. Experimental The ALOISA analysis chamber (base pressure in the 1011 mbar range) and its detection system for both ED and GIXRD is described in details elsewhere [10]. For the surface preparation, the sample is translated in the preparation chamber (base pressure of 1–2 1010 mbar) equipped with facilities for Arþ sputtering, evaporation cells and a in situ RHEED monitoring system. Before each Fe deposition, the Cu(1 1 0) single crystal substrate was cleaned by Arþ sputtering at 1.0 keV and subsequent flash to about 700 K. This resulted in a well ordered surface displaying a sharp (1 1) RHEED pattern. The photoemission surveys at grazing incidence were used periodically to check the absence of contamination due to impurities such as carbon or oxygen. Four iron films of different thickness were grown at room temperature by e-beam evaporation from an Omicron cell at a rate of about 0.1 nm/min. The deposition flux was calibrated with a quartz microbalance, while the film thickness was monitored during the evaporation by X-ray reflectivity (XRR) measurements. In a sequence of experiments, Fe evaporation was stopped at different points of the reflectivity deposition curve, thus ensuring a reliable estimate of the coverage. The ED polar scans were measured by rotating the electron analyser in the plane defined by the surface normal and the beam axis, while keeping fixed the grazing angle, the surface azimuthal orientation relative to the scattering plane and the photon polarisation orientation. We considered emission along the two main symmetry directions of the substrate, [0 0 1] and [1 1 0]. The photon energy was set to about 950 eV, in order to look at several photoelectron and
Auger peaks of both Fe and Cu. The emission intensities have been measured at the maximum of the photoemission line and at suitable chosen energies aside the peak, for a proper subtraction of the background of secondary electrons. In order to compare the ED patterns taken at different thickness, the diffractive contribute to the polar scans has been singled out from the experimental data by evaluating the anisotropy vðhÞ function defined as: vðhÞ ¼ ½IðhÞ ISOðhÞ=ISOðhÞ, where h is the polar angle, IðhÞ is the measured intensity and ISOðhÞ is the smooth non-diffractive isotropic background which can be expressed in an analytical form [12]. The v-functions have been extracted for each scan by subtracting the corresponding ISOðhÞ, as obtained by a fit to the experimental intensities. 3. Results and discussion We have taken GIXRD radial scans in the Cu reciprocal lattice across the (0 0 2) in-plane substrate reflection. They are shown in Fig. 2 as a
Fig. 2. In-plane radial scans in the Cu reciprocal lattice, as a function of the momentum transfer along the [0 0 1] substrate direction, obtained at fixed scattering geometry by varying the photon energy. The data are shown as a function of the reciprocal lattice unit of the fcc Cu(1 1 0) substrate for four thickness Fe films (0.8, 1.6, 3.2 and 6.4 nm). The full and dashed vertical lines indicate the position of the substrate (0 0 2) peak and that of the Fe bulk (2 2 0) peak, respectively.
S. Tacchi et al. / Surface Science 507–510 (2002) 324–329
function of the parallel momentum transfer along the substrate [0 0 1] direction for four different thickness Fe films. It can be seen that only the (0 0 2) diffraction peak, corresponding to the Cu lattice parameter a ¼ 0:3615 nm, is present at low coverage (0.8 nm). This is consistent with the pseudomorphic fcc Fe growth previously observed by LEED [3,7]. At Fe thickness larger than 0.8 nm, an additional peak is observed, whose corresponding parallel wave vector decreases as the thickness is increased. We attribute the appearance of the extra peak to the formation of a new Fe phase. The corresponding values of the lateral lattice spacing are reported in Table 1, together with an estimate of the domain size, as obtained from the diffraction peak full width at half maximum. It should be noted that, by increasing the film thickness, the in-plane lattice spacing of the new Fe phase approaches the value of 0.4053 nm, which corresponds to the diagonal [1 1 0] of the square bcc(1 0 0) lattice cell of bulk Fe. These findings suggest that, at a thickness between 0.8 and 1.6 nm, the Fe film undergoes a structural transition from a pseudomorphic fcc phase to a bcc-like one, with the Fe bcc [1 1 0] direction oriented along the [0 0 1] substrate direction. In order to discriminate between the possible growth directions of the bcc-like phase, we also measured radial scans along the substrate [1 1 0] direction, but we did not detect any extra diffraction peak around the (2 2 0) in-plane reflection of the substrate. Taking into account the commonly accepted high level of film roughness, the absence of this extra-peak might be attributed to the absence of long range order correlation for the Fe films along the [1 1 0] substrate direction, even if a non-tetragonal lattice distortion of the new bcclike phase cannot be excluded.
Table 1 Experimental values of the lateral lattice spacing and estimate of the domain size obtained by GIXRD on Fe films of different thickness along the substrate [0 0 1] direction Film thickness (nm) Lattice spacing (nm) Domain size (nm)
1.6 0.3977 5.0
3.2 0.4005 5.5
The error for the lattice spacing is 0.0005 nm.
6.4 0.4028 6.0
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Fig. 3. Experimental polar ED v-functions of the Fe LMM Auger (K.E. 642 eV) taken for the four Fe thickness films (same markers of Fig. 2). The polar angle is measured with respect to the surface normal. The vertical lines denote the angular position of the main forward scattering peaks characteristic of an fcc(1 1 0) surface. Upper panel: polar scan taken along the substrate [0 0 1] direction. Lower panel: polar scan taken along the [1 1 0] substrate direction.
A further insight into the structure of the Fe films was obtained from the analysis of the ED scans taken along the two main symmetry directions [0 0 1] and [1 1 0] of the substrate. The Fe LMM and Cu LVV Auger v-functions are reported in Figs. 3 and 4, respectively, for different values of the nominal film thickness. The Fe ED scans relative to the 0.8 nm thick Fe film exhibit an anisotropy resembling that expected for an fcc(1 1 0) surface, in fact, the main intensity maxima are observed along the surface normal and at polar angles of about 55° and 45° for the [0 0 1] and [1 1 0] scans, respectively, where the forward scattered electrons are focussed by the high density atom rows. In addition, the Fe ED
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Fig. 4. Experimental polar ED v-functions of the Cu LVV Auger (K.E. 913 eV) taken for the four Fe thickness films (same markers of Fig. 2). The polar angle is measured with respect to the surface normal. The vertical lines denote the angular position of the main forward scattering peaks characteristic of the fcc(1 1 0) surface. Upper panel: polar scan taken along the substrate [0 0 1] direction. Lower panel: polar scan taken along the [1 1 0] substrate direction.
scans at 0.8 nm are very similar to the corresponding Cu ED scans (Fig. 4), thus confirming previous observations of a pseudomorphic film in this thickness range. For higher coverage, the Fe signal modulation changes dramatically. By visual inspection of these high coverage polar scans, it is evident the disappearance of the ED peak at 45° along the [1 1 0] substrate direction, and the appearance of a strong and asymmetric ED peak between 25° and 30° along the [0 0 1] substrate direction. While multiple scattering calculations are required for a quantitative structure determination, we can understand the symmetry of the growing surface by comparing the experimentally observed polar directions of forward scattering
with the orientation of the high density atom rows for a few model surfaces, such as (1 0 0), (1 1 0) and (1 1 1). Simple geometrical considerations show that only a bcc(1 0 0) surface along its [1 1 0] direction presents high density atom rows, which are compatible with the anisotropy peaks displayed by the Fe ED scans taken along the [0 0 1] substrate direction (Fig. 3, upper panel). In particular, the broad ED peak with the maximum at about 27° and a marked shoulder at about 37° and the peak at 55° would correspond to the [1 1 3], [1 1 2] and [1 1 1] directions (in the bcc(1 0 0) reference system), which yield angles of 25.24°, 35.26° and 54.74°, respectively. For such a bcc(1 0 0)-oriented Fe film (with the [1 1 0] bcc axis oriented along the substrate [0 0 1] direction), the polar scans along the [1 1 0] and the [0 0 1] substrate direction should be equivalent, thanks to the fourfold cubic symmetry. On the contrary, in the polar scan along the [1 1 0] substrate direction (lower panel of Fig. 3), marked minima are visible not only at 30°, but also along the surface normal. This is a clear fingerprint of a strong faceting in the [1 1 0] substrate direction. In this case the main forward scattering scattering peak is no longer oriented along the surface normal, but along the normal to the facet, thus modifying the whole polar scan. As a result, the Fe ED anisotropy along the [1 1 0] substrate direction does not allow a simple visual analysis and multiple scattering calculations are needed. By combining the Fe ED informations with those of GIXRD, we envisage a surface morphology characterised by Fe islands elongated about 5 nm in the [0 0 1] substrate direction with extended faceting in the [1 1 0] substrate direction. This rough morphology would also explain the ED Cu scans taken at high Fe coverage, which are shown in Fig. 4. In fact, the Cu Auger v-function obtained after the deposition of 6.4 nm of Fe is still displaying a strong anisotropy along the [1 1 0] substrate direction, indicating that the Cu signal is originated by atoms close to the surface. Moreover, the anisotropy along the [1 1 0] substrate direction is very similar to that obtained for the thinner films, this observation exclude the Cu signal to be originated by segregated atoms (which would retain the bcc-like Fe structure) and
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requires the layers close to the interface to be partly uncovered. The reduced lateral connectivity of the Fe film might be the origin of the magnetic anisotropy, which shows the easy magnetisation axis to be parallel to the [0 0 1] substrate direction. At present, the possible contribute to the magnetic anisotropy by a non-tetragonal distortion of the Fe lattice cell cannot be excluded, and more extended GIXRD studies are needed to settle this structural issue. In conclusion, the combined use of GIXRD and ED indicates that fcc-Fe grows pseudomorphically on Cu(1 1 0) up to 0.8 nm films thickness. For higher coverage a new phase sets in, which displays a distorted bcc(1 0 0) surface symmetry with its [1 1 0] axis oriented along the [0 0 1] substrate direction. This new phase gradually evolves towards the Fe bulk bcc structure as the film thickness is increased. We suggest that the appearance of the new phase is also accompanied by morphological modifications of the growing surface, with the formation of facets in the [1 1 0] substrate direction. This morphology may be responsible for the strong in-plane uniaxial magnetic anisotropy observed in spin waves studies. A quantitative analysis of the ED scans, taking into account the lateral lattice spacing as given by GIXRD, is currently in progress and should allow to disentangle the interplay between the structure and morphological properties of the Fe film.
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