- Email: [email protected]
Magnetic properties of step-decorated Fe nanostripes and dots grown on Mo(1 1 0)
Journal of Magnetism and Magnetic Materials 242–245 (2002) 565–567
Magnetic properties of step-decorated Fe nanostripes and dots grown on Mo(1 1 0) P.-O. Jubert*, O. Fruchart, C. Meyer Laboratoire Louis N!eel, CNRS, BP 166, 38042 Grenoble, France
Abstract Arrays of epitaxial Fe(1 1 0) nanostripes 1–5 nm thick and B100 nm wide are fabricated under UHV by pulsed laser deposition on a stepped Mo(1 1 0) surface. This step decoration process results from surface diffusion limitations at 500 K, or alternatively from the occurrence of a metastable 6-atomic-layer thickness upon annealing a continuous film grown at room temperature. The stripe orientation and spacing are uniform over the whole sample as controlled by the 0.051 miscut of the substrate. Room temperature magnetic properties are investigated by vibrating sample magnetometry. In the case of steps parallel to Fe[0 0 1], all sources of anisotropy contribute to align the magnetization parallel to the step direction, yielding a rather square hysteresis loop with a coercive field of 70 mT. Average single particle behavior is extracted by differentiating the reversible and the irreversible magnetic contributions deduced from a series of minor hysteresis loops. A model with no interaction between the particles well reproduces the experimental results. r 2002 Published by Elsevier Science B.V. Keywords: Step edge decoration; Magnetic nanoparticles; Epitaxy; Pulsed laser deposition
The fabrication of magnetic nanostructures is of interest for both fundamental research and potential applications in magnetic recording. Low dimensional magnetic structures reveal new properties when their dimensions are comparable to some magnetic length scale such as the exchange length or the Bloch wall width which amount to B10 and B100 nm, respectively. Selfassembly/self-organization are promising methods to produce 1D and 0D model systems of such dimensions. It allows the fabrication of epitaxial nanoscale patterns over a large area, in a fast single process and with atomic ultimate size. Trade off with respect to other techniques such as e-beam lithography or scanning probe writing is of course the availability of patterns and a finite width of size distribution of the self-organized structures. Step decoration of self-organized periodically stepped surfaces has been used to grow arrays of nanowires and stripes [1–4]. This approach usually results in the formation of ultrathin structures (1 or 2 AL high) yielding strongly time-dependent magnetic behavior *Corresponding author. Fax: +33-4-76-88-11-91. E-mail address: [email protected] (P.-O. Jubert).
and usually superparamagnetic at room temperature (RT) [1–3]. We report a different approach that allow one to grow thicker nanostripes. We take advantage of diffusion kinetic limitations and of the occurrence of a previously unreported metastable 6 AL thickness for the Fe on Mo(1 1 0) system. The epitaxial growth of Fe(1 1 0) on Mo(1 1 0) was performed under ultra high vacuum by pulsed laser deposition (PLD). The samples were characterized by RHEED and STM in situ, while the morphology was also characterized on larger scales using AFM in air. The Mo(1 1 0) surface is that of a 10 nm thick single crystalline buffer layer deposited on a sapphire substrate Al2O3(1 1–2 0) following an optimized temperature gradient method [5]. Such a Mo(1 1 0) surface exhibits an array of parallel atomic steps (inset of Fig. 1b). The terrace width and orientation are uniform over the whole sample, with very small distribution. They are related to the sapphire miscut to the (1 1–2 0) surface of about 0.051, resulting in 200 nm wide terraces. Fe is ( grown with a mean deposition rate of 0.5 A/min as measured with a calibrated quartz microbalance. The substrate temperature during the growth can be tuned
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 3 8 - 1
566
P.-O. Jubert et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 565–567
Fig. 1. 10 mm 10 mm AFM images showing (a) 5 nm high wire shaped islands obtained after deposition at 500 K followed by annealing at 700 K, (b) self-organized 0.8 nm high nanostripes obtained after annealing at 700 K from a film grown at room temperature. Inset: 950 nm 950 nm STM image of the Mo(1 1 0) surface before Fe deposition.
from RT up to 800 K without any Fe–Mo intermixing [6]. The samples are finally capped with 3 nm of Mo grown at RT to avoid oxidation during ex situ experiments. Above 600 K, no influence of the atomic steps is observed. The growth proceeds in a Stranski–Krastanov mode resulting in self-assembled compact epitaxial islands elongated along Mo,Fe[0 0 1] BCC direction, on top of one pseudo-morphic atomic layer. The islands have a lateral aspect ratio around 2 and display an ingot shape with atomically flat facets. The growth proceeds close to the thermodynamic equilibrium. The shape and orientation of the islands are well understood in terms of surface and interface energies [7,8]. At 500 K, the growth of Fe on Mo(1 1 0) still follows a Stranski–Krastanov mode as observed by STM at all stages of the growth. Due to kinetic limitations (1) the wetting layer amounts to 2 atomic planes and (2) surface diffusion is hindered and becomes dependent on the presence of atomic steps. Elongated nanostructures parallel to the atomic steps of the Mo buffer layer thus form, together with a small amount of compact islands. Further annealing at 700 K results in smoothening and slight improving of the regularity of the stripes. Fig. 1a shows an AFM image of such nanostripes after deposition of 1.5 nm Fe on Mo(1 1 0) at 500 K followed by annealing at 700 K. Typical height and width for the elongated islands are 5 and 140 nm, respectively. In the range 300–400 K, kinetic limitations are even stronger resulting in the formation of a continuous thin Fe film. The elastic strain induced by the 9% misfit is partly relaxed by the introduction of a 2D dislocation network between the first and second atomic layer (AL) but the growth mode remains layer by layer up to the filling of the 4th atomic plane. Note that 2D growth is improved by PLD compared to MBE [11]. In the range of nominal thickness Y=2–6 AL and upon annealing at 700 K the films break into flat patches of constant height
(4–5 AL) above two wetting AL. Schematically, depending on the filling ratio (Y2)/4, the patches go from disconnected to percolated for Y going from 2 to 6 AL. For Y>6 AL, 3D compact islands grow at the expense of the 6 AL thick layer. This gives evidence of the metastability of 6 AL structures for the Fe/Mo(1 1 0) system. We believe that this critical thickness is associated with modifications in the dislocation network to reduce the residual stress. The morphology of the 6 AL patches are influenced by the Mo atomic steps, lower step edge acting as a favored site for the formation of the patches. For an optimum range of Y[2.5 MC, 4.5 MC], the annealing process results in the formation of an array of continuous epitaxial nanostripes on top of two wetting AL, with controlled orientation and width ws=wt(Y2 AL)/4 AL, wt being the Mo terrace width (see Fig 1b). The magnetic properties of elongated islands grown at 500 K and aligned along Fe[0 0 1] (Fig. 1a) were investigated at RT by vibrating sample magnetometry along different directions in the sample plane. [1 –1 0] is the in plane hard axis with an anisotropy field H a=200 mT, whereas [0 0 1] is the in plane easy axis with large remanence and H c=70 mT (inset Fig. 2a). This could be expected as interface, magnetocrystalline and shape anisotropy all favor magnetization aligned parallel to [0 0 1] for Fe/Mo(1 1 0) [9]. Besides, H c/H a=0.35 is considerably higher than in continuous films with same thickness (B0.06) [9] owing to the break of exchange between the islands. From measurements along [0 0 1], the details of magnetic reversal in the islands can be investigated in more detail as follows. The variation of magnetization with applied field consists of irreversible and reversible contributions. Irreversible events arise from the switching in nearly single domain particles or from nucleation or annihilation of domains. The reversible part of the magnetization variation arises from continuous rotation of magnetic moments before switching or from domain wall motion. For an assembly of particles, a distribution of switching fields has to be taken into account and the macroscopic magnetization curve is the sum of the contributions of all particles. In the simple case of independent particles, a deconvolution procedure can be applied to extract the average single particle reversal loop from measurements over the assembly [10]. This requires the knowledge of the experimental reversible and irreversible contributions to the magnetization reversal that were extracted from a series of minor hysteresis loops (see Fig. 2). wrev ðHÞ was determined from the slope at the beginning of the recoil curves. The irreversible magnetization variation (Dmrass)(H) is the difference between the major hysteresis loop variation dM/dH(H) and the reversible contribution wrev ðHÞ: The reversible contribution is peaked close to zero field so that one can assume that reversible and
(a)
1
M/MS
P.-O. Jubert et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 565–567
M/M S
µ 0H (T)
0
-1 (b)
30
dM/dH(H)
χ(-H0)
dM/dH, χ, ρ
∆m.ρass(H) 20
Xrev(H) 10
0 -0.2
-0.1
0.0
µ 0 H (T)
0.1
0.2
Fig. 2. (a) 300 K major and minor loops measured along [0 0 1] for sample in Fig. 1a. Dots: measurements, lines: minor loops reconstructed after the deconvolution analysis. Inset: Major hysteresis loops measured along [0 0 1] (full squares) and [1 –1 0] (open circles). (b) Data used for the deconvolution analysis: (dM/dH)(H), wrev ðHÞ and (Dmrass)(H)=(dM/dH) (H)wrev ðHÞ: For wrev ðHÞ; crosses are experimental data and the dashed line is the fit used for minor loop reconstruction.
irreversible events are independent, the irreversible contribution being related to the switching field distribution of particles presenting nearly square hysteresis loops. With these assumptions, the minor hysteresis curves can be rebuilt following the relation: 8H > H0 ; MðHÞ ¼ Mmaj ðH0 Þ þ þ
Z
Z
H H0
wrev ðhÞ dh
H H0
ðDmrass ÞðhÞHe ðh H0 Þ dh
ð1Þ
with M maj the magnetization value for the major hysteresis loop and H e the Heaviside function (H e(x)=0 when xo0 and 1 when x>0). A very good agreement is found between the experimental and simulated minor loops as shown in
567
Fig. 2a. The assumptions thus well describe the reversal behavior of the nanostructures. This also shows that interactions between the islands are negligible, yielding the following description of the system. Two types of independent magnetic contributions are evidenced. The first type corresponds to reversal with square hysteresis loop and mean switching field equal to 70 mT. It is attributed to the elongated islands. The second type of magnetic behavior is essentially reversible. This later magnetic contribution may arise either from a small fraction of compact islands with almost no in plane shape anisotropy (Fig. 1a), or from the 2 AL wetting layer which is expected to be superparamagnetic at 300 K [9]. Note that Rwrev ðHÞ is not fully symmetric which H is the reasonR why ð H0 wrev ðhÞ dhÞ is used in Eq. (1) H instead of ð H0 wrev ðhÞ dhÞ: Such as dissymmetry in wrev ðHÞ may be a signature of a small coupling of the supposed superparamagnetic wetting layer with the nanostripes. In conclusion, we have demonstrated the possibility of growing multi-AL thick Fe elongated nanostructures and nanostripes on stepped Mo(1 1 0)/Al2O3(1 1–2 0) with various orientations. Preliminary magnetic measurements performed on the 5 nm high step-decorated elongated islands aligned parallel to [0 0 1] present full remanence at RT, with mean H c=70 mT, in contrast with 1–2 AL thick conventional step decoration.
References . [1] H.J. Elmers, J. Hauschild, H. Hoche, U. Gradmann, . H. Betghe, D. Heuer, U. Kohler, Phys. Rev. Lett. 73 (1994) 898. [2] J. Shen, R. Skomski, M. Klaua, H. Jenniches, S. Sundar Manoharan, J. Kirschner, Phys. Rev. B 56 (1997) 2340. [3] J. Hauschild, H.J. Elmers, U. Gradmann, Phys. Rev. B 57 (1998) R677. [4] P. Gambardella, M. Blanc, H. Brune, K. Kuhnke, K. Kern, Phys. Rev. B 61 (2001) 2254. [5] O. Fruchart, S. Jaren, J. Rothman, Appl. Surf. Sci. 135 (1998) 218. [6] M. Tikhov, E. Bauer, Surf. Sci. 232 (1990) 73. [7] P.-O. Jubert, O. Fruchart, C. Meyer, J. Magn. Magn. Mater. 226 (2001) 1842. [8] P.-O. Jubert, O. Fruchart, C. Meyer, Phys. Rev. B 64 (2001) 115419. [9] J. Maltzbender, M. Przybylski, J. Giergiel, J. Kirschner, Surf. Sci. 414 (1998) 187. [10] O. Fruchart, J.-P. Nozieres, B. Kevorkian, J.-C. Toussaint, D. Givord, F. Rousseaux, D. Decanini, F. Carcenac, Phys. Rev. B 57 (1998) 2596. [11] O. Fruchart, J.-P. Nozieres, D. Givord, J. Magn. Magn. Mater. 207 (1999) 158.
Magnetic properties of step-decorated Fe nanostripes and dots grown on Mo(1 1 0) P.-O. Jubert*, O. Fruchart, C. Meyer Laboratoire Louis N!eel, CNRS, BP 166, 38042 Grenoble, France
Abstract Arrays of epitaxial Fe(1 1 0) nanostripes 1–5 nm thick and B100 nm wide are fabricated under UHV by pulsed laser deposition on a stepped Mo(1 1 0) surface. This step decoration process results from surface diffusion limitations at 500 K, or alternatively from the occurrence of a metastable 6-atomic-layer thickness upon annealing a continuous film grown at room temperature. The stripe orientation and spacing are uniform over the whole sample as controlled by the 0.051 miscut of the substrate. Room temperature magnetic properties are investigated by vibrating sample magnetometry. In the case of steps parallel to Fe[0 0 1], all sources of anisotropy contribute to align the magnetization parallel to the step direction, yielding a rather square hysteresis loop with a coercive field of 70 mT. Average single particle behavior is extracted by differentiating the reversible and the irreversible magnetic contributions deduced from a series of minor hysteresis loops. A model with no interaction between the particles well reproduces the experimental results. r 2002 Published by Elsevier Science B.V. Keywords: Step edge decoration; Magnetic nanoparticles; Epitaxy; Pulsed laser deposition
The fabrication of magnetic nanostructures is of interest for both fundamental research and potential applications in magnetic recording. Low dimensional magnetic structures reveal new properties when their dimensions are comparable to some magnetic length scale such as the exchange length or the Bloch wall width which amount to B10 and B100 nm, respectively. Selfassembly/self-organization are promising methods to produce 1D and 0D model systems of such dimensions. It allows the fabrication of epitaxial nanoscale patterns over a large area, in a fast single process and with atomic ultimate size. Trade off with respect to other techniques such as e-beam lithography or scanning probe writing is of course the availability of patterns and a finite width of size distribution of the self-organized structures. Step decoration of self-organized periodically stepped surfaces has been used to grow arrays of nanowires and stripes [1–4]. This approach usually results in the formation of ultrathin structures (1 or 2 AL high) yielding strongly time-dependent magnetic behavior *Corresponding author. Fax: +33-4-76-88-11-91. E-mail address: [email protected] (P.-O. Jubert).
and usually superparamagnetic at room temperature (RT) [1–3]. We report a different approach that allow one to grow thicker nanostripes. We take advantage of diffusion kinetic limitations and of the occurrence of a previously unreported metastable 6 AL thickness for the Fe on Mo(1 1 0) system. The epitaxial growth of Fe(1 1 0) on Mo(1 1 0) was performed under ultra high vacuum by pulsed laser deposition (PLD). The samples were characterized by RHEED and STM in situ, while the morphology was also characterized on larger scales using AFM in air. The Mo(1 1 0) surface is that of a 10 nm thick single crystalline buffer layer deposited on a sapphire substrate Al2O3(1 1–2 0) following an optimized temperature gradient method [5]. Such a Mo(1 1 0) surface exhibits an array of parallel atomic steps (inset of Fig. 1b). The terrace width and orientation are uniform over the whole sample, with very small distribution. They are related to the sapphire miscut to the (1 1–2 0) surface of about 0.051, resulting in 200 nm wide terraces. Fe is ( grown with a mean deposition rate of 0.5 A/min as measured with a calibrated quartz microbalance. The substrate temperature during the growth can be tuned
0304-8853/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 0 3 8 - 1
566
P.-O. Jubert et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 565–567
Fig. 1. 10 mm 10 mm AFM images showing (a) 5 nm high wire shaped islands obtained after deposition at 500 K followed by annealing at 700 K, (b) self-organized 0.8 nm high nanostripes obtained after annealing at 700 K from a film grown at room temperature. Inset: 950 nm 950 nm STM image of the Mo(1 1 0) surface before Fe deposition.
from RT up to 800 K without any Fe–Mo intermixing [6]. The samples are finally capped with 3 nm of Mo grown at RT to avoid oxidation during ex situ experiments. Above 600 K, no influence of the atomic steps is observed. The growth proceeds in a Stranski–Krastanov mode resulting in self-assembled compact epitaxial islands elongated along Mo,Fe[0 0 1] BCC direction, on top of one pseudo-morphic atomic layer. The islands have a lateral aspect ratio around 2 and display an ingot shape with atomically flat facets. The growth proceeds close to the thermodynamic equilibrium. The shape and orientation of the islands are well understood in terms of surface and interface energies [7,8]. At 500 K, the growth of Fe on Mo(1 1 0) still follows a Stranski–Krastanov mode as observed by STM at all stages of the growth. Due to kinetic limitations (1) the wetting layer amounts to 2 atomic planes and (2) surface diffusion is hindered and becomes dependent on the presence of atomic steps. Elongated nanostructures parallel to the atomic steps of the Mo buffer layer thus form, together with a small amount of compact islands. Further annealing at 700 K results in smoothening and slight improving of the regularity of the stripes. Fig. 1a shows an AFM image of such nanostripes after deposition of 1.5 nm Fe on Mo(1 1 0) at 500 K followed by annealing at 700 K. Typical height and width for the elongated islands are 5 and 140 nm, respectively. In the range 300–400 K, kinetic limitations are even stronger resulting in the formation of a continuous thin Fe film. The elastic strain induced by the 9% misfit is partly relaxed by the introduction of a 2D dislocation network between the first and second atomic layer (AL) but the growth mode remains layer by layer up to the filling of the 4th atomic plane. Note that 2D growth is improved by PLD compared to MBE [11]. In the range of nominal thickness Y=2–6 AL and upon annealing at 700 K the films break into flat patches of constant height
(4–5 AL) above two wetting AL. Schematically, depending on the filling ratio (Y2)/4, the patches go from disconnected to percolated for Y going from 2 to 6 AL. For Y>6 AL, 3D compact islands grow at the expense of the 6 AL thick layer. This gives evidence of the metastability of 6 AL structures for the Fe/Mo(1 1 0) system. We believe that this critical thickness is associated with modifications in the dislocation network to reduce the residual stress. The morphology of the 6 AL patches are influenced by the Mo atomic steps, lower step edge acting as a favored site for the formation of the patches. For an optimum range of Y[2.5 MC, 4.5 MC], the annealing process results in the formation of an array of continuous epitaxial nanostripes on top of two wetting AL, with controlled orientation and width ws=wt(Y2 AL)/4 AL, wt being the Mo terrace width (see Fig 1b). The magnetic properties of elongated islands grown at 500 K and aligned along Fe[0 0 1] (Fig. 1a) were investigated at RT by vibrating sample magnetometry along different directions in the sample plane. [1 –1 0] is the in plane hard axis with an anisotropy field H a=200 mT, whereas [0 0 1] is the in plane easy axis with large remanence and H c=70 mT (inset Fig. 2a). This could be expected as interface, magnetocrystalline and shape anisotropy all favor magnetization aligned parallel to [0 0 1] for Fe/Mo(1 1 0) [9]. Besides, H c/H a=0.35 is considerably higher than in continuous films with same thickness (B0.06) [9] owing to the break of exchange between the islands. From measurements along [0 0 1], the details of magnetic reversal in the islands can be investigated in more detail as follows. The variation of magnetization with applied field consists of irreversible and reversible contributions. Irreversible events arise from the switching in nearly single domain particles or from nucleation or annihilation of domains. The reversible part of the magnetization variation arises from continuous rotation of magnetic moments before switching or from domain wall motion. For an assembly of particles, a distribution of switching fields has to be taken into account and the macroscopic magnetization curve is the sum of the contributions of all particles. In the simple case of independent particles, a deconvolution procedure can be applied to extract the average single particle reversal loop from measurements over the assembly [10]. This requires the knowledge of the experimental reversible and irreversible contributions to the magnetization reversal that were extracted from a series of minor hysteresis loops (see Fig. 2). wrev ðHÞ was determined from the slope at the beginning of the recoil curves. The irreversible magnetization variation (Dmrass)(H) is the difference between the major hysteresis loop variation dM/dH(H) and the reversible contribution wrev ðHÞ: The reversible contribution is peaked close to zero field so that one can assume that reversible and
(a)
1
M/MS
P.-O. Jubert et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 565–567
M/M S
µ 0H (T)
0
-1 (b)
30
dM/dH(H)
χ(-H0)
dM/dH, χ, ρ
∆m.ρass(H) 20
Xrev(H) 10
0 -0.2
-0.1
0.0
µ 0 H (T)
0.1
0.2
Fig. 2. (a) 300 K major and minor loops measured along [0 0 1] for sample in Fig. 1a. Dots: measurements, lines: minor loops reconstructed after the deconvolution analysis. Inset: Major hysteresis loops measured along [0 0 1] (full squares) and [1 –1 0] (open circles). (b) Data used for the deconvolution analysis: (dM/dH)(H), wrev ðHÞ and (Dmrass)(H)=(dM/dH) (H)wrev ðHÞ: For wrev ðHÞ; crosses are experimental data and the dashed line is the fit used for minor loop reconstruction.
irreversible events are independent, the irreversible contribution being related to the switching field distribution of particles presenting nearly square hysteresis loops. With these assumptions, the minor hysteresis curves can be rebuilt following the relation: 8H > H0 ; MðHÞ ¼ Mmaj ðH0 Þ þ þ
Z
Z
H H0
wrev ðhÞ dh
H H0
ðDmrass ÞðhÞHe ðh H0 Þ dh
ð1Þ
with M maj the magnetization value for the major hysteresis loop and H e the Heaviside function (H e(x)=0 when xo0 and 1 when x>0). A very good agreement is found between the experimental and simulated minor loops as shown in
567
Fig. 2a. The assumptions thus well describe the reversal behavior of the nanostructures. This also shows that interactions between the islands are negligible, yielding the following description of the system. Two types of independent magnetic contributions are evidenced. The first type corresponds to reversal with square hysteresis loop and mean switching field equal to 70 mT. It is attributed to the elongated islands. The second type of magnetic behavior is essentially reversible. This later magnetic contribution may arise either from a small fraction of compact islands with almost no in plane shape anisotropy (Fig. 1a), or from the 2 AL wetting layer which is expected to be superparamagnetic at 300 K [9]. Note that Rwrev ðHÞ is not fully symmetric which H is the reasonR why ð H0 wrev ðhÞ dhÞ is used in Eq. (1) H instead of ð H0 wrev ðhÞ dhÞ: Such as dissymmetry in wrev ðHÞ may be a signature of a small coupling of the supposed superparamagnetic wetting layer with the nanostripes. In conclusion, we have demonstrated the possibility of growing multi-AL thick Fe elongated nanostructures and nanostripes on stepped Mo(1 1 0)/Al2O3(1 1–2 0) with various orientations. Preliminary magnetic measurements performed on the 5 nm high step-decorated elongated islands aligned parallel to [0 0 1] present full remanence at RT, with mean H c=70 mT, in contrast with 1–2 AL thick conventional step decoration.
References . [1] H.J. Elmers, J. Hauschild, H. Hoche, U. Gradmann, . H. Betghe, D. Heuer, U. Kohler, Phys. Rev. Lett. 73 (1994) 898. [2] J. Shen, R. Skomski, M. Klaua, H. Jenniches, S. Sundar Manoharan, J. Kirschner, Phys. Rev. B 56 (1997) 2340. [3] J. Hauschild, H.J. Elmers, U. Gradmann, Phys. Rev. B 57 (1998) R677. [4] P. Gambardella, M. Blanc, H. Brune, K. Kuhnke, K. Kern, Phys. Rev. B 61 (2001) 2254. [5] O. Fruchart, S. Jaren, J. Rothman, Appl. Surf. Sci. 135 (1998) 218. [6] M. Tikhov, E. Bauer, Surf. Sci. 232 (1990) 73. [7] P.-O. Jubert, O. Fruchart, C. Meyer, J. Magn. Magn. Mater. 226 (2001) 1842. [8] P.-O. Jubert, O. Fruchart, C. Meyer, Phys. Rev. B 64 (2001) 115419. [9] J. Maltzbender, M. Przybylski, J. Giergiel, J. Kirschner, Surf. Sci. 414 (1998) 187. [10] O. Fruchart, J.-P. Nozieres, B. Kevorkian, J.-C. Toussaint, D. Givord, F. Rousseaux, D. Decanini, F. Carcenac, Phys. Rev. B 57 (1998) 2596. [11] O. Fruchart, J.-P. Nozieres, D. Givord, J. Magn. Magn. Mater. 207 (1999) 158.