- Email: [email protected]
Fe trilayers (x=0.4–1.0)
Journal of Magnetism and Magnetic Materials 240 (2002) 235–237
Very strong interlayer exchange coupling in epitaxial Fe/Fe1xSix/Fe trilayers (x=0.4–1.0) R.R. Gareev*, D.E. Burgler, . M. Buchmeier, R. Schreiber, P. Grunberg . Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . . Festkorperforschung, . .
Abstract Fe/Fe1xSix/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J1 ) and biquadratic (J2 ) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J ¼ J1 þ J2 increases dramatically with increasing x and reaches values in excess of 6 mJ/m2. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films – trilayer; Exchange coupling – interlayer
During the last decade Fe/Si/Fe exchange coupled structures are attracting permanent interest due to unusual coupling [1–6,7]. However, the reason for the strong coupling in these structures is still unclear. The presence of interdiffusion at the Fe/Si interfaces possibly with the formation of different iron-silicides makes the behaviour of the interlayer coupling sensitive to deposition procedures. Due to interdiffusion, spacer layers tend to crystallize in CsCl-type Fe0.5Si0.5 [8], especially at elevated temperatures [2,3], but exhibit an unusual exponential decay of coupling versus spacer thickness [3,7]. In contrast, the quantum interference model (QIM) of exchange coupling [4] predicts an exponential decay of the coupling only for insulating spacers, whereas metallic spacers reveal an oscillatory coupling. Recently, we prepared epitaxial Fe/FeSi/Fe trilayers by co-deposition of Fe and Si with the spacer composition close to Fe0.5Si0.5 [5]. In order to induce the formation of epitaxial iron-silicides we deposited the spacer layer at an elevated temperature of 473 K and obtained (i) a coupling strength of less than 1 mJ/m2, (ii) long-period oscillatory coupling at all temperatures from 20 to 300 K ( (corresponding to 12 and with maxima at 18 and 39 A ( when the volume contraction during the alloy 26 A formation is taken into account), and (iii) increasing *Corresponding author. Tel.: +49-2461-613148; fax: +492461-614443. E-mail address: [email protected] (R.R. Gareev).
coupling strength with decreasing temperature in good agreement with the QIM for metallic spacers [5]. Here, we use co-deposition of Fe and Si in order to produce homogeneous and epitaxial Fe1xSix spacers with a welldefined structure and variable nominal composition x: Epitaxial Fe/FeSi-wedge/Fe sandwiches are grown in a molecular beam epitaxy system onto a GaAs(1 0 0)/ Fe(1 nm)/Ag(150 nm) substrate-buffer system described elsewhere [9]. In order to prevent Ag segregation the first ( four monolayers of the 50 A-thick bottom Fe layer are grown at room temperature (RT) and the remaining Fe at 473 K. The wedge-shaped Fe1xSix spacers are codeposited from two separate e-guns at RT and at low ( deposition rates (0.1 A/s) for both Fe and Si. Thicknesses, deposition rates, and relative atomic flux are controlled using calibrated quartz-crystal monitors. The nominal thickness of the spacer layer is given here as the sum of the quartz-crystal readings for Fe and Si. These values are not recalculated to correct for the volume contraction occurring during silicide formation [3,5] because the conversion requires precise knowledge about the silicides formed in the spacers. The nominal ( A 50 A-thick ( spacer thickness varies from 0 to 50 A. ( upper Fe layer and a 500 A-thick ZnS coating are deposited at RT. The composition and the structure of the trilayers are controlled in situ by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), respectively. A well-defined LEED (0 0) spot (at 75 eV)
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 7 7 - 6
R.R. Gareev et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 235–237
Frequency (GHz)
40 (a) 20 J = -5.6 mJ/m2 0 40
(b)
20 J = -0.9 mJ/m2 0
0
200 400 Magnetic field H (kA/m)
600
Fig. 1. Spin-wave frequencies of the optic (black) and acoustic ( Fe0.2Si0.8(t)/ (grey) modes versus magnetic field H for Fe(50 A)/ ( trilayer with (a) t ¼ 9:1 A ( and (b) t ¼ 15:1 A. ( Dots Fe(50 A) show experimental data and lines fitted curves yielding (a) J1 ¼ 4:5 mJ/m2, J2 ¼ 1:1 mJ/m2 and (b) J1 ¼ 0:8 mJ/ m2, J2 ¼ 0:1 mJ/m2. Pairs of arrows indicate the directions of magnetic moments.
MOKE signal (a.u.)
indicating epitaxial growth is observed for both Fe layers and for the whole range of spacer thicknesses. The spacer composition is calculated from Fe and Si deposition rates as well as from AES spectra. Both methods agree within the error of o5% and confirm the homogeneous composition of the FeSi spacers up to a ( nominal thickness of 50 A. The magnetic properties are checked by the longitudinal magneto-optic Kerr effect (MOKE) and by Brillouin light scattering (BLS) as described earlier [10,11]. The external magnetic field of up to H=560 kA/m is applied in the sample plane. BLS experiments are performed at RT using a tandem multipass Fabry–Perot interferometer in the back-scattering geometry. The spin-wave frequencies of the optic and acoustic modes are calculated from the spin-wave dispersion relations. The contributions of the bilinear (J1 ) and the biquadratic (J2) terms to the exchange coupling are determined following the approach described in Ref. [11]. The coupling strengths J1 and J2 are obtained from fitting the dependencies of optic and acoustic Stokes and anti-Stokes modes on H applied along an easy-axis for all spacer thicknesses t of interest. Whenever the saturation field is o560 kA/m, we derive J1 and J2 also from the fitting of easy-axis longitudinal MOKE hysteresis loops using the standard procedure of minimizing for each value of H the free energy with respect to the orientations of the magnetic moments. We determine the coupling parameters for several SixFe1x spacers with different compositions ranging from x=0.4170.02 to x=1.0, i.e. nominally pure Si. Typical dependencies of the BLS mode frequencies on H are presented in Fig. 1 for different coupling strengths. In Fig. 1 we only present optic and acoustic modes of the Stokes side of the spectra (anti-Stokes side is symmetric). The experimental and fitted BLS curves reveal good agreement for the bulk values of the magnetizations (1.7 106 A/m) and the magnetocrystalline anisotropy constants (45 kJ/m3) indicating good epitaxial growth for both Fe layers. For antiparallel alignment the frequency of the optic mode (identified by its lower intensity) is higher than for the acoustic mode. The rotation of the magnetic moments into the field direction is accompanied by a crossing of the spin-wave modes. For parallel alignment the acoustic mode is observed at higher frequencies than the optic mode. Switching from antiparallel to parallel alignment is seen in Fig. 1(b) for a coupling strengths of about 1 mJ/m2. Typical experimental and simulated MOKE hysteresis loops are shown in Fig. 2. From both BLS and MOKE experiments we determine J1 and J2 as well as the spacer thickness, tmax ; of maximum J1 for a variety of spacer compositions. The dependencies of J1 and J2 on t for a nominally pure Si and a Fe0.2 Si0.8 spacer are presented
MOKE signal (a.u.)
236
1 (a)
0
-1 1 (b)
0
-1 -300
0 Magnetic field H (kA/m)
300
( ( Fig. 2. MOKE hysteresis loops for Fe(50 A)/Si(t)/Fe(50 A) ( and (b) t ¼ 15:6 A. ( Circles show trilayers with (a) t ¼ 17:2 A experimental data and crosses fitted curves yielding (a) J1 ¼ 0:24 mJ/m2, J2 ¼ 0:03 mJ/m2 and (b) J1 ¼ 0:60 mJ/m2, J2 ¼ 0:09 mJ/m2. Pairs of arrows indicate the directions of magnetic moments.
in Fig. 3. For the nominally pure Si spacer and sufficiently large t; where J1 prevails, J1 decreases exponentially with spacer thickness. The decay length ( (inset of Fig. 3). A possible mechanism for is t0D1.7 A biquadratic coupling is described in Ref. [7] and is
R.R. Gareev et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 235–237
3 BLS:
1
6 (a)
-J (mJ/m2)
4 (a) (b) J1
-J1 (mJ/m2)
exp(-t/t0) t0=(1.7±0.1)Å
4 2
0
2 MOKE:
14
16 18 20 22 Spacer thickness t (Å)
14
(b)
tmax (Å)
-J1, -J2 (mJ/m2)
5
237
12
1 0
10
(c) (d) J2
8 0.4
6
8
10 12 14 16 18 Spacer thickness t (Å)
20
22
Fig. 3. J1 and J2 versus spacer thickness t as derived from BLS ( ( (a,c) and and MOKE experiments for Fe(50 A)/Si(t)/Fe(50 A) ( ( Fe(50 A)/Fe 0.2Si0.8 (t)/Fe(50 A) (b,d). (a,b) and (c,d) show J1 and J2 for both compositions, respectively. Inset: exponential decay of the coupling with spacer thickness t for a nominally pure Si spacer layer.
beyond the scope of this paper. The decrease of the coupling strength at small thicknesses, totmax ; is related to the presence of ferromagnetic coupling due to pinholes [3]. We observe antiferromagnetic exchange ( coupling for Si spacer thicknesses up to 22 A. The dependencies of the total coupling J ¼ J1 þ J2 and of tmax on the nominal Si content x are shown in Fig. 4. The coupling strength increases strongly with x; and tmax shifts to smaller thicknesses. The total coupling strength is >6 mJ/m2 for a nominally pure Si spacer and is the largest value ever reported for the Fe/Si/Fe system. ( and decreases to For a Si-rich spacer, tmax is near 10 A ( for x ¼ 1:0: It is important to note that (7.770.3) A these tmax values from spacers grown at RT are strikingly smaller than for metallic Fe0.5Si0.5 spacer ( corresponds to 12 A ( layers formed at 473 K (tmax=18 A effective spacer thickness corrected for the volume contraction as given in Refs. [3,5]). If the volume contraction is taken into account, the tmax values of our spacers become even smaller (e.g. by a factor 1.5 for x ¼ 0:5). This result indicates strong suppression of iron-silicide formation and a better homogeneity of our epitaxial spacers. The observed non-oscillatory but exponentially decaying coupling corresponds to the QIM for non-metallic spacers [4]. This interpretation is further supported by measurements of the resistivity r of Fe/Si/Fe trilayers in current-perpendicular-plane geometry, which yield r>106 mO cm, i.e. about 105 times bigger than resistivity of Fe. We conclude that the coupling strength increases strongly with increasing nominal Si content in the spacer layer. We relate the very strong exchange coupling and
0.6 0.8 Nominal Si content x
1.0
Fig. 4. (a) Total coupling J and (b) position of coupling ( maximum tmax versus Si content x for an epitaxial Fe(50 A)/ ( trilayer. Fe1xSix(t)/Fe(50 A)
its exponential decay for nominally pure Si spacers to the growth of highly resisitive layers that we obtain at smaller spacer thicknesses than reported before. A theoretical reasoning for this connection is lacking at present. However, our previous results for metallic-type Fe0.5Si0.5 spacers [5] and the results presented here exclude diffusive formation of metallic iron-silicide spacers to be the reason for the observed strong coupling.
References [1] E.E. Fullerton, J.E. Mattson, S.R. Lee, C.H. Sowers, Y.Y. Huang, G. Felcher, S.D. Bader, J. Magn. Magn. Mater. 117 (1992) L301. [2] A. Chaiken, R.P. Michel, M.A. Wall, Phys. Rev. B 53 (1996) 5518. [3] J.J. de Vries, J. Kohlhepp, F.J.A. den Broeder, R. Coehoorn, R. Jungblut, A. Reinders, W.J.M. de Jonge, Phys. Rev. Lett. 78 (1997) 3023. [4] P. Bruno, Phys. Rev. B 52 (1995) 411. [5] R.R. Gareev, D.E. Burgler, . M. Buchmeier, D. Olligs, R. Schreiber, P. Grunberg, . Phys. Rev. Lett. 87 (2001) 157202. [6] Y. Endo, O. Kitakami, Y. Shimada, Phys. Rev. B 59 (1999) 4279. [7] G.J. Strijkers, J.T. Kohlhepp, H.J.M. Swatgen, W.J.M. de Jonge, J. Appl. Phys. 87 (2000) 5452. [8] G.J. Strijkers, J.T. Kohlhepp, H.J.M. Swatgen, W.J.M. de Jonge, Phys. Rev. B 60 (1999) 9583. [9] D.E. Burgler, . C.M. Schmidt, J.A. Wolf, T.M. Schaub, H.-J. Guntherodt, . Surf. Sci. 366 (1996) 295. [10] P. Grunberg, . in: M. Cardona, G. Guntherodt . (Eds.), Topics in Applied Physics, Vol. 66, Springer, 1989, p. 303. [11] S.M. Rezende, C. Chesman, M.A. Lucena, A. Azevedo, F.M. de Aguiar, J. Appl. Phys. 84 (1998) 958.
Very strong interlayer exchange coupling in epitaxial Fe/Fe1xSix/Fe trilayers (x=0.4–1.0) R.R. Gareev*, D.E. Burgler, . M. Buchmeier, R. Schreiber, P. Grunberg . Institut fur Forschungszentrum Julich GmbH, D-52425 Julich, Germany . . Festkorperforschung, . .
Abstract Fe/Fe1xSix/Fe (x=0.4–1.0) wedge-type epitaxial trilayers with improved homogeneity are grown by co-evaporation from two electron-beam sources. The coupling strengths of the bilinear (J1 ) and biquadratic (J2 ) coupling terms are derived from Brillouin light scattering (BLS) spectra and longitudinal MOKE hysteresis loops. The total coupling strength J ¼ J1 þ J2 increases dramatically with increasing x and reaches values in excess of 6 mJ/m2. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Thin films – trilayer; Exchange coupling – interlayer
During the last decade Fe/Si/Fe exchange coupled structures are attracting permanent interest due to unusual coupling [1–6,7]. However, the reason for the strong coupling in these structures is still unclear. The presence of interdiffusion at the Fe/Si interfaces possibly with the formation of different iron-silicides makes the behaviour of the interlayer coupling sensitive to deposition procedures. Due to interdiffusion, spacer layers tend to crystallize in CsCl-type Fe0.5Si0.5 [8], especially at elevated temperatures [2,3], but exhibit an unusual exponential decay of coupling versus spacer thickness [3,7]. In contrast, the quantum interference model (QIM) of exchange coupling [4] predicts an exponential decay of the coupling only for insulating spacers, whereas metallic spacers reveal an oscillatory coupling. Recently, we prepared epitaxial Fe/FeSi/Fe trilayers by co-deposition of Fe and Si with the spacer composition close to Fe0.5Si0.5 [5]. In order to induce the formation of epitaxial iron-silicides we deposited the spacer layer at an elevated temperature of 473 K and obtained (i) a coupling strength of less than 1 mJ/m2, (ii) long-period oscillatory coupling at all temperatures from 20 to 300 K ( (corresponding to 12 and with maxima at 18 and 39 A ( when the volume contraction during the alloy 26 A formation is taken into account), and (iii) increasing *Corresponding author. Tel.: +49-2461-613148; fax: +492461-614443. E-mail address: [email protected] (R.R. Gareev).
coupling strength with decreasing temperature in good agreement with the QIM for metallic spacers [5]. Here, we use co-deposition of Fe and Si in order to produce homogeneous and epitaxial Fe1xSix spacers with a welldefined structure and variable nominal composition x: Epitaxial Fe/FeSi-wedge/Fe sandwiches are grown in a molecular beam epitaxy system onto a GaAs(1 0 0)/ Fe(1 nm)/Ag(150 nm) substrate-buffer system described elsewhere [9]. In order to prevent Ag segregation the first ( four monolayers of the 50 A-thick bottom Fe layer are grown at room temperature (RT) and the remaining Fe at 473 K. The wedge-shaped Fe1xSix spacers are codeposited from two separate e-guns at RT and at low ( deposition rates (0.1 A/s) for both Fe and Si. Thicknesses, deposition rates, and relative atomic flux are controlled using calibrated quartz-crystal monitors. The nominal thickness of the spacer layer is given here as the sum of the quartz-crystal readings for Fe and Si. These values are not recalculated to correct for the volume contraction occurring during silicide formation [3,5] because the conversion requires precise knowledge about the silicides formed in the spacers. The nominal ( A 50 A-thick ( spacer thickness varies from 0 to 50 A. ( upper Fe layer and a 500 A-thick ZnS coating are deposited at RT. The composition and the structure of the trilayers are controlled in situ by Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED), respectively. A well-defined LEED (0 0) spot (at 75 eV)
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 7 7 - 6
R.R. Gareev et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 235–237
Frequency (GHz)
40 (a) 20 J = -5.6 mJ/m2 0 40
(b)
20 J = -0.9 mJ/m2 0
0
200 400 Magnetic field H (kA/m)
600
Fig. 1. Spin-wave frequencies of the optic (black) and acoustic ( Fe0.2Si0.8(t)/ (grey) modes versus magnetic field H for Fe(50 A)/ ( trilayer with (a) t ¼ 9:1 A ( and (b) t ¼ 15:1 A. ( Dots Fe(50 A) show experimental data and lines fitted curves yielding (a) J1 ¼ 4:5 mJ/m2, J2 ¼ 1:1 mJ/m2 and (b) J1 ¼ 0:8 mJ/ m2, J2 ¼ 0:1 mJ/m2. Pairs of arrows indicate the directions of magnetic moments.
MOKE signal (a.u.)
indicating epitaxial growth is observed for both Fe layers and for the whole range of spacer thicknesses. The spacer composition is calculated from Fe and Si deposition rates as well as from AES spectra. Both methods agree within the error of o5% and confirm the homogeneous composition of the FeSi spacers up to a ( nominal thickness of 50 A. The magnetic properties are checked by the longitudinal magneto-optic Kerr effect (MOKE) and by Brillouin light scattering (BLS) as described earlier [10,11]. The external magnetic field of up to H=560 kA/m is applied in the sample plane. BLS experiments are performed at RT using a tandem multipass Fabry–Perot interferometer in the back-scattering geometry. The spin-wave frequencies of the optic and acoustic modes are calculated from the spin-wave dispersion relations. The contributions of the bilinear (J1 ) and the biquadratic (J2) terms to the exchange coupling are determined following the approach described in Ref. [11]. The coupling strengths J1 and J2 are obtained from fitting the dependencies of optic and acoustic Stokes and anti-Stokes modes on H applied along an easy-axis for all spacer thicknesses t of interest. Whenever the saturation field is o560 kA/m, we derive J1 and J2 also from the fitting of easy-axis longitudinal MOKE hysteresis loops using the standard procedure of minimizing for each value of H the free energy with respect to the orientations of the magnetic moments. We determine the coupling parameters for several SixFe1x spacers with different compositions ranging from x=0.4170.02 to x=1.0, i.e. nominally pure Si. Typical dependencies of the BLS mode frequencies on H are presented in Fig. 1 for different coupling strengths. In Fig. 1 we only present optic and acoustic modes of the Stokes side of the spectra (anti-Stokes side is symmetric). The experimental and fitted BLS curves reveal good agreement for the bulk values of the magnetizations (1.7 106 A/m) and the magnetocrystalline anisotropy constants (45 kJ/m3) indicating good epitaxial growth for both Fe layers. For antiparallel alignment the frequency of the optic mode (identified by its lower intensity) is higher than for the acoustic mode. The rotation of the magnetic moments into the field direction is accompanied by a crossing of the spin-wave modes. For parallel alignment the acoustic mode is observed at higher frequencies than the optic mode. Switching from antiparallel to parallel alignment is seen in Fig. 1(b) for a coupling strengths of about 1 mJ/m2. Typical experimental and simulated MOKE hysteresis loops are shown in Fig. 2. From both BLS and MOKE experiments we determine J1 and J2 as well as the spacer thickness, tmax ; of maximum J1 for a variety of spacer compositions. The dependencies of J1 and J2 on t for a nominally pure Si and a Fe0.2 Si0.8 spacer are presented
MOKE signal (a.u.)
236
1 (a)
0
-1 1 (b)
0
-1 -300
0 Magnetic field H (kA/m)
300
( ( Fig. 2. MOKE hysteresis loops for Fe(50 A)/Si(t)/Fe(50 A) ( and (b) t ¼ 15:6 A. ( Circles show trilayers with (a) t ¼ 17:2 A experimental data and crosses fitted curves yielding (a) J1 ¼ 0:24 mJ/m2, J2 ¼ 0:03 mJ/m2 and (b) J1 ¼ 0:60 mJ/m2, J2 ¼ 0:09 mJ/m2. Pairs of arrows indicate the directions of magnetic moments.
in Fig. 3. For the nominally pure Si spacer and sufficiently large t; where J1 prevails, J1 decreases exponentially with spacer thickness. The decay length ( (inset of Fig. 3). A possible mechanism for is t0D1.7 A biquadratic coupling is described in Ref. [7] and is
R.R. Gareev et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 235–237
3 BLS:
1
6 (a)
-J (mJ/m2)
4 (a) (b) J1
-J1 (mJ/m2)
exp(-t/t0) t0=(1.7±0.1)Å
4 2
0
2 MOKE:
14
16 18 20 22 Spacer thickness t (Å)
14
(b)
tmax (Å)
-J1, -J2 (mJ/m2)
5
237
12
1 0
10
(c) (d) J2
8 0.4
6
8
10 12 14 16 18 Spacer thickness t (Å)
20
22
Fig. 3. J1 and J2 versus spacer thickness t as derived from BLS ( ( (a,c) and and MOKE experiments for Fe(50 A)/Si(t)/Fe(50 A) ( ( Fe(50 A)/Fe 0.2Si0.8 (t)/Fe(50 A) (b,d). (a,b) and (c,d) show J1 and J2 for both compositions, respectively. Inset: exponential decay of the coupling with spacer thickness t for a nominally pure Si spacer layer.
beyond the scope of this paper. The decrease of the coupling strength at small thicknesses, totmax ; is related to the presence of ferromagnetic coupling due to pinholes [3]. We observe antiferromagnetic exchange ( coupling for Si spacer thicknesses up to 22 A. The dependencies of the total coupling J ¼ J1 þ J2 and of tmax on the nominal Si content x are shown in Fig. 4. The coupling strength increases strongly with x; and tmax shifts to smaller thicknesses. The total coupling strength is >6 mJ/m2 for a nominally pure Si spacer and is the largest value ever reported for the Fe/Si/Fe system. ( and decreases to For a Si-rich spacer, tmax is near 10 A ( for x ¼ 1:0: It is important to note that (7.770.3) A these tmax values from spacers grown at RT are strikingly smaller than for metallic Fe0.5Si0.5 spacer ( corresponds to 12 A ( layers formed at 473 K (tmax=18 A effective spacer thickness corrected for the volume contraction as given in Refs. [3,5]). If the volume contraction is taken into account, the tmax values of our spacers become even smaller (e.g. by a factor 1.5 for x ¼ 0:5). This result indicates strong suppression of iron-silicide formation and a better homogeneity of our epitaxial spacers. The observed non-oscillatory but exponentially decaying coupling corresponds to the QIM for non-metallic spacers [4]. This interpretation is further supported by measurements of the resistivity r of Fe/Si/Fe trilayers in current-perpendicular-plane geometry, which yield r>106 mO cm, i.e. about 105 times bigger than resistivity of Fe. We conclude that the coupling strength increases strongly with increasing nominal Si content in the spacer layer. We relate the very strong exchange coupling and
0.6 0.8 Nominal Si content x
1.0
Fig. 4. (a) Total coupling J and (b) position of coupling ( maximum tmax versus Si content x for an epitaxial Fe(50 A)/ ( trilayer. Fe1xSix(t)/Fe(50 A)
its exponential decay for nominally pure Si spacers to the growth of highly resisitive layers that we obtain at smaller spacer thicknesses than reported before. A theoretical reasoning for this connection is lacking at present. However, our previous results for metallic-type Fe0.5Si0.5 spacers [5] and the results presented here exclude diffusive formation of metallic iron-silicide spacers to be the reason for the observed strong coupling.
References [1] E.E. Fullerton, J.E. Mattson, S.R. Lee, C.H. Sowers, Y.Y. Huang, G. Felcher, S.D. Bader, J. Magn. Magn. Mater. 117 (1992) L301. [2] A. Chaiken, R.P. Michel, M.A. Wall, Phys. Rev. B 53 (1996) 5518. [3] J.J. de Vries, J. Kohlhepp, F.J.A. den Broeder, R. Coehoorn, R. Jungblut, A. Reinders, W.J.M. de Jonge, Phys. Rev. Lett. 78 (1997) 3023. [4] P. Bruno, Phys. Rev. B 52 (1995) 411. [5] R.R. Gareev, D.E. Burgler, . M. Buchmeier, D. Olligs, R. Schreiber, P. Grunberg, . Phys. Rev. Lett. 87 (2001) 157202. [6] Y. Endo, O. Kitakami, Y. Shimada, Phys. Rev. B 59 (1999) 4279. [7] G.J. Strijkers, J.T. Kohlhepp, H.J.M. Swatgen, W.J.M. de Jonge, J. Appl. Phys. 87 (2000) 5452. [8] G.J. Strijkers, J.T. Kohlhepp, H.J.M. Swatgen, W.J.M. de Jonge, Phys. Rev. B 60 (1999) 9583. [9] D.E. Burgler, . C.M. Schmidt, J.A. Wolf, T.M. Schaub, H.-J. Guntherodt, . Surf. Sci. 366 (1996) 295. [10] P. Grunberg, . in: M. Cardona, G. Guntherodt . (Eds.), Topics in Applied Physics, Vol. 66, Springer, 1989, p. 303. [11] S.M. Rezende, C. Chesman, M.A. Lucena, A. Azevedo, F.M. de Aguiar, J. Appl. Phys. 84 (1998) 958.