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CoAg trilayer films: Effect of Ag impurities on the exchange coupling
Journal of Magnetism and Magnetic Materials 156 (1996) 267-268
~4
Jeurnalof magnetism and magnetic materials
ELSEVIER
Interlayer magnetic coupling in Co/Ru/CoAg and Co/Cu/CoAg trilayer films" effect of Ag impurities on the exchange coupling K. Ounadjela a.*, Li Zhou b, R. Stamps b M. Hehn a Z. Zhang b, p. Wigen b J. Gregg c a IPCMS, CNRS, UMR46, 23, rue du Loess, F-67037 Strasbourg, France Department of Physics, Ohio State Uniuersi~. Columbus, OH 43210, USA c Clarendon Laboratory, Universi~, of Oxfi~rd, Parks Road, Oxford OXI 3PU, UK
Abstract We discuss here the consequences on the interlayer exchange coupling of the addition of small amounts of Ag within the cobalt layers of high crystalline quality C o / C u / C o A g and C o / R u / C o A g trilayers. We show that the Ag impurities induce a change in the average potential seen by the 's' and 'd' electrons in the cobalt. This is at the origin of phase shifts in the oscillatory interlayer coupling behaviour.
By varying the concentration of Ag impurities in the cobalt layers of high-quality molecular-beam-epitaxy grown C o / C u / C o A g trilayer films, we have observed a rapid decrease in the strength of the coherent antiferromagnetic (AFM) spin structure which changes sign at 5% Ag. The dependence of the exchange coupling on the spacer layer thickness measured by ferromagnetic resonance (FMR) experiments is shown in Fig. 1 for both C o / C u / C o and the C o / C u / C o 9 5 A g 5 series of samples. The oscillatory behavior found for the C o / C u / C o trilayer series indicates that the exchange interaction oscillates between antiferromagnetic and ferromagnetic coupling depending on the Cu spacer layer thickness, while for the C o / C u / C o 9 5 A g 5 series, the ferromagnetic exchange coupling decays almost linearly to zero coupling with the Cu layer thickness. In order to identify the crucial parameter responsible for the loss of the antiferromagnetic coupling in this system, we have varied the concentration of Ag impurities in the Co layers in a series of films for which Cu and Ru are the interlayer media. The samples were prepared by molecular beam epitax~¢ on a smooth, clean single-crystalline hcp (0001) 150A thick Ru buffer layer at a pressure below 2 × 10 ~0 mbar. The film growth was monitored by high-reflection energy electron diffraction (RHEED). The substrate temperature
Corresponding author. kamel @taranis.u-strasbg.fr.
Fax:
+33-88-10-72-49;
email:
was held at 240 K to favor small clusters of the Ag within the cobalt matrix, as revealed by cross-sectional transmission electron microscopy. The cobalt grows epitaxially on the Ru buffer with the hexagonal basal plane parallel to the surface. The RHEED patterns obtained during sample growth reveal well defined structures which suggest good crystalline quality. Using 3D RHEED diffraction analysis on the cobalt layers, we observed that during growth the crystal structure fluctuates between fcc (I, I, 1) and hcp
05
•
0.4
\
•
Co/Cu/C°95Ag5
\
0.3 "
- - i - - Co/Cu/Co
1).2
~.~ 0.1 0.1
. i
I ,0
. ~ . . • ~
.:
i
~ i
15 /0 Cu thickness (A)
i 25
.1 i ~()
Fig. 1. Variation of the exchange coupling constant as a function of the Cu spacer layer thickness for two series of samples: Co(24A)/Cu/Co(24A) and Co(24A)/Cu/CoAg trilayer structures. The second cobalt layer contains about 5% Ag impurities and the cobalt content is the same as in the first cobalt layer (24,~.).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 6 4 - 0
268
K. Ounadjela et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 267-268
(0, 0, 0, 1). Moreover, FMR studies have shown a dominant hcp structure with a large number of stacking faults. The use of Ag as an impurity within the second cobalt layer avoids modification of the band structure of the cobalt since the Ag is totally immiscible in Co and forms small clusters in the Co matrix. Likewise, the high structural quality of the cobalt structure is also maintained in the presence of Ag contamination, which minimizes nonmagnetic impurity scattering of the electrons. Our results suggest that the Ag impurity causes a significant modification of the RKKY interaction. At the same time, the addition of successive atomic layers of pure cobalt at the [Co93Ag7]magnetic layer interfaces progressively restores the AF exchange coupling. It is well known that the propagation of the electrons in both the magnetic layers and the spacer layer strongly influences the AFM coupling mechanism [1-3]. The addition of a small number of Ag impurities may change the average potential seen by the 's' and 'd' electrons in the cobalt as electrons rearrange in order to screen the Ag nuclei and establish a common Fermi level. For elements such as Cu near the end of the transition metal series, the Fermi energy lies above the top of the 'd'-band and consequently the magnetic coupling is primarily mediated by electrons at the Fermi level in mostly 's'-like energy states producing a weaker coupling. Thus, a slight shift in the cobalt potential may induce a rapid collapse of the AF exchange coupling. In order to confirm this hypothesis, we have performed a similar series of experiments using a Ru interlayer, which has a much broader 'd'-band than Cu and has 'd'-states available at the Fermi level. In the case of C o / R u / C o A g trilayer films, the results indicate that the period and phase of the oscillatory coupling show strong variations with the addition of a small percentage of Ag, as shown in Fig. 2. Indeed, for the series of samples with 3% Ag in the second cobalt layer, well defined oscillations in the saturation field are observed similar to those found for samples with pure cobalt layers prepared under similar conditions. The data for pure Co (shown for comparison in Fig. 2) indicate that there is a significant change in the phase (10 ,~) and the period of the oscillation (shorter with the inclusion of Ag impurities). For higher Ag concentrations, the same tendency is observed. A more significant effect occurs in the thickness region between 12 and 17 A, for which the coupling is clearly parallel for the C o / R u / C o trilayer series, whereas it changes sign to become clearly antiparallel for the C o / R u / C o 9 2 A g 8 seties. However, the amplitude of the magnetic coupling is
~1~5
-41-- Co/Ru/Co ] - @- CtdRu/Co97Ag 3
IIi~~
..A ~ com~UC%Ag~
.~3
~2
0
I 5
i 10
i 15
n 20
n 25
i 30
35
Ru thickness (A)
Fig. 2. Variation of the exchange coupling constant as a function of the Ru spacer layer thickness for three series of samples: Co(32,~)/Ru/Co(32,~) (reference), Co(32 A)/Ru/C097Ag 3 and Co(32A)/Ru/Co92Ag 8 trilayer structures. In the second and third series of samples the cobalt contents of the first and second layers are the same (32,~).
preserved which confirms that the change in the phase is due mostly to the mediation of d-like electrons in Ru. Indeed, theoretical models predict that the amplitude, phase and strength of the exchange interaction will depend on the relative positions of the 's' and 'd' electrons in the cobalt. A small shift of the position of the Co 'd'- or 's'-band, as determined by the average electron potential affected by the presence of Ag, changes the phase of the coupling without significantly affecting the strength or oscillation period [4,5]. The strong exponential decay at small decay of Cu and Ru may arise from localization effects at the interface. Bruno [3] has predicted the possibility of both exponential decay and oscillatory contributions to the magnetic coupling due to such electronic states. In conclusion, the phase shifts in the oscillatory behavior, and the exponential decay are certainly related to changes in the electronic potentials introduced by the addition of Ag. References
[1] S.N. Okuno and K. Inomata, Phys. Rev. Lett. 70 (1993) 1711; 72 (1994) 1553. [2] P.J.H. Bloemen et al., Phys. Rev. Lett. 72 (1994) 764. [3] P. Bruno, Europhys. Lett. 23 (1993) 615; Phys. Rev. B 52 (1995) 411. [4] R. Stamps et al., Phys. Rev. B, submitted. [5] J. Barnas, J. Magn. Magn. Mater. 128 (1994) 171.
~4
Jeurnalof magnetism and magnetic materials
ELSEVIER
Interlayer magnetic coupling in Co/Ru/CoAg and Co/Cu/CoAg trilayer films" effect of Ag impurities on the exchange coupling K. Ounadjela a.*, Li Zhou b, R. Stamps b M. Hehn a Z. Zhang b, p. Wigen b J. Gregg c a IPCMS, CNRS, UMR46, 23, rue du Loess, F-67037 Strasbourg, France Department of Physics, Ohio State Uniuersi~. Columbus, OH 43210, USA c Clarendon Laboratory, Universi~, of Oxfi~rd, Parks Road, Oxford OXI 3PU, UK
Abstract We discuss here the consequences on the interlayer exchange coupling of the addition of small amounts of Ag within the cobalt layers of high crystalline quality C o / C u / C o A g and C o / R u / C o A g trilayers. We show that the Ag impurities induce a change in the average potential seen by the 's' and 'd' electrons in the cobalt. This is at the origin of phase shifts in the oscillatory interlayer coupling behaviour.
By varying the concentration of Ag impurities in the cobalt layers of high-quality molecular-beam-epitaxy grown C o / C u / C o A g trilayer films, we have observed a rapid decrease in the strength of the coherent antiferromagnetic (AFM) spin structure which changes sign at 5% Ag. The dependence of the exchange coupling on the spacer layer thickness measured by ferromagnetic resonance (FMR) experiments is shown in Fig. 1 for both C o / C u / C o and the C o / C u / C o 9 5 A g 5 series of samples. The oscillatory behavior found for the C o / C u / C o trilayer series indicates that the exchange interaction oscillates between antiferromagnetic and ferromagnetic coupling depending on the Cu spacer layer thickness, while for the C o / C u / C o 9 5 A g 5 series, the ferromagnetic exchange coupling decays almost linearly to zero coupling with the Cu layer thickness. In order to identify the crucial parameter responsible for the loss of the antiferromagnetic coupling in this system, we have varied the concentration of Ag impurities in the Co layers in a series of films for which Cu and Ru are the interlayer media. The samples were prepared by molecular beam epitax~¢ on a smooth, clean single-crystalline hcp (0001) 150A thick Ru buffer layer at a pressure below 2 × 10 ~0 mbar. The film growth was monitored by high-reflection energy electron diffraction (RHEED). The substrate temperature
Corresponding author. kamel @taranis.u-strasbg.fr.
Fax:
+33-88-10-72-49;
email:
was held at 240 K to favor small clusters of the Ag within the cobalt matrix, as revealed by cross-sectional transmission electron microscopy. The cobalt grows epitaxially on the Ru buffer with the hexagonal basal plane parallel to the surface. The RHEED patterns obtained during sample growth reveal well defined structures which suggest good crystalline quality. Using 3D RHEED diffraction analysis on the cobalt layers, we observed that during growth the crystal structure fluctuates between fcc (I, I, 1) and hcp
05
•
0.4
\
•
Co/Cu/C°95Ag5
\
0.3 "
- - i - - Co/Cu/Co
1).2
~.~ 0.1 0.1
. i
I ,0
. ~ . . • ~
.:
i
~ i
15 /0 Cu thickness (A)
i 25
.1 i ~()
Fig. 1. Variation of the exchange coupling constant as a function of the Cu spacer layer thickness for two series of samples: Co(24A)/Cu/Co(24A) and Co(24A)/Cu/CoAg trilayer structures. The second cobalt layer contains about 5% Ag impurities and the cobalt content is the same as in the first cobalt layer (24,~.).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 6 4 - 0
268
K. Ounadjela et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 267-268
(0, 0, 0, 1). Moreover, FMR studies have shown a dominant hcp structure with a large number of stacking faults. The use of Ag as an impurity within the second cobalt layer avoids modification of the band structure of the cobalt since the Ag is totally immiscible in Co and forms small clusters in the Co matrix. Likewise, the high structural quality of the cobalt structure is also maintained in the presence of Ag contamination, which minimizes nonmagnetic impurity scattering of the electrons. Our results suggest that the Ag impurity causes a significant modification of the RKKY interaction. At the same time, the addition of successive atomic layers of pure cobalt at the [Co93Ag7]magnetic layer interfaces progressively restores the AF exchange coupling. It is well known that the propagation of the electrons in both the magnetic layers and the spacer layer strongly influences the AFM coupling mechanism [1-3]. The addition of a small number of Ag impurities may change the average potential seen by the 's' and 'd' electrons in the cobalt as electrons rearrange in order to screen the Ag nuclei and establish a common Fermi level. For elements such as Cu near the end of the transition metal series, the Fermi energy lies above the top of the 'd'-band and consequently the magnetic coupling is primarily mediated by electrons at the Fermi level in mostly 's'-like energy states producing a weaker coupling. Thus, a slight shift in the cobalt potential may induce a rapid collapse of the AF exchange coupling. In order to confirm this hypothesis, we have performed a similar series of experiments using a Ru interlayer, which has a much broader 'd'-band than Cu and has 'd'-states available at the Fermi level. In the case of C o / R u / C o A g trilayer films, the results indicate that the period and phase of the oscillatory coupling show strong variations with the addition of a small percentage of Ag, as shown in Fig. 2. Indeed, for the series of samples with 3% Ag in the second cobalt layer, well defined oscillations in the saturation field are observed similar to those found for samples with pure cobalt layers prepared under similar conditions. The data for pure Co (shown for comparison in Fig. 2) indicate that there is a significant change in the phase (10 ,~) and the period of the oscillation (shorter with the inclusion of Ag impurities). For higher Ag concentrations, the same tendency is observed. A more significant effect occurs in the thickness region between 12 and 17 A, for which the coupling is clearly parallel for the C o / R u / C o trilayer series, whereas it changes sign to become clearly antiparallel for the C o / R u / C o 9 2 A g 8 seties. However, the amplitude of the magnetic coupling is
~1~5
-41-- Co/Ru/Co ] - @- CtdRu/Co97Ag 3
IIi~~
..A ~ com~UC%Ag~
.~3
~2
0
I 5
i 10
i 15
n 20
n 25
i 30
35
Ru thickness (A)
Fig. 2. Variation of the exchange coupling constant as a function of the Ru spacer layer thickness for three series of samples: Co(32,~)/Ru/Co(32,~) (reference), Co(32 A)/Ru/C097Ag 3 and Co(32A)/Ru/Co92Ag 8 trilayer structures. In the second and third series of samples the cobalt contents of the first and second layers are the same (32,~).
preserved which confirms that the change in the phase is due mostly to the mediation of d-like electrons in Ru. Indeed, theoretical models predict that the amplitude, phase and strength of the exchange interaction will depend on the relative positions of the 's' and 'd' electrons in the cobalt. A small shift of the position of the Co 'd'- or 's'-band, as determined by the average electron potential affected by the presence of Ag, changes the phase of the coupling without significantly affecting the strength or oscillation period [4,5]. The strong exponential decay at small decay of Cu and Ru may arise from localization effects at the interface. Bruno [3] has predicted the possibility of both exponential decay and oscillatory contributions to the magnetic coupling due to such electronic states. In conclusion, the phase shifts in the oscillatory behavior, and the exponential decay are certainly related to changes in the electronic potentials introduced by the addition of Ag. References
[1] S.N. Okuno and K. Inomata, Phys. Rev. Lett. 70 (1993) 1711; 72 (1994) 1553. [2] P.J.H. Bloemen et al., Phys. Rev. Lett. 72 (1994) 764. [3] P. Bruno, Europhys. Lett. 23 (1993) 615; Phys. Rev. B 52 (1995) 411. [4] R. Stamps et al., Phys. Rev. B, submitted. [5] J. Barnas, J. Magn. Magn. Mater. 128 (1994) 171.