- Email: [email protected]
Ag(100) artificial superlattices
Journal of Magnetism and Magnetic Materials 126 (1993) 125-127 North-Holland
Ai41
RHEED intensity oscillations in the growth of A g / F e / A u and Au/Fe/Ag(100) artificial superlattices Y. Suzuki ,,b, H. T a k e s h i t a c, H. Kikuchi c, M. T a n i n a k a c, T. K a t a y a m a a and S. Y o s h i d a a a Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan h National Institute for AdL,anced Interdisciplinary Research, Tsukuba, Ibaraki 305, Japan " Nihon University, Funabashi, Chiba 274, Japan
Intensity oscillations of reflective high-energy electron diffraction (RHEED) were observed during UHV epitaxial growth of Fe/Au, Fe/Ag, A g / F e / A u and Au/Fe/Ag(100) superlattices. It is found that the stacking structure of all systems except F e / A g can be controlled using the RHEED intensity oscillation. Differences in the magnetic properties of these systems are discussed in terms of the growth mechanism of metallic multilayers.
The intensity oscillation of the reflective high-energy electron diffraction ( R H E E D ) due to the layerby-layer growth of atomic layers (van der Merwe mode) during molecular beam epitaxy (MBE) is one of the most useful p h e n o m e n a to control stacking structures of artificial superlattices. By counting the R H E E D intensity oscillations, well defined artificial superlattices have been obtained for semiconductor systems [1] and also metallic systems [2-5]. Bcc Fe(100)/fcc Au(100) artificial superlattices were synthesized by this method [5] of phase-locked epitaxy (PLE). The artificial period, however, did not coincide with the value expected from the periods of R H E E D intensity oscillations. The difference is thought to be due to the irregular periods of the R H E E D intensity oscillation at the beginning of the heteroepitaxial growth of each layer. It is also pointed out that the R H E E D intensity oscillation attenuates with the continued growth of the Au layers. In this paper, we report on an approach to solve these problems by putting an Ag layer on top of each Fe layer. Ag has the lowest surface free e n e r g y ('~Ag) of the above-mentioned metals (Fe, Au, Ag) and wets the Fe surface well. These three metals have almost the same two-dimensional lattice constants for (100) stack~ ing. It is thought that the surface and interface free energies are most important parameters for the heteroepitaxial growth of these metals. The surface free energy of Fe is largest, that of Au is second and Ag the smallest: YFe > 3'A. > TAg-
A significant difference between Ag ~ Au -~ Fe ~ Ag ... stacking and Au ~ Ag ~ Fe -~ Au ~ ... stacking is therefore expected. A comparison of the R H E E D intensity oscillations and magnetic properties of A g / F e / A u and A u / F e / A g structures is given in this paper. The films were epitaxially grown by the M B E method. The background pressure during growth was on the order of 10 - 9 Torr. MgO(100) cleaved surface was employed as substrate. Fig. 1 shows a schematic cross sectional structure of the samples. An Ag buffer layer was first deposited to provide a flat metal surface. After that, A g / F e / A u , A u / F e / A g , F e / A u or F e / A g artificial lattices were deposited at room temperature. The growth rate was about 0.1 ,A/s. During deposition, an intensity of the specular spot in the R H E E D pattern was detected by a photodiode and displayed on a computer screen. By observing the graph of the intensity change, the shutter system was controlled by hand. The primary energy of the electron beam of R H E E D was 20 keV and the glancing angle was about 0.3 ° for
Ag 2ML Fe 4ML Au 2ML
Ag
(1)
Correspondence to: Dr Y. Suzuki, Electrotechnical Laboratory, Umezono 1-1-4, Tsukuba, Ibaraki 305, Japan. Tel: 81298-58-5432; fax: 81-298-58-5434.
2000A
MgO (001) Fig. 1. Schematic cross section of a A g / F e / A u artificial superlattice. The notation A / B / C corresponds to successive depositions of C ~ B ~ A ~ C --, B ~ A ~ ....
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
126
Y. Suzuki et al. / RHEED oscillation of Ag / Fe / A u and Au / Fe /Ag(lO0)
Fe Au Fe
Au Fe Au Fe
Au Fe Au Fe
Ag
e-
.=.
(a) ~ z
o
0 Fe4 Aul Fe4 A u l F e 4 A u l F e 4
500
(b)
z
growth time (see) Fig. 3. RHEED intensity oscillation during the growth of an F e / A g 4/4 artificial superlattice at room temperature. The primary beam is 20 keV in energy and [011] azimuth. The glancing angle of incidence is 0.6 °,
00
600 GROWTH TIME(see) Fig. 2. RHEED intensity oscillation during the growth of Fe/Au 4/4 and F e / A u 4/1 artificial superlattices at room temperature. The primary beam is 20 keV in energy and [011] azimuth. The glancing angle of incidence is 0.6 °. the A g / F e / A u and A u / F e / A g superlattices and 0.6 ° for F e / A u and F e / A g superlattices. The electron beam was parallel to the [010] azimuth of Ag for the A g / F e / A u and A u / F e A g superlattices and [011] for F e / A u and F e / A g superlattices. After growth, the magnetic hysteresis loop was measured by a magnetic Kerr rotation spectrometer. Fig. 2 shows the R H E E D intensity oscillations during the growth of F e / A u 4 / 4 and F e / A u 4 / 1 superlattices, where n / m denotes the numbers of periods counted. As reported in ref. [5], the R H E E D intensity oscillation continues to be long period for the F e / A u 4 / 4 superlattice, but attenuates rapidly for the F e / A u 4 / 1 case. Actually, the film becomes polycrystalline after the deposition of several hundred angstroms in this case. On the other hand, the oscillation during the growth of Fe is continuous for F e / A u 4 / 3 , 4 / 4 and 4 / 1 6 superlattices. It is therefore thought that the Au layer plays an essential role in keeping a flat surface in the growth of the F e / A u superlatticc. Since Au has a lower surface free energy than Fe, a repairing of the flatness by successive depositions of Au should occur. To test this point, wc made F e / A g superlattices, where Ag has lowest surface free energy of the three metals. Fig. 3 shows the R H E E D intensity oscillation during the growth of an F e / A g 4 / 4 superlattice. The oscillation vanishes after the growth of several atomic
layers. From observations of R H E E D patterns, it is found that the deposition of Ag on an Fe surface provides a flat surface, but the deposition of Fc on an Ag surface yields a rough surface. As a result, the surface becomes rougher as the growth of the superlattice continues, and the oscillation vanishes. From these experiments, it is found that the growth of Fe on Au and the growth of Ag on Fe provide the best flat surfaces. We then prepared A g / F e / A u superlattices and compared them with A u / F e / A g superlattices. Fig. 4 shows the R H E E D intensity oscillations during the growth of A g / F e / A u 2/4/2 and
Fe
Ag 2
Au
Fe
Ag 2
Aul
(a) Fc
Au
Ag 2
Fe
Au Ag
._~
0
600 growth time (see)
Fig. 4. RHEED intensity oscillation during the growth of A g / F e / A u 2 / 4 / 2 and A u / F e / A g 2 / 4 / 2 artificial superlatrice at room temperature. The primary beam is 20 keV in energy and [010] azimuth. The glancing angle of incidence is 0.3°.
Y. Suzuki et al. / RHEED oscillation of Ag / Fe / A u and Au / Fe /Ag(lO0)
Table 1 Comparison of the saturation fields determined from polar Kerr hysteresis loops and imaginary parts of e~,~. at 4 eV determined from Kerr rotation spectra for Au/Fe 2ML/Au, Ag/Fe 2ML/Au and Au/Fe 2ML/Ag sandwich structures
Hs(kOe) exy(at4eV)
Au/Fe 2ML/Au
Ag/Fe 2ML/Au
Au/Fe 2ML/Ag
1.4 0.19
2.2 0.09
9.7 0.002
A u / F e / A g 2 / 4 / 2 superlattices. Continuous oscillations are observed for both cases. The phases and periods of the oscillation, however, are irregular for the case of A u / F e / A g ; this irregularity may becaused by the complex growth mode in this system. In contrast, the growth of an A g / F e / A u superlattice shows a regular phase and constant period of the oscillation. This implies a nearly complete layer-by-layer growth mode of the system. Table 1 lists the saturation magnetic fields, Hs, of the sandwiched Fe for the perpendicular magnetization to the film plane at room temperature. Here, a small value of H s means that the film has a higher uniaxial and perpendicular magnetic anisotropy. From this idea, the A u / F e / A u structure has the largest anisotropy. There is also an effect of the difference in Curie temperatures (Tc). Tc of F e / A g is lower than that of F e / A u from surface magneto-optical Kerr effect (SMOKE) measurements [6]. In this case, the
127
effect of T c is to increase the anisotropy of F e / A u system. Large differences between these structures are also observed in the magneto-optical Kerr spectra (see table 1), where the structure could affect the formation of quantum well states in the Fe layer [7]. In conclusion, the layered structures are controlled by the R H E E D intensity oscillation for F e / A u , A g / F e / A u and A u / F e / A g superlattices. The A g / F e / A u superlattice provides a regular and continuous oscillation. The growth mode affects sensitively the magnetic anisotropy and magneto-optical properties. References
[1] For example, T. Sakamoto, H. Funabashi, K. Ohta, T. Nakagawa, N.J. Kawai and T. Kojima, Jpn. J. Appl. Phys. 23 (1984) L657. [2] T. Kaneko, M. Imafuku, C. Kokubu, R. Yamamoto and M. Doyama, J. Phys. Soc. Jpn. 55 (1986) 2903. [3] C. Koziol, G. Lilienkamp and E. Bauer, Appl. Phys. Lett. 51 (1987) 901. [4] S.T. Purcell, A.S. Arrott and B. Heinrich, J. Vac. Sci. Technol. B6 (1988) 794. [5] Y. Suzuki, H. Kikuchi, M. Taninaka, T. Katayama and S. Yoshida, Appl. Surf. Sci. 60/61 (1992) 820. [6] Y. Suzuki, M. Taninaka, T. Katayama and S. Yoshida, unpublished. [7] Y. Suzuki, T. Katayama, A. Thiaville, K. Sato, M. Taninaka and S. Yoshida, J. Magn. Magn. Mater. 121 (1993) 539.
Ai41
RHEED intensity oscillations in the growth of A g / F e / A u and Au/Fe/Ag(100) artificial superlattices Y. Suzuki ,,b, H. T a k e s h i t a c, H. Kikuchi c, M. T a n i n a k a c, T. K a t a y a m a a and S. Y o s h i d a a a Electrotechnical Laboratory, Tsukuba, Ibaraki 305, Japan h National Institute for AdL,anced Interdisciplinary Research, Tsukuba, Ibaraki 305, Japan " Nihon University, Funabashi, Chiba 274, Japan
Intensity oscillations of reflective high-energy electron diffraction (RHEED) were observed during UHV epitaxial growth of Fe/Au, Fe/Ag, A g / F e / A u and Au/Fe/Ag(100) superlattices. It is found that the stacking structure of all systems except F e / A g can be controlled using the RHEED intensity oscillation. Differences in the magnetic properties of these systems are discussed in terms of the growth mechanism of metallic multilayers.
The intensity oscillation of the reflective high-energy electron diffraction ( R H E E D ) due to the layerby-layer growth of atomic layers (van der Merwe mode) during molecular beam epitaxy (MBE) is one of the most useful p h e n o m e n a to control stacking structures of artificial superlattices. By counting the R H E E D intensity oscillations, well defined artificial superlattices have been obtained for semiconductor systems [1] and also metallic systems [2-5]. Bcc Fe(100)/fcc Au(100) artificial superlattices were synthesized by this method [5] of phase-locked epitaxy (PLE). The artificial period, however, did not coincide with the value expected from the periods of R H E E D intensity oscillations. The difference is thought to be due to the irregular periods of the R H E E D intensity oscillation at the beginning of the heteroepitaxial growth of each layer. It is also pointed out that the R H E E D intensity oscillation attenuates with the continued growth of the Au layers. In this paper, we report on an approach to solve these problems by putting an Ag layer on top of each Fe layer. Ag has the lowest surface free e n e r g y ('~Ag) of the above-mentioned metals (Fe, Au, Ag) and wets the Fe surface well. These three metals have almost the same two-dimensional lattice constants for (100) stack~ ing. It is thought that the surface and interface free energies are most important parameters for the heteroepitaxial growth of these metals. The surface free energy of Fe is largest, that of Au is second and Ag the smallest: YFe > 3'A. > TAg-
A significant difference between Ag ~ Au -~ Fe ~ Ag ... stacking and Au ~ Ag ~ Fe -~ Au ~ ... stacking is therefore expected. A comparison of the R H E E D intensity oscillations and magnetic properties of A g / F e / A u and A u / F e / A g structures is given in this paper. The films were epitaxially grown by the M B E method. The background pressure during growth was on the order of 10 - 9 Torr. MgO(100) cleaved surface was employed as substrate. Fig. 1 shows a schematic cross sectional structure of the samples. An Ag buffer layer was first deposited to provide a flat metal surface. After that, A g / F e / A u , A u / F e / A g , F e / A u or F e / A g artificial lattices were deposited at room temperature. The growth rate was about 0.1 ,A/s. During deposition, an intensity of the specular spot in the R H E E D pattern was detected by a photodiode and displayed on a computer screen. By observing the graph of the intensity change, the shutter system was controlled by hand. The primary energy of the electron beam of R H E E D was 20 keV and the glancing angle was about 0.3 ° for
Ag 2ML Fe 4ML Au 2ML
Ag
(1)
Correspondence to: Dr Y. Suzuki, Electrotechnical Laboratory, Umezono 1-1-4, Tsukuba, Ibaraki 305, Japan. Tel: 81298-58-5432; fax: 81-298-58-5434.
2000A
MgO (001) Fig. 1. Schematic cross section of a A g / F e / A u artificial superlattice. The notation A / B / C corresponds to successive depositions of C ~ B ~ A ~ C --, B ~ A ~ ....
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. (North-Holland)
126
Y. Suzuki et al. / RHEED oscillation of Ag / Fe / A u and Au / Fe /Ag(lO0)
Fe Au Fe
Au Fe Au Fe
Au Fe Au Fe
Ag
e-
.=.
(a) ~ z
o
0 Fe4 Aul Fe4 A u l F e 4 A u l F e 4
500
(b)
z
growth time (see) Fig. 3. RHEED intensity oscillation during the growth of an F e / A g 4/4 artificial superlattice at room temperature. The primary beam is 20 keV in energy and [011] azimuth. The glancing angle of incidence is 0.6 °,
00
600 GROWTH TIME(see) Fig. 2. RHEED intensity oscillation during the growth of Fe/Au 4/4 and F e / A u 4/1 artificial superlattices at room temperature. The primary beam is 20 keV in energy and [011] azimuth. The glancing angle of incidence is 0.6 °. the A g / F e / A u and A u / F e / A g superlattices and 0.6 ° for F e / A u and F e / A g superlattices. The electron beam was parallel to the [010] azimuth of Ag for the A g / F e / A u and A u / F e A g superlattices and [011] for F e / A u and F e / A g superlattices. After growth, the magnetic hysteresis loop was measured by a magnetic Kerr rotation spectrometer. Fig. 2 shows the R H E E D intensity oscillations during the growth of F e / A u 4 / 4 and F e / A u 4 / 1 superlattices, where n / m denotes the numbers of periods counted. As reported in ref. [5], the R H E E D intensity oscillation continues to be long period for the F e / A u 4 / 4 superlattice, but attenuates rapidly for the F e / A u 4 / 1 case. Actually, the film becomes polycrystalline after the deposition of several hundred angstroms in this case. On the other hand, the oscillation during the growth of Fe is continuous for F e / A u 4 / 3 , 4 / 4 and 4 / 1 6 superlattices. It is therefore thought that the Au layer plays an essential role in keeping a flat surface in the growth of the F e / A u superlatticc. Since Au has a lower surface free energy than Fe, a repairing of the flatness by successive depositions of Au should occur. To test this point, wc made F e / A g superlattices, where Ag has lowest surface free energy of the three metals. Fig. 3 shows the R H E E D intensity oscillation during the growth of an F e / A g 4 / 4 superlattice. The oscillation vanishes after the growth of several atomic
layers. From observations of R H E E D patterns, it is found that the deposition of Ag on an Fe surface provides a flat surface, but the deposition of Fc on an Ag surface yields a rough surface. As a result, the surface becomes rougher as the growth of the superlattice continues, and the oscillation vanishes. From these experiments, it is found that the growth of Fe on Au and the growth of Ag on Fe provide the best flat surfaces. We then prepared A g / F e / A u superlattices and compared them with A u / F e / A g superlattices. Fig. 4 shows the R H E E D intensity oscillations during the growth of A g / F e / A u 2/4/2 and
Fe
Ag 2
Au
Fe
Ag 2
Aul
(a) Fc
Au
Ag 2
Fe
Au Ag
._~
0
600 growth time (see)
Fig. 4. RHEED intensity oscillation during the growth of A g / F e / A u 2 / 4 / 2 and A u / F e / A g 2 / 4 / 2 artificial superlatrice at room temperature. The primary beam is 20 keV in energy and [010] azimuth. The glancing angle of incidence is 0.3°.
Y. Suzuki et al. / RHEED oscillation of Ag / Fe / A u and Au / Fe /Ag(lO0)
Table 1 Comparison of the saturation fields determined from polar Kerr hysteresis loops and imaginary parts of e~,~. at 4 eV determined from Kerr rotation spectra for Au/Fe 2ML/Au, Ag/Fe 2ML/Au and Au/Fe 2ML/Ag sandwich structures
Hs(kOe) exy(at4eV)
Au/Fe 2ML/Au
Ag/Fe 2ML/Au
Au/Fe 2ML/Ag
1.4 0.19
2.2 0.09
9.7 0.002
A u / F e / A g 2 / 4 / 2 superlattices. Continuous oscillations are observed for both cases. The phases and periods of the oscillation, however, are irregular for the case of A u / F e / A g ; this irregularity may becaused by the complex growth mode in this system. In contrast, the growth of an A g / F e / A u superlattice shows a regular phase and constant period of the oscillation. This implies a nearly complete layer-by-layer growth mode of the system. Table 1 lists the saturation magnetic fields, Hs, of the sandwiched Fe for the perpendicular magnetization to the film plane at room temperature. Here, a small value of H s means that the film has a higher uniaxial and perpendicular magnetic anisotropy. From this idea, the A u / F e / A u structure has the largest anisotropy. There is also an effect of the difference in Curie temperatures (Tc). Tc of F e / A g is lower than that of F e / A u from surface magneto-optical Kerr effect (SMOKE) measurements [6]. In this case, the
127
effect of T c is to increase the anisotropy of F e / A u system. Large differences between these structures are also observed in the magneto-optical Kerr spectra (see table 1), where the structure could affect the formation of quantum well states in the Fe layer [7]. In conclusion, the layered structures are controlled by the R H E E D intensity oscillation for F e / A u , A g / F e / A u and A u / F e / A g superlattices. The A g / F e / A u superlattice provides a regular and continuous oscillation. The growth mode affects sensitively the magnetic anisotropy and magneto-optical properties. References
[1] For example, T. Sakamoto, H. Funabashi, K. Ohta, T. Nakagawa, N.J. Kawai and T. Kojima, Jpn. J. Appl. Phys. 23 (1984) L657. [2] T. Kaneko, M. Imafuku, C. Kokubu, R. Yamamoto and M. Doyama, J. Phys. Soc. Jpn. 55 (1986) 2903. [3] C. Koziol, G. Lilienkamp and E. Bauer, Appl. Phys. Lett. 51 (1987) 901. [4] S.T. Purcell, A.S. Arrott and B. Heinrich, J. Vac. Sci. Technol. B6 (1988) 794. [5] Y. Suzuki, H. Kikuchi, M. Taninaka, T. Katayama and S. Yoshida, Appl. Surf. Sci. 60/61 (1992) 820. [6] Y. Suzuki, M. Taninaka, T. Katayama and S. Yoshida, unpublished. [7] Y. Suzuki, T. Katayama, A. Thiaville, K. Sato, M. Taninaka and S. Yoshida, J. Magn. Magn. Mater. 121 (1993) 539.