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Cu multilayers
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e969–e970
Structural and magnetic properties of epitaxial Fe/Cu multilayers Y. Harada*, Y. Nakanishi, N. Yoshimoto, A. Yamaguchi, M. Nakamura, M. Yoshizawa Department of Material Science and Engineering, Iwate University, Ueda 4-3-5, Morioka 020-8551,Japan
Abstract We have grown FCC-Fe/Cu multilayers by molecular beam epitaxy method. The structural and magnetic properties were studied by RHEED, XRD and magnetoresistance measurement (MR). The RHEED images confirmed that Fe/Cu multilayers were epitaxially grown on Cuð1 0 0Þ: Furthermore, a clear negative MR was observed. The buffer layer condition for MR effect will be discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 75.70.Cn Keywords: Magnetic multilayers; Magnetoresistance; MBE; Epitaxial growth
Magnetic intermetallic multilayers have attracted the scientific curiosity due to the possibility of discovering new magnetic materials by man-made structures with tailored properties. Especially, face-centered-cubic (FCC) Fe is one of the most interesting systems for studying the origin of the magnetism due to the possibility diverse magnetic phases it may manifest. In previous reports FCC-Fe/Cu multilayer had been grown on Cuð0 0 1Þ substrates [1] and MgOð0 0 1Þ substrates [2]. However, they have shown some serious problems. For example, in MgO substrates, there exists a large mismatch of 14% between bulk Cu and substrate. In order to solve such problem, a thick buffer is necessary to grow epitaxially the multilayers. Then we used SrTiO3 ð0 0 1Þ STO substrates because STO has 7% mismatch between bulk Cu . To elucidate explicity the thickness dependence of its magnetic property, we have grown epitaxially FCC-Fe/Cu multilayers by molecular beam epitaxy (MBE) on the MgOð0 0 1Þ and STO substrates under various deposition conditions. In this paper, we have investigated structural and magnetic *Corresponding author. Tel.: +81-19-621-6356; fax: +8119-621-6373. E-mail address: [email protected] (Y. Harada).
properties by reflected high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and magnetoresistance (MR) measurement. The relationship between the buffer layer deposition conditions and the MR ratio will be discussed. We have grown a series of [FCC-Fe/Cu] multilayers were prepared on MgOð0 0 1Þ and STO single crystal substrates in UHV. Fe and Cu were deposited by a Knudsen cell. The base pressure was below 5 1010 Torr: A series of films with Fe layer thickness ranging from 1 to 5 monolayer (ML) and Cu layer ( The deposition rate of 0.04– thickness of 10 and 26 A: ( 0:1 A=s was monitored by a quartz crystal oscillator. The Fe seed layer was deposited at 473 K: The others were deposited at 673 K: The Type 1 buffer layer was deposited at 423 K: The other buffers were deposited at 473 K: All multilayers were deposited at 333 K: The outof plane crystallographic structures of the prepared samples were characterized by XRD using CuKa radiation with the scattering vector perpendicular to the film plane. The in-plane structure were characterized by in situ RHEED measurements. The magnetoresistance was measured by a conventional four-lead method. Fig. 1 shows RHEED patterns taken for [FCC( multilayers. The RHEED pattern Feð5 MLÞ=Cuð10 AÞ]
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.269
ARTICLE IN PRESS e970
Y. Harada et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e969–e970
0
MR ratio(%)
-0.2
-0.4
-0.6
-0.8 -12
-8
-4
0
4
8
12
H(kOe) Fig. 1. RHEED patterns taken for (A) Cu buffer layer grown on STO substrate and (B) and (C) 1st Fe and Cu layer and (D) 15th Cu layer subsequently grown on the Cu buffer layers, respectively. Incident electron beam of 15 kV is along Cu½1 1 0:
( Fig. 2. Magnetoresistance of ½Feð5 MLÞ=Cuð10 AÞ 15 multilayers at 77 K: The buffer layer conditions is Type 3 on STO substrate.
for Cuð1 0 0Þ surface in Fig. 1 (A) indicates twodimensional growth of the Cu buffer layer. Multilayers were deposited on the Cu buffer layer. From the RHEED image for multilayers shown in Fig. 1 (B), (C), and (D), it is found that Fe/Cu multilayers are epitaxially grown on Cuð1 0 0Þ: This in-plane lattice parameter of multilayers are estimated to be about 3% smaller than that in bulk Cu and FCC-Fe forms. We have studied the out-of-plane lattice parameter by XRD. The designed artificial periodicities of the samples were in agreement with the experimental values estimated from the low-angle diffraction peaks and highangle satellite peaks with in an error of 5%. ( MR curve at 77 K for a ½Feð5 MLÞ=Cuð10 AÞ 15 is shown in Fig. 2. The buffer layer condition is Type 3 on STO substrate. The magnetic field is applied perpendicular to the film plane. MR ratio is estimated to be of 0.78%. Table 1 shows the buffer layer condition dependence of MR effect. The MR effect with STO is larger than that with MgO indicating that MR ratio are sensitive to the buffer layer in-plane lattice parameters because the mismatch between MgO and Cu are two times larger than that between STO and Cu. For the Type 4, the largest MR effect was observed. However, MR effect disappears on thicker buffer (Type 1 and 2). This result may be ascribed to the fact that the little electric current flow into multilayers but mainly current flow into Cu buffers.
Table 1 Buffer layer dependence of MR ratio Layer condition
Type1 Type2 Type3 Type4
MR ratio (%)
( Seed layer ðAÞ
( Buffer layer ðAÞ
STO
MgO
Cu(150) Cu(850) Fe(10) Cu(100)
Cu(850) Cu(150) Cu(240) Cu(150)
0 0 0.78 0.95
0 0 0.64 0.5
The magnetic field is applied perpendicular to the film plane.
We have investigated structural and magnetic properties of [FCC-Fe/Cu] multilayers prepared on single crystal MgO and STO substrates by MBE. The present results indicate that the MR effect is sensitive to the buffer layer condition. Thin buffer layer, in addition to the low mismatch between substrate and buffer layer, seems to be relevant factor to enhance the MR effect in this system.
References [1] S. Mitani, et al., J. Magn. Magn. Mater. 126 (1993) 79. [2] M. Matsui, et al., Surf. Sci. 493 (2001) 737.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e969–e970
Structural and magnetic properties of epitaxial Fe/Cu multilayers Y. Harada*, Y. Nakanishi, N. Yoshimoto, A. Yamaguchi, M. Nakamura, M. Yoshizawa Department of Material Science and Engineering, Iwate University, Ueda 4-3-5, Morioka 020-8551,Japan
Abstract We have grown FCC-Fe/Cu multilayers by molecular beam epitaxy method. The structural and magnetic properties were studied by RHEED, XRD and magnetoresistance measurement (MR). The RHEED images confirmed that Fe/Cu multilayers were epitaxially grown on Cuð1 0 0Þ: Furthermore, a clear negative MR was observed. The buffer layer condition for MR effect will be discussed. r 2003 Elsevier B.V. All rights reserved. PACS: 75.70.Cn Keywords: Magnetic multilayers; Magnetoresistance; MBE; Epitaxial growth
Magnetic intermetallic multilayers have attracted the scientific curiosity due to the possibility of discovering new magnetic materials by man-made structures with tailored properties. Especially, face-centered-cubic (FCC) Fe is one of the most interesting systems for studying the origin of the magnetism due to the possibility diverse magnetic phases it may manifest. In previous reports FCC-Fe/Cu multilayer had been grown on Cuð0 0 1Þ substrates [1] and MgOð0 0 1Þ substrates [2]. However, they have shown some serious problems. For example, in MgO substrates, there exists a large mismatch of 14% between bulk Cu and substrate. In order to solve such problem, a thick buffer is necessary to grow epitaxially the multilayers. Then we used SrTiO3 ð0 0 1Þ STO substrates because STO has 7% mismatch between bulk Cu . To elucidate explicity the thickness dependence of its magnetic property, we have grown epitaxially FCC-Fe/Cu multilayers by molecular beam epitaxy (MBE) on the MgOð0 0 1Þ and STO substrates under various deposition conditions. In this paper, we have investigated structural and magnetic *Corresponding author. Tel.: +81-19-621-6356; fax: +8119-621-6373. E-mail address: [email protected] (Y. Harada).
properties by reflected high-energy electron diffraction (RHEED), X-ray diffraction (XRD) and magnetoresistance (MR) measurement. The relationship between the buffer layer deposition conditions and the MR ratio will be discussed. We have grown a series of [FCC-Fe/Cu] multilayers were prepared on MgOð0 0 1Þ and STO single crystal substrates in UHV. Fe and Cu were deposited by a Knudsen cell. The base pressure was below 5 1010 Torr: A series of films with Fe layer thickness ranging from 1 to 5 monolayer (ML) and Cu layer ( The deposition rate of 0.04– thickness of 10 and 26 A: ( 0:1 A=s was monitored by a quartz crystal oscillator. The Fe seed layer was deposited at 473 K: The others were deposited at 673 K: The Type 1 buffer layer was deposited at 423 K: The other buffers were deposited at 473 K: All multilayers were deposited at 333 K: The outof plane crystallographic structures of the prepared samples were characterized by XRD using CuKa radiation with the scattering vector perpendicular to the film plane. The in-plane structure were characterized by in situ RHEED measurements. The magnetoresistance was measured by a conventional four-lead method. Fig. 1 shows RHEED patterns taken for [FCC( multilayers. The RHEED pattern Feð5 MLÞ=Cuð10 AÞ]
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.269
ARTICLE IN PRESS e970
Y. Harada et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e969–e970
0
MR ratio(%)
-0.2
-0.4
-0.6
-0.8 -12
-8
-4
0
4
8
12
H(kOe) Fig. 1. RHEED patterns taken for (A) Cu buffer layer grown on STO substrate and (B) and (C) 1st Fe and Cu layer and (D) 15th Cu layer subsequently grown on the Cu buffer layers, respectively. Incident electron beam of 15 kV is along Cu½1 1 0:
( Fig. 2. Magnetoresistance of ½Feð5 MLÞ=Cuð10 AÞ 15 multilayers at 77 K: The buffer layer conditions is Type 3 on STO substrate.
for Cuð1 0 0Þ surface in Fig. 1 (A) indicates twodimensional growth of the Cu buffer layer. Multilayers were deposited on the Cu buffer layer. From the RHEED image for multilayers shown in Fig. 1 (B), (C), and (D), it is found that Fe/Cu multilayers are epitaxially grown on Cuð1 0 0Þ: This in-plane lattice parameter of multilayers are estimated to be about 3% smaller than that in bulk Cu and FCC-Fe forms. We have studied the out-of-plane lattice parameter by XRD. The designed artificial periodicities of the samples were in agreement with the experimental values estimated from the low-angle diffraction peaks and highangle satellite peaks with in an error of 5%. ( MR curve at 77 K for a ½Feð5 MLÞ=Cuð10 AÞ 15 is shown in Fig. 2. The buffer layer condition is Type 3 on STO substrate. The magnetic field is applied perpendicular to the film plane. MR ratio is estimated to be of 0.78%. Table 1 shows the buffer layer condition dependence of MR effect. The MR effect with STO is larger than that with MgO indicating that MR ratio are sensitive to the buffer layer in-plane lattice parameters because the mismatch between MgO and Cu are two times larger than that between STO and Cu. For the Type 4, the largest MR effect was observed. However, MR effect disappears on thicker buffer (Type 1 and 2). This result may be ascribed to the fact that the little electric current flow into multilayers but mainly current flow into Cu buffers.
Table 1 Buffer layer dependence of MR ratio Layer condition
Type1 Type2 Type3 Type4
MR ratio (%)
( Seed layer ðAÞ
( Buffer layer ðAÞ
STO
MgO
Cu(150) Cu(850) Fe(10) Cu(100)
Cu(850) Cu(150) Cu(240) Cu(150)
0 0 0.78 0.95
0 0 0.64 0.5
The magnetic field is applied perpendicular to the film plane.
We have investigated structural and magnetic properties of [FCC-Fe/Cu] multilayers prepared on single crystal MgO and STO substrates by MBE. The present results indicate that the MR effect is sensitive to the buffer layer condition. Thin buffer layer, in addition to the low mismatch between substrate and buffer layer, seems to be relevant factor to enhance the MR effect in this system.
References [1] S. Mitani, et al., J. Magn. Magn. Mater. 126 (1993) 79. [2] M. Matsui, et al., Surf. Sci. 493 (2001) 737.