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Fe multilayers
Journal of Magnetism and Magnetic Materials 226}230 (2001) 1740}1742
Weak stripe domains in Co/Fe multilayers G. Ausanio , V. Iannotti , L. Lanotte , M. Carbucicchio *, M. Rateo Dipartimento di Scienze Fisiche, Fac. Ingegneria, Universita% di Napoli **Federico II++ and INFM, 80125 Napoli, Italy Dipartimento di Fisica, Universita% di Parma, and INFM, Parco Area delle Scienze 7/A, 43100 Parma, Italy
Abstract Ultra-high-vacuum electron-beam-evaporated Co/Fe multilayers (Co /Fe with x"5, 10, 15 nm) were studied by V V atomic and magnetic force microscopy (AFM, MFM), vibrating sample magnetometry (VSM) and conversion electron MoK ssbauer spectroscopy (CEMS). MFM investigation showed long, dense and parallel domain regions (stripe domains) with the average width decreasing with the multilayer thickness. VSM and CEMS measurements showed the presence of an out-of-plane anisotropy, whose origin is ascribed to a planar stress at the interfaces, and which can give account of the observed narrow stripe domain pattern. 2001 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Domain pattern; Magnetostriction
In the last years a growing interest was addressed to the realization of magnetic nanocomposites which combine soft and hard magnetic characteristics via the exchange interactions. This coupling can lead to remanence and energy product enhancement, and can be achieved with the constituting phases "nely distributed on a nanometric scale. Recently, e!orts have been made to obtain planar nanocomposite in the form of multilayers [1]. A series of Co/Fe multilayers, namely 5[Co /Fe ] V V with x"5, 10, 15 nm, with an additional Co layer on the top and protected by an outermost 4 nm thick Mo capping layer, was electron-beam evaporated in ultra-high vacuum (10\ mbar starting vacuum and 10\ mbar operating vacuum) onto Si(1 0 0) and Si(1 1 1) single crystals, without native oxide removal. Grazing incidence X-ray di!raction revealed that the samples have an isotropic polycrystalline structure with BCC Fe and HCP Co [2]. Surface topography was investigated by atomic force microscopy (AFM) and it was found that, as the layer thickness decreases, the surface grain size increases and the roughness decreases (Fig. 1).
* Corresponding author. Fax: #39-0521-905223. E-mail address: carbucicchio@"s.unipr.it (M. Carbucicchio).
The domain patterns were observed by magnetic force microscopy (MFM). Hysteresis loops were measured by a vibrating sample magnetometer (VSM), applying the magnetic "eld both parallel and perpendicular to the "lm plane. The magnetization direction was determined by conversion electron MoK ssbauer spectroscopy (CEMS). MFM investigation showed very regular long and parallel domain regions (stripe domains) whose average width is proportional to the overall multilayer thickness (Fig. 2). The occurrence of stripe domains may be explained only if a spontaneous magnetization component perpendicular to the "lm plane is present. This out-of-plane component was indeed revealed by both magnetic and MoK ssbauer measurements. Magnetization cycles (Fig. 3) showed (i) the absence of in-plane magnetic anisotropy, and (ii) an easier than expected magnetization direction along the Z-axis. On the basis of the line intensities ratios, CEMS measurements in turn allowed to determine that the magnetization average direction is inclined with an angle of about 403 with respect to the "lm plane. Table 1 summarizes the main sample parameters and properties. The presence of the out-of-plane anisotropy may be justi"ed by the presence of a planar stress at the interface. In e!ect, the magnetoelastic energy density E "(1/2) (M /M )
0 1
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 7 8 - 7
G. Ausanio et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1740}1742
1741
Fig. 1. Surface topography images for Co /Fe with x"5, 10 and 15 nm: (a), (b) and (c), respectively. Areas of one square micron are V V reported. On the orthogonal axis 50 nm/div are shown.
Fig. 2. MFM domain patterns for Co /Fe with x"5, 10 and 15 nm: (a), (b) and (c), respectively. Images relative to (10 m) V V sample area. Table 1 Samples parameters and properties
Fig. 3. Hysteresis cycles of Co /Fe . X and Y are two mutually perpendicular directions in the sample plane. Z is the axis perpendicular to the surface. It is possible to see that the X and Y orientation are practically equivalent and Z is not the hard direction, in comparison to X and Y, considering the high shape anisotropy. Therefore, the reported results are in agreement with the presence of a perpendicular magnetic anisotropy.
(where is the isotropic magnetostriction constant) can produce an out-of-plane easy magnetization axis if the signs of and are opposite [3]. Due to the mis"t between iron and cobalt lattice parameters, assuming isotropic crystallographic orientation, at
Sample code 5[Co /Fe ]
5[Co /Fe ]
5[Co /Fe ]
D (nm) R (nm) d (nm) H (mT) H (mT) M /M 0 1 (deg)
40 6 250 19 100 0,52 443
40 5 300 11 70 0,58 453
60 2 180 19 40 0,68 423
D, average size of surface grain as detected by AFM (Fig. 1); R, surface roughness; d, average thickness of the stripe domains (Fig. 2); H , coercive "eld; H , saturation "eld; M /M , remanent 0 1 magnetization/saturation magnetization; , average angle of magnetization direction with respect to the sample plane.
the interface the iron lattice is under tensile stress ( '0) while that of cobalt is under compressive one ( (0). This means that in iron, where '0, perpendicular an isotropy induction due to E is not e!ective. On the
contrary, since above 2 nm a positive value of is ex pected for cobalt [4], the presence of a compressive stress ( (0) produces in Co layers a negative E value and,
therefore, a perpendicular easy axis of magnetization that gives the out-of-plane anisotropy of the whole multilayer.
1742
G. Ausanio et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1740}1742
References [1] E.E. Fullerton, J.S. Jiang, S.D. Bader, J. Magn. Magn. Mater. 200 (1999) 392. [2] G. Asti, M. Carbucicchio, F. D'Orazio, M. Ghidini, F. Lucari, M. Rateo, G. Ruggiero, M. Solzi, J. Appl. Phys. 87 (2000) 6689.
[3] A. Hubert, R. SchaK fer, Magnetic Domains The Analysis of Magnetic Microstructures, Springer, Berlin, 1998. [4] H. Takahashi, S. Tsunashima, S. Iwata, S. Uchiyama, Jpn. J. Appl. Phys. 32 (1993) L 1328.
Weak stripe domains in Co/Fe multilayers G. Ausanio , V. Iannotti , L. Lanotte , M. Carbucicchio *, M. Rateo Dipartimento di Scienze Fisiche, Fac. Ingegneria, Universita% di Napoli **Federico II++ and INFM, 80125 Napoli, Italy Dipartimento di Fisica, Universita% di Parma, and INFM, Parco Area delle Scienze 7/A, 43100 Parma, Italy
Abstract Ultra-high-vacuum electron-beam-evaporated Co/Fe multilayers (Co /Fe with x"5, 10, 15 nm) were studied by V V atomic and magnetic force microscopy (AFM, MFM), vibrating sample magnetometry (VSM) and conversion electron MoK ssbauer spectroscopy (CEMS). MFM investigation showed long, dense and parallel domain regions (stripe domains) with the average width decreasing with the multilayer thickness. VSM and CEMS measurements showed the presence of an out-of-plane anisotropy, whose origin is ascribed to a planar stress at the interfaces, and which can give account of the observed narrow stripe domain pattern. 2001 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Domain pattern; Magnetostriction
In the last years a growing interest was addressed to the realization of magnetic nanocomposites which combine soft and hard magnetic characteristics via the exchange interactions. This coupling can lead to remanence and energy product enhancement, and can be achieved with the constituting phases "nely distributed on a nanometric scale. Recently, e!orts have been made to obtain planar nanocomposite in the form of multilayers [1]. A series of Co/Fe multilayers, namely 5[Co /Fe ] V V with x"5, 10, 15 nm, with an additional Co layer on the top and protected by an outermost 4 nm thick Mo capping layer, was electron-beam evaporated in ultra-high vacuum (10\ mbar starting vacuum and 10\ mbar operating vacuum) onto Si(1 0 0) and Si(1 1 1) single crystals, without native oxide removal. Grazing incidence X-ray di!raction revealed that the samples have an isotropic polycrystalline structure with BCC Fe and HCP Co [2]. Surface topography was investigated by atomic force microscopy (AFM) and it was found that, as the layer thickness decreases, the surface grain size increases and the roughness decreases (Fig. 1).
* Corresponding author. Fax: #39-0521-905223. E-mail address: carbucicchio@"s.unipr.it (M. Carbucicchio).
The domain patterns were observed by magnetic force microscopy (MFM). Hysteresis loops were measured by a vibrating sample magnetometer (VSM), applying the magnetic "eld both parallel and perpendicular to the "lm plane. The magnetization direction was determined by conversion electron MoK ssbauer spectroscopy (CEMS). MFM investigation showed very regular long and parallel domain regions (stripe domains) whose average width is proportional to the overall multilayer thickness (Fig. 2). The occurrence of stripe domains may be explained only if a spontaneous magnetization component perpendicular to the "lm plane is present. This out-of-plane component was indeed revealed by both magnetic and MoK ssbauer measurements. Magnetization cycles (Fig. 3) showed (i) the absence of in-plane magnetic anisotropy, and (ii) an easier than expected magnetization direction along the Z-axis. On the basis of the line intensities ratios, CEMS measurements in turn allowed to determine that the magnetization average direction is inclined with an angle of about 403 with respect to the "lm plane. Table 1 summarizes the main sample parameters and properties. The presence of the out-of-plane anisotropy may be justi"ed by the presence of a planar stress at the interface. In e!ect, the magnetoelastic energy density E "(1/2) (M /M )
0 1
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 8 7 8 - 7
G. Ausanio et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1740}1742
1741
Fig. 1. Surface topography images for Co /Fe with x"5, 10 and 15 nm: (a), (b) and (c), respectively. Areas of one square micron are V V reported. On the orthogonal axis 50 nm/div are shown.
Fig. 2. MFM domain patterns for Co /Fe with x"5, 10 and 15 nm: (a), (b) and (c), respectively. Images relative to (10 m) V V sample area. Table 1 Samples parameters and properties
Fig. 3. Hysteresis cycles of Co /Fe . X and Y are two mutually perpendicular directions in the sample plane. Z is the axis perpendicular to the surface. It is possible to see that the X and Y orientation are practically equivalent and Z is not the hard direction, in comparison to X and Y, considering the high shape anisotropy. Therefore, the reported results are in agreement with the presence of a perpendicular magnetic anisotropy.
(where is the isotropic magnetostriction constant) can produce an out-of-plane easy magnetization axis if the signs of and are opposite [3]. Due to the mis"t between iron and cobalt lattice parameters, assuming isotropic crystallographic orientation, at
Sample code 5[Co /Fe ]
5[Co /Fe ]
5[Co /Fe ]
D (nm) R (nm) d (nm) H (mT) H (mT) M /M 0 1 (deg)
40 6 250 19 100 0,52 443
40 5 300 11 70 0,58 453
60 2 180 19 40 0,68 423
D, average size of surface grain as detected by AFM (Fig. 1); R, surface roughness; d, average thickness of the stripe domains (Fig. 2); H , coercive "eld; H , saturation "eld; M /M , remanent 0 1 magnetization/saturation magnetization; , average angle of magnetization direction with respect to the sample plane.
the interface the iron lattice is under tensile stress ( '0) while that of cobalt is under compressive one ( (0). This means that in iron, where '0, perpendicular an isotropy induction due to E is not e!ective. On the
contrary, since above 2 nm a positive value of is ex pected for cobalt [4], the presence of a compressive stress ( (0) produces in Co layers a negative E value and,
therefore, a perpendicular easy axis of magnetization that gives the out-of-plane anisotropy of the whole multilayer.
1742
G. Ausanio et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 1740}1742
References [1] E.E. Fullerton, J.S. Jiang, S.D. Bader, J. Magn. Magn. Mater. 200 (1999) 392. [2] G. Asti, M. Carbucicchio, F. D'Orazio, M. Ghidini, F. Lucari, M. Rateo, G. Ruggiero, M. Solzi, J. Appl. Phys. 87 (2000) 6689.
[3] A. Hubert, R. SchaK fer, Magnetic Domains The Analysis of Magnetic Microstructures, Springer, Berlin, 1998. [4] H. Takahashi, S. Tsunashima, S. Iwata, S. Uchiyama, Jpn. J. Appl. Phys. 32 (1993) L 1328.