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Co trilayer junctions
Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505
Direct observation of domains and magnetization switching processes in NiFe/alumina/Co trilayer junctions C.C. Yu*, A.K. Petford-Long, J.P. Jakubovics Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Abstract Transmission Lorentz microscopy was used to observe the e!ect of an applied "eld on the domain structure and magnetization process of NiFe/alumina/Co junctions. The alumina layer was prepared either by oxidizing a sputtered Al layer in air, or by direct sputtering of Al O . Two distinct magnetization reversal processes, where NiFe reverses "rst, followed by the magnetization reversal of Co, were observed with increasing applied "eld. The range of "eld values over which the magnetization of NiFe and Co are in an antiparallel con"guration is approximately the same for both "lms. The alumina thickness, and waviness of the interfaces, can a!ect the magnetization process of the junctions. 1999 Elsevier Science B.V. All rights reserved. Keywords: Transmission Lorentz microscopy; Metal/insulator junction; Magnetization process; coupling
Ferromagnet/insulator/ferromagnet junctions have attracted wide attention from the viewpoint of theory and application [1}4]. The junction magnetoresistance can be understood based on Julliere's model [5]. The magnetization of the two ferromagnets (FMs) will align parallel at high "eld. When the applied "eld is reversed, the magnetization of the FM with a lower coercivity will &switch' to the reversed "eld direction, resulting in the magnetization of the two ferromagnets being antiparallel. When the reversed "eld increases further, the magnetization of the magnetically harder ferromagnet will &switch' to the reversed "eld direction, resulting in a parallel con"guration of the FM magnetization directions. In this paper, we describe the result of direct observation of the magnetization &switching' processes in NiFe/ alumina/Co junctions using transmission Lorentz microscopy. We also investigated the e!ect of alumina layer deposition process of the magnetization process.
* Corresponding author. Tel.: #44-1865-273733; fax: #441865-273789. E-mail address: [email protected] (C.C. Yu)
NiFe/Al-O/Co (T1) and NiFe/Al O /Co (T2) "lms were deposited by DC/rf magnetron sputtering. The base pressure was 3;10\ mbar and the Ar pressure during sputtering was 6;10\ mbar. The NiFe and Co layers were dc sputtered. The alumina layer in T1 was prepared by oxidizing a rf sputtered Al layer in air for 48 h before deposition of Co layer, and the alumina layer in T2 by direct rf sputtering of Al O . In situ magnetizing experi ments were carried out in a 400 kV transmission electron microscope "tted with a low-"eld objective pole-piece. The domain structure was observed using the Fresnel mode of Lorentz microscopy. The layer and interface microstructure were observed by high resolution electron microscopy (HREM) of cross-sectional specimens. Fig. 1 shows HREM images of T1 and T2 "lms. The Al-O layer in T1 is 7 nm and in the Al O layer in T2 is 2.5 nm. The NiFe and Co layers in both "lms are polycrystalline, and are clearly separated by the alumina layers. The Al-O layer (which could be oxidized from the NiFe/Al-O interface as well cause the sputtering chamber was opened after the deposition of NiFe for replacement of mask) shows an amorphous structure and no polycrystalline Al is observed (Fig. 1a). It is also unlikely to obtain amorphous Al by sputtering Al. The Al-O layer is thicker
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 9 4 - 9
504
C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505
Fig. 1. HREM cross-sectional bright "eld images of (a) NiFe / Al-O /Co (T1) and (b) NiFe /Al O /Co (T2).
Fig. 3. Fresnel images of the magnetization process of "lm T1. Direction of applied "eld H is indicated; "eld values in Oe are shown; all images are of the same area.
Fig. 2. Fresnel images showing the domain structure of "lm T1 (a & b) and "lm T2 (c & d) in the as-grown state. Two sets of walls corresponding to NiFe (A) and to Co (B) are shown. (a & c) overfocused and (b & d) underfocused. Magnetization ripple appears in all the Fresnel images.
and more uniform than the Al O layer (Fig. 1b). The interfaces in T2 are wavier than those in T1, and therefore stronger magnetostatic &orange-peel' coupling between the NiFe and the Co layers is expected for T2. Fresnel images of "lms T1 and T2 in their as-grown states are shown in Fig. 2. Two sets of domain walls (marked A and B) are observed in both "lms. In situ magnetizing experiments con"rm that set A are walls in the NiFe layer, and set B are walls in the Co layer.
Magnetization ripple, which is due to anisotropy dispersion, exists in both "lms, and the magnetization direction is everywhere normal to the ripple [6]. The magnetization processes of the T1 and T2 "lms are shown in Figs. 3a and 4, respectively. A "eld of !400 Oe is applied to both "lms initially in order to saturate the NiFe and the Co layers. When the "eld is reduced to zero magnetization ripple appears in both "lms at remanence (Figs. 3 and 4a). When the "eld is increased in a reverse direction, walls in the NiFe layer appear and remain between 9.5}18.9 Oe in T1 (Figs. 3b and 3c) and 32.4}37.8 Oe in T2 (Figs. 4b and 4c). The disappearance of the NiFe walls indicates that the magnetization reversal of the NiFe layer is complete. At this "eld, the magnetization directions of NiFe and Co are antiparallel (Figs. 3d and 4d). As the "eld is increased further, walls in the Co layer appear and remain between 51.3}67.5 Oe in T1 (Figs. 3e and 3f) and 68.9}78.3 Oe in T2 (Figs. 4e and 4f).
C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505
505
stronger than in T1, resulting in the NiFe and the Co layers in T2 &switching' at higher "elds than in T1. However, the "eld range over which the NiFe and the Co magnetization directions are antiparallel is approximately the same (32 Oe) for both "lms. It is, therefore, believed that the shift of the magnetization processes with respect to the applied "eld is due to the alumina layer thickness and the waviness of the interfaces. However, it is less clear which factor contributes more to the coupling e!ect in the present study. In summary, we used transmission Lorentz microscopy to observe two distinct magnetization reversal processes in NiFe/alumina/Co junctions corresponding to the &switching' of NiFe and Co in an increasing applied "eld. The "eld range over which the magnetization directions of the NiFe and the Co layers are antiparallel is approximately the same for "lms T1 and T2. which have di!erent alumina layers. The alumina layer thickness and waviness of the interfaces could shift the magnetization processes of the junctions with respect to the applied "eld.
Acknowledgements CCY would like to thank the Croucher Foundation Hong Kong for a scholarship, St Cross College, Oxford and the University of Oxford for a travel grant. AKPL is grateful to the Royal Society for support.
References Fig. 4. Fresnel images of the magnetization process of "lm T2. Direction of applied "eld H is indicated; "eld values in Oe are shown; all images are of the same area.
After magnetization reversal of the Co layer, the magnetization directions of NiFe and Co are parallel, and only magnetization ripple is observed (Figs. 3g and 4g). The coupling between the NiFe and the Co layers in T2 is
[1] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [2] W.J. Gallagher, S.S.P. Parkin, Y. Lu, X.P. Bian, A. Marley, K.P. Roche, R.A. Altman, S.A. Rishton, C. Jahnes, T.M. Shaw, G. Xiao, J. Appl. Phys. 81 (1997) 3741. [3] J.M. Daughton, J. Appl. Phys. 81 (1997) 3758. [4] J. Nowak, J. Rauluszkiewicz, J. Magn. Magn. Mater. 109 (1992) 79. [5] M. Julliere, Phys. Lett. A 54 (1975) 225. [6] B.D. Cullity, Introduction to Magnetic Materials, AddisonWesley, Reading, MA, 1972, p. 436.
Direct observation of domains and magnetization switching processes in NiFe/alumina/Co trilayer junctions C.C. Yu*, A.K. Petford-Long, J.P. Jakubovics Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK
Abstract Transmission Lorentz microscopy was used to observe the e!ect of an applied "eld on the domain structure and magnetization process of NiFe/alumina/Co junctions. The alumina layer was prepared either by oxidizing a sputtered Al layer in air, or by direct sputtering of Al O . Two distinct magnetization reversal processes, where NiFe reverses "rst, followed by the magnetization reversal of Co, were observed with increasing applied "eld. The range of "eld values over which the magnetization of NiFe and Co are in an antiparallel con"guration is approximately the same for both "lms. The alumina thickness, and waviness of the interfaces, can a!ect the magnetization process of the junctions. 1999 Elsevier Science B.V. All rights reserved. Keywords: Transmission Lorentz microscopy; Metal/insulator junction; Magnetization process; coupling
Ferromagnet/insulator/ferromagnet junctions have attracted wide attention from the viewpoint of theory and application [1}4]. The junction magnetoresistance can be understood based on Julliere's model [5]. The magnetization of the two ferromagnets (FMs) will align parallel at high "eld. When the applied "eld is reversed, the magnetization of the FM with a lower coercivity will &switch' to the reversed "eld direction, resulting in the magnetization of the two ferromagnets being antiparallel. When the reversed "eld increases further, the magnetization of the magnetically harder ferromagnet will &switch' to the reversed "eld direction, resulting in a parallel con"guration of the FM magnetization directions. In this paper, we describe the result of direct observation of the magnetization &switching' processes in NiFe/ alumina/Co junctions using transmission Lorentz microscopy. We also investigated the e!ect of alumina layer deposition process of the magnetization process.
* Corresponding author. Tel.: #44-1865-273733; fax: #441865-273789. E-mail address: [email protected] (C.C. Yu)
NiFe/Al-O/Co (T1) and NiFe/Al O /Co (T2) "lms were deposited by DC/rf magnetron sputtering. The base pressure was 3;10\ mbar and the Ar pressure during sputtering was 6;10\ mbar. The NiFe and Co layers were dc sputtered. The alumina layer in T1 was prepared by oxidizing a rf sputtered Al layer in air for 48 h before deposition of Co layer, and the alumina layer in T2 by direct rf sputtering of Al O . In situ magnetizing experi ments were carried out in a 400 kV transmission electron microscope "tted with a low-"eld objective pole-piece. The domain structure was observed using the Fresnel mode of Lorentz microscopy. The layer and interface microstructure were observed by high resolution electron microscopy (HREM) of cross-sectional specimens. Fig. 1 shows HREM images of T1 and T2 "lms. The Al-O layer in T1 is 7 nm and in the Al O layer in T2 is 2.5 nm. The NiFe and Co layers in both "lms are polycrystalline, and are clearly separated by the alumina layers. The Al-O layer (which could be oxidized from the NiFe/Al-O interface as well cause the sputtering chamber was opened after the deposition of NiFe for replacement of mask) shows an amorphous structure and no polycrystalline Al is observed (Fig. 1a). It is also unlikely to obtain amorphous Al by sputtering Al. The Al-O layer is thicker
0304-8853/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 1 9 4 - 9
504
C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505
Fig. 1. HREM cross-sectional bright "eld images of (a) NiFe / Al-O /Co (T1) and (b) NiFe /Al O /Co (T2).
Fig. 3. Fresnel images of the magnetization process of "lm T1. Direction of applied "eld H is indicated; "eld values in Oe are shown; all images are of the same area.
Fig. 2. Fresnel images showing the domain structure of "lm T1 (a & b) and "lm T2 (c & d) in the as-grown state. Two sets of walls corresponding to NiFe (A) and to Co (B) are shown. (a & c) overfocused and (b & d) underfocused. Magnetization ripple appears in all the Fresnel images.
and more uniform than the Al O layer (Fig. 1b). The interfaces in T2 are wavier than those in T1, and therefore stronger magnetostatic &orange-peel' coupling between the NiFe and the Co layers is expected for T2. Fresnel images of "lms T1 and T2 in their as-grown states are shown in Fig. 2. Two sets of domain walls (marked A and B) are observed in both "lms. In situ magnetizing experiments con"rm that set A are walls in the NiFe layer, and set B are walls in the Co layer.
Magnetization ripple, which is due to anisotropy dispersion, exists in both "lms, and the magnetization direction is everywhere normal to the ripple [6]. The magnetization processes of the T1 and T2 "lms are shown in Figs. 3a and 4, respectively. A "eld of !400 Oe is applied to both "lms initially in order to saturate the NiFe and the Co layers. When the "eld is reduced to zero magnetization ripple appears in both "lms at remanence (Figs. 3 and 4a). When the "eld is increased in a reverse direction, walls in the NiFe layer appear and remain between 9.5}18.9 Oe in T1 (Figs. 3b and 3c) and 32.4}37.8 Oe in T2 (Figs. 4b and 4c). The disappearance of the NiFe walls indicates that the magnetization reversal of the NiFe layer is complete. At this "eld, the magnetization directions of NiFe and Co are antiparallel (Figs. 3d and 4d). As the "eld is increased further, walls in the Co layer appear and remain between 51.3}67.5 Oe in T1 (Figs. 3e and 3f) and 68.9}78.3 Oe in T2 (Figs. 4e and 4f).
C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505
505
stronger than in T1, resulting in the NiFe and the Co layers in T2 &switching' at higher "elds than in T1. However, the "eld range over which the NiFe and the Co magnetization directions are antiparallel is approximately the same (32 Oe) for both "lms. It is, therefore, believed that the shift of the magnetization processes with respect to the applied "eld is due to the alumina layer thickness and the waviness of the interfaces. However, it is less clear which factor contributes more to the coupling e!ect in the present study. In summary, we used transmission Lorentz microscopy to observe two distinct magnetization reversal processes in NiFe/alumina/Co junctions corresponding to the &switching' of NiFe and Co in an increasing applied "eld. The "eld range over which the magnetization directions of the NiFe and the Co layers are antiparallel is approximately the same for "lms T1 and T2. which have di!erent alumina layers. The alumina layer thickness and waviness of the interfaces could shift the magnetization processes of the junctions with respect to the applied "eld.
Acknowledgements CCY would like to thank the Croucher Foundation Hong Kong for a scholarship, St Cross College, Oxford and the University of Oxford for a travel grant. AKPL is grateful to the Royal Society for support.
References Fig. 4. Fresnel images of the magnetization process of "lm T2. Direction of applied "eld H is indicated; "eld values in Oe are shown; all images are of the same area.
After magnetization reversal of the Co layer, the magnetization directions of NiFe and Co are parallel, and only magnetization ripple is observed (Figs. 3g and 4g). The coupling between the NiFe and the Co layers in T2 is
[1] J.S. Moodera, L.R. Kinder, J. Appl. Phys. 79 (1996) 4724. [2] W.J. Gallagher, S.S.P. Parkin, Y. Lu, X.P. Bian, A. Marley, K.P. Roche, R.A. Altman, S.A. Rishton, C. Jahnes, T.M. Shaw, G. Xiao, J. Appl. Phys. 81 (1997) 3741. [3] J.M. Daughton, J. Appl. Phys. 81 (1997) 3758. [4] J. Nowak, J. Rauluszkiewicz, J. Magn. Magn. Mater. 109 (1992) 79. [5] M. Julliere, Phys. Lett. A 54 (1975) 225. [6] B.D. Cullity, Introduction to Magnetic Materials, AddisonWesley, Reading, MA, 1972, p. 436.