Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505

Direct observation of domains and magnetization switchingprocesses 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 andmagnetization process of NiFe/alumina/Co junctions. The alumina layer was prepared either by oxidizing a sputtered Allayer in air, or by direct sputtering of Al

2O

3. 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 overwhich 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. ( 1999Elsevier Science B.V. All rights reserved.

Keywords: Transmission Lorentz microscopy; Metal/insulator junction; Magnetization process; coupling

Ferromagnet/insulator/ferromagnet junctions have at-tracted wide attention from the viewpoint of theory andapplication [1}4]. The junction magnetoresistance canbe understood based on Julliere's model [5]. The mag-netization of the two ferromagnets (FMs) will align paral-lel at high "eld. When the applied "eld is reversed, themagnetization of the FM with a lower coercivity will&switch' to the reversed "eld direction, resulting in themagnetization of the two ferromagnets being antiparallel.When the reversed "eld increases further, the magneti-zation of the magnetically harder ferromagnet will&switch' to the reversed "eld direction, resulting in a par-allel con"guration of the FM magnetization directions.In this paper, we describe the result of direct observationof the magnetization &switching' processes in NiFe/alumina/Co junctions using transmission Lorentzmicroscopy. We also investigated the e!ect of aluminalayer deposition process of the magnetization process.

*Corresponding author. Tel.: #44-1865-273733; fax: #44-1865-273789.

E-mail address: acc.yu@materials.oxford.ac.uk (C.C. Yu)

NiFe/Al-O/Co (T1) and NiFe/Al2O

3/Co (T2) "lms

were deposited by DC/rf magnetron sputtering. The basepressure was 3]10~6 mbar and the Ar pressure duringsputtering was 6]10~3 mbar. The NiFe and Co layerswere dc sputtered. The alumina layer in T1 was preparedby oxidizing a rf sputtered Al layer in air for 48 h beforedeposition of Co layer, and the alumina layer in T2 bydirect rf sputtering of Al

2O

3. In situ magnetizing experi-

ments were carried out in a 400 kV transmission electronmicroscope "tted with a low-"eld objective pole-piece.The domain structure was observed using the Fresnelmode of Lorentz microscopy. The layer and interfacemicrostructure were observed by high resolution electronmicroscopy (HREM) of cross-sectional specimens.

Fig. 1 shows HREM images of T1 and T2 "lms. TheAl-O layer in T1 is 7 nm and in the Al

2O

3layer in T2 is

2.5 nm. The NiFe and Co layers in both "lms are poly-crystalline, and are clearly separated by the aluminalayers. The Al-O layer (which could be oxidized from theNiFe/Al-O interface as well cause the sputtering chamberwas opened after the deposition of NiFe for replacementof mask) shows an amorphous structure and no polycrys-talline Al is observed (Fig. 1a). It is also unlikely to obtainamorphous 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

Fig. 1. HREM cross-sectional bright "eld images of (a) NiFe14 /.

/Al-O

7 /./Co

21 /.(T1) and (b) NiFe

14 /./Al

2O

3(2.5 /.)/Co

21 /.(T2).

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 ofwalls corresponding to NiFe (A) and to Co (B) are shown. (a & c)overfocused and (b & d) underfocused. Magnetization rippleappears in all the Fresnel images.

and more uniform than the Al2O

3layer (Fig. 1b). The

interfaces in T2 are wavier than those in T1, and there-fore stronger magnetostatic &orange-peel' couplingbetween the NiFe and the Co layers is expected for T2.

Fresnel images of "lms T1 and T2 in their as-grownstates are shown in Fig. 2. Two sets of domain walls(marked A and B) are observed in both "lms. In situmagnetizing experiments con"rm that set A are walls inthe NiFe layer, and set B are walls in the Co layer.

Fig. 3. Fresnel images of the magnetization process of "lm T1.Direction of applied "eld H is indicated; "eld values in Oe areshown; all images are of the same area.

Magnetization ripple, which is due to anisotropy disper-sion, exists in both "lms, and the magnetization directionis everywhere normal to the ripple [6].

The magnetization processes of the T1 and T2 "lms areshown in Figs. 3a and 4, respectively. A "eld of !400 Oeis applied to both "lms initially in order to saturate theNiFe and the Co layers. When the "eld is reduced to zeromagnetization ripple appears in both "lms at remanence(Figs. 3 and 4a). When the "eld is increased in a reversedirection, walls in the NiFe layer appear and remainbetween 9.5}18.9 Oe in T1 (Figs. 3b and 3c) and32.4}37.8 Oe in T2 (Figs. 4b and 4c). The disappearanceof the NiFe walls indicates that the magnetization rever-sal of the NiFe layer is complete. At this "eld, the mag-netization directions of NiFe and Co are antiparallel(Figs. 3d and 4d). As the "eld is increased further, walls inthe Co layer appear and remain between 51.3}67.5 Oe inT1 (Figs. 3e and 3f) and 68.9}78.3 Oe in T2 (Figs. 4e and 4f).

504 C.C. Yu et al. / Journal of Magnetism and Magnetic Materials 198}199 (1999) 503}505

Fig. 4. Fresnel images of the magnetization process of "lm T2.Direction of applied "eld H is indicated; "eld values in Oe areshown; all images are of the same area.

After magnetization reversal of the Co layer, the magnet-ization directions of NiFe and Co are parallel, and onlymagnetization ripple is observed (Figs. 3g and 4g). Thecoupling between the NiFe and the Co layers in T2 is

stronger than in T1, resulting in the NiFe and the Colayers in T2 &switching' at higher "elds than in T1. How-ever, the "eld range over which the NiFe and the Comagnetization directions are antiparallel is approxim-ately the same (32 Oe) for both "lms. It is, therefore,believed that the shift of the magnetization processeswith respect to the applied "eld is due to the aluminalayer thickness and the waviness of the interfaces. How-ever, it is less clear which factor contributes more to thecoupling e!ect in the present study.

In summary, we used transmission Lorentz micro-scopy to observe two distinct magnetization reversalprocesses in NiFe/alumina/Co junctions correspondingto the &switching' of NiFe and Co in an increasing applied"eld. The "eld range over which the magnetization direc-tions of the NiFe and the Co layers are antiparallel isapproximately the same for "lms T1 and T2. which havedi!erent alumina layers. The alumina layer thickness andwaviness of the interfaces could shift the magnetizationprocesses of the junctions with respect to the applied"eld.

Acknowledgements

CCY would like to thank the Croucher FoundationHong Kong for a scholarship, St Cross College, Oxfordand the University of Oxford for a travel grant. AKPL isgrateful to the Royal Society for support.

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