Microstructure and magnetic properties of Co/Os/Co and Fe/Os/Fe thin films

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Journal of Magnetism and Magnetic Materials 304 (2006) e349–e352

Microstructure and magnetic properties of Co/Os/Co and

Fe/Os/Fe thin ﬁlms

C.Y. Chou

a,

, Y.D. Yao

b

, P.C. Kuo

a

, S.F. Lee

b

, J.J. Chou

b

a_{Institute of Materials Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC} b_{Institute of Physics, Academia Sinica, Nankang 115 Taipei, Taiwan, ROC}

Available online 2 March 2006

Abstract

The Co/Os/Co and Fe/Os/Fe thin films were deposited on natural oxidized Si (1 0 0) substrates at room temperature by magnetron sputtering. The thickness was kept at 100 A˚ for all the magnetic layers, and was varied between 3 and 20 A˚ for the Os layer. Effects of the Os layer thickness and microstructure on the magnetic properties of the film were investigated. The results showed that the antiferromagnetic coupling occurred for the Co/Os/Co thin films with the Os thickness between 7 and 13 A˚. Due to the mismatch of Fe and Os, pinholes in the Os layer could be observed. The presence of interface roughness and pinholes reduces the antiferromagnetic interlayer exchange coupling strength and affect the dependence of the interlayer exchange coupling on the Os layer thickness. From the magnetoresistance (DMR) study, the largest DMR ratio arising from antiferromagnetic coupling for Co/Os/Co and Fe/Os/Fe thin films are about 0.45% and 0.07% respectively. The small DMR in Fe/Os/Fe system is explained by the large interface roughness and lattice mismatch effect.

PACS: 68.37.Lp; 72.25.Mk; 75.60.Ej

Keywords: Interlayer exchange coupling; Magnetoresistance; Osmium

Antiferromagnetic interlayer exchange coupling due to indirect exchange of ferromagnetic films across interlayer was observed in Fe/Cr/Fe multilayer in 1986 [1]. There-after, it has been shown to have an important influence on the electrical properties of the multilayer film. Large change in resistance with magnetic field was discovered in Fe/Cr superlattice [2]. This discovery led to an intensive research in the past few years due to its potential in commercial applications. It has been shown that the synthetic antiferromagnet structure is an important com-ponent in MRAM device [3–5]. A synthetic antiferro-magnet is a structure where two ferroantiferro-magnetic layers are antiferromagnetically coupled through a thin nonmagnetic spacer. So, it is important to understand the mechanism of interlayer exchange coupling by studying the magnetiza-tion process of two magnetic layers separated by the nonmagnetic layer. In this work, we investigate the

microstructure and magnetic properties of Co/Os/Co and Fe/Os/Fe thin ﬁlms of the Os thin ﬁlms inserted in Co or Fe magnetic layers.

The Co/Os/Co and Fe/Os/Fe thin films were deposited on natural-oxidized Si (1 0 0) substrates at room tempera-ture by DC-magnetron sputtering with a base pressure less than 1 108Torr. The thickness was kept at 100 A˚ for all the magnetic layers, and was varied between 3 and 20 A˚ for the Os layer. The microstructure of the film was investigated by high-resolution transmission electron mi-croscopy (HRTEM). Magnetic properties of the films were measured by a vibrating sample magnetometer (VSM) at room temperature. The magnetic field was applied in the film plane. The magnetoresistance was measured with a standard DC four-probe technique.

Fig. 1a shows the HRTEM image of the as-deposited Si//Co/Os/Co thin ﬁlm. The Os thickness is 9 A˚. The clear contrast difference between the Co and Os layers resulting from the scattering and absorption effects for different atoms could be observed. Besides, it can be seen that the Os

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layer was grown continually between Co layers. From the high-magnification cross-sectional image (not shown), we did observe polycrystalline structures in the Os layer, and the lattice image of Os could be clearly observed from the lower interface to upper interface. The HRTEM image also shows that the crystalline structure is continuous through-out the multilayer.Fig. 1bshows the HRTEM image of the as-deposited Si//Fe/Os/Fe thin film. The Os thickness is 9 A˚. It can be observed that the Os layer is crystalline and the interface between Fe and Os layers were relatively rough. The thickness fluctuations at the interfaces will affect the interlayer coupling. Besides, a discontinuity in the Os layer was some times existed with a Fe columnar grain. These discontinuities could be distinguished even if the TEM image is in out of focus conditions to enhance the Fresnel contrast[6]. These discontinuous pinholes could be related to the mismatch between Fe and Os. The presence of interface roughness and pinholes will reduce the antiferromagnetic coupling strength and will result in the biquadratic coupling or the orange peel coupling [7–9], which tends to the parallel alignment of the ferromagnetic layers magnetizations.

Fig. 2shows the M–H loops of as-deposited Si//Co/Os/ Co thin ﬁlms with various Os thickness. The thickness was varied between 3 and 20 A˚ for the Os layer. Nonrectangular M–H loop was observed for samples with the Os thickness between 7 and 13 A˚, which indicated that the antiferro-magnetic coupling occurred for these samples. It can be seen that the relatively large saturation ﬁeld occurred for samples roughly with the Os thickness between 7 and 11 A˚. It can be observed that the ferromagnetic coupling occurred in samples with Os layer thickness less than 5 A˚ or between 15 and 20 A˚. In plane squareness (S99) of

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Fig. 1. Cross-sectional HRTEM image of as-deposited (a) Si//Co/Os/Co and (b) Si//Fe/Os/Fe thin ﬁlm.

Os 3 A S//=0.28 Os 13 A S//=0.11 Os 11 A S//=0.09 Os 9 A S//=0.07 Os 7 A S_//=0.16 Os 5 A S//=0.51 M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) Os 20 A S//=0.90 Os 15 A S//=0.87 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 -400 -200 0 200 400 H (Oe) H (Oe)

Fig. 2. M–H loops of as-deposited Si//Co/Os/Co thin ﬁlms with various Os thickness. C.Y. Chou et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e349–e352 e350

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magnetization curves were indicated in each M–H loop, respectively. The smallest S99 of about 0.07 occured in

sample with Os thickness of 9 A˚, which is related to the antiferromagnetic coupling for samples with the Os thickness between 7 and 11 A˚.

Fig. 3shows the M–H loops of as-deposited Si//Fe/Os/ Fe thin ﬁlms with various Os thickness. The thickness was varied between 7 and 20 A˚ for the Os layer. The antiferromagnetic coupling occurred for samples with the Os thickness of 9 and 11 A˚. The M–H loop exhibits ferromagnetic exchange coupling for sample with the Os thickness of 7 A˚ and between 13 and 20 A˚. The rougher interface and pinholes as discussed from the TEM results suggest that ferromagnetic coupling may occur easily in Fe/ Os/Fe than Co/Os/Co. Large remanent magnetization and squareness are observed for samples with ferromagnetic coupling, the value of S99is roughly between 0.4 and 0.6.

The smallest S99of about 0.14 occur for the sample with the

Os thickness of 9 A˚.

The anisotropic magnetoresistance (AMR) has always existed in both trilayer systems, even for samples with the Os thickness of 9 A˚, i.e. for sample with strongest antiferromagnetic coupling. Fig. 4 shows the variations of the DMR% with Os layer thickness of the Co/Os/Co and Fe/Os/Fe thin films. The measurements have been made with the magnetic field in the film plane and parallel to the current direction. The DMR was defined as shown in the inset figure (DMR ¼ 100(RPR0)/R0,), that Rp is the

peak value of resistance due to interlayer exchange coupling. The maximum DMR as shown in Fig. 2 is roughly 0.45% and 0.07% for Co/Os/Co and Fe/Os/Fe systems, respectively. This is related to the rougher interface of Fe/Os/Fe system.

In conclusion, the Co/Os/Co and Fe/Os/Fe thin ﬁlms with Os layer varied between 3 and 20 A˚ were investigated.

The results showed that the antiferromagnetic coupl-ing occurred for the Co/Os/Co thin ﬁlms with the Os thickness between 7 and 13 A˚. Due to the mis-match of Fe and Os, pinholes in the Os layer could be observed. The presence of interface roughness and pinholes reduces the antiferromagnetic interlayer exchange coupling strength and affects the dependence of the interlayer exchange coupling on the Os layer thickness. From the magnetoresistance (DMR) study, the largest DMR ratio arising from antiferromagnetic coupling for Co/Os/Co and Fe/Os/Fe thin ﬁlms are about 0.45% and 0.07%, respectively. The small DMR in Fe/Os/Fe system is explained by the large interface roughness and lattice mismatch effect.

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M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) M (a.u.) Os 7 A S_//=0.40 Os 13 A S_//=0.44 Os 15 A S_//=0.55 Os 9 A S//=0.14 -1800 -1200 -600 0 600 1200 1800 Os 11 A S//=0.19 H (Oe) -1800 -1200 -600 0 600 1200 1800 -1800 -1200 -600 0 600 1200 1800 -1800 -1200 -600 0 600 1200 1800 -1800 -1200 -600 0 600 1200 1800 -1800 -1200 -600 0 600 1200 1800 H (Oe) Os 20 A S//=0.52

Fig. 3. M–H loops of as-deposited Si//Fe/Os/Fe thin ﬁlms with various Os thickness.

4 6 8 10 12 14 16 18 0.0 0.1 0.2 0.3 0.4 0.5 -750 -500 -250 0 250 500 750 1.099 1.106 1.113 1.120 R₀ R_P R( Ω ) H(Oe) Co(100A)/Os(9A)/Co100A, T=300K MR% Co Fe tOs(A)

Fig. 4. Relationship between DMR% and Os thickness of Si//Co/Os/Co and Si//Fe/Os/Fe thin ﬁlm.

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