Structural Studies of Sputtered Ni 80 Co 20 /Cu Multilayers
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found for relatively thick Cu layers (tc-, > 10 A.). The high-angle diffraction data were analyzed using a trapezoidal model. The results indicate that the films have a polycrystalline structure with a preferred (111) orientation with coherent interfaces of S100-240 A depending on the Cu layer thickness. The relatively large expansion of (111) spacings in NiCo alloy layers gives rise to the lower atomic ordering in NiCo/Cu multilayers. 1. Introduction Sputter-deposited metallic magnetic multilayers have been extensively studied because of their fascinating magnetic and transport properties. 'The interesting physics in the problem is generally believed to arise from the interactions at the interface between different metals[l]. Although quantitative structural examination on simple systems such as Fe/Cr and Co/Cu have attracted strong interest, there has been insufficient work devoted to a systematic structural study of multilayers containing a soft magnetic alloy, such as Nis0Co 2 O/Cu. Our previous study on NiCo/Cu multilayers has demonstrated that interesting properties such as low-field GMR[2] and weak antiferromagnetic coupling[3] are found in this system. In this paper, we present the results of quantitative examination of NiCo/Cu layered structures using x-ray diffraction. In general, low-angle x-ray reflectivity data provide information on compositional modulation such as the bilayer period A, the composition profile and interfacial roughness etc, while high-angle x-ray measurements with the scattering vector perpendicular to the film surface plane provide information on the atomic order along the growth direction. Moreover, detailed quantitative structural properties could be obtained only by modelling the multilayer structures and comparing the calculated x-ray spectra with the experimental
data[4]. 2. Grazing Angle X-Ray Diffraction The multilayer samples studied here were prepared by DC magnetron sputtering, the details of the thin film growth can be found elsewhere[2]. The low-angle reflectivity experiments 'Prcsent address: IBM Almaden Research Center, San Jose, CA95120 197 Mat. Res. Soc. Symp. Proc. Vol. 382 0 1995 Materials Research Society
were performed using a high-resolution triple-crystal diffractoineter[5], and the high-angle diffraction were carried out using a conventional diffractometer with Cu-K, radiation. The x-ray reflectivity data analysis is based on the standard optical model[6]. For an incoming x-ray beam illuminating a crystal surface, the refractive index n of most materials in the x-ray wavelength range is slightly less than 1, and can be expressed as n = 1 - 6 - i/3, where 6 and 0 can be written as
X-r, A'(f. + Af')= Af'or•A 2
- 2.r
(1)
2 A•
(Af")= A,
(2)
where re is the classical electron radius e 2/mc 2 = 2.818 x 10-" cm. AX. is the number density of atoms, A is the x-ray wavelength, f, is the atomic scattering factor at zero momentum transfer and is equal to Z, the atomic number of the considered element, Af' and Af" are the real and imaginary parts of the dispersion
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