Structural Studies and Magnetotransport Properties of Sputtered Ni/Co Multilayers
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R.W. Cochrane Departement de Physique et Groupe de recherche en physique et technologie des couches minces, Universitd de Montrdal, C.P. 6128, Succ. Centre-Ville, Montreal, Que'bec, Canada H3A 3J7. Abstract Ferromagnetic/ferromagnetic Ni/Co multilayers were prepared by DC-magnetron sputtering with component layer thicknesses ranging from 40 A down to 5 A. Structural characterizations by x-ray diffractometry show a well-defined compositional modulation along the film growth direction and a preferred (111) crystalline orientation. A longitudinal magnetoresistance AR/R over 2.7% with a sensitivity of 0.11%/Ce was measured at room temperature in small fields less than 20 Oe. The highest room temperature sensitivity obtained in this system was 0.16%/Oe. Magnetoresistive sensitivity was found to vary inversely with the number of bilayers in the multilayers. The magnetic anisotropy of the films as determined by MOKE magnetometry is correlated to the magnetoresistance and indicative of an AMR effect. I. Introduction The observation of giant magnetoresistance (GMR) in a wide class of antiferromagnetically coupled ferromagnetic/nonmagnetic multilayers[1,2] and in uncoupled granular alloy thin films[3,4] has spurred much research in this field toward a better understanding of the fundamental principles as well as to develop possible applications. In this context, it has been reported recently that ferromagnetic/ferromagnetic Ni/Co multilayers[5] grown by molecular beam epitaxy (MBE) show large magnetoresistance (MR) effects with small saturation fields, leading to high MR sensitivities which compare well to other current and potential magnetic sensor materials. In this paper, we report similar results from sputtered Ni/Co multilayers, and attribute the effect to anisotropic magnetoresistance (AMR). The AMR effect[6], which depends on the orientation of the magnetization with respect to the direction of electric current and the motion of magnetic domains in an external field, is common in ferromagnetic metals with magnitudes Ap/p greater than 5% for some bulk ferromagnetic alloys such as Ni. 70 Co.3 0 . Thin films exhibiting large AMR are currently used in the fabrication of magnetoresistance devices, and therefore the development of new materials with higher sensitivities is of practical importance. Since AMR is related to the intrinsic magnetization of the material, it is therefore important to relate the magnitude and sensitivity of the MR effect to the growth conditions and multilayer composition.
II. Experimental A series of multilayers was prepared by DC-magnetron sputtering from separate targets of Ni and Co onto glass, silicon (100), and oxidized silicon (100) substrates with individual layer thicknesses ranging from 40 A down to 5 A and total number of bilayers between 6 359 Mat. Res. Soc. Symp. Proc. Vol. 384 01995 Materials Research Society
and 48. Background pressure was < 8 x 10-' Torr. With a sputtering pressure of 8.0 mTorr of argon, the deposition rates for Ni and Co, as determined by low-angle x-ray reflect
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