Oscillations of the transport properties in Ni /Co superlattices

Although metallic multilayers have recently attracted much attention, especially since the discovery of giant magnetoresistance in Fe /Cr multilayers [1], only a small number of true superlattice effects have been experimentally observed. These are the appearance of superlattice Bragg peaks in X-ray diffraction [2], the collective behavior of magnons in magnetic/non-magnetic superlattices [3], and the opening of superlattice gaps in the electronic band structure [4]. Until now, no superlattice effect has been observed in the transport properties. In particular, for all metallic multilayers, the resistivity p increases smoothly while decreasing the modulation wavelength, A, and for some range of thickness, p a A-J , implying that in this range the electron mean free path is limited by the layer thickness, thus impeding the observation of superlattice effects [5-7]. We report here the observation of a true superlattice effect in the transport properties of N i / Co superlattices: oscillations in the magnetotransport as functions of the Ni and /or Co layers thicknesses. Since the oscillations disappear when the number of bilayers is reduced, this behaviour is obviously related to the superlattice structure of the samples. (NidNi/Coaco) N superlattices were grown by Molecular Beam Epitaxy on single-crystal sapphire substrates (AI20~(1120)). The structure of the multilayers was characterized in-situ with electron spectroscopy (LEED, RHEED and AES) and ex-situ with X-ray diffraction [8]. The results show that the multilayers grow epitaxially, both Ni and Co in the fcc structure, with the (111) direction normal to the film surface and a perpendicular * Corresponding author. Fax: +34-1-372-0623; email: immgv84 @pinar2.csic.es. structural coherence length of the order of the whole superlattice thickness ( ~ 1000 ,~), and lattice matched in the plane of the film [8]. There are four different types of in-plane domains, each one with a definite epitaxial relationship with the sapphire substrate, and whose in-plane size ranges between 100 and 200 A. In summary, the multilayers are quasi single-crystalline, i.e. single crystals in the growth direction, but with four different types of in-plane domains [8]. The transport properties were measured at T = 4.2 K on photolithographically patterned samples. Fig. 1 shows the resistivity (P0), anisotropic magnetoresistance (Ap) , ordinary Hall coefficient (R0), and spontaneous Hall coefficient (Rs), as a function of the modulation wavelength ( A ) , for a series of superlattices of the form