Role of the buffer layers in determining the antiferromagnetic coupling and magnetoresistance of NiFeCo/Cu superlattices

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The antiferromagnetic (AF) coupling and magnetoresistance in magnetron-sputtered polycrystalline NiFeCo/Cu superlattices grown on Fe, Cr, or Zr buffer layers have been studied. The results show that the buffer layers significantly affect AF coupling and magnetoresistance through modulating the growth of specifically oriented crystalline multilayered structures. Two types of buffer layer effects on AF coupling and magnetoresistance are considered: modifying interfacial roughness and producing layer-thickness fluctuations. Interfacial roughness is formulated in the first Born approximation using data obtained from specular and off-specular (diffuse) x-ray scattering measurements. For the roughness at interfaces, correlated interfacial roughness is found to predominate and estimated to be as large as 5–7 Å with a moderately large lateral correlation length scale ξ that ranges from 120 to 200 Å. Upon approximately the same AF coupling, for Fe or Cr buffer layered superlattices characteristic of well-defined, comparatively flat layer structures throughout the entire specimen, magnetoresistance turns out to increase as the correlated interfacial roughness increases. For the superlattices grown on Zr buffer layers in which the presence of columnar structures is confirmed, a constant, weak AF coupling and moderate magnetoresistance concur and are both almost independent of buffer layer thickness. These features are explained in terms of the averaging of AF coupling and magnetoresistance through parallel layer-thickness fluctuation structures approximating the columnar structures.