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Magnetic tunnel junctions ͑MTJ͒ have become of strategic importance due to the large tunneling magnetoresistance ratio ͑TMR͒ that they can achieve at room temperature. The largest TMR values observed until now were recorded in MTJs with MgO barriers and CoFeB electrodes after annealing of the junction above the recrystallization temperature of the amorphous CoFeB layers. We have used x-ray reflectivity combined with polarized neutron reflectivity to characterize the structure and the magnetism of ͓Co 60 Fe 20 B 20 / MgO͔ 14 multilayers, where the MgO layers were prepared by different methods and annealed at different temperatures. We have found that the MgO preparation method as well as the annealing temperature play a significant role in the systems. A gradient in thickness together with a variation of the scattering length density along the multilayer stacks induce a process of underoxidation or overoxidation, strictly dependent on the MgO production method.

Specular reflectivity measurements of artificial multilayers often exhibit a broadening of high order Bragg peaks and an intensity drop off that is slower than expected from the q −4 Fresnel dependence modified with a static Debye-Waller factor accounting for the interfacial roughness. This behaviour is described on the basis of a model that takes into account the finite size of the detector slit, which determines the instrumental resolution. The amplitude of the interface roughness as well as the in-and out-of-plane correlation lengths strongly affect the shape of the specular x-ray spectra. Information about in-plane and out-of-plane correlated roughness can be obtained, therefore, not only from diffuse scattering data but also from the quasi-specular reflectivity by relaxing the instrumental resolution. Moreover, for systems with short correlation lengths, quasi-specular reflectivity scans are more informative than rocking curves across Bragg reflections. The model is applied to the interpretation of x-ray scattering results on two multilayer systems: MgO(0 0 1)/[Fe(2 ML)/V(16 ML)] 30 and Si/SiO 2 /[CoFeB(3 nm)/MgO(1.5 nm)] 15 . The simulation of the experimental reflectivity scans properly describes the broadening of the higher order Bragg peaks, the damping of Kiessig fringes, and the increased intensity at higher scattering vectors as compared with the purely specular reflectivity.