Stacking dependent disordering processes in Gd/Co/Pt(111) studied with surface x-ray diffraction

Abstract

Disordering processes induced by Gd overlayers on 15 atomic layers thick Co films grown on top of a Pt(111) surface have been studied in situ by surface x-ray diffraction. They have been investigated in two structurally different Co layers deposited at different substrate temperatures: a film grown at 220 K consisting of a mixture of hcp and fcc crystallites plus some disordered stacking, and another one grown at room temperature containing more disorder and fcc crystallites only. Gd induced disordering has been found to be dependent on the structure of the Co film since cubic fcc crystallites are more sensitive to disordering than hexagonal ones. In addition, Gd deposition causes a reduction in about 0.5% in the Co interatomic distance in the layer plane, which arises from relaxation of stress in the film. The stress is mainly concentrated in fcc grains, which are more strained in the film plane than hcp ones. Furthermore, Co grains having the substrate fcc stacking sequence are majority in the Co films, and they are more stable against disordering than the corresponding fcc twin ones, suggesting some kind of long-range interaction stabilizing the majority stacking. These structural modifications induced by the Gd overlayers affect the magnetism of the film, as evidenced by the hysteresis loops measured by magneto-optical Kerr effect, probing the Co magnetization, and by resonant x-ray scattering at the Pt ${L}_{\text{III}}$ edge, sensitive to the Pt magnetization at the Co/Pt interface. It has been found that the Gd overlayer changes the magnetization of the whole Co film, increasing significantly its coercive field for thick enough Gd overlayers and reducing the amplitude of the loops. These changes are compatible with the formation of a disordered Co-Gd alloy with reduced magnetic moment for the Co atoms, and they affect the whole Co film down to the Pt interface. All these results illustrate how surface x-ray diffraction can be used to characterize in detail alloying processes happening in thin films of binary systems, especially in those containing heterogeneous mixtures of different phases.