Microstructural evolution of Ni/Ti multilayers doped with nitrogen at different d-spacings for neutron supermirrors

This paper presents the microstructural evolution of a Ni/Ti supermirror with m = 3 based on the investigation of the interfacial roughness, crystallisation, and stress of Ni/Ti multilayers doped with nitrogen at different d-spacings. The performance of the samples was characterized using grazing incidence X-ray reflectivity, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The roughness and compressive stress of the samples increased as the d-spacing increased from 4 nm to 12 nm; thereafter, the values varied gradually as the d-spacing further increased to 20 nm. The grain size of the crystallised nitrogen-doped Ni/Ti multilayers was always smaller than the layer thickness. The nitrogen atoms concentrated at interfaces due to their attraction to titanium atoms, and the formed TiN barrier layer effectively prevented diffusion between adjacent layers, resulting in a smooth interface. The film growth of the nitrogen-doped Ni/Ti supermirror showed three distinct stages, and three corresponding growth models were proposed to explain the microstructural evolution of the films.