Simultaneous measurement of volume and grain boundary diffusivity separately in Fe thin film with stable nanostructure using polarized neutron reflectivity

Abstract

Polarized neutron reflectivity (PNR) measurements have been used for simultaneous measurement of volume and grain boundary diffusivity separately in stable nanocrystalline Fe thin film at very low homologous temperature (0.2 Tm < T < 0.3 Tm). PNR measurements were done on Si (substrate)/Fe (150 nm)/[57Fe (3 nm)/natFe (9 nm)]x10 thin film system with periodic 57Fe isotope modulation. PNR from as deposited film shows strong Bragg peaks due to neutron scattering length contrast between 57Fe and natFe layers. Atomic Diffusivity was measured from decrease in the intensity of the Bragg peak due to interdiffusion of 57Fe and natFe layers after annealing the film at three different temperatures 418 K, 483 K and 548 K, respectively for different time intervals starting from 30 min to several hours. The change in the nanostructure of the film after annealing is characterized using grazing incidence x-ray diffraction. No appreciable grain growth within error bar is observed in the film after annealing indicating that the diffusion measurements were done in stable nanostructure. It is observed that the grain boundary diffusivity is two orders of magnitude higher than the volume diffusivity. However, the mechanism of atomic diffusion is similar in both grain and grain boundary.