Self-Diffusion in 57 Fe/ nat Fe Multilayers by In Situ Neutron Reflectometry
- PDF / 756,090 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 80 Downloads / 203 Views
elf-Diffusion in 57Fe/natFe Multilayers by In Situ Neutron Reflectometry Szilárd Sajtia, László Bottyána, Jean-François Moulinb, and Amitesh Paulc,d, * a
Functional Nanostructures Research Group, Wigner Research Centre for Physics, Budapest, 1525 Hungary Neutron Scattering, Helmholtz-Zentrum Geesthacht Outstation at Heinz Maier-Leibnitz Zentrum, Garching, 85748 Germany c Technische Universität München, Physik Department E21, Lehrstuhl für Neutronenstreuung, Garching, 85748 Germany d Guangdong Technion-Israel Institute of Technology, Shantou , 515063, Guangdong, China *e-mail: [email protected]
b
Received July 5, 2019; revised August 27, 2019; accepted August 29, 2019
Abstract—Time-of-flight in situ neutron reflectometry (i-NR) on Si/[57Fe(x nm)/natFe(x nm)]4/Pt with x = 4 and 8 nm multilayers during consecutive heat treatments at 423, 448, 473 and 498 K reveal an unexpected rearrangement of free volumes and an interface smoothening in the isotopic Fe multilayer below 473 K, before the regime of regular Bragg intensity decay starts. The bilayer period dependence of the diffusivities at around 500 K, however, does not follow predictions of Harrison’s theory for the C-type regime representing grain boundary diffusion. Keywords: self diffusion, iron, grain boundary diffusion, neutron reflectometry, in situ DOI: 10.1134/S1027451020070460
INTRODUCTION Diffusion along grain boundaries is a key feature for application-oriented properties of nanocrystalline materials [1]. Due to their metastability and increased defect concentration, the bulk diffusion coefficient is inappropriate to describe diffusion in thin films since atomic migration via grain boundaries, dislocations and free surfaces lead to orders of magnitude faster paths compared to the bulk diffusivity [2]. Harrison’s scheme lays out three different regimes of diffusion, A, B and C [3]. At low temperatures in Ctype diffusion, the length Ld = (2Dt)1/2 ! δ. Here, D denotes the volume diffusivity during an isothermal annealing time t and δ is the grain boundary diameter (typically 0.5 nm). At low enough temperature the volume diffusion is negligible compared to the diffusion along grain boundaries (GB), and the effective diffusion coefficient, D, is related to the diffusivity DGB, via the grain boundary density factor, D = DGB(4δ/d). In this model, one assumes columnar close packed grains with grain diameter d equal to the film thickness. In nanocrystalline metals, grain boundaries can be engineered by controlling the grain size which in a multilayer is limited by the layer thickness. Self-diffusion measurements have been performed in a series of alloys, some of which are also iron based. Most of these studies have been done using radiotracer techniques, secondary ion mass spectroscopy, Rutherford backscattering, Auger electron spectroscopy and lately using neutron reflectivity [4–6]. Grain size evolution is common during diffusion, which, in turn,
affects diffusivity of grain boundaries. In order to realize diffusion along grain boundaries with no grain size
Data Loading...
