Diffusion jumps of single atoms into vacancies in an iron monolayer studied by nuclear resonant scattering

Abstract

We study the structure of a model surface, i.e., an Fe monolayer on W͑110͒ by nuclear resonant scattering of x rays. We conclude that at room temperature the structure is nearly perfect, whereas when increasing the temperature up to 770 K an increasing fraction of the Fe atoms feels the appearance of defects manifested by a beat structure due to an electric field gradient. The field gradient is caused by an asymmetry of the neighborhood of part of the atoms and attributable to an increasing fraction of vacancies in the monolayer. With increasing temperature the beat structure gets smeared due to the onset of dynamics in the surface, i.e., the motion of the defects and consequently of the atoms. From the temperature dependence of the amplitude of the beat we deduce the vacancy concentration, thus the vacancy formation energy ͓0.17͑5͒ eV͔ and from the broadening with less accuracy we deduce the vacancy migration energy ͓0.16͑6͒ eV͔ resulting in 0.33͑11͒ eV for the activation energy of diffusion.

Future diffusion studies with new X-ray sources

“…Forward scattering from 57 Fe jumping in an iron monolayer on tungsten. Left: data taken at room temperature, right: data at 723 K. From [8] Assuming that the change in the spectrum is caused by defects jumping in the surface, from the temperature dependence of the size of the beat and the steepness of the temporal decay, activation energies of formation and migration of the defects can be estimated.…”

In SituMössbauer Spectroscopy With Synchrotron Radiation on Thin Films

“…The surface atomic diffusion was experimentally investigated on a pseudomorphic monolayer of Fe(110) grown on atomically flat W(110) substrate [99]. The NRS time spectra were measured at an incidence angle of 5 mrad, where the maximum of the delayed scattered radiation was found.…”

“…The derived values for the concentration and jump frequencies of vacancies as well as the diffusion coefficients from the fitted time spectra are summarized in Table 1.2 [99]. …”

“…An Fe monolayer on the W(110) surface is an established model system to study a variety of structural [113], magnetic [114], and diffusion [91,98,99] phenomena at the nanoscale. A flat pseudomorphic monolayer is stabilized by deposition of Fe on the atomically clean W(110) surface at 520 K. At room temperature, the Fe atoms are in paramagnetic state experiencing a quadrupole interaction due to an electric field gradient induced by the broken translational symmetry at the surface [81].…”

Condensed Matter Physics Using Nuclear Resonant Scattering

“…Noteworthy examples include the use of SR to study thin films, interfaces, and surfaces 37,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] in a superconductor 55) and in high-pressure experiments, [56][57][58][59] as well as spin ice 60) and diffusion. [61][62][63][64][65][66][67][68][69] The application of external perturbations that are synchronized to the pulse timing of SR is a unique method. [70][71][72] Studies of slow dynamics have also been performed using SR. [73][74][75] …”