Dynamical Diffraction Simulations in FePt—I

Abstract

A series of multislice simulations to quantify the effect of various degrees of order, composition, and thickness on the electron diffracted intensities were performed using the L1₀ FePt system as the case study. The dynamical diffraction studies were done in both a convergent electron beam diffraction and selected area electron diffraction condition. The L1₀ symmetry demonstrated some peculiar challenges in the simulation, in particular between the {111} plane normal and the <111> direction, which are not equivalent because of tetragonality. A hybrid weighting function atom of Fe-Pt was constructed to account for S < 1 or nonequiatomic compositions. This statistical approach reduced the complexity of constructing a crystal with the probability that a particular atom was at a particular lattice site for a given order parameter and composition. Considerations of accelerating voltage, convergent angle, and thermal effects are discussed. The simulations revealed significant differences in intensity ratios between films of various compositions but equivalent unit cell numbers and degree of order.

Comparison of Simulated and Experimental Order Parameters in FePt—II

“…For specimen H, an intensity ratio of 0.86 corresponded to an S = 0.3 ± 0.01. Although the simulation process is straightforward for SAED patterns (Torres et al, 2011), experimentally the quantification of the scattered intensities proved more challenging than CBED. The larger region of interest for SAED patterns incorporated the effects of the substrate for both specimen G and specimen H. The use of a small focused probe on a region of interest was more amenable to quantifying S in this study.…”

“…Knowledge of the film's composition and thickness are required for order parameter determination (Torres et al, 2011). The film thicknesses, tabulated in Table 1, were determined from cross-sectional foils, prepared by focus ion beam (FIB) milling in a dual beam FEI Quanta 3D FIB (Giannuzzi et al, 1998).…”

“…The simulation conditions and resultant diffracted intensity outputs are a function of degree of long-range order S , composition, thickness, orientation, and thermal effects in the L 1 0 FePt system (Torres et al, 2011). Simulations were required to determine the order parameter in electron diffraction because of dynamical diffraction events (Kirkland, 1998).…”

“…X-ray diffraction, because of its simplicity in determining order parameter (Warren, 1990), was adopted. However, experimental X-ray diffraction intensities from small volumes can be difficult to quantify because of the low diffracted intensities (Torres et al, 2011). This concern can be reduced or eliminated by using a synchrotron source.…”