Statistical Nature of Atomic Disorder in Irradiated Crystals

Abstract

Atomic disorder in irradiated materials is investigated by means of x-ray diffraction, using cubic SiC single crystals as a model material. It is shown that, besides the determination of depth-resolved strain and damage profiles, x-ray diffraction can be efficiently used to determine the probability density function (PDF) of the atomic displacements within the crystal. This task is achieved by analyzing the diffraction-order dependence of the damage profiles. We thereby demonstrate that atomic displacements undergo Lévy flights, with a displacement PDF exhibiting heavy tails [with a tail index in the γ=0.73-0.37 range, i.e., far from the commonly assumed Gaussian case (γ=2)]. It is further demonstrated that these heavy tails are crucial to account for the amorphization kinetics in SiC. From the retrieved displacement PDFs we introduce a dimensionless parameter f_{D}^{XRD} to quantify the disordering. f_{D}^{XRD} is found to be consistent with both independent measurements using ion channeling and with molecular dynamics calculations.

The amorphization of 3C-SiC irradiated at moderately elevated temperatures as revealed by X-ray diffraction

“…In the present work, we use XRD to study damage buildup in 3C-SiC bombarded at moderately elevated temperatures (25-200 °C) with 500 keV Ar ions. Numerical simulations [23,24] of XRD data allow us to retrieve depth-profiles of both lattice strain and damage and to compare them with the damage profiles previously measured by RBS/C [19]. In agreement with previous RBS/C studies [19], we find that dynamic annealing is enhanced with increasing sample temperature, even at these moderate temperatures, requiring higher ion fluences to reach amorphization.…”

“…In the following, the damage is defined as 1 -DW, which lies in the [0,1] range, where 0 corresponds to a perfect crystal, and 1 corresponds to a fully damaged crystal. It should be noted that the DW factor, and, hence, the corresponding damage value, is a function of Qz and it is, therefore, not an absolute measure of lattice damage [24]. Instead, it provides a convenient quantitative comparison of lattice damage in different samples, provided that the same lattice planes are probed in the XRD experiment.…”

Recent developments in the manipulation of magnetic domain walls in CoFeB–MgO wires for applications to high-density nonvolatile memories

“…As shown in Figure 11.14, we can either determine the necessary current densities by directly imaging the nucleated domains and domain walls (device from Figure 11.14) or by electrically detecting the walls (Figure 11.6a) (Boulle et al, 2008). As shown in Figure 11.14, we can either determine the necessary current densities by directly imaging the nucleated domains and domain walls (device from Figure 11.14) or by electrically detecting the walls (Figure 11.6a) (Boulle et al, 2008).…”

“…Recently, it was shown that more efficient current-induced domain wall (CIDW) motion can be obtained in asymmetric magnetic multilayers due to novel spin-orbit torques compared to conventional spin-transfer torques (Boulle, 2011). A first observation of the spin-orbit torque on the DW motion in Pt/Co/AlOx nanowire was suggested by Miron et al (2010), as the presence of structure inversion asymmetry (SIA) gives rise to an effective field (due to the Rashba effect, Gambardella et al, 2011) perpendicular to both the current flow direction and the magnetic easy-axis, which makes it energetically easier to rotate the magnetization inside the magnetic layer so that DW velocities have been detected up to 400 m/s.…”

“…Recently, another contribution necessary for the fast CIDW motion has been explored theoretically and experimentally, namely the Dzyaloshinskii-Moriya interaction (DMI) (Thiaville et al, 2012;Emori et al, 2013a,b;Boulle et al, 2013;Ryu et al, 2013). It was realized that the spin-orbit torques are not sufficient to move a domain wall that is usually a Bloch wall but one needs Néel DWs all with the same chirality, so that the SHE torque efficiency is maximized and synchronous motion is obtained.…”