Vibrational and magnetic signatures of extended defects in Fe

Abstract

Defects change the phonon spectrum and also the magnetic properties of bcc-Fe. Using molecular dynamics simulation, the influence of defects – vacancies, dislocations, and grain boundaries – on the phonon spectra and magnetic properties of bcc-Fe is determined. It is found that the main influence of defects consists in a decrease of the amplitude of the longitudinal peak, PL, at around 37 meV. While the change in phonon spectra shows only little dependence on the defect type, the quantitative decrease of PL is proportional to the defect concentration. Local magnetic moments can be determined from the local atomic volumes. Again, the changes in the magnetic moments of a defective crystal are linear in the defect concentrations. In addition, the change of the phonon density of states and the magnetic moments under homogeneous uniaxial strain are investigated. Graphical abstract

Changes in the phonon density of states of Fe induced by external strain

“…The change in P L is considerably more pronounced in the simulated sample than in experiment; we consider that this is due to the larger grain size of the experimental sample. In previous work [8], it was shown that the P L decrease correlates linearly with an increase in dislocation density in the material.…”

“…To analyze and interpret the changes of the pDOS under strain, we calculate data for the pDOS of Fe using molecular dynamics simulation.These data were obtained in a way as described in [8] for a polycrystalline (pc) Fe sample with a grain size of around 10 nm; the sample contains around N = 250,000 atoms. The strained sample was produced with a strain rate of 10 10 s −1 .…”

“…It was convoluted with a gaussian of width 0.632 ps in order to reduce noise within the signal. Meyer et al [8] also studied the influence of the interatomic interaction potential on the pDOS, which was found to be surprisingly large. The potential by Ackland et al [12] was found to show the closest agreement with experimental data and was therefore used in the present study.…”

“…The physical reason for the decrease of the peak heights lies in a depopulation of mid-frequency modes in the strained samples due to defect formation [8]. The modes are redistributed from the mid-frequency region to the high-and low-frequency tails.…”

“…The data focus on the changes of the pDOS induced by straining an Fe sample. To analyze the data, we use molecular dynamics data of the pDOS in homogeneously strained Fe, on the basis of previous theoretical analyses [8]. The comparison will allow us to interpret the changes induced by strain and to discuss the origin of the remaining differences between experiment and simulation.…”