Efficient calculation methods for the diffusion coefficient of interstitial solutes in dilute alloys

In the example of oxygen diffusion in dilute ferritic iron alloys it is shown that the calculation of the diffusion coefficient can be separated into a contribution related to the migration in the interaction region between oxygen and the substitutional solute and a part related to diffusion in pure body centered cubic (bcc) Fe. The corresponding diffusion times are determined by analytical expressions using Density-Functional-Theory (DFT) data for the respective binding energies. The diffusion coefficient in the interaction region must be determined by atomistic kinetic Monte Carlo (AKMC) simulations with DFT values for the migration barriers as input data. In contrast to previous calculations, AKMC simulation must only be performed for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations in a very efficient manner. This leads to a tremendous decrease of computational efforts. Under certain conditions it is even possible to use analytical expressions where merely DFT data for the binding energies are needed. The limits of applicability of the presented calculation procedures are discussed in detail. The methods presented in this work can be generalized to interstitial diffusion in other host materials with small concentrations of substitutional solutes

Annual Report 2021 - Institute of Ion Beam Physics and Materials Research

The year 2021 was still overshadowed by waves of the COVID-19 pandemic, although the arrival of efficient vaccinations together with the experience of the preceding year gave us a certain routine in handling the situation. By now the execution of meetings in an online mode using zoom and similar video conference systems has been recognized as actually being useful in certain situations, e.g. instead of flying across Europe to attend a three-hours meeting, but also to be able to attend seminars of distinguished scientists which otherwise would not be easily accessible. The scientific productivity of the institute has remained on a very high level, counting 190 publications with an unprecedented average impact factor of 8.0. Six outstanding and representative publications are reprinted in this Annual Report. 16 new third-party projects were granted, among them 7 DFG projects, but very remarkably also an EU funded project on nonlinear magnons for reservoir computing with industrial participation of Infineon Technologies Dresden and GlobalFoundries Dresden coordinated by Kathrin Schultheiß of our Institute. The scientific success was also reflected in two HZDR prizes awarded to the members of the Institute: Dr. Katrin Schultheiß received the HZDR Forschungspreis for her work on “Nonlinear magnonics as basis for a spin based neuromorphic computing architecture”, and Dr. Toni Hache was awarded the Doktorandenpreis for his thesis entitled “Frequency control of auto-oscillations of the magnetization in spin Hall nano-oscillators”. Our highly successful theoretician Dr. Arkady Krasheninnikov was quoted as Highly Cited Researcher 2021 by Clarivate. The new 1-MV facility for accelerator mass spectrometry (AMS) has been ordered from NEC (National Electrostatics Corporation). Design of a dedicated building to house the accelerator, the SIMS and including additional chemistry laboratories for enhanced sample preparation capabilities has started and construction is planned to be finished by mid 2023, when the majority of the AMS components are scheduled for delivery. In the course of developing a strategy for the HZDR - HZDR 2030+ Moving Research to the NEXT Level for the NEXT Gens - six research focus areas for our institute were identified. Concerning personalia, it should be mentioned that the long-time head of the spectroscopy department PD Dr. Harald Schneider went into retirement. His successor is Dr. Stephan Winnerl, who has been a key scientist in this department already for two decades. In addition, PD Dr. Sebastian Fähler was hired in the magnetism department who transferred several third-party projects with the associated PhD students to the Institute and strengthens our ties to the High Magnetic Field Laboratory, but also to the Institute of Fluid Dynamics. Finally, we would like to cordially thank all partners, friends, and organizations who supported our progress in 2021. First and foremost we thank the Executive Board of the Helmholtz-Zentrum Dresden-Rossendorf, the Minister of Science and Arts of the Free State of Saxony, and the Ministers of Education and Research, and of Economic Affairs and Climate Action of the Federal Government of Germany. Many partners from universities, industry and research institutes all around the world contributed essentially, and play a crucial role for the further development of the institute. Last but not least, the directors would like to thank all members of our institute for their efforts in these very special times and excellent contributions in 2021

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii-Moriya interaction

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii--Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall plane. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls.Comment: 5 pages, 3 figures, supplementary material

Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii–Moriya interaction

The orientation of a chiral magnetic domain wall in a racetrack determines its dynamical properties. In equilibrium, magnetic domain walls are expected to be oriented perpendicular to the stripe axis. We demonstrate the appearance of a unidirectional domain wall tilt in out-of-plane magnetized stripes with biaxial anisotropy and Dzyaloshinskii–Moriya interaction (DMI). The tilt is a result of the interplay between the in-plane easy-axis anisotropy and DMI. We show that the additional anisotropy and DMI prefer different domain wall structure: anisotropy links the magnetization azimuthal angle inside the domain wall with the anisotropy direction in contrast to DMI, which prefers the magnetization perpendicular to the domain wall. Their balance with the energy gain due to domain wall extension defines the equilibrium magnetization the domain wall tilting. We demonstrate that the Walker field and the corresponding Walker velocity of the domain wall can be enhanced in the system supporting tilted walls

Laser-induced positional and chemical lattice reordering generating ferromagnetism

Atomic scale reordering of lattices can induce local modulations of functional material properties, such as reflectance and ferromagnetism. Pulsed femtosecond laser irradiation enables lattice reordering in the picosecond range. However, the dependence of the phase transitions on the initial lattice order as well as the temporal dynamics of these transitions remain to be understood. This study investigates the laser‐induced atomic reordering and the concomitant onset of ferromagnetism in thin Fe‐based alloy films with vastly differing initial atomic orders. The optical response to single femtosecond laser pulses on selected prototype systems, one that initially possesses positional disorder, Fe60V40, and a second system initially in a chemically ordered state, Fe60Al40, has been tracked with time. Despite the vastly different initial atomic orders the structure in both systems converges to a positionally ordered but chemically disordered state, accompanied by the onset of ferromagnetism. Time‐resolved measurements of the transient reflectance combined with simulations of the electron and phonon temperatures reveal that the reordering processes occur via the formation of a transient molten state with an approximate lifetime of 200 ps. These findings provide insights into the fundamental processes involved in laser‐induced atomic reordering, paving the way for controlling material properties in the picosecond range

Efficient Calculation Methods for the Diffusion Coefficient of Interstitial Solutes in Dilute Alloys

In the example of oxygen diffusion in dilute ferritic iron alloys it is shown that the calculation of the diffusion coefficient can be separated into a contribution related to the migration in the interaction region between oxygen and the substitutional solute and a part related to diffusion in pure body centered cubic (bcc) Fe. The corresponding diffusion times are determined by analytical expressions using Density-Functional-Theory (DFT) data for the respective binding energies. The diffusion coefficient in the interaction region must be determined by atomistic kinetic Monte Carlo (AKMC) simulations with DFT values for the migration barriers as input data. In contrast to previous calculations, AKMC simulation must only be performed for one concentration of the substitutional solute, and the obtained results can be employed to obtain data for other concentrations in a very efficient manner. This leads to a tremendous decrease of computational efforts. Under certain conditions it is even possible to use analytical expressions where merely DFT data for the binding energies are needed. The limits of applicability of the presented calculation procedures are discussed in detail. The methods presented in this work can be generalized to interstitial diffusion in other host materials with small concentrations of substitutional solutes