Danny Thonig

We study the ultrafast demagnetization dynamics of L n Rh 2 Si 2 ( L n = Pr , Nd, Sm, Gd, Tb, Dy, Ho) antiferromagnets after excitation by a laser pulse, using a combination of density functional theory and atomistic spin and spin-lattice dynamics simulations. In the first step, we calculate the Heisenberg interactions using the magnetic force theorem and compare two approaches, where the 4 f states of the rare earths are treated as frozen core states or as valence states with added correlation corrections. We find marked quantitative differences in terms of predicted Curie temperature for most of the systems, especially for those with a large orbital moment of the rare-earth cations. This can be attributed to the importance of indirect interactions of the 4 f states through the Si states, which depends on the binding energy of the 4 f states and coexists with Ruderman-Kittel-Kasuya-Yosida–type interactions mediated by the conduction states. However, qualitatively both approaches agree in terms of the predicted antiferromagnetic ordering at low temperature, which is in line with previous experiments. In the second step, the atomistic dynamics simulations are used in combination with a heat-conserving two-temperature model, which allows for the calculation of spin and electronic temperatures during the magnetization dynamics simulations. Our simulations demonstrate that despite quite different demagnetization times, magnetization dynamics of all studied L n Rh 2 Si 2 antiferromagnets exhibit similar two-step behavior, in particular the first fast drop followed by slower demagnetization. In addition, we observe that the demagnetization amplitude depends linearly on laser fluence, for low fluences, something that is also in agreement with experimental observations. We also investigate the impact of lattice dynamics on ultrafast demagnetization using coupled atomistic spin-lattice dynamics simulations and the heat-conserving three-temperature model, which confirm the linear dependence of magnetization on laser fluence. The microscopic mechanisms behind these behaviors are investigated in detail.

Magnetoelasticity plays a crucial role in numerous magnetic phenomena, including magnetocalorics, magnon excitation via acoustic waves, and ultrafast demagnetization, or the Einstein–de Haas effect. Despite a long-standing discussion on anisotropy-mediated magnetoelastic interactions of relativistic origin, the exchange-mediated magnetoelastic parameters within an atomistic framework have only recently begun to be investigated. As a result, many of their behaviors and values for real materials remain poorly understood. Therefore, by using a proposed simple modification of the embedded cluster approach that reduces the computational complexity, we critically analyze the properties of exchange-mediated spin-lattice coupling parameters for elemental 3 d ferromagnets (bcc Fe, fcc Ni, and fcc Co), comparing methods used for their extraction and relating their realistic values to symmetry considerations and orbitally decomposed contributions. Additionally, we investigate the effects of noncollinearity (spin temperature) and applied pressure on these parameters. For Fe, we find that single-site rotations, associated with spin temperatures around 100 K, induce significant modifications, particularly in Dzyaloshinskii-Moriya-type couplings; in contrast, such interactions in Co and Ni remain almost configuration independent. Moreover, we demonstrate a notable change in the exchange-mediated magnetoelastic constants for Fe under isotropic contraction. Finally, the conversion between atomistic, quantum-mechanically derived parameters and the phenomenological magnetoelastic theory is discussed, which can be a useful tool towards larger and more realistic dynamics simulations involving coupled subsystems. Published by the American Physical Society 2025

The Landau-Lifshitz-Gilbert (LLG) and Landau-Lifshitz (LL) equations play an essential role for describing the dynamics of magnetization in solids. While a quantum analog of the LL dynamics has been proposed in [], the corresponding quantum version of LLG remains unknown. Here, we propose such a quantum LLG equation that inherently conserves purity of the quantum state. We examine the quantum LLG dynamics of a dimer consisting of two interacting spin- 1 2 particles. Our analysis reveals that, in the case of ferromagnetic coupling, the evolution of initially uncorrelated spins mirrors the classical LLG dynamics. However, in the antiferromagnetic scenario, we observe pronounced deviations from classical behavior, underscoring the unique dynamics of becoming a spinless state, which is nonlocally correlated. Moreover, when considering spins that are initially entangled, our study uncovers an unusual form of revival-type quantum correlation dynamics, which differs significantly from what is typically seen in open quantum systems. Published by the American Physical Society 2024

The impact of the local chemical environment on the Gilbert damping in the binary alloy Fe 100 − x Co x is investigated, using computations based on density functional theory. By varying the alloy composition x as well as Fe-Co atom positions we reveal that the effective damping of the alloy is highly sensitive to the nearest-neighbor environment, especially to the amount of Co and the average distance between Co-Co atoms at nearest-neighbor sites. Both lead to a significant local increase (up to an order of magnitude) of the effective Gilbert damping, originating mainly from variations of the density of states at the Fermi energy. In a global perspective (i.e., making a configuration average for a real material), those differences in damping are masked by statistical averages. When low-temperature explicit atomistic dynamics simulations are performed, the impact of short-range disorder on local dynamics is observed to also alter the overall relaxation rate. Our results illustrate the possibility of local chemical engineering of the Gilbert damping, which may stimulate the study of new ways to tune and control materials aiming for spintronics applications. Published by the American Physical Society 2024

Magnetic skyrmions have garnered attention for their potential roles in spintronic applications, such as information carriers in computation, data storage, and nano-oscillators due to their small size, topological stability, and the requirement of small electric currents to manipulate them. Two key challenges in harnessing skyrmions are the stabilization requirement through a strong out-of-plane field, and the skyrmion Hall effect (SkHE). Here, we present a systematic model study of skyrmions in ferromagnetic/antiferromagnetic (FM/AFM) multilayer structures by employing both atomistic Monte Carlo and atomistic spin dynamics simulations. We demonstrate that skyrmions stabilized by exchange bias have superior stability to field-stabilized skyrmions due to the formation of a magnetic imprint within the AFM layer. Additionally, stacking two skyrmion hosting FM layers between two AFM layers suppresses the SkHE and enables the transport of AFM-coupled skyrmions with high velocity in the order of a few km/s. This proposed multilayer configuration could serve as a pathway to overcome existing limitations in the development of skyrmion-based devices, and the insights obtained through this study contribute significantly to the broader understanding of topological spin textures in magnetic materials. Published by the American Physical Society 2024

Multireservoir models are widely used for modeling and interpreting ultrafast magnetization dynamics. Here we introduce an alternative formulation to existing three-temperature models for the treatment of spin, electron, and lattice temperatures in magnetization dynamics simulations. In contrast to most existing models of calculations of energy transfer between reservoirs in these types of simulations, the heat distribution of the spin and lattice subsystems is evaluated during the simulation instead of being calculated a priori. The model is applied to investigate the demagnetization and remagnetization of fcc Ni, when subjected to a strong laser pulse. In particular, our model results in a fast interplay between the electron and spin subsystems which reproduces the main features of experimental data for fcc Ni significantly better than most reported three-temperature models. We also show that the way in which the electron, spin, and lattice heat capacities are described can have a significant impact on the simulated ultrafast dynamics. By introducing spin-lattice couplings in the simulation, it is shown that these explicit interactions only have a marginal impact on the magnetization dynamics of fcc Ni, albeit it is more pronounced for higher laser pulse powers.

Ultrafast manipulation of magnetism bears great potential for future information technologies. While demagnetization in ferromagnets is governed by the dissipation of angular momentum^1–3, materials with multiple spin sublattices, for example antiferromagnets, can allow direct angular momentum transfer between opposing spins, promising faster functionality. In lanthanides, 4fmagnetic exchange is mediated indirectly through the conduction electrons⁴(the Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction), and the effect of such conditions on direct spin transfer processes is largely unexplored. Here, we investigate ultrafast magnetization dynamics in 4fantiferromagnets and systematically vary the 4foccupation, thereby altering the magnitude of the RKKY coupling energy. By combining time-resolved soft X-ray diffraction with ab initio calculations, we find that the rate of direct transfer between opposing moments is directly determined by this coupling. Given the high sensitivity of RKKY to the conduction electrons, our results offer a useful approach for fine tuning the speed of magnetic devices.