This year the discovery of femtosecond demagnetization by laser pulses is 20 years old. For the first time this milestone work by Bigot and coworkers gave insight in a very direct way into the time scales of microscopic interactions that connect the spin and electron system. While intense discussions in the field were fueled by the complexity of the processes in the past, it now became evident that it is a puzzle of many different parts. Rather than giving an overview that has been presented in previous reviews on ultrafast processes in ferromagnets, this perspective will show that with our current depth of knowledge the first real applications are on their way: THz spintronics and all-optical spin manipulation are becoming more and more feasible. The aim of this perspective is to point out where we can connect the different puzzle pieces of understanding gathered over 20 years to develop novel applications. based on many observations in a large number of experiments. Differences in the theoretical models arise from the localized and delocalized nature of ferromagnetism. Transport effects are intrinsically non-local in spintronic devices and at interfaces. We review the need for multiscale modeling to address processes starting from electronic excitation of the spin system on the picometer length scale and sub-femtosecond time scale, to spin wave generation, and towards the modeling of ultrafast phase transitions that altogether determine the response time of the ferromagnetic system. Today, our current understanding gives rise to the first real applications of ultrafast spin physics for ultrafast magnetism control: THz spintronic devices. This makes the field of ultrafast spin-dynamics an emerging topic open for many researchers right now.

Perspective: Ultrafast magnetism and THz spintronics

Antiferromagnet materials have recently gained renewed interest due to their possible use in spintronics technologies, where spin transport is the foundation of their functionalities. In that respect metallic antiferromagnets are of particular interest, since they enable complex interplays between electronic charge transport, spin, optical, and magnetization dynamics. Here we review phenomena where the metallic conductivity provides unique perspectives for the practical use and fundamental properties of antiferromagnetic materials.

Perspective on Metallic Antiferromagnets

Atomistic spin models describe a class of models which approach a discrete limit of magnetic materials, where each atom is ascribed a localized atomistic magnetic moment or spin. These atomic spins interact principally by the Heisenberg exchange interaction, leading to long-range magnetic order and resulting in macroscopic magnetic properties. Here we review the theoretical foundations and recent developments of atomistic spin dynamics and their application to advanced magnetic materials, thermal and ultrafast spin dynamics. R. F. L. Evans ( ) The Department of Physics, The University of York, Heslington, York, UK e-mail: richard.evans@york.ac.uk © Springer International Publishing AG, part of Springer Nature 2018 W. Andreoni, S. Yip (eds.), Handbook of Materials Modeling, https://doi.org/10.1007/978-3-319-50257-1_147-1 1

Atomistic Spin Dynamics

We report ab initio simulations of the quantum dynamics of electronic charge and spins when subjected to intense laser pulses. By performing these purely electron-dynamics calculations for a thin film and for the bulk of Ni, we conclude that formation of surfaces has a dramatic influence of amplifying the laser induced demagnetization. The reason for this amplification is enhanced spin-currents on the surface of the thin films. We show that the underlying physics of demagnetization for bulk is dominated by spin-flips induced by spin-orbit coupling. In the case of thin films, the dominant cause of demagnetization is a combination of the flow of spin-currents and spin-flips. Furthermore, a comparison of our results with experimental data shows that below  ∼120 fs processes of demagnetization are entirely dominated by purely electronic processes followed by which dissipative effects like the Elliott-Yafet mechanism start to contribute significantly.

https://iopscience.iop.org/article/10.1088/1361-648X/aa66f2/pdf

Ultrafast demagnetization in bulk versus thin films: an ab initio study.

We study the mapping between time-dependent densities and potentials for noninteracting electronic systems on lattices. As discovered recently by Baer [J. Chem. Phys. 128, 044103 (2008)], there exist well-behaved time-dependent density functions on lattices which cannot be associated with any real time-dependent potential. This breakdown of time-dependent V-representability can be tracked down to problems with the continuity equation which arise from discretization of the kinetic-energy operator. Examples are given for lattices with two points and with N points, and implications for practical numerical applications of time-dependent density-functional theory are discussed. In the continuum limit, time-dependent noninteracting V-representability is restored.

http://absimage.aps.org/image/MAR08/MWS_MAR08-2007-001868.pdf

Time-dependent V-representability on lattice systems

The ultrafast demagnetization of small iron clusters initiated by an intense optical excitation is studied from the time-dependent spin density functional theory (TDSDFT). In particular we investigate the effect of the spin-orbit interaction on the onset of the demagnetization process. It is found that demagnetization occurs locally, in the vicinity of the atomic sites, and the initial rate of spin loss, coherent with the laser field, is proportional to the square of the ionic spin-orbit coupling strength $\ensuremath{\lambda}$. A simplified quantum spin model comprising spin-orbit interaction and a time-dependent magnetic field is found to be the minimal model able to reproduce our ab initio results. The model predicts the ${\ensuremath{\lambda}}^{2}$ dependence of the onset rate of demagnetization when it is solved either analytically for the small $t$ regime, or numerically integrated in the time domain. Our findings are supported by additional TDSDFT simulations of clusters made of Co and Ni.

/pdf/role-of-spin-orbit-interaction-in-the-ultrafast-29kuxfthfq.pdf

Role of spin-orbit interaction in the ultrafast demagnetization of small iron clusters

We examine the ultrafast demagnetization process of iron-based materials, namely, ${\mathrm{Fe}}_{6}$ clusters and bulk bcc Fe, with time-dependent spin-density functional theory (TDSDFT). The magnetization continuity equation is reformulated and the torque due to the spin-current divergence is written in terms of an effective time-dependent kinetic magnetic field, an object already introduced in the literature. Its time evolution, as extracted from the TDSDFT simulations, is identified as one of the main sources of the local out-of-equilibrium spin dynamics and it plays a major role in the demagnetization process in combination with the spin orbit interaction. Such demagnetization is particularly strong in hot spots where the kinetic torque is maximized. Finally, we find the rate of demagnetization in ${\mathrm{Fe}}_{6}$ to be strongly dependent on the direction of polarization of the exciting electric field and this can be linked to the out-of-equilibrium distribution of the kinetic field in two comparative cases.

/pdf/ultrafast-demagnetizing-fields-from-first-principles-206m97mpgy.pdf

Ultrafast demagnetizing fields from first principles

The study of defect centers in silicon has been recently reinvigorated by their potential applications in optical quantum information processing. A number of silicon defect centers emit single photons in the telecommunication $O$-band, making them promising building blocks for quantum networks between computing nodes. The two-carbon G-center, self-interstitial W-center, and spin-$1/2$ T-center are the most intensively studied silicon defect centers, yet despite this, there is no consensus on the precise configurations of defect atoms in these centers, and their electronic structures remain ambiguous. Here we employ \textit{ab initio} density functional theory to characterize these defect centers, providing insight into the relaxed structures, bandstructures, and photoluminescence spectra, which are compared to experimental results. Motivation is provided for how these properties are intimately related to the localization of electronic states in the defect centers. In particular, we present the calculation of the zero-field splitting for the excited triplet state of the G-center defect as the structure is transformed from the A-configuration to the B-configuration, showing a sudden increase in the magnitude of the $D_{zz}$ component of the zero-field splitting tensor. By performing projections onto the local orbital states of the defect, we analyze this transition in terms of the symmetry and bonding character of the G-center defect which sheds light on its potential application as a spin-photon interface.

Effect of localization on photoluminescence and zero-field splitting of silicon color centers

Abstract Defect engineering is foundational to classical electronic device development and for emerging quantum devices. Here, we report on defect engineering of silicon with ion pulses from a laser accelerator in the laser intensity range of 10 19 W cm −2 and ion flux levels of up to 10 22 ions cm −2 s −1 , about five orders of magnitude higher than conventional ion implanters. Low energy ions from plasma expansion of the laser-foil target are implanted near the surface and then diffuse into silicon samples locally pre-heated by high energy ions from the same laser-ion pulse. Silicon crystals exfoliate in the areas of highest energy deposition. Color centers, predominantly W and G-centers, form directly in response to ion pulses without a subsequent annealing step. We find that the linewidth of G-centers increases with high ion flux faster than the linewidth of W-centers, consistent with density functional theory calculations of their electronic structure. Intense ion pulses from a laser-accelerator drive materials far from equilibrium and enable direct local defect engineering and high flux doping of semiconductors. 

https://www.nature.com/articles/s43246-023-00349-4.pdf

Defect engineering of silicon with ion pulses from laser acceleration

https://link-springer-com-443.webvpn.jmu.edu.cn/content/pdf/10.1007%2F978-3-319-42913-7_70-1.pdf

Jacopo Simoni

Papers

Role of spin-orbit interaction in the ultrafast demagnetization of small iron clusters

Ultrafast demagnetizing fields from first principles

Effect of localization on photoluminescence and zero-field splitting of silicon color centers

Defect engineering of silicon with ion pulses from laser acceleration

Time-Dependent Density Functional Theory for Spin Dynamics