Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet.

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L_{3} resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.

Element Specificity of Transient Extreme Ultraviolet Magnetic Dichroism.

In this work we combine theory and experiment to study transient magnetic circular dichroism (TRMCD) in the extreme ultraviolet spectral range in bulk Co and CoPt. We use the ab initio method of real-time time-dependent density functional theory to simulate the magnetization dynamics in the presence of short laser pulses. From this we demonstrate how TRMCD may be calculated using an approximation to the excited-state linear response. We apply this approximation to Co and CoPt and show computationally that element-specific dynamics of the local spin moments can be extracted from the TRMCD in the extreme ultraviolet energy range, as is commonly assumed. We then compare our theoretical prediction for the TRMCD for CoPt with experimental measurement and find excellent agreement at many different frequencies including the M_{23} edge of Co and N_{67} and O_{23} edges of Pt.

Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy.

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.

Ultrafast optically induced spin transfer in ferromagnetic alloys.

The vision of using light to manipulate electronic and spin excitations in materials on their fundamental time and length scales requires new approaches in experiment and theory to observe and understand these excitations. The ultimate speed limit for all-optical manipulation requires control schemes for which the electronic or magnetic subsystems of the materials are coherently manipulated on the time scale of the laser excitation pulse. In our work, we provide experimental evidence of such a direct, ultrafast, and coherent spin transfer between two magnetic subsystems of an alloy of Fe and Ni. Our experimental findings are fully supported by time-dependent density functional theory simulations and, hence, suggest the possibility of coherently controlling spin dynamics on subfemtosecond time scales, i.e., the birth of the research area of attomagnetism.

Extending Solid-State Calculations to Ultra-Long-Range Length Scales.

We present a method that enables solid-state density functional theory calculations to be applied to systems of almost unlimited size. Computations of physical effects up to the micron length scale but which nevertheless depend on the microscopic details of the electronic structure, are made possible. Our approach is based on a generalization of the Bloch state, which involves an additional sum over a finer grid in reciprocal space around each k point. We show that this allows for modulations in the density and magnetization of arbitrary length on top of a lattice-periodic solution. Based on this, we derive a set of ultra-long-range Kohn-Sham equations. We demonstrate our method with a sample calculation of bulk LiF subjected to an arbitrary external potential containing nearly 3500 atoms. We also confirm the accuracy of the method by comparing the spin density wave state of bcc Cr against a direct supercell calculation starting from a random magnetization density. Furthermore, the spin spiral state of γ-Fe is correctly reproduced.

Magneto-Optical Functions at the 3p Resonances of Fe, Co, and Ni: Ab initio Description and Experiment.

We present experimental data and a complete theoretical description of the magneto-optical contributions to the complex refractive index in the extreme ultraviolet (XUV) range covering the 3p resonances of Fe, Co, and Ni. The direct comparison of the two allows us to conclude that many-body corrections to the ground state and local field effects are crucial for an accurate description of M-edge spectra. Our results are relevant for investigation of static magnetization, via XUV spectroscopy of multielement systems, as well as the dynamics of magnetization, as needed in the study of femtomagnetism and spintronics.

Light-wave dynamic control of magnetism.

The enigmatic interplay between electronic and magnetic phenomena observed in many early experiments and outlined in Maxwell's equations propelled the development of modern electromagnetism1. Today, the fully controlled evolution of the electric field of ultrashort laser pulses enables the direct and ultrafast tuning of the electronic properties of matter, which is the cornerstone of light-wave electronics2-7. By contrast, owing to the lack of first-order interaction between light and spin, the magnetic properties of matter can only be affected indirectly and on much longer timescales, through a sequence of optical excitations and subsequent rearrangement of the spin structure8-16. Here we introduce the regime of ultrafast coherent magnetism and show how the magnetic properties of a ferromagnetic layer stack can be manipulated directly by the electric-field oscillations of light, reducing the magnetic response time to an external stimulus by two orders of magnitude. To track the unfolding dynamics in real time, we develop an attosecond time-resolved magnetic circular dichroism detection scheme, revealing optically induced spin and orbital momentum transfer in synchrony with light-field-driven coherent charge relocation17. In tandem with ab initio quantum dynamical modelling, we show how this mechanism enables the simultaneous control of electronic and magnetic properties that are essential for spintronic functionality. Our study unveils light-field coherent control of spin dynamics and macroscopic magnetic moments in the initial non-dissipative temporal regime and establishes optical frequencies as the speed limit of future coherent spintronic applications, spin transistors and data storage media.

Competing Spin Transfer and Dissipation at Co/Cu(001) Interfaces on Femtosecond Timescales.

By combining interface-sensitive nonlinear magneto-optical experiments with femtosecond time resolution and ab initio time-dependent density functional theory, we show that optically excited spin dynamics at Co/Cu(001) interfaces proceeds via spin-dependent charge transfer and back transfer between Co and Cu. This ultrafast spin transfer competes with dissipation of spin angular momentum mediated by spin-orbit coupling already on sub 100 fs timescales. We thereby identify the fundamental microscopic processes during laser-induced spin transfer at a model interface for technologically relevant ferromagnetic heterostructures.

Real time scissor correction in TD-DFT.

We demonstrate how the scissor correction to the optical band gap, common in linear-response time-dependent density functional theory (TD-DFT), may be extended to the domain of real-time TD-DFT. This requires modifying both the eigenvalues and momentum matrix elements of the underlying basis set. It provides a simple and computationally economical approach for calculating accurate electron dynamics in solids. We demonstrate the importance of this correction for prototypical semiconductors, diamond and silicon, where the energy absorption in both the linear and non-linear regimes is examined. We also show that for a particular system, ZnSe, using the adiabatic local density approximation together with a scissor correction can be advantageous over other approximations, as the underlying quasi-particle band structure is more accurate.

Source-Free Exchange-Correlation Magnetic Fields in Density Functional Theory.

Spin-dependent exchange-correlation energy functionals in use today depend on the charge density and the magnetization density: Exc[ρ, m]. However, it is also correct to define the functional in terms of the curl of m for physical external fields: Exc[ρ,∇ × m]. The exchange-correlation magnetic field, Bxc, then becomes source-free. We study this variation of the theory by uniquely removing the source term from local and generalized gradient approximations to the functional. By doing so, the total Kohn-Sham moments are improved for a wide range of materials for both functionals. Significantly, the moments for the pnictides are now in good agreement with experiment. This source-free method is simple to implement in all existing density functional theory codes.

Spin Flips versus Spin Transport in Nonthermal Electrons Excited by Ultrashort Optical Pulses in Transition Metals.

A joint theoretical and experimental investigation is performed to understand the underlying physics of laser-induced demagnetization in Ni and Co films with varying thicknesses excited by 10 fs optical pulses. Experimentally, the dynamics of spins is studied by determining the time-dependent amplitude of the Voigt vector, retrieved from a full set of magnetic and nonmagnetic quantities performed on both sides of films, with absolute time reference. Theoretically, ab initio calculations are performed using time-dependent density functional theory. Overall, we demonstrate that spin-orbit induced spin flips are the most significant contributors with superdiffusive spin transport, which assumes only that the transport of majority spins without spin flips induced by scattering does not apply in Ni. In Co it plays a significant role during the first ∼20 fs only. Our study highlights the material dependent nature of the demagnetization during the process of thermalization of nonequilibrium spins.

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

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.

Ultrafast laser induced local magnetization dynamics in Heusler compounds.

The overarching goal of the field of femtomagnetism is to control, via laser light, the magnetic structure of matter on a femtosecond time scale. The temporal limits to the light-magnetism interaction are governed by the fact that the electron spin interacts indirectly with light, with current studies showing a laser induced global loss in the magnetic moment on a time scale of the order of a few 100 s of femtoseconds. In this work, by means of ab-initio calculations, we show that more complex magnetic materials - we use the example of the Heusler and half-Heusler alloys - allow for purely optical excitations to cause a significant change in the local moments on the order of 5 fs. This, being purely optical in nature, represents the ultimate mechanism for the short time scale manipulation of spins. Furthermore, we demonstrate that qualitative behaviour of this rich magnetic response to laser light can be deduced from the ground-state spectrum, thus providing a route to tailoring the response of some complex magnetic materials, like the Heuslers, to laser light by the well established methods for material design from ground-state calculations.

Excitons in Organics Using Time-Dependent Density Functional Theory: PPV, Pentacene, and Picene.

We apply the bootstrap kernel within time-dependent density functional theory to study the one-dimensional chain of polymer polyphenylenevinylene and molecular crystals of picene and pentacene. The absorption spectra of poly(p-phenylenevinylene) has a bound excitonic peak that is well-reproduced. Pentacene and picene, electronically similar materials, have remarkably different excitonic physics, and this difference is also well captured. We show that the inclusion of local-field effects dramatically changes the spectra of both picene and pentacene but not for poly(p-phenylenevinylene).

Laser-induced demagnetization at ultrashort time scales: predictions of TDDFT.

Time-dependent density functional theory (TDDFT) is implemented in an all electron solid-state code for the case of fully unconstrained noncollinear spins. We use this to study intense, short, laser pulse-induced demagnetization in bulk Fe, Co, Ni and find that demagnetization can take place on time scales of <20 fs. It is demonstrated that this form of demagnetization is a two-step process: excitation of a fraction of electrons followed by spin-flip transitions mediated by spin­orbit coupling of the remaining localized electrons. We further show that it is possible to control the moment loss by tunable laser parameters, including frequency, duration, and intensity.

Calculating electron momentum densities and Compton profiles using the linear tetrahedron method.

A method for computing electron momentum densities and Compton profiles from ab initio calculations is presented. Reciprocal space is divided into optimally-shaped tetrahedra for interpolation, and the linear tetrahedron method is used to obtain the momentum density and its projections such as Compton profiles. Results are presented and evaluated against experimental data for Be, Cu, Ni, Fe3Pt, and YBa2Cu4O8, demonstrating the accuracy of our method in a wide variety of crystal structures.

Optical response of extended systems using time-dependent density functional theory.

In this chapter, time-dependent density functional theory is introduced and an outline of the Runge-Gross theorem is presented. An equation for linear response within time-dependent density functional theory is derived. A key ingredient of this equation is the exchange-correlation kernel for which several modern-day approximation exist. These approximations are investigated for their ability to capture the excitonic physics in absorption and electron loss spectra. To this end, results for medium (Si and diamond) to large (LiF and Ar) band-gap insulators are presented, which exhibit excitonic physics to varying degrees.

Spectral density and metal-insulator phase transition in Mott insulators within reduced density matrix functional theory.

We present a method for calculating the spectrum of periodic solids within reduced density matrix functional theory. This method is validated by a detailed comparison of the angular momentum projected spectral density with that of well-established many-body techniques, finding very good agreement in all cases. The physics behind the pressure induced insulator-metal phase transition in MnO is investigated. The driving mechanism of this transition is identified as increased crystal field splitting with pressure, resulting in a charge redistribution between the Mn e(g) and t(2)g symmetry projected states.

Bootstrap approximation for the exchange-correlation kernel of time-dependent density-functional theory.

A new parameter-free approximation for the exchange-correlation kernel f(xc) of time-dependent density-functional theory is proposed. This kernel is expressed as an algorithm in which the exact Dyson equation for the response, as well as an approximate expression for f(xc) in terms of the dielectric function, are solved together self-consistently, leading to a simple parameter-free kernel. We apply this to the calculation of optical spectra for various small band gap (Ge, Si, GaAs, AlN, TiO(2), SiC), large band gap (C, LiF, Ar, Ne), and magnetic (NiO) insulators. The calculated spectra are in very good agreement with the experiment for this diverse set of materials, highlighting the universal applicability of the new kernel.