Abstract
We propose an efficient and non-perturbative scheme to compute magnetic excitations for extended systems employing the framework of time-dependent density functional theory. Within our approach, we drive the system out of equilibrium using an ultrashort magnetic kick perpendicular to the ground-state magnetization of the material. The dynamical properties of the system are obtained by propagating the time-dependent Kohn–Sham equations in real time, and the analysis of the time-dependent magnetization reveals the transverse magnetic excitation spectrum of the magnet. We illustrate the performance of the method by computing the magnetization dynamics, obtained from a real-time propagation, for iron, cobalt, and nickel and compare them to known results obtained using the linear-response formulation of time-dependent density functional theory. Moreover, we point out that our time-dependent approach is not limited to the linear-response regime, and we present the first results for nonlinear magnetic excitations from first principles in iron.
Highlights
Collective magnetic excitations play a central role in our understanding of the stability of magnetic materials; for example, they determine the Curie (Neé l) temperature of ferromagnets.[1]
There are two approaches to study the spin magnetization dynamics: (1) A phenomenological approach, where the spin degrees of freedom are encoded in a model Hamiltonian, such as the Heisenberg model, and the magnetization dynamics is computed, for example, by solving the classical Landau−Lifshitz−Gilbert equation.[9] (2) The first-principles approach, based on the solution of the microscopic Pauli−Schrödinger equation for the electrons, where magnetism emerges due to the intrinsic magnetic moment of the interacting electrons
In Appendix A, we provide a more in-depth discussion of the TD-density functional theory (DFT) linearresponse formalism, and in Appendix B, we discuss the socalled generalized Bloch theorem, which allows for the efficient simulation of long-wavelength magnons using the chemical unit cell
Summary
Collective magnetic excitations play a central role in our understanding of the stability of magnetic materials; for example, they determine the Curie (Neé l) temperature of (anti-) ferromagnets.[1]. We employ instead the exact reformulation of quantum mechanics based on density functional theory (DFT).[10−12] Since its inception, DFT has evolved into the most widespread approach to numerically study extended systems from first principles. The linear-response formulation of TD-DFT has been used to study optical properties and magnetic excitations such as magnons, which are collective fluctuations of the spin magnetization.[16−23]. The ultrafast demagnetization due to intense laser pulses[24] has been successfully investigated using TD-DFT approaches.[25−29] In the present work, we are interested in modeling transverse magnetic excitations, magnons, which are long-wavelength collective excitations of magnetic materials with a typical energy of tens to hundreds of milli-electronvolts. In Appendix A, we provide a more in-depth discussion of the TD-DFT linearresponse formalism, and in Appendix B, we discuss the socalled generalized Bloch theorem, which allows for the efficient simulation of long-wavelength magnons using the chemical unit cell
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