Ultrafast-light- or current-driven classical and quantum nonequilibrium states of magnons in magnetic heterostructures

Abstract

Spin and charge pumping—current generation in the absence of a bias voltage along with the ensuing terahertz (THz) radiation from laser-irradiated magnetic materials, is a key phenomenon in spintronics with significant implications for designing novel table-top THz emitters and ultrafast magnetic memory devices. The interplay between localized magnetic moments (LMMs) and conduction electrons plays a significant role in these processes, influencing material properties including Giant Magnetoresistance (GMR) and Tunneling Magnetoresistance (TMR), which are central to modern spintronic devices like magnetic tunnel junctions (MTJs) used in hard disk drives and non-volatile magnetic random access memories (MRAM). In magnetic materials, the collective excitation in the ordering of the LMMs is called spin wave (SW), and its quantized form is known as a magnon. Magnon-based logic gates and information processing have been explored as potential avenues for low-power, high-frequency computing due to the ability of spin waves to transmit information without the joule heating associated with charge transport. ☐ Recent advancements in spintronics have led to the development of efficient THz emitters from magnetic materials that output a broad, gapless spectrum in the range of 1-30 THz. However, a detailed microscopic mechanism of THz generation—specifically, the dynamics of conduction electrons and LMMs excited by the light, as well as effects of spin and charge pumping due to the dynamics of LMMs and the back action of conduction electrons on the LMMs is not well understood even after decades of intense research. To unravel the mechanism of THz generation from magnetic materials, in this thesis, we develop a multiscale quantum-classical hybrid formalism in which conduction electrons are described by quantum master equation (QME) that includes the effects of dissipation by external bosonic bath and the dynamics of LMMs is modeled classically using atomistic spin dynamics governed by the Landau-Lifshitz-Gilbert (LLG) equation. The emitted electromagnetic radiation is then computed from the time-dependent charge densities and bond currents using Jefimenko's equations. On applying this framework to a bilayer of Mn$_3$Sn and spin-orbit (SO)-coupled nonmagnetic material like Platinum (Pt), we demonstrate that---charge pumping by local magnetization of Mn3Sn in the presence of its own SO coupling is far more important than standardly assumed spin current generation and subsequent spin-to-charge conversion within the adjacent nonmagnetic SO-coupled material, in the process of THz generation from magnetic materials. ☐ Furthermore, to investigate electron-magnon interactions, we employ a recently developed numerical exact quantum-classical hybrid scheme of time-dependent nonequilibrium Green functions (TDNEGF)+LLG formalism---applied to a two-terminal device hosting a SW attached to two semi-infinite normal metal leads. The attached leads provide a continuous spectrum, allowing for the exchange of energy and particles from the system. We show that SW pumps chiral spin and charge current into leads, with the flow of current tied to the direction of the propagation of SW and current scaling linearly with the frequency of precession of the SW. In contrast to previous literature, we also showed that chiral charge current can be pumped from the SW even in the absence of SO coupling. This is due to time-retardation effects---motion of localized magnetic moment affects conduction electron spin in a retarded way so that it takes a finite time until the electron spin reacts to the motion of the classical vector. ☐ Nonetheless, in both QME+LLG+Jefimenko and TDNEGF+LLG formalism, the nonequilibrium dynamics of LMMs is described by the LLG equation, treating localized spins as classical vectors of fixed length. However, spin is a genuine quantum degree of freedom, and even though quantum effects become progressively less important for spin value S > 1, they exist for all S < ∞. To test the validity of this approximation, we compare the dynamics of LMMs interaction with conduction electrons via $sd$ exchange interaction in fully quantum-many body treatment and quantum-classical approach. We were able to show that quantum-classical dynamics can faithfully reproduce fully quantum dynamics in the ferromagnetic (F) metallic case, but only when spin S and Heisenberg exchange between localized spins and sd exchange are sufficiently small. Increasing any of these three parameters can lead to substantial deviations, which are explained by the dynamical buildup of entanglement between localized spins and/or between them and electrons. In the antiferromagnetic (AF) metallic case, substantial deviations appear even at early times, which therefore poses a challenge to how to rigorously justify the wide usage of the LL equation in phenomenological modeling of antiferromagnetic spintronics experiments.