Charge and spin current pumping by ultrafast demagnetization dynamics

Abstract

The surprising discovery of ultrafast demagnetization -- where electric field of femtosecond laser pulse couples to electrons of a ferromagnetic (FM) layer causing its magnetization vector {\em to shrink while not rotating}, is also assumed to be accompanied by generation of spin current in the direction orthogonal to electric field. However, understanding of the microscopic origin of such spin current and how efficiently it can be converted into charge current, as the putative source of THz radiation, is lacking despite nearly three decades of intense studies. Here we connect the standard pumping phenomena driven by microwave precession of magnetization vector replacing periodic time-dependence of magnetization precession with nonperiodic time-dependence of demagnetization, as obtained from experiments on ultrafast-light-driven Ni layer. Applying time-dependent nonequilibrium Green's functions, able to evolve such setup with arbitrary time dependence, reveals how demagnetization dynamics pumps both charge and spin currents in directions both parallel and orthogonal to electric field of laser pulse, even in the absence of spin-orbit coupling and thereby induced spin-to-charge conversion mechanisms. Although pumped currents follow $d{M}_z/dt$ in some setups, this becomes obscured when NM layers are disconnected and pumped currents start to reflect from FM boundaries (as is the case of experimental setups). Finally, we use the Jefimenko equations to compute electromagnetic radiation by charge current pumped in disconnected setup during demagnetization, or later during its slow recovery, unraveling that radiated electric field only in the former time interval exhibits features in 0.1--30 THz frequency range probed experimentally or explored for applications of spintronic THz emitters.

8 pages, 4 figures; Supplemental Material with two additional figure is available from https://wiki.physics.udel.edu/qttg/Publications ; Highlighlight at physics.aps.org is available from https://physics.aps.org/articles/v17/s97

References in corpus (15)

• Subterahertz spin pumping from an insulating antiferromagnet
• Polarized phonons carry the missing angular momentum in femtosecond demagnetization
• Thouless pumping and topology
• Imaging mesoscopic spin Hall flow: Spatial distribution of local spin currents and spin densities in and out of multiterminal spin-orbit coupled semiconductor nanostructures
• Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten
• Magnonic Charge Pumping via Spin-Orbit Coupling
• Spin and charge pumping in magnetic tunnel junctions with precessing magnetization: A nonequilibrium Green function approach
• Probing optical spin-currents using THz spin-waves in noncollinear magnetic bilayers
• Where does the spin angular momentum go in laser induced demagnetisation?
• Ab initio investigation of laser-induced ultrafast demagnetization of L1${}_0$ FePt: Intensity dependence and importance of electron coherence
• Ultrafast Demagnetization through Femtosecond Generation of Non-thermal Magnons
• Quantum interference and the time-dependent radiation of nanojunctions
• High-harmonic generation in spin and charge current pumping at ferromagnetic or antiferromagnetic resonance in the presence of spin-orbit coupling
• Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn${}_3$Sn
• Spin pumping from antiferromagnetic insulator spin-orbit-proximitized by adjacent heavy metal: A first-principles Floquet-nonequilibrium Green's function study

Cited by in corpus (4)

• Origins of electromagnetic radiation from spintronic terahertz emitters: A time-dependent density functional theory plus Jefimenko equations approach
• Deciphering the origin of spin current in spintronic terahertz emitters and its imprint on their electromagnetic radiation via time-dependent density functional theory
• Nonclassical dynamics of Néel vector and magnetization accompanied by THz and high-harmonic radiation from ultrafast-light-driven NiO antiferromagnet insulator
• Optimizing spin-based terahertz emission from magnetic heterostructures