Laser-Driven Multiferroics and Ultrafast Spin Current Generation

Abstract

We propose an ultrafast way to generate spin chirality and spin current in a class of multiferroic magnets using a terahertz circularly polarized laser. Using the Floquet formalism for periodically driven systems, we show that it is possible to dynamically control the Dzyaloshinskii-Moriya interaction in materials with magnetoelectric coupling. This is supported by numerical calculations, by which additional resonant phenomena are found. Specifically, when a static magnetic field is applied in addition to the circularly polarized laser, a large resonant enhancement of spin chirality is observed resembling the electron spin resonance. Spin current is generated when the laser is spatially modulated by chiral plasmonic structures and could be detected using optospintronic devices. DOI: 10.1103/PhysRevLett.117.147202 Introduction.-Control of emergent collective phenomena by external fields is an important problem in condensed matter. Multiferroic magnets (for a review, see Refs. [1][2][3]) are opening new possibilities in this direction since the local spins are coupled not only to magnetic fields but to electric fields through the magnetoelectric (ME) coupling. Laser control of materials is attracting interest with a goal of realizing ultrafast and noncontact manipulation [4][5][6][7][8][9][10][11][12][13][14]. In the research community of magnetic systems, control of magnetism using a laser is being studied in the context of spin-pumping and spintronics [4][5][6][7]. On the other hand, in the field of electronic systems, periodically driven quantum systems draw the interest of many researchers. When the Hamiltonian is time periodic, the system can be described by the so-called Floquet states [15,16], a temporal analog of the Bloch states, and it is possible to control their quantum nature. For noninteracting systems, the control of the band topology has been studied theoretically [8][9][10] and experimentally [13,14]. It is possible to understand the effect of a laser through a mapping from the time-dependent Hamiltonian to a static effective Hamiltonian using the Floquet theory, and the change of quantum state, e.g., topology and symmetry, is attributed to the emergent terms in the static effective Hamiltonian. This framework can also be applied to quantum magnets. Laser-induced magnetization growth in general quantum magnets [17,18] as well as laser-driven topological spin states [18,19], a quantum spin versions of Floquet topological insulators, were proposed recently.In the current work, we apply the Floquet theory to quantum multiferroics and study the synthetic interactions appearing in the effective Floquet Hamiltonian

Floquet topological magnons

“…This formalism is mostly dominated by electronic systems [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] and also optical bosonic systems [60][61][62][63][64][65]. Moreover, an applied laser field provides a means for coherent control of the magnetization in 1D quantum magnets [66][67][68][69][70].…”

High-harmonic generation by electric polarization, spin current, and magnetization

“…In addition to this, various experimental studies of THz-wave driven magnetic phenomena have been done: intense THz-laser driven magnetic resonance in an antiferromagnet 43,45 , magnon resonances in multiferroic magnets with the electric field of THz wave or laser [46][47][48] , spin control by THzlaser driven electron transitions 49 , dichroisms driven by THz vortex beams in a ferrimagnet 50 , etc. These experimental studies have stimulated theorists in many fields of condensed-matter physics, and as a result, several ultrafast magnetic phenomena driven by THz or GHz waves have been proposed and predicted: THz-wave driven inverse Faraday effect 51,52 , Floquet engineering of magnetic states such as chirality ordered states 53 and a spin liquid state 54 , applications of topological light waves to magnetism [55][56][57][58] , control of exchange couplings in Mott insulators with low-frequency pulses 59,60 , optical control of spin chirality in multiferroic materials 61 , and rectification of dc spin currents in magnetic insulators with THz or GHz waves 62,63 . Very recently, Takayoshi et al 64 numerically calculate the HHG spectra in quantum spin models, assuming that the applied THz laser is extremely strong beyond the current technique.…”

“…From the experimental viewpoint, the HHG may be one of the simplest phenomena in nonlinear optical effects in magnetic insulators. Therefore, estimating required strength of THz or GHz waves for the HHG contributes not only to deepen the understanding of the HHG itself, but also to give us a reference value of required ac fields for the realization of the other nonlinear phenomena such as optical control of magnetism 55,56,61 , Floquet engineering of magnets [51][52][53][54] , and spin current rectification 62,63 .…”

Impact of high-frequency pumping on anomalous finite-size effects in three-dimensional topological insulators

“…At the same time, using the coupling to an external electromagnetic field opens up interesting opportunities to study exotic states of hybrid light-matter nature [30][31][32][33][34][35][36][37][38][39][40] . For highly intense lasers light-matter coupling is sufficiently strong to cause the breakdown of standard perturbative expansion.…”