Ultrafast Double Pulse All-Optical Reswitching of a Ferrimagnet

Abstract

All-optical re-switching has been investigated in the half-metallic Heusler ferrimagnet Mn 2 Ru 0.9 Ga, where Mn atoms occupy two inequivalent sites in the XA-type structure. The effect of a second 200 fs 800 nm pump pulse that follows a first pulse, when both are above the threshold for switching, is studied as a function of t 12 , the time between them. The aims are to identify the physical mechanisms involved and to determine the minimum time needed for re-switching. The time trajectory of the switching process on a plot of sublattice angular momentum, S 4a vs S 4c , is in three stages; When t < 0.1 ps, the sublattice moments are rapidly disordered, but not destroyed, while conserving net angular momentum via optical spin-wave excitations. This leads to transient parallel alignment of the residual Mn spins in the first quadrant. The net angular momentum associated with the majority sublattice then flips in about 2 ps, and a fully-reversed ferrimagnetic state is then established via the spin-lattice interaction, which allows re-switching provided t 12 > 10 ps.Single-pulse all-optical switching of magnetization (SP-AOS) is of both fundamental and technological interest [1][2][3]. Despite intense scrutiny over the last two decades, the microscopic origin of the effect is still poorly understood, but the possibility of switching the magnetisation of a thin film between two stable states on a picosecond timescale without recourse to an external magnetic field is intriguing and technologically relevant in the quest for ever-faster and more energy-efficient information technologies [4][5][6]. Here we establish the minimum time that must elapse between two pulses, if the second one is to re-establish the original state. Our results advance the fundamental understanding of SP-AOS and highlight its potential for future application in technology.

Realistic micromagnetic description of all-optical ultrafast switching processes in ferrimagnetic alloys

“…For circularly polarized pulses (𝜎𝜎 ± = ±1) the images correspond to alternative switching and the HD-AOS without the central demagnetized (multidomain) state. Again, these results are in good agreement with recent experimental observations (see figures in [28,36,37]).…”

“…Note that these results correspond to a FiM alloy Gdx(FeCo)1-x with 𝑥𝑥=0. 25 Moreover, the inclusion of the MCD in our eLLB model allows us to explain the experimental observation of rings [28,36,37] which appear after the application of a second laser pulse. This is illustrated in FIG.…”

Helicity-independent all-optical switching of magnetization in ferrimagnetic alloys

“…In this alloy, the sublattice-specific magnetic moments are similar (ratio ≈1:1.2) and are strongly coupled (J 4a−4c =−55 meV). Thus, decoupled Bloch relaxation at sublattice-specific rates, as expected from a femtosecond excitation, does not substantially dominate on the timescale before electronlattice equilibration (τ e−l < 1 ps [65,66]). The system instead primarily relaxes under non-equilibrium via exchange-driven demagnetization, conserving the total angular momentum [67].…”

Parametrically driven THz magnon-pairs: predictions towards ultimately fast and minimally dissipative switching

“…The explosive growth of digital data is fuelling the quest for faster and more energy-efficient ways to process and store digital information in magnetic storage devices. Following the discovery of all-optical switching (AOS) in ferrimagnetic GdFeCo 1 , which was subsequently demonstrated in related and other ferrimagnetic alloys [2][3][4][5] , ultrafast manipulation of magnetic order has emerged into a cutting edge approach towards both faster and more energy efficient switching. Despite that the mechanism relies on ultrafast heating [6][7][8] , in GdFeCo a write-read cycle was performed lasting only about 30 ps, with an unprecedented energy dissipation of 16 fJ when scaled to nanometer dimensions 9 .…”