Pathways for Single-Shot All-Optical Switching of Magnetization in Ferrimagnets

Abstract

Single-shot helicity-independent all-optical switching of magnetization in ferrimagnets represents the fastest known approach for deterministic data recording. Recently, it was shown that 15-ps-long optical pulses could suffice in triggering the magnetic switching in certain Gd-Fe-Co alloys, generating enormous controversy about the underlying mechanism. Here, we demonstrate how the exact composition of the ferrimagnet affects the kinetics of the reversal process and facilitates the use of thermal pulses with a duration spanning all relevant timescales within the nonadiabatic limit. By modelling a generic ferrimagnet as two coupled macrospins, we show that the magnetization reversal can occur via distinctly different pathways, depending on the duration of the heater. We experimentally reveal that pulses with a duration below and above a critical pulse width respectively enable and disable the capability of all-optical magnetization switching in Gd-Fe-Co alloys, and that modest change of the alloy composition leads to drastic variation of the critical pulse width, by almost 2 orders of magnitude. Our interpretation and results resolve an urgent and outstanding technologically relevant controversy, and provide crucial but previously overlooked guidelines for how to engineer deterministic all-optical switching of magnetization in suitable ferrimagnets.

Single pulse all-optical toggle switching of magnetization without gadolinium in the ferrimagnet Mn2RuxGa

“…Therefore, initially, the angular momentum of the 4c sublattice must be higher than that of 4a to switch successfully. This strongly suggests that the driving mechanism for switching in MRG is exchange scattering with conservation of angular momentum within the spin systems, which is in agreement with current understanding of the exchange-driven part of the switching in GdFeCo 31,32 and recent studies of the switching in MRG with pulses of various durations and wavelengths from a free-electron laser 33 . In order to switch, the sublattice with the stronger intra-sublattice exchange, Mn(4a) in our case, must cross zero before the weaker.…”

Exchange-driven all-optical magnetic switching in compensated 3d ferrimagnets

“…First, single-shot AOS was achieved in Mn 2 Ru 0.75 Ga across a very wide spectral range, with photon energies as low as 50 meV, and second, there is a critical pulse duration τ c ≈ 2.5 ps whereby any single pulse longer or shorter than this threshold induces either demagnetization or single-shot AOS. This latter feature is independent of the photon energy, and has recently also been observed for GdFeCo [21]. On repeating these measurements for the other alloy, Mn 2 Ru 0.80 Ga, we observe the same behavior except for a downward shift in the duration threshold to τ c ≈ 1 ps (see Supplemental Material Note 3) [37].…”

“…We demonstrate single-shot AOS for photon energies varying by a factor 30, from 1.55 down to 0.05 eV, and we show that pulse durations ranging from tens to several thousands of femtoseconds are able to achieve switching. As in GdFeCo [21,31], we identify a critical pulse-duration threshold τ c below which AOS is enabled that depends strongly on the sample temperature but, unlike GdFeCo, we find that switching vanishes completely when the initial temperature is above the compensation point. Finally, we propose a common framework for single-shot AOS that allows us to account for the similarities and differences between the two ferrimagnetic systems.…”

“…Based on this alone, we might expect that intense ultrashort optical pulses would initially excite and demagnetize the 4c sublattice and the 4a sublattice would follow later through energy transfer via the lattice [16]. MRG's unusual band structure therefore raises the question of how optical irradiation can simultaneously demagnetize both sublattices [17][18][19], generating the transient nonequilibrium state from which single-shot AOS emerges [20,21].…”

“…Discussion. The process of AOS in both MRG and GdFeCo can be understood using a common framework, involving an interplay of relaxation pathways that coexist and dominate on different timescales [5,21]. The strong laser-pulse-induced nonequilibrium state generated in these ferrimagnets results in three limiting cases of relaxation: (i) "ultrafast" demagnetization ∂S i /∂t ∝ α i /μ i , where the sublattices i demagnetize independently, irrespective of the exchange interaction between them, at a rate determined by their atomic moment and damping [26]; (ii) demagnetization with conservation of spin angular momentum by exchange between the sublattices, i.e., ∂S 1 /∂t = −∂S 2 /∂t; and (iii) "slow" magnetization changes, when the sublattices have a common temperature and their magnetizations are at all times in equilibrium with the temperature of the lattice.…”