Two types of all-optical magnetization switching mechanisms using femtosecond laser pulses

Abstract

Magnetization manipulation in the absence of an external magnetic field is a topic of great interest, since many novel physical phenomena need to be understood and promising new applications can be imagined. Cutting-edge experiments have shown the capability to switch the magnetization of magnetic thin films using ultrashort polarized laser pulses. In 2007, it was first observed that the magnetization switching for GdFeCo alloy thin films was helicity-dependent and later helicityindependent switching was also demonstrated on the same material. Recently, all-optical switching has also been discovered for a much larger variety of magnetic materials (ferrimagnetic, ferromagnetic films and granular nanostructures), where the theoretical models explaining the switching in GdFeCo films do not appear to apply, thus questioning the uniqueness of the microscopic origin of all-optical switching. Here, we show that two different all-optical switching mechanisms can be distinguished; a "single pulse" switching and a "cumulative" switching process whose rich microscopic origin is discussed. We demonstrate that the latter is a two-step mechanism; a heat-driven demagnetization followed by a helicity-dependent remagnetization. This is achieved by an all-electrical and timedependent investigation of the all-optical switching in ferrimagnetic and ferromagnetic Hall crosses via the anomalous Hall effect, enabling to probe the all-optical switching on different timescales. arXiv:1602.08525v2 [cond-mat.mtrl-sci]

Ab InitioTheory of Coherent Laser-Induced Magnetization in Metals

“…For ferro-or ferri-magnets this initial process will again be alike for both laser helicities. Once the magnetic order in the material is largely quenched (and, being similar to paramagnetic) the opposite helicities of the subsequent laser pulses will induce IFE magnetizations with opposite sign, which, in a second step, will push the re-magnetization process in one or the other direction, consistent with recent experiments [31].…”

“…Recently, all-optical helicity-dependent switching was observed for a variety of metallic materials including synthetic ferrimagnets and ferromagnets [6][7][8]30]. To achieve switching in these experiments repeated laser pulses were applied [7,30,31]. This "two-step" mechanism appearing after multi-shot laser pulses [31] can be understood from our calculations as well: the initial laser pulses lead to demagnetization and spin moment disorder, in a first step.…”

“…Our calculations explain puzzling aspects of all-optical magnetization switching experiments [7,20,30,31]. Alloptical magnetization reversal triggered by single laser …”

“…To achieve switching in these experiments repeated laser pulses were applied [7,30,31]. This "two-step" mechanism appearing after multi-shot laser pulses [31] can be understood from our calculations as well: the initial laser pulses lead to demagnetization and spin moment disorder, in a first step. For ferro-or ferri-magnets this initial process will again be alike for both laser helicities.…”

Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers

“…To date, GdFeCo is the only material system where such deterministic switching is observed. When extended to ferromagnetic systems, which are of greater interest in many technological applications, only a partial effect can be achieved, which in turn requires repeated laser pulses for full switching [9][10][11] .…”