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The field of ultrafast magnetism explores how flashes of light can manipulate a material's magnetization in trillionths of a second. In the process called all-optical switching (AOS), a single laser pulse of several femtoseconds (≈10^-15 seconds) duration flips tiny magnetic regions without the need for an externally applied magnetic field.

Enabling such an ultrafast control over magnetization, orders of magnitude faster than what can be achieved using a conventional magnet-based read/write head as in a magnetic hard drive, AOS is a promising candidate for novel spintronics devices that use magnetic spins with their associated magnetic moments as information carriers. Such devices typically consist of a stack of nanometer-thin materials, with the actual magnetic material being one of them.

Until now, the switching process was thought to happen uniformly in the magnetic material wherever the laser pulse deposits a sufficient amount of energy. In a study recently in Nature Communications, researchers from the Max Born Institute together with collaborators from Berlin and Nancy revealed that this is not the case. Instead, there is an ultrafast propagation of a magnetization boundary into the depth of the material.

Combining ultrashort infrared (IR) excitation with table-top femtosecond soft-X-ray spectroscopy, the scientists studied a 9.4 nm thin gadolinium-cobalt (GdCo) film in a typical stack with platinum and copper layers on top and a tantalum layer below. Using broadband X-rays tuned to an atomic resonance of the rare earth atom Gd, they applied a technique recently developed at MBI that allows following magnetization changes along the depth of the sample in time. The result is a movie of the magnetization as it evolves along the film's depth, with femtosecond temporal resolution.

In this movie, the researchers could observe what had been hidden so far: Immediately after the arrival of the infrared pulse of 27 fs duration, the entire GdCo layer first heats up and its magnetization drops nearly uniformly, in line with the conventional thinking.

But after two picoseconds, two domains of opposite magnetization appear: The top region—receiving an additional stimulus from the more strongly heated up platinum layer on top of the GdCo—flips first, while the magnetization direction at the bottom remains unchanged. A boundary between these two domains is formed and subsequently propagates downward at about 2,000 m/s, sweeping through the entire GdCo layer in roughly 4.5 ps.

In particular, only the surface-near slice of the GdCo is initially excited strong enough to overcome the threshold required for AOS; nevertheless, the switching succeeds as the rest of the film follows due to the propagating boundary.

This discovery forces a rethink of AOS as a combination of local and non-local processes, challenging the current understanding of the process by the established theoretical models. The moving boundary, possibly driven by a combination of angular-momentum exchange between the switched and unswitched regions and the thermal gradients across the heterostructure established on an ultrashort time scale, ultimately determines both the switching speed and the final magnetic state.

Looking forward, these insights open new routes to engineer light-actuated magnetic devices. By choosing different surrounding layers in addition to altering film thickness and composition, one can control where the boundary nucleates and how fast it travels. Such design freedom could enable fast and energy-efficient memory and logic elements that exploit light-driven magnetization reversal.