For faster magnetic switching—destroy and rebuild

Magnetic data storage technology and the everincreasing speed of information processing have brought enormous changes to our daily life. These developments naturally lead us to ask if there is a physical limit to the speed at which magnetic moments can be switched [1]—a topic that has caused no shortage of controversy in the scientific community. Exploring this limit is complicated, partly because switching the magnetization from one direction to the other can occur in multiple ways and along different paths. For example, magnetic and electric fields, electric currents, and laser pulses can all stimulate magnetic switching and the trajectory of the magnetization vector from its initial to its final state will vary with each of these switching mechanisms. Kadir Vahaplar and colleagues at Radboud University Nijmegen in The Netherlands, in collaboration with scientists in Germany, the UK, Japan, and Russia have made a dramatic leap forward in exploring the limits to magnetic switching. Writing in Physical Review Letters, they demonstrate a magnetic write-read event that occurs on times as short as 30 picoseconds (ps), which is the fastest magnetic switching process observed so far [2]. But the work by Vahaplar et al. is much more than the demonstration of high-speed magnetic switching. By combining sophisticated experimental methods with theoretical tools that fully account for the magnetization on many length scales (from the continuum to the atomic and electronic limit), their study leads to important insight and detailed understanding of what fundamental processes allow ultrafast magnetic switching to occur. So far, groups have mainly looked at ways of turning and redirecting the magnetization continuously, typically by causing it to precess with magnetic field pulses [3]. Using purely optical methods, Vahaplar et al. show that a faster way to switch the magnetization is to temporarily quench it [4], that is, reduce it to zero, and restore it immediately afterwards in the opposite direction, a scheme they aptly call a linear reversal (Fig. 1). Their experiments are an ingenious combination of the different effects by which light interacts with magnetic moments. These effects are usually categorized as optomagnetic or magneto-optical, depending on whether they describe the influence of the light pulse on the magnetization or vice versa. In their setup, Vahaplar et al. first stimulate the magnetization of amorphous 20 nm ferromagnetic films made of GdxFe100−x−yCoy with a short and intense circularly polarized (pump) laser pulse and then image the magnetization with a second, equally short but linearly polarized (probe) laser pulse. The first laser pulse has two effects on the magnetization. First, it rapidly pumps energy into the film, locally heating the material and demagnetizing it [5]. The energy of the laser pulse is primarily absorbed by the electrons, which reach a temperature of about 1200 K within the first few hundred femtoseconds (fs) after the pulse. Changes in the electronic temperature affect the magnetic properties on sub-ps time scales. Most importantly, the magnitude of the magnetization M decreases as the temperature of the electronic system approaches the Curie temperature TC (the temperature at which the material undergoes a phase transition from a ferromagnet to a paramagnet, at equilibrium). Vahaplar et al. show that the magnetization can in fact be temporarily “destroyed” down to a value of zero about 500 fs after applying a sufficiently strong laser pulse. The first laser pulse also affects the magnetization via the inverse Faraday effect [6]: as the circularly polarized electromagnetic field pulse traverses the sample, it acts as an effective magnetic field along the pulse’s propa-