Understanding the Flow of Angular Momentum for Improved Data Storage

A team led by scientists at Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) sought to better understand the flow of angular momentum during ultrafast optical demagnetization in a ferromagnetic material. Working with scientists from Helmholtz Zentrum Berlin and Nihon University, the MBI scientists followed the flow of angular momentum in detail for an iron-gadolinium alloy. In this material, adjacent iron and gadolinium atoms have magnetization with opposite direction.

Wolfgang Pauli and Niels Bohr watching a spinning top in 1954. Erik Gustafson photograph. Courtesy of AIP Emilio Segre Visual Archives, Margrethe Bohr Collection.

A ferromagnetic material can be quickly demagnetized by illuminating it with an ultrashort laser pulse. For example, in iron, cobalt, and nickel, the magnetization is extinguished within about one picosecond after the laser pulse has hit the material. On a microscopic level, magnetization is connected to the angular momentum of the electrons in the material.

The researchers used ultrashort x-ray pulses to monitor the absorption of circularly polarized x-rays by the iron and gadolinium atoms as a function of time after previous laser excitation. This approach allowed the researchers to track the magnetic moment during the ultrafast demagnetization at both types of atoms individually. When the respective absorption spectra were analyzed, this approach also made it possible to distinguish angular momentum stored in the orbital motion versus in the spin of the electrons.

The scientists found that the demagnetization process at the gadolinium atoms in the alloy was significantly faster than in pure gadolinium. “We understand the accelerated response of gadolinium as a consequence of the very high temperatures generated among the electrons within the alloy,” researcher Martin Hennecke said.

At the outset, gadolinium (Gd) possess no angular momentum (L = 0) and no accumulation is seen during the demagnetization after the laser pulse has hit the sample at time zero. In iron (Fe), both spin (S) and orbital moment (L) decrease at the same rate, with no reshuffling between S and L detectable. Courtesy of MBI Berlin.

So how does the angular momentum flow? “Obviously, all angular momentum is fully transferred to the atomic lattice,” Hennecke said. “In line with recent theoretical predictions, the spin angular momentum is first transferred to the orbital motion at the same atom via the spin-orbit interaction, but we cannot see it accumulate there as it is directly moving on to the atomic lattice.” The latter process has recently been theoretically predicted to be as fast as 1 femtosecond. The team’s detailed experiments now confirm that this last transfer step is not a bottleneck in the overall flow of angular momentum.

Given that short laser pulses can be used to permanently switch magnetization and thus write bits for magnetic data recording, insight into the dynamics of these fundamental mechanisms could be useful in the development of new approaches that would allow data to be written to mass storage media much faster than possible today.

The research was published in Physical Review Letters (https://doi.org/10.1103/PhysRevLett.122.157202).