Switching ferromagnetic spins by an ultrafast laser pulse: Emergence of giant optical spin-orbit torque

Abstract

-Faster magnetic recording technology is indispensable to massive data storage and big data sciences. All-optical spin switching offers a possible solution, but at present it is limited to a handful of expensive and complex rare-earth ferrimagnets. The spin switching in more abundant ferromagnets may significantly expand the scope of all-optical spin switching. Here by studying 40,000 ferromagnetic spins, we show that it is the optical spin-orbit torque that determines the course of spin switching in both ferromagnets and ferrimagnets. Spin switching occurs only if the effective spin angular momentum of each constituent in an alloy exceeds a critical value. Because of the strong exchange coupling, the spin switches much faster in ferromagnets than weakly-coupled ferrimagnets. This establishes a paradigm for all-optical spin switching. The resultant magnetic field (65 T) is so big that it will significantly reduce high current in spintronics, thus representing the beginning of photospintronics.

Spin-orbit torque-mediated spin-wave excitation as an alternative paradigm for femtomagnetism

“…We also understand why spin flips. Figure 5(c) shows that spin-orbit torque [39] τ z = λ(L×S) z around 0 fs is positive and large, so the spin flips from the −z axis to the +z axis. Figure 5(d) illustrates the key idea of our alternative paradigm: It is this spin-orbit torque that flips the spin, generates a highly noncollinear spin wave and cancels the spins at different lattice sites.…”

“…1). We wonder whether our prior model that was developed for all-optical spin switching [37][38][39][40][41] could help. This model is very much similar to the traditional t − J model, where the spin degree of freedom is taken into account by the Heisenberg exchange interaction, and the charge degree of freedom is taken into account by the hopping term between neighboring atoms.…”

“…We numerically solve the Heisenberg equation of motion [39] for the spin and other observables at site i as follows:…”

All-optical spin switching under different spin configurations

“…Different from other studies, we augment a spin-orbit coupling term, λL i · S i , to the Heisenberg exchange model, where λ is the spin-orbit coupling constant, L i and S i are the orbital and spin angular momenta at site i. Our Hamiltonian is [15,[19][20][21]…”

“…1(b). We choose two different spin angular momenta S 0 = 1h and 2h since it is known the spin value affects how laser switches spins [20]. Figure 4 Fig.…”