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

Abstract

Laser-induced femtosecond demagnetization, femtomagnetism, offers a potential route to develop faster magnetic storage devices. It is generally believed that the traditional spin-wave theory, which is developed for thermally driven slow demagnetization, can not explain this rapid demagnetization by design. Here we show that this traditional spin-wave theory, once augmented by laser-induced spin-orbit torque, provides a highly efficient paradigm for demagnetization, by capturing lowenergy spin-wave excitation that is absent in existing mechanisms. Our paradigm is different from existing ones, but does not exclude them. Microscopically, we find that optical spin-orbit torque generates massive spin waves across several hundred lattice sites, collapsing the long-range spin-spin correlation within 20 fs. Our finding does not only explain new experiments, but also establishes an alternative paradigm for femtomagnetism. It is expected to have far-reaching impacts on future research.

An attempt to simulate laser-induced all-optical spin switching in a crystalline ferrimagnet

“…For a weak exchange interaction case, regardless of the magnitude of the laser field amplitude, the spin switching is not observed. This points out an entirely different scenario from ferromagnetic cases [19] and demagnetization [20]. The rich picture that is found here reveals a crucial effect of the effect of exchange interaction on AOS, and should motivate further experimental and theoretical investigations in the future.…”

“…In order to compute the spin change, we solve the Heisenberg equation of motion [20] for each spin operator at every site under laser excitation [17]. Figure 1(a) shows the spin z component at two Mn sites as a function of time.…”

“…There is a narrow window that one can switch spins. demagnetization in a system with a small spin moment [20]. Figure 2(a) shows the spin change at Mn 1 (the situation is similar at Mn 2 ), where the solid, dotted, and dashed lines denote the x, y, and z components, respectively.…”

Spin Berry points as crucial for ultrafast demagnetization

“…Orbital momentum change mainly occurs when the laser field is on. 13 Once the laser field is over, the system evolves within the relative subspaces of spin and orbital. The second difficulty is experimental.…”

“…This brings us to the third picture: the spin wave or magnon picture. 19,13 Here spins are situated at a lattice site and do not move, but their orientations can be changed in space. This directly generates a long-wavelength spin waves across many hundred lattice sites.…”