Spin-Orbit Interaction and Electron Motion central for the Magnetization Dynamics in Transition-Metal Alloys

The laser-induced magnetization dynamics in binary ferromagnetic transition-metal alloys has been theoretically investigated. The fundamental question about the origin of a spin angular-momentum transfer between the inhomogeneous magnetic sublattices has been addressed in the framework of our purely electonic many-body model taking into accout the fundamental electronic hopping, local Coulomb interaction, and spin-orbit coupling (SOC) on the same footing. We demonstrate how the ultrafast energy and spin-density redistributions among the alloy components resulting from inhomogeneities in the microscopic parameters are mediated by the itinerant d-electrons responsible of magnetism. Moreover transient and steady-state dynamical regimes, element-specific demagnetization times and delays in the onset of the local demagnetizations and ultrafast redistributions of the spin-polarized density reveal a strong correlation between the magnetization dynamics of the alloy components and demonstrate that the interatomic spin angular-momentum transfer between the different elements is one of the fastest processes of manipulating the magnetic order. The results highlight the central role played by both the local SOC and the spin-conserving electronic hybridizations which control the time scale of the electronic hopping. For instance, the demagnetization time of the elemental spin sublattices depend on the SOC strengths of both the local and adjacent sublattices as well as on the spin-density flow between the sublattices mediated by the electron hopping (see the figure below). Understanding the microscopic mechanisms of ultrafast demagnetization, provides not only unique insights into the fundamental quantum mechanical processes occurring in itinerant magnetism, but also features a new generation of  magnetic storage devices which are orders of magnitude faster, smaller and in particular more energy efficient than the currently available ones.

Figure: Ultrafast magnetization dynamics in a binary alloy. The time dependence of the local spin polarizations S_z^A and S_z^B shows a comparable but time-delayed demagnetization of the different alloy components.

Reference: submitted to the Physical Review B