Long-distance coupling between magnetic centers in ultrafast spin transfer: Dynamic Goodenough-Kanamori-Anderson rules

Using state-of-the-art quantum chemistry, we present a microscopic mechanism for ultrafast laser-induced long-range spin transfer in carbon chains to which two magnetic atoms (Ni) are attached. Here, the Ni atoms function as spin carriers, while the carbons provide the delocalized conjugated {p}_z -character orbitals to bridge them. First, we establish that the key to the whole process is the virtual excitations between the localized Ni atomic orbitals, and the conjugated orbitals, through which the two Ni atoms can indirectly couple to each other. Second, the efficiency of dynamic interaction depends on the orbital phases of carbon atoms bonded to Ni, with longer-distance transfers benefiting from favorable degeneracies and phase alignments. Third, even when conjugated orbitals interact with only one Ni atom, their coherent superposition facilitates long-distance spin coupling. Fourth, the different structural characters between and configurations result in distinct spin-transfer behavior. Fifth, the spin-transfer speed and efficiency are tunable based on the position of magnetic centers. We name this set of interaction mechanisms dynamical Goodenough-Kanamori-Anderson (GKA) rules, because it resembles the well-known static GKA rules, in which the nature of the indirect metal-to-metal coupling (i.e., ferromagnetic versus antiferromagnetic) is dictated by the occupancy of the degenerate {p}_z orbitals of the bridging oxygen. Unlike in the static case, the spin density here is dynamically transferred between two Ni atoms via well-designed laser pulses.