  All-optical spin switching (AOS) represents a new frontier in magnetic
storage technology -- spin manipulation without a magnetic field, -- but its
underlying working principle is not well understood. Many AOS ferrimagnets such
as GdFeCo are amorphous and renders the high-level first-principles study
unfeasible. The crystalline half-metallic Heusler Mn$_2$RuGa presents an
opportunity. Here we carry out hitherto the comprehensive density functional
investigation into the material properties of Mn$_2$RuGa, and introduce two
concepts - the spin anchor site and the optical active site - as two pillars
for AOS in ferrimagnets. In Mn$_2$RuGa, Mn$(4a)$ serves as the spin anchor
site, whose band structure is below the Fermi level and has a strong spin
moment, while Mn$(4c)$ is the optical active site whose band crosses the Fermi
level. Our magneto-optical Kerr spectrum and band structure calculation jointly
reveal that the delicate competition between the Ru-$4d$ and Ga-$4p$ states is
responsible for the creation of these two sites. These two sites found here not
only present a unified picture for both Mn$_2$RuGa and GdFeCo, but also open
the door for the future applications. Specifically, we propose a
Mn$_2$Ru$_x$Ga-based magnetic tunnel junction where a single laser pulse can
control magnetoresistance.
