Low-Energy, Ultrafast Spin Reorientation at Competing Hybrid Interfaces with Tunable Operating Temperature

Information can be stored in magnetic materials by encoding with the direction of the magnetic moment. A figure of merit for these systems is the energy needed to rewrite the information by changing the magnetic moment. Organic molecules offer a playground to manipulate spin order, with metallo-molecular interfaces being a promising direction for sustainable devices. Here, a spin reorientation transition is demonstrated in molecular interfaces of 3d ferromagnetic films due to a competition between a perpendicular magnetic anisotropy (PMA) induced by a heavy metal that dominates at high temperatures, and an in-plane anisotropy generated by molecular coupling at low temperatures. The transition can be tuned around room temperature by varying the ferromagnet thickness (1.4 - 1.9 nm) or the choice of molecular overlayer, with the organic molecules being C₆₀, hydrogen, and metal (Cu, Co) phthalocyanines. Near the transition temperature, the magnetisation easy axis can be switched with a small energy input, either electrically with a current density of 10⁵ A cm^-2, or optically by a fs laser pulse of fluence as low as 0.12 mJ cm^-2, suggesting heat assisted technology applications. Magnetic dichroism measurements point toward a phase transition at the organic interface being responsible for the spin reorientation transition.