  Magnetic anisotropy and magnetic exchange interactions are crucial parameters
that characterize the hybrid metal-organic interface, key component of an
organic spintronic device. We show that the incorporation of 4$f$ RE atoms to
hybrid metal-organic interfaces of CuPc/REAu$_2$ type (RE= Gd, Ho) constitutes
a feasible approach towards on-demand magnetic properties and functionalities.
The GdAu$_2$ and HoAu$_2$ substrates differ in their magnetic anisotropy
behavior. Remarkably, the HoAu$_2$ surface boosts the inherent out-of-plane
anisotropy of CuPc, owing to the match between the anisotropy axis of substrate
and molecule. Furthermore, the presence of RE atoms leads to a spontaneous
antiferromagnetic (AFM) exchange coupling at the interface, induced by the
3$d$-4$f$ superexchange interaction between the unpaired 3$d$ electron of CuPc
and the 4$f$ electrons of the RE atoms. We show that 4$f$ RE atoms with
unquenched quantum orbital momentum ($L$), as it is the case of Ho, induce an
anisotropic interfacial exchange coupling.
