  The long spin-diffusion length, spin-lifetimes and excellent optical
absorption coefficient of graphene provide an excellent platform for building
opto-electronic devices as well as spin-based logic in a nanometer regime. In
this study, by employing density functional theory and its time-dependent
version, we provide a detailed analysis of how the size and shape of graphene
nanoflakes can be used to alter their magnetic structure and optical
properties. As the edges of zigzag graphene nanoribbons are known to align
anti-ferromagnetically and armchair nanoribbons are typically non-magnetic, a
combination of both in a nanoflake geometry can be used to optimize the
ground-state magnetic structure and tailor the exchange coupling decisive for
ferro- or anti-ferromagnetic edge magnetism, thereby offering the possibility
to optimize the external fields needed to switch magnetic ordering. Most
importantly, we show that the magnetic state alters the optical response of the
flake leading to the possibility of opto-spintronic applications.
