  Exploring giant magnetic anisotropy in small magnetic nanostructures is of
both fundamental interest and technological merit for information storage. To
prevent spin flipping at room temperature due to thermal fluctuation, large
magnetic anisotropy energy (MAE) over 50 meV in magnetic nanostructure is
desired for practical applications. We chose one of the smallest magnetic
nanostructures-Ir2 dimer, to investigate its magnetic properties and explore
possible approach to engineer the magnetic anisotropy. Through systematic
first-principles calculations, we found that the Ir2 dimer already possesses
giant MAE of 77 meV. We proposed an effective way to enhance the MAE of the Ir2
dimer to 223~294 meV by simply attaching a halogen atom at one end of the Ir-Ir
bond. The underlying mechanism for the record high MAE is attributed to the
modification of the energy diagram of the Ir2 dimer by the additional
halogen-Ir bonding, which alters the spin-orbit coupling Hamiltonian and hence
the magnetic anisotropy. Our strategy can be generalized to design other
magnetic molecules or clusters with giant magnetic anisotropy.
