Combining density-functional theory calculations and microscopic tight-binding models, we investigate theoretically the electronic and magnetic properties of individual substitutional transition-metal impurities (Mn and Fe) positioned in the vicinity
of the (110) surface of GaAs. For the case of the $[rm Mn^{2+}]^0$ plus acceptor-hole (h) complex, the results of a tight-binding model including explicitly the impurity $d$-electrons are in good agreement with approaches that treat the spin of the impurity as an effective classical vector. For the case of Fe, where both the neutral isoelectronic $[rm Fe^{3+}]^0$ and the ionized $[rm Fe^{2+}]^-$ states are relevant to address scanning tunneling microscopy (STM) experiments, the inclusion of $d$-orbitals is essential. We find that the in-gap electronic structure of Fe impurities is significantly modified by surface effects. For the neutral acceptor state $[{rm Fe}^{2+}, h]^0$, the magnetic-anisotropy dependence on the impurity sublayer resembles the case of $[{rm Mn}^{2+}, h]^0$. In contrast, for $[{rm Fe}^{3+}]^{0}$ electronic configuration the magnetic anisotropy behaves differently and it is considerably smaller. For this state we predict that it is possible to manipulate the Fe moment, e.g. by an external magnetic field, with detectable consequences in the local density of states probed by STM.
Using first-principles methods we study theoretically the properties of an individual ${Fe_4}$ single-molecule magnet (SMM) attached to metallic leads in a single-electron transistor geometry. We show that the conductive leads do not affect the spin
ordering and magnetic anisotropy of the neutral SMM. On the other hand, the leads have a strong effect on the anisotropy of the charged states of the molecule, which are probed in Coulomb blockade transport. Furthermore, we demonstrate that an external electric potential, modeling a gate electrode, can be used to manipulate the magnetic properties of the system. For a charged molecule, by localizing the extra charge with the gate voltage closer to the magnetic core, the anisotropy magnitude and spin ordering converges to the values found for the isolated ${Fe_4}$ SMM. We compare these findings with the results of recent quantum transport experiments in three-terminal devices.
