چكيده به لاتين
Diluted magnetic semiconductors abbreviated as DMS bridge the two fascinating fields of semiconductors and magnetism. A diluted magnetic semiconductor is a non-magnetic semiconductor doped with a small amount of magnetic impurity. Low magnetic ion concentration leads to new spin functionalities while keeping the host semiconducting properties. This allows the use of conventional semiconductor technology as a compatible underlying technology for spintronic applications. Magnetic measurements have also revealed ferromagnetism in organic semiconductors. The analysis of these materials showed magnetic impurity in a very dilute limit. Based on the content of magnetic impurity and the similarities with inorganic magnetic semiconductors, the new class of diluted organic magnetic semiconductors (DOMS) is introduced. A clear understanding of the origin and mechanism of magnetic interactions is fundamental for successful synthesis of a high temperature ferromagnetic semiconductor for practical applications. In this regard the structural, electronic and magnetic properties of ZnO-based and some organic diluted magnetic semiconductors are studied using relativistic density functional theory.
Apart from magnetic impurities, point defects such as vacancies can induce magnetism in semiconductors. The interactions between magnetic impurity and point defects in the form of transition metal-vacancy near-neighbour pair are investigated in the two ZnO-based DMSs, ZnO:(Co,vacancy) and ZnO:(Mn,vacancy) by the scalar relativistic approach. The GGA+U method is also considered to take into account the electron correlations. Our magnetic calculations revealed that the cation vacancy preserves its triplet spin state and couples ferromagnetically with cobalt magnetic impurity in ZnO:(Co,vacancy). However, magnetic properties of ZnO:(Mn,vacancy) showed sensitivity to small changes in the position of the atoms as well as the magnitude of Coulomb repulsion parameter U. While the effective coupling between manganese and vacancy magnetic moments is ferromagnetic in unrelaxed structures, the structural relaxation leads to an effective antiferromagnetic coupling.
In organic semiconductors, the magnetic impurity can be incorporated in or adsorbed on the organic material. Binding energy calculations showed that the 3d transition metals (Sc-Cu) are strongly bound to phthalocyanine to form transition metal-phthalocynine (TMPc) molecules. Regarding the practical importance of magnetic anisotropy and orbital magnetism, an orbital polarization correction to the GGA exchange-correlation functional is applied for the first time on the TMPc molecules. The good agreement between the obtained results for FePc and the available experimental data indicates the ability of this correction to reproduce the experimental results. A large in-plane orbital moment (M=1.38𝜇B) corresponding to a high magnetic anisotropy energy of 51.5 meV is calculated in the full relativistic approach with OP correction for VPc. In the case of organic semiconductors with magnetic adatoms, Fe and Co atoms adsorbed on coronene molecule were investigated. Both of the complexes show a spin transition from low to high spin states by changing the adatom-coronene distance. The GGA calculations indicate a strong covalent binding with a low-spin state for both of the Fe-coronene and Co-coronene complexes. However, a van der Waals-like interaction with high-spin state is observed by inclusion of Coulomb repulsion between 3d electrons in terms of Hubbard U parameter. The adsorption of 3d transition metals on the two organic semiconductors, olympicene and corannulene are also studied. Ni-olymicene with a slightly curved strusture is the most stable among 3d TM-olympecene structures. Moreover, Fe showed the largest magnetic moment of 0.18 𝜇B in the TM-olympicene family. In the case of bowl-shaped corannulene, all of the 3d transition metals are found to bind preferably on the convex surface. Furthermore, all TM-corannulene structures are magnetized except Ni-corannulene.
