Nanospintronics with Molecular Magnets - Tunneling and Spin-Electric Coupling

Sammanfattning: This dissertation investigates theoretically electric control of the magnetic properties of molecular magnets. Two classes of magnetic molecules are considered. The first class consists of molecules that are spin frustrated. As a consequence of the frustration, the ground-state manifold of these molecules is characterized by states of different spin chirality, which can be coupled by an external electric field. Electric control of these spin states can be used to encode and manipulate quantum information. The second class comprises molecules known as single-molecule magnets, which are characterized by a high spin and a large magnetic anisotropy. Here the main goal is to control and manipulate the magnetic properties, such as the anisotropy barriers, by adding and subtracting individual electrons, as achieved in tunneling transport. Papers I, II and III deal with spin-electric coupling in spin frustrated molecules. Spin density functional theory is used to evaluate the parameters that control the strength of this coupling. Paper I reports the electronic and magnetic properties of the triangular antiferromagnet Cu3. It is found that an external electric field couples to the spin chirality of the system. The strength of this coupling is large enough to allow efficient spin-electric manipulation with electric fields generated by a scanning tunneling microscope. Paper II investigates the zero-field splitting in the ground-state manifold of the triangular Cu3 molecular magnet caused by the Dzyaloshinskii-Moriya (DM) interaction. It employs a Hubbard model approach to elucidate the connection between the spin-orbit and the DM interaction. It is shown that the DM interaction constant D can be expressed in terms of the microscopic Hubbard-model parameters, which are calculated by first principles methods. Paper III investigates systematically the spin-electric coupling in several triangular molecular magnets, such as V3 and Cu3O, and its dependence on different types of magnetic atoms, distances between magnetic centers and exchange paths between magnetic atoms. A generalization of the spin-electric coupling for a V15 molecular magnet, comprising fifteen magnetic centers, is also reported in this paper. In Paper IV first-principles methods are employed to study theoretically the properties of an individual Fe4 single-molecule magnet attached to metallic leads in a single-electron transistor geometry. It is demonstrated that an external electric potential, modeling a gate electrode, can be used to manipulate the magnetic properties of the system by adding or subtracting electrons to the molecule. In Paper V quantum transport via a triangular molecular magnet such as Cu3 is investigated. It is proposed that Coulomb-blockade transport experiments can be used to determine the spin-electric coupling strength in triangular molecular magnets. The theoretical analysis, based on a Hubbard model, is supported by master-equation calculations of quantum transport in the cotunneling regime.