Electron Transport via Single Molecule Magnets with Magnetic Anisotropy

Single molecule magnets (SMMs) are molecules of mesoscopic scale which exhibit quantum properties such as quantum tunneling of magnetization, quantum interference, spin filtering effects, strong spin-phonon coupling and strong hyperfine Stark effects. These effects allow applications of SMMs to high-density information storage, molecular spintronics, and quantum information science. Therefore, SMMs are of interest to physicists, chemists, and engineers. Recently, experimental fabrication of individual SMMs within transistor set-ups have been achieved, offering a new method to examine magnetic properties of individual SMMs. In this thesis, two types of SMMs, specifically Eu2(C8H8)3 and Ni9Te6(PEt3)8, are theoretically investigated by simulating their electron transport properties within three-terminal transistor set-ups.

An extended metal atom chain (EMAC) consists of a string of metallic atoms with organic ligands surrounding the string. EMACs are an important research field for nanoelectronics. Homometallic iron-based EMACs are especially attractive due to the high spin and large magnetic anisotropy of iron(II). We explore the exchange coupling of iron atoms in two EMACs: [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2].

Chapter 1 provides an introduction to SMMs, electron transport experiments via SMMs and an introduction to density functional theory (DFT).

Chapter 2 presents a theoretical study of electron transport via Eu2(C8H8)3. This type of molecule is interesting since its magnetic anisotropy type changes with oxidation state. The unique magnetic properties lead to spin blockade effects at zero and low bias. In other words, the current through this molecule is completely suppressed until the bias voltage exceeds a certain value.

Chapter 3 discusses a theoretical study of electron transport via Ni9Te6(PEt3)8. The magnetic anisotropy of this magnetic cluster has cubic symmetry, which is higher than most SMMs. With appropriate magnetic anisotropy parameters, in the presence of an external magnetic field, uncommon phenomena such as low-bias blockade effects, negative conductance and discontinuous conductance lines, are observed. In Chapter 2 and 3 DFT-calculated magnetic anisotropy parameters are used and electron transport properties are calculated by solving master equations at low temperature.

Chapter 4 examines the exchange coupling between iron ions in EMACs [Fe2(mes)2(dpa)2] and [Fe4(tpda)3Cl2]. The exchange coupling constants are calculated by using the least-squares fitting method, based on the DFT-calculated energies from different spin configurations.