Spin Selected Electron Transport In Single-molecule Magnet Tunnel Junctions

Single molecule magnets (SMM) are a class of molecules exhibiting magnetic prop-erties similar to those observed bulk magnets, but of molecular origin. The first of these molecules was discovered 17 years ago, and then a series of multinuclear transition-metal complexes have been reported acting as SMMs. These special molecular mate-rials have been proposed as potential candidates for several technological application, such as molecular spintronics circuit and memory devices. By the development of re-sent experiment technical, it is possible for attach magnetic molecules between two gold surface which can described as a metal-SMM-metal junction(NM/SMM/NM). Further more, with the advent of spin-polarized scanning tunneling microscopy(SP-STM), the structure composed by only one magnetic molecule sandwiched between ferromagnetic electrodes (FM/SMM/FM junction) is achieved and detect different tun-neling electron current correspond to the magnetic molecule's different spin-states. Owing to these experiments, SMMs seem to be suitable base for memory cell of future information storage and processing technology. And electrons transport through SMMs is also become one of the central themes which is relation with a series of interesting physics phenomena, such as negative differential conductance (NDC) or magnetic re-versal in magnetic molecule.It is already well know that when a spin polarized current flows through a mag-net, spin moment carried from current can affect the magnet, leading to the interesting current-driven domain wall motion phenomena in experiment and can be welly ex-plained by spin transfer torque. By contraries, when a none polarized electron current flows through a magnet, the spin moment from magnet can also react to the current, which we can use ferromagnetic electrodes for produce polarized electron current. To a single magnet molecule, it can exhibit magnetic properties in low temperature regime due to its bistability energy structure. We discussed that for this special energy structure, the SMM can also act like bulk magnet to produce polarized current from NM/SMM/NM junction. In a certain bias window, electron current polarization is extremely high and can reach satiable plateau. The result clearly show that electrons transport through the LUMO level on molecule in low bias regime has been strictly selected by the SMM's spin state. By another aspect, the spin polarized current can also react on the SMM's spin state. A typical effect is the well-known spin-transfer torque (STT), which has been recently used to probe the spin state of the nanoscale sin-gle magnetic atom or SMM via the spin-polarized scanning tunneling microscope and an inelastic electron tunneling spectroscopy. The driving polarized electronic current is generated by an external bias voltage exerted across the magnetic tip and surface. Owing to the spin exchange coupling between conductive electrons and the magnetic particle, the spin orientation of these magnetic particles can be rotated to be parallel or antiparallel to the tip spin direction.This thesis is composed of two parts as follows:(1) We investigate the elastic electron tunneling process in NM/SMM/NM junctions . Owing to the advancement of recent experiment and unique bistable spectrum of the single-molecular-magnets, we propose to fabricate a SMM connect on a normal metal electrode which can extract a 100% spin polarized current and inject into another normal electrode. Both spin up and spin down electrons can be injected into the output lead without applying spin-bias and external magnetic field, only by changing the gate voltage. We find that present device works well and is a good candidate for spin-filter's scheme in the future spintronic circuits.Instead of applied external bias voltage, we have studied the spin transport through an SMM (such as Mn12-ac) connected to two normal metal electrode with a small temperature bias at low temperatures. It is found that there exists a pure spin current and it can be increased/decreased by the external magnetic field. The 100% spin-polarized currents can be always generated in a suitable window of?0, no matter whether the magnetic field and/or U are present. The present spin generator needs neither bias voltage nor magnetic electrodes, and it can be controlled by fully electrical manner. The present device is a good candidate for novel future spintronic devices, e. g., the thermally driven spin generator.(2)We investigate the inelastic electron tunneling process in a FM/SMM/NM junc-tion. It is shown that the orientation of the SMM's spin in the junction can be manipu-lated by the temperature bias/gate voltage to be parallel or antiparallel to the magnetiza-tion of the FM lead. Such a manipulation is driven by a temperature difference between the two leads, and needs neither external magnetic field nor bias voltage exerted across the junction. The reversal of the SMM's spin arises from the thermal spin-transfer torque effect, and it is realized via a set of transitions between the SMM's states, which must satisfy the basic selection rules,?m= 1/2 and?n= 1. The M vs?0 curve exhibits a hysteresis loop with bistable magnetic states, and the sign reversal of M cor-responds to certain local maximal thermally driven charge current. The SMMs with the gate voltage controlled bistable magnetic states are expected to be new information units in the future.