| Spintronics, mainly focusing on how to utilize spin degree of freedom as the ultimate carrier of information, has achieved great success and tremendous development since the late 80s of last century. Organic spintronics is a branch of spintronics developed in the direction of new material and also a combination of two important research fields: organic electronics and spin electronics. Compared to traditional inorganic materials, Organic materials have several significant features and advantages:first, due to the weak van der Waals interactions between molecules, electron wave functions are mainly localized within a single molecule and barely overlap. The mechanism of charge-carrier transport is dominated by hopping and therefore, organic semiconductors (OSCs) present very low mobility in comparison with inorganic semiconductors, in which the band transport mechanism dominates. For example, the mobility of amorphous tris(8-hydroxyquinoline) aluminum (Alq3) is on the order of ～10-5 cm2V-1s-1 at room temperature, compared with 450 cm2V-1s-1 for p-type Si. Second, owing to the constituents of light elements (such as C, H, N, and O), and C12 possessing zero nuclear spin moment, organic materials are characterized by weak spin orbital coupling strength and hyperfine interaction. This makes the spin relaxation time extremely long and OSCs suitable as the spin transport materials. Finally, the type of organic material is very rich and various functional groups can construct a variety of organic molecules. In addition, the preparation of organic thin films is relatively simpler compared to inorganic materials, we can use spin coating, low-temperature thermal evaporation to prepare organic thin films with high quality. In the meantime, organic material is flexible, can be bent and twisted, such as curved OLED display. Organic spintronics introducing the spin degree of freedom into the organic system exploit special properties of organic materials and combine developed organic electronics with spin electronics. All of above makes organic materials great candidate for spintronics applications.We first briefly introduce the troditional inorganic spintronics. There are four widely accepted spin relaxation mechanisms in inorganic systems, including the Elliott-Yafet mechanism, the D’yakonov-Perel’mechanism, the Bir-Aronov-Pikus mechanism, and the hyperfine interaction mechanism. Then, we review the development and some breakthroughs in the field of organic spintronics in the past ten years. We also summarize the current research progress of spin relaxation mechanisms in organic spin valves, including the hyperfine interaction and spin-orbit coupling, and emphasize some unresolved issues in this topic. Finally, we discuss some fundamental open questions in the field of organic spintronics, such as magnetoresistance temperature dependence, magnetoresistance bias dependence, experimental demonstration of the spin-orbit coupling spin relaxation mechanism of transport carriers, manipulation of spin relaxation strength on carriers in organic materials and so on. My research mainly focuses on some of the above problems.This thesis concerns on the role of organic molecules in spin relaxation and its manipulation.First, we optimized the fabrication of bottom LSMO electrodes. Single crystal LSMO films were deposited using PLD system, and then annealed in flowing pure oxygen gas at atmosphere pressure. The Curie temperature (Tc) of LSMO increased from 320 K to 360 K and atomic flat surface was achieved. The measured MR of Alq3 organic spin valve can be as high as 2.2% at room temperature when the optimal annealed LSMO was used. Then, we fabricated Alq3-based organic spin valves (OSVs) using as-grown La0.67Sr0.33MnO3 (LSMO) with Tc of 320 K, annealed LSMO with Tc of 360 K, and La0.67Ca0.33MnO3 with Tc of 250 K as the bottom electrode and found that below 100 K, the temperature dependence of normalized MR was almost identical with these three electrodes despite the huge difference in Curie temperature (from 250 K to 360 K). Concerning the identical resistance temperature dependence of the three different organic spin valves below 100 K, we attributed this similar MR temperature dependence to the spin relaxation in Alq3.It is generally believed that spin-orbit coupling (SOC) strength and the associated spin relaxation can be enhanced by introducing heavy metal ions in organic semiconductors. Here, in order to study the role of heavy metal ions on spin transport in organic semiconductors, we fabricated OSVs using small molecules of tris(2-phenylpyridine)iridium [Ir(ppy)3] and Alq3, which have similar chemical structures, but one with light metal ion Al3+(Z= 13) and the other with heavy metal ion Ir3+(Z= 77). The SOC strength of Ir(ppy)3 is expected to be much stronger than that of Alq3 due to the much heavier Ir3+ ion. As expected, the photoluminescence spectroscopy measurements show that the SOC strength in Ir(ppy)3 is several orders of magnitude larger than in Alq3. Surprisingly, the spin diffusion length in Ir(ppy)3, deduced from magnetoresistance measurements in Ir(ppy)3-based organic spin valves, is longer than in Alq3. Considering the lower carrier mobility in Ir(ppy)3, the spin relaxation time in Ir(ppy)3 is much longer than in Alq3, implying that the SOC strength in Ir(ppy)3 is weaker than in Alq3. The seemingly contradictory results of photoluminescence spectroscopy and magneto-transport can be explained by the SOC strength depending on the electronic states of a material. In the organic complex, the ligand field is generally very strong, Ir(ppy)3 is a 5d transition metal octahedral complexes which leads to the d orbitals splitting into a doubly degenerate eg level with higher energy and a triply degenerate t2g level with lower energy. Heavy metal complexes are in completely different electronic states when emitting light and transporting carriers. In the light emitting process, photon comes from the transition of exciton which is assigned to the metal-to-ligand-charge-transfer (MLCT) state in Ir(ppy)3, a bound state of an electron localizes on the ligand and a hole from the 5d orbital of the Ir4+ ion. Therefore, the Ir ion participates in the light emission and the exciton transforms Ir3+ into Ir4+ in the MLCT state. Because Ir4+ is in d5 configuration, 5 electrons are filled in the t2g level with one uncompensated spin, i.e., non-zero spin and orbital moment. Accordingly, the SOC strength is increased due to the heavy Ir atom and plays a significant role in the light emission process. Therefore, the Ir ion enhanced SOC strength can be observed in the phenomena with light emission. While during carrier transport, heavy metal ion is in a ground state with zero orbital and zero spin angular momentum, SOC on spin carriers transporting through π orbitals is almost zero, the spin relaxation is weak.Finally, we studied the effect of defect states on electrical transport and magnetic transport of OSVs. Trap states can be introduced into organic films by molecule doping and will strongly affect the current-voltage I-V characteristics of organic electronic devices because of the relatively low density of intrinsic thermal activated free charge carriers in organic solids. We fabricated ZnPc doped Alq3 organic spin valves (OSVs) to study the trap effects on spin transport in OSVs. The electrical transport was firstly investigated. Memory effect was successfully realized in this device. The waiting time in every molecule taken by carriers can be written as:π= 1/ωij ∠exp [(εj-εi)/kBT]. By comparing the total time taken by charge carriers to transport through the middle organic layer, we thought that: the resistance of the device when carriers go through trap states is much larger than that when carriers hop around a trap filled with like repulsive charges. Due to the spin transport and charge transport are closely related, spin-dependent effects were further studied. Lower magnetoresistance ratio was measured when device in higher resistance state. This phenomenon could be qualitatively explained as follows. Carriers will spend more time on each trap and precess under the magnetic field induced by the hyperfine interaction, thus spin relaxation effect becomes stronger and magnetoresistance ratio smaller. By applying external voltage bias, the resistance of the organic spin valve device can be switched between high and low states, thus realizing the manipulation of spin relaxation strength on carriers in organic films. |
