Film Growth And Magnetic Characterization For Metal-free Phthalocyanine Spin Valve

Organic spintronics is an emerging research field based on the broader field of spintronics and organic semiconductors(OSCs).The addition of spin freedom in spintronics greatly enriched the research content of microelectronics,because these new devices utilized not only electron charge transport,but also carrier spin transport.OSCs have been extensively explored,mainly because their high structural flexibility,low cost,and large area production.In particular,OSCs are expected to feature longer spin relaxation time owing to weak spin-orbit coupling strength and weak hyperfine interaction,which contribute to the development of next generation nano-electronic devices.In the last decade,the magnetoresistance effects(MR)including giant magetoresistance(GMR)and tunneling magnetoresistance,have been observed in organic spin valves(OSVs).Significant progress has been made in understanding spin-dependent transport phenomenon,such as spin injection,tunneling,diffusion and detection in organic spintronics.However,to date,organic spintronic materials with high efficiency to prepare organic spin valve devices for commercial applications are still lacking.In this work,we first studied the magnetic properties of thin film electrodes including Fe,Co and CoFe,and investigated the growth character for the ferromagnetic(FM)film electrodes.Then,we studied the stepped-like hysteresis loops of spin valve structures.Fe,Co and their alloys possess small spin polarization but high Curie temperature compared to those fully spin-polarized materials.Together with convenient preparation methods,they are more suitable as spin valve electrodes for future commercial applications.We also found that the organic semiconductor films used in organic spintronic devices are mainly amorphous structure.Spin carriers transport among disordered molecules would limit the spin diffusion length and spin life time.The energy barrier and defects caused by disorder structure in the organic layer can cause a spin flip,which reduces the transport efficiency of spin carriers.Furthermore,the conductance mismatch problem between organic layer and ferromagnetic electrode interface would also lead to a significant spin scattering.Therefore,optimization of the growth condition of the organic layer is necessary to improve device performance.The influences of substrate temperature for the growth of H2Pc film layer were studied.The H2Pc films grown on different substrates were also investigated.The research results are summarized as follow:(1)We have successfully fabricated Co,Fe and CoFe magnetic thin film electrodes,and the effects of growth conditions(including growth pressure and sputtering power)on the in-plane coercivity of the FM films were investigated.The FM/H2Pc/FM sandwich structure were successfully prepared and a stepped hysteresis loop of the sandwich structure was obtained at room temperature,suggesting that the heterostructure is a spin valve.A 1%GMR signal was obtained at low temperature,which is one of the rarely reported magnetoresistance effect based on phthalocyanine compounds.It was also found that the MR ratio showed strong temperature dependence,which is likely due to the flip of spin carriers in H2Pc.(2)In order to improve the above device performance,the growths of H2Pc films with high orientation are explored.The growth of H2Pc films on glass substrate was optimized by controlling substrate temperature.It was found that the H2Pc films remains ? phase and show a preferred texture along(200)orientation as the substrate temperature increased.The UV-vis absorption spectra showed that the Q absorption band with the peak at 632 nm red-shifted with increasing substrate temperatures,indicating that the conjugated degree of ? electrons is enhanced.In addition,the Al/H2Pc/Al structures(H2Pc deposited with different substrate temperatures)were prepared for electrical characterization.The obtained nonlinear I-V curves showed strong temperature dependence,indicating that carriers'transportation relies on thermal activation in H2Pc films.It was found that the thermal activation energy decreased with increasing substrate temperatures,further confirming that the films become more ordered.(3)H2Pc films were fabricated on both organic semiconductor(PTCDA)and ferromagnetic metal substrates.It was found that the organic semiconductor PTCDA template layer can change the crystallographic orientation of H2Pc molecules,showing the molecules become lying flat along the substrate.Meanwhile,the absorption constant of H2Pc films grown on PTCDA substrate was increased by 1.6 times than that grown on glass,which may be related to the closer molecular arrangement.This change in optical properties of H2Pc film could be more conducive to the application of optoelectronic devices.Then,H2Pc were also grown on cobalt substrate.H2Pc molecules usually absorbed onto the Co surface by strong chemisorption because of a strong interaction at the interface.The interaction changed the electronic states of the first monolayer of H2Pc films,resulting in H2Pc molecules tend to lying flat.For the cobalt substrate,the saturation magnetization reduced by a factor of 7%after capped by a H2Pc layer.Our analysis showed that the interface coupling between FM and H2Pc layers plays a dominant role for the magnetic change,which may have a influence on spin injection efficiency.