Materials Design And Devices Preparation Of Organic Ferroelectric Spin Valve

The developments of spintronics materials and devices have a significant impact on the progress of society.The manipulation of spin opens a new dimension of electrons.The technology of generation and detection of electron spins has made tremendous progress.Organic materials have longer spin relaxation times due to the small spin-orbit coupling and weak hyperfine interactions.The properties of ferroelectric materials could be manipulated by electric fields.The introduction of ferroelectric materials into spin valves achieves multiple configurations of device performance because the performance of devices could be controlled by both magnetic and electric fields.This is very important for the development of next generation electronic devices with device size down to nanoscale and faster speed.The computational materials science has attracted more and more attentions in recent years.The design and improvement materials by theoretical calculations are in favour of environmentally friendly development.As device sizes further reduce,the study of magnetic coupling at the nanoscale interface is critical for device miniaturization.Although the magnetic properties of materials controlled by the ferroelectric materials have been greatly investigated,the magnetoresistance(MR)of the device controlled by the organic ferroelectric material was only in the theoretical stage and the experimental study is very rare.Thus,the fabrication and investigation of organic ferroelectric spin valves controlling by the extra electric field was interesting and important for the practical application.Magnetic coupling in multilayer films was important to the devices performance.Especially,at the nanoscale,quantum effects will be significant and dominate some magnetic properties.The MR of Polyvinylidene Difluoride(PVDF)spin valve could be controlled by the polarization direction of PVDF in previous calculations.So the electric field can be used to control the device performance of the organic spin valves.Here,the PVDF films were prepared by the Langmuir-Blodgett(LB)method and the properties were studied.The PVDF films were found to be ferroelectric ?-phase.Then,spin valves were prepared using PVDF as the spin transportant layer.The MR of PVDF spin valves is systematically studied and the spin transportant mechanism was discussed.Since the electrode has an important influence on the performance of spin valves and the MnGa material has a potential application in the spin transfer torque.Thus,the magnetic couplings between L10-MnGa and different thickness of Co or Fe are studied by first-principles calculations based on density functional theory.It is found that the quantum effect has an important influence on the magnetic coupling.The main research results are summarized as follow:1.A high-quality PVDF film was prepared by LB method.The infrared spectra and ferroelectric analysis showed that the films were ferroelectric ?-phase.The MR of PVDF spin valve was obtained over 2%at room temperature.This proved the feasibility using PVDF as the spacer layer to prepare the spin valves.2.It was found that the MR of PVDF spin valves decreases with increasing both the organic layer thickness and measurement temperature.The increase of the temperature and the organic layer thickness would increase the spin scattering of the interface,leading to reduce the MR.As the PVDF thickness increases,the resistance of the device increases nonlinearly.With the PVDF thickness increase,the mechanism of spins transport changed from quantum tunneling to hopping between molecules.3.The surface energies of L10-MnGa(001)and interface energies of L10-MnGa/Co(F e)with different terminal were calculated by first-principles calculations.It was found that the surface and interface with Ga terminal was more steady.The magnetic coupling between different thicknesses of Co(Fe)and L10-MnGa was calculated.The Co and Fe have different coupling states with L10-MnGa.Fe and Llo-MnGa was ferromagnetic coupling,but the coupling strength was oscillation with increasing the thickness of Fe layer.Interestingly,the coupling between Co and L10-MnGa changed from ferromagnetic and antiferromagnetic alternately as the layer thickness of Co increases.4.The period of magnetic coupling oscillation is consistent with the oscillation of magnetic crystal anisotropic energy in the Co film.Through the energy bands,quantum well states(QWS)energies and Fermi surface cross sections analysis,it is confirmed that the oscillation of magnetic coupling does not originate from the spin down ?5 and ?2 bands of Co(Fe).By further analyzing the Co(Fe)Fermi surface cross sections and the L10-MnGa/Co(Fe)band diagram,it is determined that the QWS formed from the electrons at X point result in L10-MnGa/Co(Fe)magnetic coupling oscillations.Last,one-dimensional finite square potential well models were proposed to explain the different numbers of band for different states Co or Fe film at X point.