Study On Carrier Characteristics In Organic Ferromagnets

Organic spintronics,as an emerging research field,has received extensive attention from physics and chemical scientists.Organic materials have many advantages in spintronics.On the one hand,the organic material is composed of C,H,O,N and S atoms,and the nucleus spin of the C atom is zero.Compared with inorganic materials,the spin-orbit and hyperfine interactions in organic materials are weak.And the spin-relaxation time is long,which is advantaged for the spin transport and storage.On the other hand,organic materials have "softness" characteristic,which is easy to attached to a metal electrode and facilitate electron spin injection.The "softness" also reduces the interface barrier,and modifies the interfacial spin polarization.Furthermore,the electron-lattice interaction in organic materials is strong,where the injected electrons(holes)exist in localized states,and form various nonlinear excitations,such as solitons,polarons,and bipolarons.And the forms of spin transport are also more abundant than inorganic materials.Therefore,the injection,transport and spin functionality of carriers in organic polymers have been investigated extensively in organic spin devices.At present,the organic spin devices can be divided into two categories: one is the ferromagnetic/organic semiconductor/ferromagnetic sandwich structure device,where the spin injection,transport and magnetoresistance from the ferromagnetic metal to the organic semiconductor have been studied.The second is a spin valve structure based on an organic ferromagnets(OFs)as the intermediate layer,which utilizes itself magnetic properties of the organic magnetic molecule to achieve spin functionality.OFs are usually fabricated artificially.The design of organic devices with the utilization of OFs has triggered lots of interests,since they combine both the organic and ferromagnetic properties.However,most previous pioneer studies on electron transport through OFs were based on the neutral ground state of isolated OFs.And the study on the nonlinear carrier properties is lack,which is necessary for understanding the electron transport of OFs.Based on the Anderson+SSH(Su-Schrieffer-Heeger)model,we theoretically investigatedthe polaron characteristics of OFs.And the interfacial spin polarization and charge spin transfer characteristic are researched when the OFs are attached to a metal electrode.The specific research contents of this paper are as follows:1.Based on the Anderson model including the electron-lattice interaction and the electron-electron interaction term,we investigated theoretically the ground state of the poly-BIPO and the spin-charge characteristic of the polaron state doping an electron.The results show that,the organic ferromagnet lattice dimerizes in the ground state.And the spin density wave exists in the main chain,and the side radicals spins is the ferromagnetic state.In the presence of an additional electron,distortions of spin and charge densities are observed both in the main chain and radicals.A polaron-like lattice distortion confined with fractional charges and spins be formed in the main chain.And a spin-charge disparity of the polaron is observed.In addition,the electron-electron interaction intensity,on-site energy of radicals and the electron hopping integral between the main chain and the side radicals are also further discussed.2.Based on the Anderson model including the electron-lattice and electron-electron interaction terms,the ground-state properties of the metal/poly-BIPO heterojunction were investigated.Here both nonmagnetic and ferromagnetic metals are considered.The results reveal a picture of electron injection and the induced excitations in the OFs,which is divided into two regimes with the increase of injected electrons: the first solitonlike interfacial state and the subsequent coexistence of the interfacial state and polaron states.In the case of the ferromagnetic metal,the effect of the magnetization orientation alignment between the metal and the organic ferromagnet is discussed,where the lowest injection barrier is found when the magnetization orientations of the two components are parallel.An injection-induced degradation of the magnetism of the organic ferromagnet is also obtained.