Effects Of Bipolarons On Spin Polarized Transport In Magnetic Permeated Sublayer Of Organic Spin Devices

In 2002, Dediu’s group firstly reported spin injection and transport in a sandwich nanostructure ’La0.7Sr0.3MnO3/T6/La0.7Sr0.3MnO3’.It has been found that spin injected in the organic layer, and the current is spin polarized. In recent years, organic spintronics as a very potential research area of condensed physics has attracted a lot of interests. The studies of spintronics include investigation of spin injection, transport and detection in electronic materials, as well as spin manipulation. Compared with conventional semiconductors, soft organic semiconductors (OSCs) have an opportunity to form a good interface with ferromagnetic metal (FM) or half-metal contacts, reducing the probability of spin scattering at the interface. The spin relaxation time is much longer than that of conventional semiconductors due to the weak spin-orbital and hyperfine interactions. Otherwise, the carriers in OSCs are some "quasi-particles" such as polarons and bipolarons. They have more complex spin-charge relation, which will result in more abundant properties of an organic spin device.As reported in experiments, magnetic atoms(Co) will penetrate into the soft organic layer and form a magnetic permeated sublayer (MPS) in an organic spin device, such as "Co/OSC/La0.7Sr0.3MnO3". Zhang Y B et al. has studied the effect of the MPS on the spin-polarized transport and the magnetoresistance of the device. However, they neglected the effect of bipolarons on transport in the MPS. Transition between spin polarons and spinless bipolarons is an important factor that affects the relaxation of spin in OSC, and the other factor is the spin-flipping of spin polarons. Spinless bipolarons have not contribution to the spin polarization, but they will influence the distribution of polarons, further affect the spin-polarized transport. In this paper, based on the drift-diffusion equation, combining with the characteristics of MPS, we investigate the effect of bipolarons on the spin-polarized transport in the MPS. The results are as follows:(1) Consider the effects of mobilities of carriers on spin-polarized transport. Mobility of polaron and bipolarons in the MPS will reduce due to the scattering of the Co atoms. It is found that, with the increase of mobility of carriers, the position that the density of bipolarons reaches saturation will increase, which will be helpful to the spin transport, due to the transition between polarons and bipolarons. Moreover, we find that small mobility of bipolarons in the MPS will result in quick spin relaxation when the mobility of polarons is constant.(2) Consider the effect of the splitting of spin-flip time. Due to the spin related scattering, the spin flip-time of polarons that carries different spin will be different to each other. It is found that bigger splitting of spin-flip time will result in the slower decay of the spin polarization, which is similer with there is no bipolarons. Further, we find that the splitting of spin-flip time is the principal element which cause the spin relaxation, and the polarons-bipolarons interaction is the secondary reason.(3)Consider the effect of the intensity of transition between polaron and bipolaron. It is found that the large intensity of transition between polaron and bipolaron is helpful to the spin-polarized transport. We also consider the effect of thickness of the MPS, the results suggest that the large thickness of MPS is helpful to spin polarized transport, which is similar with there is no bipolaron.