Spin-polarization of organic molecules at the ferromagnetic surface

Study on organic/ferromagnetic interface is helpful for understanding the effects of magnetoresistance in organic spin-valve, because one of the reasons of leading to this phenomenon is due to the spin injection at the interface. However, the interactions at the organic/ferromagnetic interface are complicated and full of possibilities, and the effects are still under debate till now. One possible cause is that the adsorption of organic molecules on the ferromagnetic surface is random, which leads to various adsorbing configurations. Therefore, in this paper we select some typical adsorbing configurations of benzene/Co system to reveal the effect of spin-polarization of organic molecules at the ferromagnetic surface by using first-principles calculations. It is obtained that the spin degenerated electronic states of benzene molecule will be broken due to the coupling between the 3d electrons of Co atoms and the 2p electrons of benzene molecule. The density of states at the Fermi level becomes spin related and a spin polarization appears in the benzene molecule. For both of the configurations T1T2 and T1H12, from the projected density of states we can find that the majority-spin electrons of the benzene molecule is oriented in opposition to the direction of the ferromagnetic electrode at the Fermi level, which means that the organic molecules filter and reverse the original spin direction of the injected electrons from the ferromagnetic electrode. As mentioned above, the adsorbing configurations are different, so we consider three kinds of configurations with different adsorbing distances for further studying the spin polarization at the interface. On the basis of the configuration T1T2, distances of 2.0 angstrom, 2.2 angstrom and 2.4 angstrom are studied, where 2.0 angstrom is the equilibrium position we obtained with full relaxation. It should be noted that we do not relax the geometric structure of the system in this part of study. It is found that the spin polarization is sensitively dependent on the distance between benzene and Co surface. The spin-polarization near the Fermi level even changes its direction from positive to negative with the increase of the distance in such a small range. Our studies reflect the complexity of organic molecule/ferromagnetic electrode interfaces, and enrich the understanding of this field.