Theoretical Investigation of the Interfaces and Mechanisms of Induced Spin Polarization of 1D Narrow Zigzag Graphene- and h-BN Nanoribbons on a SrO-Terminated LSMO(001) Surface

Abstract

The structure of the interfaces and the mechanisms of induced spin polarization of 1D infinite and finite narrow graphene-and h-BN zigzag nanoribbons placed on SrO-terminated La 1-x Sr x MnO 3 (LSMO) (001) surface were studied using Density Functional (DFT) electronic structure calculations. It was found that the p-conjugated nanofragments are bounded with LSMO(001) surface by weak disperse interactions. The types of coordination of the fragments, the strength of bonding and the rate of spin polarization depend upon the nature of the fragments. Infinite and finite graphene narrow zigzag nanoribbons are characterized by the lift of the spin degeneracy and strong spin-polarization caused by interface-induced structural asymmetry and oxygen-mediated indirect exchange interactions with Mn ions of LSMO support. Spin polarization changes the semiconducting nature of infinite graphene nanoribbons to half-metallic state with visible spin-up density of states at the Fermi level. The h-BN nanoribbon binding energy is weaker than graphene nanoribbon ones with noticeably shorter interlayer distance. The asymmetry effect and indirect exchange interactions cause spin polarization of h-BN nanoribbon as well with formation of embedded states inside the band gap. The results show a possibility to use one-atom thick nanofragments to design LSMO-based heterostructures for spintronic nanodevices with h-BN as an inert spacer to develop different potential barriers.Keywords: LSMO thin film, pentacene, spin states, induced spin polarization, graphene and h-BN nanoribbons, heterostructures, DFT+U calculations.* Author to whom correspondence should be addressed.The progress in developing of graphene-based spintronic nanodevices is related with weak spin-orbital interactions of carbon atoms, which enable one to utilize extremely high mobility of electrons together with long spin transport length. [1][2][3] The effectiveness of spin injection in carbon nanostructures by ferromagnetic supports (e.g. Ni(111)) has already been discovered experimentally and interpreted theoretically. 4,5 In particular, high magnetoresistance has been detected in p-conjugated organic media on ferromagnetic supports. 6-10 Isolating h-BN fragments located between graphene and ferromagnetic metallic supports have been used for chemical passivation of the surfaces and creation of tunnel contacts to promote graphene spin injection. [11][12][13] It was demonstrated that in 2D nanoheterostructures the injection coherence length could achieve up to 350 µm 14,15 due to interface-related Buchkov-Rashba effect. 16,17,18 Bychkov-Rashba effect is caused by the lift of spin degeneracy due to structure inverse asymmetry (SIA) 18 in 2D electron gas (2DEG) at the interfaces. 2DEG is confined by the effective electric field oriented in [001] direction caused by SIA effect, which, in many cases, is much stronger than the bulk symmetrical features. 19 For nonmagnetic surfaces, the Rashba field lifts the spin degeneracy of the surface states, 19 whereas for magnetic ones, it le...