We proposed a single-molecule magnetic junction (SMMJ), composed of a dissociated amine-ended benzene sandwiched between two Co tip-like nanowires. To better simulate the break junction technique for real SMMJs, the first-principles calculation associated with the hard-hard coupling between a amine-linker and Co tip-atom is carried out for SMMJs with mechanical strain and under an external bias. We predict an anomalous magnetoresistance (MR) effect, including strain-induced sign reversal and bias-induced enhancement of the MR value, which is in sharp contrast to the normal MR effect in conventional magnetic tunnel junctions. The underlying mechanism is the interplay between four spin-polarized currents in parallel and anti-parallel magnetic configurations, originated from the pronounced spin-up transmission feature in the parallel case and spiky transmission peaks in other three spin-polarized channels. These intriguing findings may open a new arena in which magnetotransport and hard-hard coupling are closely coupled in SMMJs and can be dually controlled either via mechanical strain or by an external bias.

The first-principles calculation with the nonequilibrium Green's function formalism is employed to comprehensively demonstrate that the mechanical strain and anchoring group are two crucial impacts on spin transport in single-molecule magnetic junctions. For the dissociated amineended benzene contacted to cobalt electrodes, we present the strain-enhanced spin injection efficiency, including sign reversal and nearly perfect spin injection under junction stretching process. The underlying mechanism is the strain-assisted movement of pronounced and broad spin-up transmission feature toward the Fermi energy. This intriguing finding reveals the superior spin transfer in amine-ended single-molecule magnetic junction, which is in sharp contrast to the better charge transfer between gold electrodes in traditional thiol-ended molecular junctions. Our calculation results may pave the way for promising tunability of spin injection efficiency under mechanical stimulus of break junction technique.

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