What Determines the Sign Reversal of Magnetoresistance in a Molecular Tunnel Junction?

Abstract

The observations of both positive and negative signs in tunneling magnetoresistance (TMR) for the same organic spin-valve structure have baffled researchers working in organic spintronics. In this article, we provide an answer to this puzzle by exploring the role of metal-molecule interface on TMR in a single molecular spin-valve junction. A planar organic molecule sandwiched between two nickel electrodes is used to build a prototypical spin-valve junction. A parameter-free, single-particle Green's function approach in conjunction with a posteriori, spin-unrestricted density functional theory involving a hybrid orbital-dependent functional is used to calculate the spin-polarized current. The effect of external bias is explicitly included to investigate the spin-valve behavior. Our calculations show that only a small change in the interfacial distance at the metal-molecule junction can alter the sign of the TMR from a positive to a negative value. By changing the interfacial distance by 3%, the number of participating eigenchannels as well as their orbital characteristics changes for the antiparallel configuration, leading to the sign reversal in TMR.

Voltage polarity manipulation of the magnetoresistance sign in organic spin valve devices

“…Moreover, the electrode-molecule distance is critical for this effect, meaning that thermal fluctuation can easily smear out this effect. 25 This mechanism cannot explain our results.…”

“…25 In our case, the electric field is $0.03 V/nm at 1 V, which is too small to induce an observable change. Moreover, the electrode-molecule distance is critical for this effect, meaning that thermal fluctuation can easily smear out this effect.…”

Interface effects on tunneling magnetoresistance in organic spintronics with flexible amine–Au links

“…In the case of molecules bonded directly to the magnetic leads, the magnetic proximity effect 35 for molecules next to the magnet would lead to spin polarization of the molecule, which in turn would be extremely sensitive to the metal-molecule interface. …”

“…35 Furthermore, the predicted TMR is in general quite large compared to that computed for benzene-dithiol-Ni junctions. 36,37 Comparing the transmission across all 8 structures, we see that in fact, the spin up transmission in the P configuration lies in a narrow range, with standard deviation of 0.6, which is one order of magnitude smaller than that of spin down transmission, i.e.…”

Electrical tuning of spin current in a boron nitride nanotube quantum dot

“…For example, using molecular quantum dot, researchers have already demonstrated giant magnetoresistance effect in molecular tunnel (MT) devices. 3,6,7 However, the strong sensitivity of magnetoresistance to the junction structure 21 and the difficulty in achieving atomic level controls at the interface make the implementation of the MT device an arduous task. Other promising nano-structures being investigated as spin-carriers are carbon nanotube quantum dots (CNTQD) coupled to ferromagnetic electrodes.…”