Giant magnetoresistance due to orbital-symmetry mismatch in transition metal benzene sandwich molecules

Achieving large magnetoresistance (MR) effects in atomic-scale junctions is of great importance in the field of molecular spintronics. In this work, using ab initio quantum transport calculations, we report on the spin transport properties of molecular junctions made of single transition metal benzene sandwich molecules $\left(\mathrm{TM}{\mathrm{Bz}}_2\right)$ bridged by ferromagnetic electrodes. We find that relative magnetic orientations of two electrodes and the molecule can dramatically affect low-bias transport properties of the junction. While the $\mathrm{TM}{\mathrm{Bz}}_2$ are insulating in the gas phase, their corresponding molecular junctions bridged between the two electrodes are half-metallic minority-spin conductors (i.e., almost 100% spin polarization) and display very large MR. More interestingly, even larger molecular MR, corresponding to a reversal of molecular spin moment, are found due to the spin-valve behavior of the molecule controlled by the magnetic impurity. Such a particular performance is attributed to strong spin-selective hybridization of electronic states at the molecule/metal interface due to orbital-symmetry mismatch between the electrode's and molecular states. We also discuss the importance of the shape of electrodes on spin transport efficiency.

Figure 1

DFT energy level diagram in the gas phase for a $\mathrm{Co}{\mathrm{Bz}}_2$ molecule with a symmetric sandwich structure. Spin-up and spin-down components are plotted in black and red lines, respectively. The HOMO and LUMO for both spin channels are depicted together with their relative isosurfaces.

Figure 2

Ni chain/$\mathrm{Co}{\mathrm{Bz}}_2$ junctions: Spin-polarized transmission spectra for $\mathrm{Co}{\mathrm{Bz}}_2$ single-molecule bridging the two semi-infinite 1D chains as shown on the inset in the top panels with three different spin configurations, namely $\uparrow \uparrow \uparrow$ (a), $\uparrow \downarrow \uparrow$ (b), and $\uparrow \uparrow \downarrow$ (c). Black and red lines denote spin-up and spin-down channels, respectively.

Figure 3

$\mathrm{Ni}\left(111\right)/\mathrm{Co}{\mathrm{Bz}}_2$ junctions: Spin-polarized transmission spectra for $\mathrm{Co}{\mathrm{Bz}}_2$ bridging two Ni(111) electrodes as shown on the top panel: (a) $\uparrow \uparrow \uparrow$, (b) $\uparrow \downarrow \uparrow$, and (c) $\uparrow \uparrow \downarrow$ spin configurations. The distance between a Ni apex atom and the Co atom is 4.0 Å. Black and red lines denote spin-up and spin-down channels, respectively.

Figure 4

Spin-up PDOS on Ni apex atoms (a) and molecule (b) for a $\mathrm{Ni}\left(111\right)/\mathrm{Co}{\mathrm{Bz}}_2$ junction in $\uparrow \downarrow \uparrow$ configuration as shown in Fig. 3. A sharp peak, originating from Co-$d_{xz,yz}$ and Ni-$d_{xz,yz}$, is found just above the Fermi energy.

Figure 5

Adatom$+\mathrm{Ni}\left(111\right)/\mathrm{Co}{\mathrm{Bz}}_2$ junctions: The same as Fig. 3, but with Ni(111) electrodes plus one outside atom connecting to the apex.

Figure 6

The same as Fig. 2 but with perpendicular molecular connections.

Figure 7

Comparison between $QuantumEspresso\left(QE\right)$ and $Siesta$ results. Calculated spin-dependent PDOS on Co by $QE$ (top) and $Siesta$ (bottom) for the $\mathrm{Ni}\left(111\right)/\mathrm{Co}{\mathrm{Bz}}_2$ junction with $\uparrow \uparrow \uparrow$ spin configuration. A good qualitative agreement is found between both sets of results, ensuring the accuracy of the basis sets parameters used in $Siesta$.

Figure 8

Projected DOS of Co in Ni chain/$\mathrm{Co}{\mathrm{Bz}}_2$ junction with $\uparrow \uparrow \uparrow$ calculated by DFT (top) and $\mathrm{DFT}+U$ (bottom).

Figure 9

Molecular orbital localization of spin-down LUMO and $\mathrm{LUMO}+1$ in the gas phase of $\mathrm{Co}{\mathrm{Bz}}_2$ in the $XY$ plane. These molecular orbitals will be orthogonal, by symmetry, to the $s$ electrons of Ni electrodes.