Spin-dependent transport in Fe-doped carbon nanotubes

We report on a first principles calculation of spin-dependent quantum transport in Fe-doped single-wall carbon nanotube (CNT) junctions. For junctions of a pristine $(9,0)$ CNT in contact with Fe-doped $(9,0)$ CNT leads, the total current in parallel configuration of the moment is larger than that of antiparallel configuration under lower bias voltage. For higher bias voltages the opposite happens. A tunnel magnetoresistance ration as large as 40% is found at zero bias, it decays with bias, and eventually goes to negative values at larger bias. Similar results are obtained for junctions with pristine $(10,0)$, $(8,0)$ CNT in contact with Fe-doped $(10,0)$ or $(8,0)$ CNT leads. The spin-dependent transport features can be understood by analyzing microscopic details of the transmission coefficients.

Figure 1

(Color online) (a) Schematic plot of a CNT MTJ where a pristine $(9,0)$ single-wall CNT is connected with the left- and right-hand CNT leads. The CNT leads are themselves $(9,0)$ single-wall CNTs but they are doped with Fe atoms. (b) Unit cell cross sections of an $(8,0)$ CNT filled with four Fe atoms; $(9,0)$ CNT filled with six Fe atoms; and $(10,0)$ CNT filled with six Fe atoms.

Figure 2

(a) Spin currents $J_{\uparrow }$ (circles) and $J_{\downarrow }$ (squares) versus bias voltage in PC, for Fe-filled $(9,0)$ CNT MTJ. Inset: DOS of the Fe-filled $(9,0)$ lead. The upper curve represents the spin-up channel and the lower curve represents the spin-down channel. (b) Spin currents in APC.

Figure 3

(a) Total conductance of both spin-up and spin-down channels for Fe-doped (9,0) CNT MTJ versus bias voltage in PC (squares) and APC (circles). (b) TMR of the same device as a function of bias.

Figure 4

Transmission coefficient of Fe-doped (9,0) CNT MTJ projected to every eigenchannel, as a function of energy. Left-hand panels are (a,b,c) are for PC and right-hand panels (d,e,f) are for APC. Bias voltages are $0.05\mathrm{V}$ for (a,d); $0.27\mathrm{V}$ for (b,e); $0.49\mathrm{V}$ for (c,f). Solid line, dashed line, dotted line, dashed-dotted line, and solid-dotted line are for different eigenchannels, which are labeled using numerals 1,2, 3,4 in all panels. The upper halves of each panel are for eigenchannels of spin-up transmission, the lower halves are for spin-down transmission.

Figure 5

Contribution to spin-current from six dominating eigenchannels which contribute more than 98% of the total. The left-hand panels (a, b, c) are for PC, right-hand panels (d, e, f) are for APC. Bias voltages are $0.05\mathrm{V}$ for (a,d); $0.27\mathrm{V}$ for (b,e); $0.49\mathrm{V}$ for (c,f). Empty bars represent contributions to spin-up current; filled bars to spin-down current.

Figure 6

Spin currents $J_{\sigma }$ of $\mathrm{Fe}6$-doped $(10,0)$ CNT MTJ versus bias. (a) PC, (b) APC. Circles, $J_{\uparrow }$; squares, $J_{\downarrow }$. Insets in (a) and (b) are $J_{\sigma }$ for $\mathrm{Fe}4$-doped $(8,0)$ MTJ for PC and APC, respectively. In the insets, the solid line is for $J_{\uparrow }$; the dotted line is for $J_{\downarrow }$.

Figure 7

(a) Conductance of Fe-doped $(10,0)$ MTJ versus bias. Squares, PC; circles, APC. Inset of (a) plots conductance of Fe-doped $(8,0)$ MTJ versus bias, solid line for PC, dotted line for APC. (b) TMR of Fe-doped $(10,0)$ MTJ versus bias. Inset of (b) is TMR for Fe-doped $(8,0)$ MTJ.