Emergence of high spin polarization of conductance in atomic-size Co-Au contacts

We present a first-principles study of spin-dependent electron transport through gold nanochains suspended between cobalt electrodes aimed at elucidating the electronic origin of the high magnetoresistance recently observed experimentally in such nanojunctions. Our nonequilibrium Green's function based calculations confirm the occurrence of high spin polarization of conductance in Co/Au/Co nanocontacts, which in some cases reaches 90%. Our analysis allows to relate such behavior to the hybridization of electronic orbitals of neighboring cobalt and gold atoms of the nanocontact. The results obtained give clear evidence of the presence of spin injection from Co electrodes into paramagnetic gold contacts.

Figure 1

(a) Geometry of the nanocontact used in calculations: an Au wire between Co electrodes, side view. (b) Unit cell geometry of the electrode, top view. (c) Dependence of relaxed interlayer/interatomic distances $h_i$ on the contact on the distance between electrodes $d$.

Figure 2

(a) Conductance of the nanocontact for different inter-electrode distances. The inset shows the spin polarization of conductance at the Fermi level. (b)–(d) Minority $s$ and $d$ projected densities of states for Co, tip Au, and central Au atoms for different distances between electrodes. Filled areas: $s$ states, thick curves: $d_{z^2}$ states. $s$-states curves for different distances are color- and hatching-coded (see the legend for details).

Figure 3

(a) Projected density of $d_{z^2}$ states of Co atoms in contact with the Au wire (dashed line) and $d_{z^2}$ states of the same atoms in absence of the Au chain (solid line). (b) $s$ states of tip Au atom and (c) $s$ states of central Au atom for different distances between electrodes. Positions of peaks around the fermi level are highlighted by vertical black lines. The splitting of the band/levels is shown by horizontal arrows.

Figure 4

(a) Sketch of the nanojunction structure used for magnetoresistance calculations: Co/Au nanocontact attached to Cu(111) bulk leads. (b) Two spin alignments of electrodes used for magnetoresistance calculations.

Figure 5

Comparison of the spin-polarized conductance of a system with semi-infinite Co electrodes (dashed blue line) and our model system for MR calculations—Co slabs supported on Cu (solid red line).

Figure 6

Total conductance for parallel and antiparallel spin orientations of cobalt electrodes.

Figure 7

Conductance of systems with varying geometry and chemical composition/stoichiometry. Sketches of considered configurations are shown in the top part of the figure (atom colors the same as in Fig. 1). Corresponding conductance curves can be found in the graph in the bottom part.