Ballistic conductance and magnetism in short tip suspended Ni nanowires

Electronic and transport properties of a short Ni nanowire suspended between two semi-infinite ferromagnetic Ni leads are explored in the framework of density-functional theory. The spin-dependent ballistic conductance of the nanowire is calculated using a scattering-based approach and the Landauer-Büttiker formula. The total calculated conductance in units of $G_0=2e^2∕h$ is around 1.6, in fairly good agreement with the broad peak observed around $\sim 1.5$ for the last conductance step in the break junctions. Separating contributions from different spins, we find nearly $0.5G_0$ from the majority spin $s$-like channel, whereas the remaining minority spin conductance of $1.1G_0$ contains significant contributions from several $d$ states, but much less than $0.5G_0$ from $s$ states. The influence of the structural relaxation on the magnetic properties and the ballistic conductance of the nanowire is also studied.

Figure 1

Supercell used for simulating a three-atom Ni wire suspended between two semi-infinite $\mathrm{Ni}\left(001\right)$ leads.

Figure 2

The $xy$ integrated magnetization as a function of $z$ for the tip suspended three-atom Ni wire in unrelaxed (a) and relaxed (b) configurations. Magnetic moments (in the Bohr magneton) for some atoms are also given.

Figure 3

Transmission vs energy for the minority spin channel of the (repeated) Ni nanowire in a relaxed configuration, calculated at different ${\mathbf{k}}_{\perp }$. Selected ${\mathbf{k}}_{\perp }$ values are shown in the insets. The numerical value of transmission at the Fermi energy—representative of ballistic conductance at zero voltage—is also given for each curve. The transmission function obtained by averaging over the full set of ten special ${\mathbf{k}}_{\perp }$ points, closest to the single nanowire conductance, is plotted as the dashed curve on the lowest panel.

Figure 4

Supercell band structure for the majority spin of the relaxed three-atom Ni wire calculated along the $z$ direction at the $\overline{\mathrm{M}}$ point of the 2D BZ. Each electron state can be labeled by a certain $\mid m\mid$, where $m$ is the projection of the angular momentum on the nanowire axis (see text). The circles mark the states of $m=0$ symmetry having the charge on the wire larger than some threshold value (see text) while the states of $\mid m\mid =1$ and $\mid m\mid =2$ symmetry are not presented since they are irrelevant for electron transport. For each $m=0$ state, the charge on the contact and central atoms of the wire is given by vertical lines on the LDOS panel. The electron transmission of the $m=0$ channel is also shown.

Figure 5

The same as in Fig. 4 but for the minority spin channel. In addition to $m=0$ states (a), the corresponding data for the states of $\mid m\mid =1$ (b) and of $\mid m\mid =2$ (c) symmetry (which now contribute to the transmission around the Fermi energy) are also shown.

Figure 6

Spin-dependent transmission function of the Ni nanowire in the fully relaxed configuration (upper panel). The lower panels display the symmetry decomposition of the transmission function and of the LDOS at the mid-nanowire atom.

Figure 7

The same as in Fig. 6 but for the Ni nanowire in an unrelaxed configuration.