Search for magnetoresistance in excess of 1000% in Ni point contacts: Density functional calculations

Electronic transport in nickel magnetic point contacts is investigated with a combination of density functional theory and the nonequilibrium Green’s function method in the linear response limit. In particular, we address the possibility of huge ballistic magnetoresistance in impurity-free point contacts and the effects of oxygen impurities. On-site corrections over the local spin density approximation for the exchange and correlation potential, namely, the $\mathrm{LDA}+U$ method, are applied in order to account for low coordination and strong correlations. We show that impurity-free point contacts present magnetoresistance never in excess of 50%. This value can raise up to about 450% in the case of oxygen contamination. These results suggest that magnetoresistance in excess of 1000% cannot have solely electronic origin.

Figure 1

(Color online) Ball and stick diagram of a nickel point contact formed by two pyramidal tips joined by a single Ni atom. The 11 atoms included in the box correspond to the tip atoms (three atoms forming the atomic chain and four atoms on either side forming the apexes). $d$ is the distance between the two outermost planes.

Figure 2

(Color online) Ball and stick representation of the relaxed atomic arrangement of a nickel point contact with one oxygen atom bridging the gap between the two tips. Color code: gray (blue) Ni; black (red) O.

Figure 3

(Color online) PDOS for impurity-free MPC as calculated with the LSDA: (a) entire simulation cell, (b) 11 atoms in the central region (apexes plus atomic chain), (c) apex atoms, and (d) the 3 Ni atoms forming the monoatomic chain.

Figure 4

(Color online) Transmission coefficients as a function of energy for Ni point contacts formed by a single-atom chain as calculated with the LSDA: (a) parallel and (b) antiparallel magnetic configurations.

Figure 5

(Color online) Schematic representation of the atomic orbitals responsible for the electronic structure of a Ni monoatomic wire. The $d_{x^2\text{−}{y}^2}$ and $d_{xy}$ orbitals are perpendicular to the axis of the wire and therefore weakly coupled.

Figure 6

(Color online) Band structures of a one-dimensional nickel chain (lattice spacing of $2.24\mathrm{\AA }$). (a) LSDA, (b) ASIC, and (c) $\mathrm{LDA}+U$ with $U=6\mathrm{eV}$ and $J=1\mathrm{eV}$. The panels to the left (right) correspond to majority (minority) spins.

Figure 7

(Color online) Transmission coefficients as a function of energy using $\mathrm{LDA}+U$ for Ni MPC. (a) Parallel and (b) antiparallel alignments of the magnetic moments of the electrodes. The values of $U$ and $J$ are 6 and $1\mathrm{eV}$, respectively.

Figure 8

(Color online) Band structures (left panel) and projected density of states (right panel) for an infinite one-dimensional NiO wire in the antiferromagnetic configuration calculated with the LSDA. In this case the two-spin subbands are degenerate.

Figure 9

(Color online) Band structures and projected density of states for an infinite one-dimensional NiO wire in the ferromagnetic configuration calculated with the LSDA. The left (right) panels are for the minority (majority) spins.

Figure 10

(Color online) Zero-bias transmission coefficients of oxygen contaminated Ni point contacts calculated with the LSDA: (a) parallel and (b) antiparallel alignments. In the PA the solid (dashed) line represents the majority (minority) spins, while the two spins are degenerate in the AA.

Figure 11

(Color online) Band structures and projected density of states for an infinite one-dimensional NiO wire in the ferromagnetic configuration calculated with the $\mathrm{LDA}+U$. The left (right) panels are for the minority (majority) spins. The values of $U$ and $J$ are set to 6 and $1\mathrm{eV}$, respectively.

Figure 12

(Color online) Zero-bias transmission coefficients of oxygen contaminated Ni point contacts calculated with the $\mathrm{LDA}+U$: (a) parallel and (b) antiparallel alignments. In the PA the solid (dashed) line represents the majority (minority) spins, while the two spins are degenerate in the AA.