Magnetoresistance of Co nanoconstrictions fabricated by means of electron beam lithography

Co nanowires of T-shaped geometry with and without nanoconstrictions are fabricated by means of electron beam lithography (EBL) followed by electron beam evaporation. We have succeeded in minimizing the width of the nanoconstriction, i.e., the nanocontact, down to 6 nm. In a subsequent second EBL process, gold contact leads are attached as close as possible to the nanoconstrictions, which allows us to measure the magnetoresistance (MR) of the sample as a function of the nanocontact width. We found that the MR of the T-shaped nanowires without a nanoconstriction can exclusively and quantitatively be explained by the anisotropic magnetoresistance of the Co nanowires. This allows us to separate the MR contribution of the nanoconstriction from the total MR of the sample quantitatively. We find that the resistance of the nanoconstriction is independent of the width of the nanocontact, whereas the corresponding MR contributions fluctuate depending on the various shapes of the nanoconstriction.

Figure 1

(Color online) Contact geometry for the four-terminal measurement of the resistance of Co nanowires with T-shaped geometry. The inset shows the vicinity of the nanoconstriction.

Figure 2

SEM images of typical nanoconstrictions of different shapes and different nanocontact widths. The width of the nanocontact is determined by the number and the configuration of grains at the junction.

Figure 3

(Color online) MFM image of T-shaped nanowires with a nanoconstriction after presaturation in a field of $B=2\text{T}$. In remanence, clear dark and bright spots occur at the wire ends and at the nanoconstriction.

Figure 4

(Color online) Magnetoresistance of a nanoconstriction with a nanocontact width of $w=12\text{nm}$ (left) and $w=6\text{nm}$ (right) at $T=4.2\text{K}$. The procedure of the measurement is indicated by arrows. An image of the nanoconstriction as obtained by SEM is showed in the inset. The magnetic field is oriented in plane, which is perpendicular to the comblike structure.

Figure 5

(Color online) Magnetoresistance of Co nanowires with T-shaped geometry at $T=4.2\text{K}$. For the sweep from the negative to the positive direction of the magnetic field (red curve), the local direction of the magnetization and the current direction are illustrated by diagrams.

Figure 6

(Color online) Resistance of the nanoconstriction as a function of the nanocontact width. The dashed line represents a constant resistance of $R_{\text{NC}}=255\Omega$. The inset indicates the nanoconstriction (black) and the nanowire sections between the voltage-sensing leads (gray).

Figure 7

(Color online) MR effect at the nanoconstriction as a function of the nanocontact width.