Very Large Magnetoresistance in Lateral Ferromagnetic (Ga,Mn)As Wires with Nanoconstrictions

We have fabricated (Ga,Mn)As nanostructures in which domain walls can be pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can switch the regions on either side of the constriction to either parallel or antiparallel magnetization. All samples exhibit a positive magnetoresistance, consistent with domain-wall trapping. For metallic samples, we find a magnetoresistance up to $8\%$, which can be understood from spin accumulation. In samples where, due to depletion at the constriction, a tunnel barrier is formed, we observe a magnetoresistance of up to $2000\%$.

Figure 1

False-color SEM picture (side view) of a double constriction showing part of the outer wires with the voltage leads. Note the resist that is still present on the wire. The insets show the relative magnetization of the parts (left) and the resulting schematic MR trace for sweep-up (solid line) and sweep-down (dashed line).

Figure 2

MR of a representative sample as fabricated (a) and after further etching (b). The inset in (a) shows the MR of a wire without constrictions, whereas the inset in (b) shows that the MR is reduced exponentially with bias voltage.

Figure 3

(b) MR in a sample with tunnel barriers at the constrictions. The top panel (a) depicts schematically the tunneling events taking place in the sample.