Detecting Spin-Polarized Currents in Ballistic Nanostructures

We demonstrate a mesoscopic spin polarizer/analyzer system that allows the spin polarization of current from a quantum point contact in a large in-plane magnetic field to be measured. A transverse electron focusing geometry is used to couple current from an emitter point contact into a collector point contact. At large in-plane fields, with the point contacts biased to transmit only a single spin ($g<e^2/h$), the voltage across the collector depends on the spin polarization of the current incident on it. Spin polarizations of $>70\%$ are found for both emitter and collector at $300\mathrm{m}\mathrm{K}$ and $7\mathrm{T}$ in-plane field.