Magnetic-field-assisted spin-transfer switching in nonlocal spin valves*

Nonlocal spin valve (NLSV) is a spin injection and detection device consisting of a nonmagnetic metal N connected to ferromagnetic spin injector F$_{1}$ and detector F$_{2}$. We fabricate NLSV devices, using 100 nm thick Cu as N, 10 -- 20 nm thick Co as F$_{1}$, and 3 nm thick Co as F$_{2}$. The widths of the Cu and Co wires are $\sim $ 150 nm. To ensure the electrical continuity of the F$_{2}$ electrode, a 5 nm thick Cu layer is placed underneath. The center-to-center separations between F$_{1}$ and F$_{2}$ are 200 - 350 nm. The nonlocal spin signals at 4.2 K are 2 - 4 milliohms. The spin-transfer switching has been achieved with the assistance of a magnetic field. The F$_{1}$ and F$_{2}$ electrodes are set in an anti-parallel configuration, and the magnetic field is set to a value smaller but close to the switching field of F$_{2}$. A d.c. current pulse with appropriate polarity is injected through F$_{1}$ to induce the spin-transfer. A small d.c. current ($<$ 0.5 mA) is sufficient to switch F$_{2}$ into being parallel with F$_{1}$. Analysis has been done to rule out possible artifacts due to Oersted fields.