To date, theoretical predictions and experimental observations have been based on a trilayer system (e.g., Co/Cu/Co) consisting of two ferromagnetic layers separated by a metallic layer. The two Co layers have different thicknesses so that one acts as a spin-polarizer while the other acts as a spin-analyzer or detector. The magnetization of the thicker layer is fixed, while that of the thinner one is free to rotate.

We have recently observed spin-transfer torque effects using a point-contact geometry (Ag) on a single ferromagnetic Co layer. The signatures of spin-wave excitations as revealed by peaks in the differential resistance dV/dI (Fig. 1a), have the same charactristics as those observed in trilayers and multilayers. The critical current for exciting spin waves depends linearly on the external magnetic field as shown in Fig. 1b, and this line demarkates the phase boundary for the onset of spin transfer torque effects

The reason for the unanticipated result is that there is a gradient in the current density when current is injected through a point contact. Because of the spin-transfer torque effects, the single ferromagnetic layer separates into "free" and "fixed" regions even though physically there is only one Co layer.

These results demonstrate that the spin-transfer torque effects can be explored and exploited without using lithography-intensive nanopillar structures. It also raises the questions whether similar single-layer effects also exist in trilayers and multilayers.

Fig. 2: Schematic picture of the Co layer separating into "free" and "fixed" regions due to the large gradient in the current density of the injected current from a tip.