has never been observed, although $g_{\parallel}<0$ can occur in principle and is predicted theoretically [@13] for the Co/Cu/Co(111) system with a switching magnet consisting of a single atomic plane of Co. It follows from Eq. (\[crbias\]) that $|v_{{\rm P}\rightarrow{\rm AP}}/v_{{\rm AP}\rightarrow{\rm
    P}}|=|g_{\parallel}(\pi)/g_{\parallel}(0)|$ in zero external field. Experimentally this ratio, essentially the same as the ratio of critical currents, may be considerably less than 1 ([*e.g.*]{} $<0.5$ [@albert]), greater than 1 ([*e.g.*]{} $\approx 2$ [@19]) or close to 1 [@16]. Usually the field dependence of the critical current is found to be stronger than that predicted by Eq. (\[crbias\]) [@albert; @16].

We now discuss the reversible behaviour shown in Fig. \[fig5\](b) which occurs for $|h_{\rm ext}|>1$. The transition from hysteretic to reversible behaviour at a critical external field seems to have been first seen in pillar structures by Katine [*et al.*]{} [@21]. Curves similar to the lower one in Fig. \[fig5\](b) are reported with $|v_{{\rm P}\rightarrow{\rm AP}}|$ increasing with increasing $h_{\rm ext}$, as expected from Eq. (\[crbias\]). Plots of the differential resistance ${\rm d}V/{\rm d}I$ show a peak near the point of maximum gradient of the dashed curve. Similar behaviour has been reported by several groups [@22; @23; @24]. It is 