to the $z$-axis as shown in Fig. \[fig1\].

![Schematic picture of a magnetic layer structure for current-induced switching (magnetic layers are darker, non-magnetic layers lighter).[]{data-label="fig1"}](Fig1.eps){width="11cm"}

The pinning of the magnetisation of the polarising magnet can be due to its large coercivity (thick magnet) or a strong uniaxial anisotropy. The role of the polarising magnet is to produce a stream of spin-polarised electrons, [*i.e.*]{} spin current, that is going to exert a torque on the magnetisation of the switching magnet whose magnetisation lies in the general direction of a unit vector ${\bf
m}$. The orientation of the vector ${\bf m}$ is defined by the polar angles $\alpha$, $\phi$ shown in Fig. \[fig1\]. There is a non-magnetic metallic layer inserted between the two magnets whose role is merely to separate magnetically the two magnetic layers and allow a strong charge current to pass. The total thickness of the whole trilayer sandwiched between two non-magnetic leads must be smaller than the spin diffusion length $l_{\rm sf}$ so that there are no spin flips due to impurities or spin-orbit coupling. A typical junction in which current-induced switching is studied experimentally [@albert] is shown schematically in Fig. \[fig2\]. The thickness of the polarising magnet is 