40nm, that of the switching magnet 2.5nm and the non-magnetic spacer is 6nm thick. The materials for the two magnets and the spacer are cobalt and copper, respectively, which are those most commonly used. The junction cross section is oval-shaped with dimensions 60nm$\times$130nm. A small diameter is necessary so that the torque due to the Oersted field generated by a charge current of $10^{7}$-$10^8$ A/cm$^2$, required for current-induced switching, is much smaller than the spin-transfer torque we are interested in.

The aim of most experiments is to determine the orientation of the switching magnet moment as a function of the current (applied bias) in the junction. Sudden jumps of the magnetisation direction, [*i.e.*]{} current-induced switching, are of particular interest. The orientation of the switching magnet moment ${\bf m}$ relative to that of the polarising magnet ${\bf p}$, which is fixed, is determined by measuring the resistance of the junction. Because of the GMR effect, the resistance of the junction is higher when the magnetisations of the two magnets are anti-parallel than when they are parallel, In other words, what is observed are hysteresis loops of resistance versus current. A typical experimental hysteresis loop of this type [@16] is reproduced in 