of the angular dependence of $T_{\perp}$ and $T_{\parallel}$ so that $g_{\perp}$ and $g_{\parallel}$ may be regarded as constant parameters for the given structure. In the next section we use Eqs. (\[torque2\]) for the spin-transfer torque in a phenomenological theory of current-induced switching of magnetisation. This phenomenological treatment enables us to understand most of the available experimental data. It is more usual in experimental works to relate spin-transfer torque to current rather than bias. However in theoretical work, based on the Landauer or Keldysh approach, bias is more natural. In practice the resistance of the system considered is rather constant (the GMR ratio is only a few percent) so that bias and current are in a constant ratio.

Phenomenological treatment of current-induced switching of magnetisation {#tre}
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In this section we explore the consequences of the spin-transfer torque acting on a switching magnet using a phenomenological Landau Lifshitz equation with Gilbert damping (LLG equation). This is essentially a generalisation of the approach used originally by Slonczewski [@3] and Sun [@sun]. We assume that there is a polarising magnet whose magnetisation is pinned in the $xz$-plane in the direction of a unit vector ${\bf p}$, which is at general fixed angle $\theta$ 