Spin transfer and current-induced switching in a ferromagnetic single-electron transistor

We propose a theoretical model of current-induced magnetization (CIMS) switching in a ferromagnetic single electron tunneling (FM-SET) transistor. The CIMS effect arises from the transfer of spin angular momentum from the net spin accumulation Delta S on the island electrode to the local magnetic moments via s-d exchange coupling. Based on the single-domain model, we derive an analytical expression for the critical spin accumulation Delta S-sw on the island for CIMS, and calculate the M-Delta S hysteresis curves which represent the effect of Delta S on the island moments. This magnetization response is then related to the charge and spin transport model in the SET transistor. We extend the Korotkov scheme spin-dependent "orthodox" theory of single charge tunneling, by linking the transport I-V and Delta S-V characteristics to the M-Delta S hysteresis. We thus determine Delta S as a function of external bias or current and hence obtain the switching current density j(sw) for CIMS. For a typical spin polarization P=60% of the source electrode, j(sw) is calculated to be of the order of 10(5) A/cm(2), and this falls to just similar to 3x10(4) A/cm(2) when a near half-metal (P=90%) is used. This value is several orders of magnitude smaller than j(sw) observed in multilayer and magnetic tunnel junction structures. The SET transistor is an ideal device for the CIMS effect since (i) a small amount of moments (on the island) need to be switched to generate a large change in conduction and (ii) the island electrode being isolated from the rest of the circuit by spin-dependent tunnel barriers effectively confines the spin accumulation Delta S in the vicinity of these moments.