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

Abstract

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 ΔS 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 ΔSsw on the island for CIMS, and calculate the M-ΔS hysteresis curves which represent the effect of ΔS 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 ΔS-V characteristics to the M-ΔS hysteresis. We thus determine ΔS as a function of external bias or current and hence obtain the switching current density jsw for CIMS. For a typical spin polarization P=60% of the source electrode, jsw is calculated to be of the order of 105A/cm2, and this falls to just ∼3×104A/cm2 when a near half-metal (P=90%) is used. This value is several orders of magnitude smaller than jsw 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 ΔS in the vicinity of these moments. © 2005 The American Physical Society.