Magnetic tunnel junctions 90 to 300 nm wide and of aspect ratio ≈2 are studied using high-speed pulse fields with regard to the soft-layer magnetization reversal under thermal agitation. It is found that the larger cells, 200-300 nm wide, reverse through nonuniform magnetization states with the energy barriers to thermal activation an order of magnitude smaller than those expected for single-domain magnets. The single-domain limit is reached for the smallest cells, having elliptical soft layers approximately 90 nm wide and 150-200 nm long. The magnetization decay in the small cell limit is well described by the StonerWohlfarth single-domain model and the Arrhenius activation law. The results demonstrate that the penalty due to the smaller magnetic volume is compensated by a larger relative energy barrier to activation as the junction size is reduced to ∼ 90 nm. This determines the important length scale for geometric scaling of such technologies as magnetic random access memory. © 2006 IEEE.