Thermally activated switching in nanoscale magnetic tunnel junctions

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 Stoner–Wohlfarth 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. Index Terms—Magnetic memories, magnetization reversal, magnetoresistive devices, thermal activation. T HE magnetic tunnel junction (MTJ) is the primary candi-date for use in such large-scale technological applications as magnetic random access memory (MRAM) [1]. The storage element of the MTJ memory cell is a patterned layer of ferro-magnetic material, designed to have two stable magnetization states representing the “0 ” and “1 ” digital memory states, and is often referred to as the soft or free layer. The data are writte