A serial magnetic mass storage device and associated data storage method of the kind in which data is encoded in single magnetic domains in nanowires. In the invention, the nanowires (10) are provided with a large number of notches (12) along their length to form domain wall pinning sites. Moreover, the notches are addressed in groups (A, B, C) by heating electrodes. By alternately heating the notches hosting head-to-head and tail-to-tail domain walls (16, 18) in synchrony with alignment and anti-alignment of an operating field (H) along the nanowire the magnetic domains (14) are moved along the nanowire by alternate movement of the head-to-head and tail-to-tail domain walls in caterpillar or worm-like motion in which the domains are incrementally lengthened and shortened by one inter-notch distance as they move along the nanowires under the joint coordinated action of the heating and alternating operating field. From an interconnect and fabrication standpoint, the scheme can be scaled almost without restriction out of the plane of the substrate to provide hundreds or thousands of stacked layers of nanowires, thus allowing very dense three- dimensional networks of stored information to be realised.