This memory concept gets around the size issue because the dimensions of the ferromagnetic dots and their distance from the Josephson junctions do not compromise the operation of the cell. The dots can be very small and very close to the junctions, and the cell will still function properly.
The group tested their concept by fabricating a memory chip out of the metal niobium (Nb). The chip consists of a pattern of several Nb Josephson junction arrays, with eight junctions per array. Each junction is coupled to a ferromagnetic dot that is 6 micrometers wide, 9 micrometers long, and 600 nanometers thick. There are many ferromagnetic materials that would work, but for this first test the group chose a nickel-iron compound commonly known as Permalloy.