(Top) Electrons tunnel between the tip of a STM microscope and a single-layer nanoisland of atoms deposited on . Because the tip polarizes the spins of these electrons, the differential conductance depends on the relative magnetic polarizations of the tip and the island: maximum when they are parallel (nanomagnet in “up” state), lower when they are antiparallel (nanomagnet in “down” state). (Bottom) During the magnetization reversal, the collinear alignment of atomic spins breaks down, as shown by the snapshot picture in the center, obtained from Monte Carlo simulations . The nucleation of a reversed magnetic domain is followed by the propagation of a domain wall across the island. The probability of this process is determined by shape. For islands elongated along the propagation axis, domain walls are short and energetically more affordable, but they have a larger probability to turn back and annihilate. For islands elongated along the anisotropy axis, many atoms at the boundary can act as nucleation centers, but the activation energy increases, as does the length of the domain wall.