Memory effects in individual submicrometer ferromagnets

We have used ballistic Hall micromagnetometry to study the magnetization of individual submicrometer nickel disks (80 nm high, 0.1--1.0μm diameter). At low temperatures, hysteresis loops of the disks no longer show inversion symmetry in a magnetic field, as if the time reversal symmetry were broken. Furthermore, the magnetization of the smallest disks can be ``frozen'' in two possible states that are characterized by hysteresis loops which are each other's inverse. At temperatures below 19.5 K a magnetic field as high as 2 T cannot switch between the states, proving that it is extremely difficult to fully polarize a small ferromagnetic particle. On the other hand, at slightly higher temperatures (only T>19.8 K), a field as low as 0.1 T appears to be enough to fully polarize the disks. We attribute this extraordinary behavior to the glass-liquid transition experienced by spins at the particle surface.