Memory effects in individual submicrometer ferromagnets

We have used ballistic Hall micromagnetometry to study the magnetization of individual submicrometer nickel disks (80 nm high, $0.1–1.0\mu \mathrm{m}$ 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 $T>\mathrm{}19.8\mathrm{}\hspace{0.5em}\mathrm{K}),$ 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.