No abstract

Temperature-dependent fluctuations in the local current passing through close-packed magnetite nanocrystal ͑NC͒ films were probed by scanning tunneling microscopy. This phenomenon, which peaked near the blocking temperature ͑T b ͒, reflects spin-polarized tunneling fluctuations due to NC magnetization switching events. The current exhibited telegraph noise patterns, switching between low and high states. Above T b both states occurred with equal probability while below it the high current state dominated, which is consistent with a superferromagnetic ground state where the NC moments are aligned.