Magnetic reversal of a quantum nanoferromagnet

When the external magnetic field applied to a ferromagnetically coupled atomic chain is reversed suddenly, the magnetization of the chain switches, due to the reversal of all the atomic magnetic moments in the chain. The quantum processes underlying the magnetization switching and the time required for the switching are analyzed for model magnetic chains adsorbed on a surface at 0 K. The sudden field reversal brings the chain into an excited state that relaxes towards the system ground state via interactions with the substrate electrons. Different mechanisms are outlined, ranging from the global stepwise rotation of the chain macrospin induced by spin-flip collisions with substrate electrons in the pure Heisenberg chain (Neel-Brown process) to a correlation-mediated direct switching process in the presence of strong magnetic anisotropies in short chains (the global spin of the chain reverses in a single electron interaction). The processes for magnetization switching induced by electrons tunneling from a scanning tunneling microscope tip are also analyzed.