Quantum spin tunneling

Quantum spin tunneling, or quantum tunneling of magnetization, is a physical phenomenon by which the quantum mechanical state that describes the collective magnetization of a nanomagnet is a linear superposition of two states with well defined and opposite magnetization. Classically, the magnetic anisotropy favors neither of the two states with opposite magnetization, so that the system has two equivalent ground states.

Because of the quantum spin tunneling, an energy splitting between the bonding and anti-bonding linear combination of states with opposite magnetization classical ground states arises, giving rise to a unique ground state separated by the first excited state by an energy difference known as quantum spin tunneling splitting. The quantum spin tunneling splitting also occurs for pairs of excited states with opposite magnetization.

As a consequence of quantum spin tunneling, the magnetization of a system can switch between states with opposite magnetization that are separated by an energy barrier much larger than thermal energy. Thus, quantum spin tunneling provides a pathway to magnetization switching forbidden in classical physics.

Whereas quantum spin tunneling shares some properties with quantum tunneling in other two level systems such as a single electron in a double quantum well or in a diatomic molecule, it is a multi-electron phenomenon, since more than one electron is required to have magnetic anisotropy. The multi-electron character is also revealed by an important feature, absent in single-electron tunneling: zero field quantum spin tunneling splitting is only possible for integer spins, and is certainly absent for half-integer spins, as ensured by Kramers degeneracy theorem. In real systems containing Kramers ions, like crystalline samples of single ion magnets, the degeneracy of the ground states is frequently lifted through dipolar interactions with neighboring spins, and as such quantum spin tunneling is frequently observed even in the absence of an applied external field for these systems.

Initially discussed in the context of magnetization dynamics of magnetic nanoparticles, the concept was known as macroscopic quantum tunneling, a term that highlights both the difference with single electron tunneling and connects this phenomenon with other macroscopic quantum phenomena. In this sense, the problem of quantum spin tunneling lies in the boundary between the quantum and classical descriptions of reality.