Magnetic reversal of a quantum nanoferromagnet

Abstract

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 (Néel-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.

A model for individual quantal nano-skyrmions

“…The skyrmion state is then stable when it is the ground stable but also when it is an excited state. However, the system described by equation (1) is not alone in space; the spin structure in Fig.1 is adsorbed on a metal surface and the inelastic collision of an electron from the substrate on one of the structure sites can induce a transition from the upper state to the lower state (electron-hole pair creation) and is responsible for the finite lifetime of the excited state (see a discussion of the decay rate induced by electron-hole pair creation in 49,51 and the application to spin decay in 52,53,54,55 ). This process can involve thermally excited electrons but the decay is already present at vanishing temperature.…”

“…However, as discussed below, highly excited states Transition between FM and Sk states is only possible via correlation. The various spins in the system are entangled, so that by touching one site in the structure, all the spins in the system can be modified (see similar situations in 54,58,59,60 ). In the present system, correlation between FM-type and Sk-type states exists but is weak and so is the probability for an electron to induce FM-Sk transitions by collision on one of the sites in the crown.…”

Quantum revivals and magnetization tunneling in effective spin systems

“…The Hamilton operator in (1) includes the standard spin operatorsŜ i , uniaxial onsite anisotropy K and a magnetic field =  ( ) B B 0, 0, z . This Hamiltonian models an effective quantum spin representing a nanoparticle, a molecule or an atomic cluster [20,24,31] = --…”

“…i with n-number of harmonics. Hence, the spin value s is not at all present in the analytical expression (20). As i is an integer and s is an integer or half-integer in (19) the fundamental frequency corresponds to the very specific choices of…”

“…Meanwhile, the quantum dynamical phenomena become increasingly important also in solid state physics. Particularly, quantum mechanical tunneling or the non-classical field dependence of magnetization has been reported for nano-magnets [20], molecules [21,22] and single atoms [23]. Therefore, instead of studies of timeaveraged properties like, e.g., magnetization curves [23,24], nowadays the emphasis is put on the time dependent behavior of magnetization [25][26][27][28][29].…”