Longitudinal magnetization reversal in ferromagnets with Heisenberg exchange and strong single-ion anisotropy

Abstract

We analyze theoretically the novel pathway of ultrafast spin dynamics for ferromagnets with high enough single-ion anisotropy. This longitudinal spin dynamics includes the coupled oscillations of the modulus of the magnetization together with the quadrupolar spin variables, which are expressed through quantum expectation values of operators bilinear on the spin components. Even for a simple single-element ferromagnet, such dynamics can lead to a magnetization reversal under the action of an ultrashort laser pulse.

Classical limit of a quantal nano-magnet in an anisotropic environment

“…, alone leads to the same evolution in both quantal and classical approaches: the precession of the magnetic moment around the applied field. However, it was also shown and clearly illustrated [35,44,45] that the presence of a quadratic term, such as the magnetic anisotropy, in the magnetic Hamiltonian (Eq. 1) resulted in a quantal evolution very different from the classical one; in particular, it appears that the modulus of the mean value of the spin, …”

“…in [35,44,45], quantal evolution results in a variation of the mean value of the angular momentum b S ! N as a function of time, at variance with the classical evolution which involves the S !…”

“…The passage between quantal and classical descriptions received quite some attention (see e.g. recent discussions in [35,44,45] and references therein). The link between the two approaches was attempted by considering the mean value of the quantal spin, b S !…”

Longitudinal spin dynamics in nickel fluorosilicate

“…37 In Ref. 29, it has been proposed to employ the dynamic effects that are not described by the standard closed-form equation for the average spin hSi or magnetization M ¼ À2l B hSi, where l B is the Bohr magneton. In what follows h…i denotes both quantum and (at nonzero temperature) thermal averaging of physical quantities.…”

“…29 it has been proposed to use such longitudinal spin dynamics associated with the quantum spin reduction to implement ultrafast switching of the magnetic moment of a ferromagnet with a considerable effect of quantum spin reduction. These studies belong to the new and promising field of the physics of magnetism, dubbed femtomagnetism, and based on the ability to manipulate the magnetization of magnetic materials using femtosecond laser pulses.…”