Excitation and control of large-amplitude standing magnetization waves

Abstract

A robust approach to excitation and control of large amplitude standing magnetization waves in an easy axis ferromagnetic by starting from a ground state and passage through resonances with chirped frequency microwave or spin torque drives is proposed. The formation of these waves involves two stages, where in the first stage, a spatially uniform, precessing magnetization is created via passage through a resonance followed by a self-phase-locking (autoresonance) with a constant amplitude drive. In the second stage, the passage trough an additional resonance with a spatial modulation of the driving amplitude yields transformation of the uniform solution into a doubly phase-locked standing wave, whose amplitude is controlled by the variation of the driving frequency. The stability of this excitation process is analyzed both numerically and via Whitham's averaged variational principle.

Narrow autoresonant magnetization structures in finite-length ferromagnetic nanoparticles

“…Autoresonant magnetic excitations of this different type were discussed in Ref. [21] and comprise spatial oscillations of θ having a finite periodicity length. Interestingly, the two types of solutions coalesce in the limit A → 0 (the dotted red line in Fig.…”

“…We used the numerical approach described in Ref. [21]. The simulations solved an equivalent system of two coupled NLS-type equations based on the quantum two-level analog due to R. Feynman [23].…”

“…In recent years, autoresonance ideas were also implemented in magnetics. Examples are autoresonant waves in magnetic materials [17,18], the switching of magnetization of single domain point-like nanoparticles [19,20], and most recently the autoresonant excitation of large amplitude standing magnetization waves in long ferromagnetic nanowires by using rotating driving fields with short wavelength spatial modulations [21]. This recent approach required two resonant stages, where in the first stage one excites a rotating, but uniform magnetization of the wire, while later, in the second resonant stage, the system develops a spatially modified standing wave profile.…”

“…FIG.4: The effective potential. Two classes of oscillations correspond to different values of the quasi-energy: A > 0 (solid horizontal line)is the case of the zero boundary conditions used in this work, A < 0 (dished horizontal line)is the standing magnetization wave of Ref [21]…”