Fluctuations in a model ferromagnetic film driven by a slowly oscillating field with a constant bias

TL;DR

This study combines simulations and theory to analyze magnetization fluctuations in a ferromagnetic film driven by an oscillating field with bias, revealing stochastic resonance effects unrelated to a true phase transition.

Contribution

It demonstrates that large fluctuations in magnetization are due to stochastic resonance, not a critical phase transition, clarifying the nature of the dynamic phase transition in ferromagnetic films.

Findings

Fluctuations produce peaks in susceptibility at bias values increasing with inverse log of period.

Large system sizes show size independence, indicating non-critical behavior.

The effect is identified as stochastic resonance, not a thermodynamic phase transition.

Abstract

We present a numerical and theoretical study that supports and explains recent experimental results on anomalous magnetization fluctuations of a uniaxial ferromagnetic film in its low-temperature phase, which is forced by an oscillating field above the critical period of the associated dynamic phase transition (DPT) [P. Riego, P. Vavassori, A. Berger, Phys. Rev. Lett. 118, 117202 (2017)]. For this purpose, we perform kinetic Monte Carlo simulations of a two-dimensional Ising model with nearest-neighbor ferromagnetic interactions in the presence of a sinusoidally oscillating field, to which is added a constant bias field. We study a large range of system sizes and supercritical periods and analyze the data using a droplet-theoretical description of magnetization switching. We find that the period-averaged magnetization, which plays the role of the order parameter for the DPT, presents large fluctuations that give rise to well-defined peaks in its scaled variance and its susceptibility with respect to the bias field. The peaks are symmetric with respect to zero bias and located at values of the bias field that increase toward the field amplitude as an inverse logarithm of the field oscillation period. Our results indicate that this effect is independent of the system size for large systems, ruling out critical behavior associated with a phase transition. Rather, it is a stochastic-resonance phenomenon that has no counterpart in the corresponding thermodynamic phase transition, providing a reminder that the equivalence of the DPT to an equilibrium phase transition is limited to the critical region near the critical period and zero bias.