Coercivity Panorama of Dynamic Hysteresis

Abstract: We study the stochastic $\phi^4$ model under periodic driving by an external field $H$ at different scales of driving rate $v_H$, where the noise strength $\sigma$ quantifies the deviation of the system size from the thermodynamic limit. For large systems with small $\sigma$, we find the coercivity $H_c=H(\langle\phi\rangle=0)$ sequentially exhibits distinct behaviors with increasing $v_H$: $v_H$-scaling increase from zero, stable plateau ($v_H^0$), $v_H^{1/2}$-scaling increase, and abrupt decline to disappearance. The $H_c$-plateau reflects the competition between thermodynamic and quasi-static limits, namely, $\lim_{\sigma\to 0}\lim_{v_H\to 0}H_c = 0$, and $\lim_{v_H\to 0}\lim_{\sigma\to 0}H_c=H^*$. Here, $H^*$ is exactly the first-order phase transition (FOPT) point. In the post-plateau slow-driving regime, $H_c-H^*$ scales with $v_H^{2/3}$. Moreover, we reveal a finite-size scaling for the coercivity plateau $H_P$ as $(H^*-H_P)\sim\sigma^{4/3}$ by utilizing renormalization-group theory. These predicted scaling relations are demonstrated in magnetic hysteresis obtained with the Curie-Weiss model. Our work provides a panoramic view of the finite-time evolution of the stochastic $\phi^4$ model, bridges dynamics of FOPT and dynamic phase transition, and offers new insights into finite-time/finite-size effect interplay in non-equilibrium thermodynamics.