Ground state optimization and hysteretic demagnetization: the random-field Ising model

TL;DR

This paper compares the ground state and demagnetized state of the random-field Ising model, analyzing their energy differences and phase transition behavior across dimensions, revealing how well the demagnetized state approximates the ground state.

Contribution

It provides a detailed comparison of the ground state and demagnetized state energies and phase transition points in the random-field Ising model across different dimensions, including exact solutions.

Findings

Significant energy difference in low disorder regimes.

Different critical points for demagnetized and ground states in d>=3.

Similar scaling behavior at the transition in d=3.

Abstract

We compare the ground state of the random-field Ising model with Gaussian distributed random fields, with its non-equilibrium hysteretic counterpart, the demagnetized state. This is a low energy state obtained by a sequence of slow magnetic field oscillations with decreasing amplitude. The main concern is how optimized the demagnetized state is with respect to the best-possible ground state. Exact results for the energy in d=1 show that in a paramagnet, with finite spin-spin correlations, there is a significant difference in the energies if the disorder is not so strong that the states are trivially almost alike. We use numerical simulations to better characterize the difference between the ground state and the demagnetized state. For d>=3 the random-field Ising model displays a disorder induced phase transition between a paramagnetic and a ferromagnetic state. The locations of the critical points R_c(DS), R_c(GS) differ for the demagnetized state and ground state. Consequently, it is in this regime that the optimization of the demagnetized stat is the worst whereas both deep in the paramagnetic regime and in the ferromagnetic one the states resemble each other to a great extent. We argue based on the numerics that in d=3 the scaling at the transition is the same in the demagnetized and ground states. This claim is corroborated by the exact solution of the model on the Bethe lattice, where the R_c's are also different.