Force Correlations in Disordered Magnets

TL;DR

This paper experimentally validates the functional renormalization group predictions for force correlations in disordered magnets, revealing universal behaviors and anti-correlations in Barkhausen noise across different universality classes.

Contribution

It provides the first experimental evidence supporting the functional renormalization group predictions for force correlations in disordered magnetic systems.

Findings

Force correlations are universal and match theoretical predictions.

Correlations grow linearly at small distances and are bounded at large distances.

Avalanches exhibit anti-correlations, indicating domain walls behave as a single wall.

Abstract

We present a proof of principle for the validity of the functional renormalization group, by measuring the force correlations in Barkhausen-noise experiments. Our samples are soft ferromagnets in two distinct universality classes, differing in the range of spin interactions, and the effects of eddy currents. We show that the force correlations have a universal form predicted by the functional renormalization group, distinct for short-range and long-range elasticity, and mostly independent of eddy currents. In all cases correlations grow linearly at small distances, as in mean-field models, but in contrast to the latter are bounded at large distances. As a consequence, avalanches are anti-correlated. We derive bounds for these anti-correlations, which are saturated in the experiments, showing that the multiple domain walls in our samples effectively behave as a single wall.