Detection of weak-order phase transitions in ferromagnets by ac resistometry

TL;DR

This paper demonstrates that ac resistometry is an effective method for detecting weak-order phase transitions in ferromagnets by analyzing impedance and permeability changes, which are not visible in dc resistivity measurements.

Contribution

It introduces ac resistometry as a sensitive technique for identifying weak-order phase transitions in ferromagnetic materials, utilizing impedance and permeability measurements.

Findings

Impedance measurements reveal anomalies at phase transitions.

Frequency dependence of permeability enhances detection sensitivity.

Method successfully applied to Gd and Ni₂MnGa samples.

Abstract

It is shown that ac resistometry can serve as an effective tool for the detection of phase transitions, such as spin reorientation or premartensitic phase transitions, which generally are not disclosed by dc resistivity measurement. Measurement of temperature dependence of impedance, $Z(T)$, allows one to unmask the anomaly, corresponding to a weak-order phase transition. The appearance of such an anomaly is accounted for by a change in the effective permeability $\mu$ of a sample upon the phase transition. Moreover, frequency dependence of $\mu$ makes it possible to use the frequency of the applied ac current as an adjusting parameter in order to make this anomaly more pronounced. The applicability of this method is tested for the rare earth Gd and Heusler alloy Ni$_2$MnGa.