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

It is shown that ac resistometry can serve as an effective tool for thedetection of phase transitions, such as spin reorientation or premartensiticphase transitions, which generally are not disclosed by dc resistivitymeasurement. Measurement of temperature dependence of impedance, $Z(T)$, allowsone to unmask the anomaly, corresponding to a weak-order phase transition. Theappearance of such an anomaly is accounted for by a change in the effectivepermeability $\mu$ of a sample upon the phase transition. Moreover, frequencydependence of $\mu$ makes it possible to use the frequency of the applied accurrent as an adjusting parameter in order to make this anomaly morepronounced. The applicability of this method is tested for the rare earth Gdand Heusler alloy Ni$_2$MnGa.Comment: 4 pages, 2 figures, to be published in J. Appl. Phys., v.94(5