Field-dependent AC susceptibility of itinerant ferromagnets

TL;DR

This study uses radio-frequency measurements to reveal field-dependent features of AC susceptibility in itinerant ferromagnets, showing deviations from existing theories and proposing a new semi-phenomenological model.

Contribution

It introduces sensitive rf susceptibility measurements in ZrZn₂ and develops a semi-phenomenological model to explain observed phenomena beyond existing theories.

Findings

Susceptibility peaks at ~15 K below T_C

Magnetic field suppresses and shifts the susceptibility maximum

Existing theories do not predict the observed maximum behavior

Abstract

Whereas dc measurements of magnetic susceptibility, $\chi$, fail to distinguish between local and weak itinerant ferromagnets, radio-frequency (rf) measurements of $\chi$ in the ferromagnetic state show dramatic differences between the two. We present sensitive tunnel-diode resonator measurements of $\chi$ in the weak itinerant ferromagnet ZrZn$_2$ at a frequency of 23 MHz. Below Curie temperature, $T_C \approx 26$ K, the susceptibility is seen to increase and pass through a broad maximum at approximately 15 K in zero applied dc magnetic field. Application of a magnetic field reduces the amplitude of the maximum and shifts it to lower temperatures. The existence and evolution this maximum with applied field is not predicted by either the Stoner or self-consistent renormalized (SCR) spin fluctuations theories. For temperatures below $T_C$ both theories derive a zero-field limit expression for $\chi$. We propose a semi-phenomenological model that considers the effect of the internal field from the polarized fraction of the conduction band on the remaining, unpolarized conduction band electrons. The developed model accurate describes the experimental data.