Field-dependent AC susceptibility of itinerant ferromagnets

Whereas dc measurements of magnetic susceptibility, χ, fail to distinguish between local and weak itinerant ferromagnets, radio-frequency (rf) measurements of χ in the ferromagnetic state show dramatic differences between the two. We present sensitive tunnel-diode resonator measurements of χ in the weak itinerant ferromagnet ZrZn2 at a frequency of 23 MHz. Below the Curie temperature, TC∼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 of this maximum with applied field is not predicted by either the Stoner or self-consistent renormalized (SCR) spin-fluctuation theories. For temperatures below TC both theories derive a zero-field limit expression for χ. 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 accurately describes the experimental data.