  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.
