Dynamic susceptibility of itinerant ferromagnets in the ordered state*

Measurements of radio-frequency dynamic susceptibility of ferromagnets exhibit striking differences between local moment and itinerant systems. Whereas local moment systems show a sharp peak at the Curie temperature ($T_c$) which evolves to higher temperatures and lower amplitudes with applied dc magnetic field, itinerant systems show a broad maximum at temperatures well below $T_c$. The itinerant system's maximum is suppressed in amplitude and shifts to lower temperatures with applied dc magnetic field. Existing Stoner or spin fluctuations theories derive strictly zero-field susceptibility and we propose a generalization of these models to incorporate the effect of applied dc field. A good agreement between our semi-phenomenological approach and experimental results obtained on several generally accepted itinerant materials with various $T_c$'s is presented.