Very Weak Itinerant Ferromagnets; Application to ZrZn2

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Abstract
The itinerant electron model is considered in the limit of very low exchange splitting energies. Magnetic isotherms giving M(H, T) are derived to cover a wide range of fields and temperatures. The isotherms are transformed so as to show that plots of M2 vs H/M at different temperatures give parallel straight lines. The differential susceptibility below and above the Curie temperature is derived as a function of T. The free energy corresponding to this model is calculated and several results concerning the specific heat of very weak itinerant ferromagnets are deduced. Some properties related to spin wave excitations are also considered. New experimental data on the material ZrZn2 are considered on the basis of the theory and shown to be in reasonable agreement with it. Several characteristic parameters of this material are deduced, for example a value of the exchange splitting energy 0.040 eV, and of the effective interation between the itinerant electrons 0.35 eV. The occurrence of very weak itinerant ferromagnetism in other substances is also discussed.

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