Itinerant ferromagnetism and superconductivity were traditionally believed to represent incompatible forms of electronic order that are well described by the Fermi liquid model of the metallic state or extensions thereof. The itinerant ferromagnet ZrZn2 exhibits in contrast superconductivity in the milli-Kelvin temperature range that vanishes above the critical pressure p c where ferromagnetism is suppressed. This suggests that itinerant ferromagnetism may be a precondition for certain forms of superconductivity. The itinerant electron magnet MnSi exhibits, on the other hand, a sharp change from a Fermi liquid T 2 resistivity to a T 3/2 temperature dependence over an exceptionally large temperature range in the normal metallic state above the critical pressure p c for which magnetic order is suppressed. The latter property is not consistent with the predictions of the Fermi liquid theory of the normal metallic state. When taken together the superconductivity and non-Fermi liquid normal state may highlight inconsistencies of the current theory of quantum criticality.
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