Heavy quasiparticles in the ferromagnetic superconductor ZrZn2

We report a study of the de Haas-van Alphen effect in the normal state of the ferromagnetic superconductor ZrZn2. Our results are generally consistent with a linear muffin-tin orbital band structure which predicts four exchange-split Fermi surface sheets. Quasiparticle effective masses are enhanced by a factor of 4.9 implying a strong coupling to magnetic excitations or phonons. ZrZn2 is unique among metallic ferromagnets in that it has a very large density of states in the ferromagnetic phase.