Neutron Diffraction Study on Stoner Gap in the Superconducting Ferromagnet UGe$_2$

We report the neutron diffraction study on the temperature dependence of Bragg peak intensities ($I_B$) in the superconducting ferromagnet UGe$_2$. When the temperature is lowered, $I_B(T)$ steeply increases below a characteristic temperature ($T_X$) in the ferromagnetic phase. We have found that it can be explained in terms of a simple Stoner model, and that the so-called Stoner gap $\Delta$ decreases with increasing pressure and collapses to zero around a critical pressure ($P_X \sim $ 1.2 GPa) where the superconducting transtion temperature displays a maximum. We speculate that a transition from the paramagnetic to perfect polarized state takes place at $T_X$~and $P_X$~in heavy electron bands.