[摘要]:We report the discovery of 180 degrees electron-spin precession in spin-polarized electron-reflection experiments on Fe films on Ag(001), the largest possible precession angle in a single electron reflection. Both experiments as a function of Fe film thickness and ab initio calculations show that the appearance of this ultimate spin precession depends with utmost sensitivity on the relaxation of the Fe surface layers during growth. Similar spin precession is also predicted for other ferromagnetic films.