Measurements of the energy loss and energy-loss straggling of relativistic bare heavy ions (Z > 18) demonstrate systematic deviations from the first Born approximation (Bethe and Bohr theories). Besides the projectile influence on the stopping characteristics, the density and the structure of the target material modifies the slowing-down properties. Measurements of the energy-loss distributions of relativistic uranium ions at 300 MeV/u in a silicon crystal under random and channeling conditions are presented. Furthermore, the small influence of ferromagnetism in a target on the stopping power for helium ions is discussed.