  Experimental measurements show that the angular dependence of the anisotropic
magnetoresistance (AMR) in L1$_0$ ordered FePt epitaxial films on the current
orientation and magnetization direction is a superposition of the corresponding
dependences of twofold and fourfold symmetries. The twofold AMR exhibits a
strong dependence on the current orientation, whereas the fourfold term only
depends on the magnetization direction in the crystal and is independent of the
current orientation. First-principles calculations reveal that the fourfold AMR
arises from the relaxation time anisotropy due to the variation of the density
of states near the Fermi energy under rotation of the magnetization. This
relaxation time anisotropy is a universal property in ferromagnetic metals and
determines other anisotropic physical properties that are observable in
experiment.
