[Co(7 nm)/Fe(7 nM)](6) multilayers were electron-beam evaporated onto Si(100) substrates in ultrahigh vacuum and irradiated at room temperature with 200-keV Xe ions, leading to ion beam mixing within the Co/Fe multilayer, but not with the Si substrate. Irradiation-induced changes in structural and magnetic properties were characterized by means of Rutherford backscattering spectroscopy, X-ray diffraction and in-plane magneto-optical Kerr effect. Irradiation with 1 X 10(16) Xe ions/cm(2) induced Co/Fe intermixing to a 1:1 atomic concentration ratio (RBS) and the formation of the Fe50Co50 permendur phase in the intermixed zone (XRD). For lower ion fluences, the coercivity decreased strongly, but then increased slowly for higher fluences. The angular pattern of the relative remanence showed a perfect uniaxial anisotropy. The magnetic energy density was parametrized with the expression E-s/M-s proportional to (K-u1/M-s) sin(2) (phi - phi(0))+(K-u2/M-s) sin(4)(phi - phi(0)), M-s being the saturation magnetization and phi(0) the symmetry angle. The second-order term K-u2/M-s was found to decrease strongly with increasing Xe fluence. (c) 2006 Elsevier B.V. All rights reserved.