Sputter deposited (Ni80Fe20-2nm/Au-2nm/Co-tCo/Au-2nm)10 multilayers are characterized by alternating in-plane (Ni80Fe20) and out-of-plane (for Co thickness in the range 0.3<tCo<1.2nm) magnetic anisotropy. The magnetic properties of the samples, before and after ion bombardment, were characterized by magnetometric and magnetoresistive measurements. The irradiation process was performed with fluences of 30keV He+ ions varied in the range 1013⩽D⩽3×1016He+/cm2. We report that for fluences in the range between 1015 and 1016He+/cm2 the effective (perpendicular) anisotropy of the Co layer decreases noticeably. The origin of the effect is related to a reduction in the surface anisotropy thus to interfaces modification. As a consequence strong reduction of magnetoresistance was also observed.