Magnetic CoNi/Pt interfaces are studied as a function of their preparation conditions by magnetization-induced second-harmonic generation (MSHG) measurements. A detailed method has been developed to decompose the total MSHG response into magnetic and crystallographic contributions for each interface. Although the bulk magnetism of the CoNi film (3 nm thick) shows only a subtle dependence on the sputtering Ar pressure, the interfaces appear to be dramatically affected. It can be shown that the crystallographic part probes the increase in the interface roughness while the magnetic one clearly reveals a maximum in the in-plane magnetization of the interface. ©1998 American Institute of Physics.