Swift heavy ion induced mixing is reported in a-Si/Mn/a-Si thin films on a silicon wafer, when irradiated by 120 MeV Au ions at three different fluences of 1 x 10(13), 3 x 10(13) and 1 x 10(14) ions cm(-2). The samples were characterized before ( pristine) and after irradiation using secondary ion mass spectroscopy ( SIMS) and Rutherford backscattering spectroscopy ( RBS). Atomic force microscopy of the samples was used to determine the surface roughness contribution to RBS and SIMS profiles. Depth profiles showed distinct changes in the interface region and it was observed that interface mixing increased linearly with the increase in the ion fluence. The mixing rate was estimated to be similar to 1000 nm(4). The mixing effect is explained in the framework of the thermal spike model. The track radius and duration of the transient melt phase have been theoretically calculated for this system to estimate the diffusivity during the transient melt stage at the interface.