Ion beam mixing of layered iron‐boron films with 200‐keV Ar ions is investigated by x‐ray diffraction, Auger depth profiling, and high‐temperature resistivity. Samples consisting of alternating layers of electron beam deposited polycrystalline iron (25 nm thick) and boron (50 nm thick) were irradiated to doses from 5×1015to 1×1016ions/cm2, at rates ranging from 0.3 to 3.3 &mgr;A cm2. The mixing extends to a depth of 150 nm and increases with dose rate. This dependence is due to beam heating of the sample. The structure of the mixed material is amorphous iron boron.