Ion implantation of lighter (4He+) and heavier ion species (121Sb+)) was studied to investigate their suitability in fabricating bit patterned media (BPM) for areal densities ≥ 5 Tbpsi. Conventional CoCrPt-SiO2 and the next generation high-anisotropy L10 FePt media were employed to understand the mass-dependent lateral straggle of the ions. First-order reversal curve measurement for CoCrPt-SiO2 media revealed an increase in exchange interaction in the implanted films with the increasing fluence. Interestingly, there was an order-disorder transformation of the phase for L10 FePt media upon implantation. Implantation using lighter ions resulted in a larger lateral straggle. This lateral movement of ions from the unmasked to masked regions during BPM fabrication will, furthermore, impede the process of bit isolation. In contrast, lateral straggle was reduced for heavier ions. However, this was achieved at the expense of the diffusion of host atoms displaced from their lattice positions which caused a reduction of anisotropy even in the unimplanted regions. Therefore, the requirement to strike a balance between the lateral straggle of the implanted species and the movement of host atoms to accomplish ultra-high densities in implantation-assisted-BPM recording is reported as a challenging problem. © 2014 IEEE.