This paper presents engineering perpendicular magnetic anisotropy in amorphous Tb-Fe-Co thin films employing irradiation of Ar+ ions at 200 and 250 keV energies with various fluences. The pristine film depicts a rectangular hysteresis loop with high perpendicular magnetic anisotropy, showing elongated, inter-connected maze-like magnetic domains. Irradiation at higher fluences (≥1015 ions cm−2) results in the transformation of magnetization reversal mechanism from rapid domain wall motion to nucleation and propagation of reverse domains due to the pinning of domain walls. The elongated magnetic domains get fragmented with reduced contrast and interestingly inhomogeneous flux-closure patterns are observed at higher fluences, indicating a spin reorientation transition. The modifications in magnetic properties are correlated with the structural investigations which reveal the crystallization of the Fe-Co phase in an amorphous Tb-Fe-Co matrix at higher energy and fluences of the projectile ions. Micromagnetic simulations complement the experimental results of domain modification. © 2020 Elsevier B.V.