Background: Shell evolution can impact the structure of the nuclei and lead to effects such as shape coexistence. The nuclei around 68 Ni represent an excellent study case, however, spectroscopic information of the neutron-rich, Z < 28 nuclei is limited. Purpose: The goal is to measure γ-ray transitions in 66 Fe, 66 Co, and 66 Ni populated in the β − decay of 66 Mn to determine absolute β feedings and relative γ-decay probabilities and to compare the results with Monte Carlo shell model calculations in order to study the influence of the relevant single neutron and proton orbital occupancies around Z = 28 and N = 40. Method: The low-energy structures of 65,66 Fe, 66 Co, and 66 Ni were studied in the β − decay of 66 Mn produced at ISOLDE, CERN. The beam was purified by means of laser resonance ionization and mass separation. The β and γ events detected by three plastic scintillators and two MiniBall cluster germanium detectors, respectively, were correlated in time to build the low-energy excitation schemes and to determine the β-decay half-lives of the nuclei. Results: The relative small β-decay ground state feeding of 66 Fe obtained in this work is at variant to the earlier studies. Spin and parity 1 + was assigned to the 66 Co ground state based on the strong ground-state feeding in the decay of 66 Fe as well as in the decay of 66 Co. Experimental log(f t) values, γ-ray de-excitation patterns, and energies of excited states were compared to Monte Carlo shell model calculations. Based on this comparison, spin and parity assignments for the selected number of low-lying states in the 66 Mn to 66 Ni chain were proposed. Conclusions: The β-decay chain starting 66 Mn toward 66 Ni, crossing N = 40, evolves from deformed nuclei to sphericity. The β-decay population of a selected number of 0 + and 2 + states in 66 Ni, which is understood within shape coexistence framework of Monte Carlo shell model calculations, reveals the crucial role of the neutron 0g 9/2 shell and proton excitations across the Z = 28 gap.