${\mathrm{ErCo}}_2$ is a compound which has recently attracted attention as a potential magnetic refrigeration material for hydrogen liquefaction. The interest in the material is due to the large magnetocaloric effect it exhibits, the tunability of the transition temperature, and the recent discovery of the tunability of the order of the transition by small substitutions of transition metals [X. Tang et al., Nat. Commun. 13, 1817 (2022)]. The reason for the change in the order of the transition is not yet understood. In this study the microscopic magnetic correlations and crystal lattice distortions in ${\mathrm{ErCo}}_{2-x}T{}_x$ ($T$=Fe and Ni) are investigated using neutron and synchrotron x-ray diffraction, as well as AC susceptibility measurements. The results indicate that in contrast to the previous study, the cubic to rhombohedral structure change was not suppressed, although the way the unit cell expansion occurred did change. No significant changes were observed in the magnetic long-range order and crystal lattice symmetry, which still formed a ferrimagnetic and rhombohedral crystal structure below the phase transition temperature $T_{\text{C}}$ for all substitutions as well as pure ${\mathrm{ErCo}}_2$. However, an increase in magnetic short-range correlations above $T_{\text{C}}$, which is strongly correlated with the entropy change and the dynamic AC susceptibility, was detected only in the substituted samples. Unlike pure ${\mathrm{ErCo}}_2$, the nucleation of the local ferrimagnetic clusters above $T_{\text{C}}$ occurs in the substituted samples, which significantly reduces the energy barrier between the higher-temperature cubic phase and the lower-temperature rhombohedral phase. We conclude that the reduction of the energy barrier due to the development of short-range order induced by chemical substitution at temperatures above $T_{\text{C}}$ is possibly the origin of the significant change in the nature of the phase transition in ${\mathrm{ErCo}}_2$.

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• Magnetocaloric effect
• Second order phase transitions
• Rare-earth magnetic materials
• AC susceptibility measurements
• Neutron diffraction
• X-ray powder diffraction