Targeted structural changes and magnetic properties study in (Ho/Er)5Ga3−x(Co/Fe)x

• Synthesis of new Ho5Ga3−xCox and Er5Ga3−xFe(Co)x phases with high purity.
• Targeted Mn5Si3-to-Cr5B3 structural changes in the (Ho/Er)5Ga3−x(Fe/Co)x systems.
• Magnetic transition and magnetocaloric effect of the new Ho5Ga3−xCox phases.

Phase transformations in the Ho5Ga3−xCox (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1), Er5Ga3−xFex (x = 0, 0.4), and Er5Ga3−xCox (x = 0, 0.4) system reveal an intimate coupling between the crystal structure and atomic size. A decrease in the effective atomic size of the Ga site through the transition metal substitution results in a transition from the Mn5Si3-type structure to Cr5B3-type one. According to the single crystal X-ray diffraction, Co and Fe substitution occurs only on the Ga 8h site. The relationship between the composition, crystal structures and magnetic properties is analyzed. Magnetization studies for pure Ho5Ga3, Ho5Ga2.9Co0.1, Ho5Ga2.7Co0.3, Ho5Ga2.6Co0.4 phases reveal an antiferromagnetic ordering for Ho5Ga3, but ferromagnetic transition for the other phases. In addition, the ferromagnetic transition temperature increases with the Co amount. The maximum magnetic entropy change of −12.7 J/kg K is obtained in Ho5Ga2.6Co0.4 at 32.5 K.