Crystal and magnetic effects of selected substitutions of Ni for Fe and for Co in the orthorhombic MnFe0.35Co0.65P compound

Abstract Both series of MnFe 0.35−x Co 0.65 Ni x P and MnFe 0.35 Co 0.65−x Ni x P orthorhombic compounds were analyzed for their structural characteristics versus x and versus temperature, as well for the changes induced to the magnetic behaviors (ordering temperature, saturation magnetization, magnetic entropy change). The operated Ni substitutions lead to decrease the Curie point closer to room temperature, especially when Ni substitutes to Co. However, in both systems, the saturation magnetization evolves unexpectedly. Even if remaining rather weak, the corresponding changes of magneto-caloric entropy at the transition are significantly increased, being more than twice and three times the initial value for Ni substituted to Fe and to Co respectively. Chemical and topological arguments are then proposed to discuss the noticeable difference of behaviors observed between the hexagonal (e.g. MnFeP 1−x As x ) and the orthorhombic (e.g. MnFe 1−x Co x P) series of polytype pnictides.