Site dependence of the magnetocaloric effect in Mn_{5- x} Fe _x Si₃

The nuclear and magnetic structures of Mn₃Fe₂Si₃ are investigated in the temperature range from 20 to 300 K. The magnetic properties of Mn₃Fe₂Si₃ were measured on a single crystal. The compound undergoes a paramagnetic to antiferromagnetic transition at T _N2 ≃ 120 K and an antiferromagnetic to antiferromagnetic transition at T _N1 ≃ 69 K. A similar sequence of magnetic phase transitions is found for the parent compound Mn₅Si₃ upon temperature variation, but the field-driven transition observed in Mn₅Si₃ is not found in Mn₃Fe₂Si₃, resulting in a strongly reduced magnetocaloric effect. Structurally, the hexagonal symmetry found for both compounds under ambient conditions is preserved in Mn₃Fe₂Si₃ through both magnetic transitions, indicating that the crystal structure is only weakly affected by the magnetic phase transition, in contrast to Mn₅Si₃ where both transitions distort the nuclear structure. Both compounds feature a collinear high-temperature magnetic phase AF2 and transfer into a non-collinear phase AF1 at low temperature. While one of the distinct crystallographic sites remains disordered in the AF2 phase in the parent compound, the magnetic structure in the AF2 phase involves all magnetic atoms in Mn₃Fe₂Si₃. These observations imply that the distinct sites occupied by the magnetic atoms play an important role in the magnetocaloric behaviour of the family.