Neutron diffraction study on hydrostatic pressure regulated magnetostructural transition and magnetocaloric effect in MnNi_(1-x)Fe_xSi_(1-y)Ge_y alloys

摘要

Ni_2In-type hexagonal compounds MM'X (M, M' = transition metals, X = main element) involving magnetostructural transition (MST) have attracted much attention due to their giant magnetocaloric effect (MCE). Physical pressure, as an effective method, has been used to adjust the MST and the resultant MCE. Enhanced and diminished MCEs by hydrostatic pressure were both reported previously, but the underlying mechanism is unclear. Here, we report our neutron diffraction study on MST and MCE regulated by hydrostatic pressure in MnNi_(1-x)Fe_xSi_(1-y)Ge_y alloys. Careful refinements indicate that the martensitic phase shows a linear ferromagnetic structure with spin moment confined on Mn sites, which remains almost unchanged at pressures lower than 5 kbar even though slight compressions of Mn-Mn bond lengths can be identified. The MST keeps sharp under pressures lower than 5 kbar, while the derived volume change (ΔV/V) across MST reduces 7%, i.e., from 2.84% (0 kbar) to 2.63% (2.1 kbar), due to the effect of pressure on two-phase coexistence. Accordingly, the estimated lattice entropy change (ΔS_(Latt)) based on the Debye approximation reduces by 10% from 37.1 J/kgK (Okbar) to 33.5 J/kgK (2.1 kbar). These ΔS_(Latt) values, driven by temperature, are all somewhat larger than the magnetic entropy change driven by a 5T magnetic field. This result may imply that a magnetic field of 5 T is not sufficient for the MST to complete. As the pressure reaches 5 kbar, the MST notably slows down. This may originate from the extended temperature region of two-phase coexistence but not the decoupling of MST.