Martensitic Transitions and Magnetocaloric Properties in ${\hbox{Ni}}_{48}{\hbox{Mn}}_{39}{\hbox{In}}_{13\hbox{-}{\rm x}}{\hbox{Ge}}_{\rm x} ({\rm x} = 1\hbox{-}3)$ Ribbons

The effects of partial Ge substitution for In on phase transformation and magnetocaloric properties in annealed Ni
₄₈
Mn
₃₉
In
_13-x
Ge
_x
(x = 1-3) ribbons were studied using X-ray diffraction and magnetization measurements. The ribbons for x ≤ 2 crystallize in a cubic L21 ordered phase structure, while Ni
₄₈
Mn
₃₉
In
₁₀
Ge
₃
ribbon is in the martensitic phase with a modulated structure at room temperature. The martensitic structural transition temperature (T
_M
) increases from 220 K for x = 1 to 320 K for x = 3 with increasing Ge content. The positive magnetic entropy changes (ΔS
_M
), 29.6, 28.0 J/kg K for x = 1, 2, respectively, were found for a magnetic field change H = 50 kOe. The hysteresis loss and net refrigeration capacity ( RC) were also observed to change significantly. The largest value of the net RC (330 J/kg) for the second order phase transition was found for x = 1 at T
_C
= 280 K, with negative ΔS
_M
= -6.5 J/kg k at ΔH=50 kOe. These values are comparable to those of rare earth based systems near room temperature.