Martensitic and room-temperature magnetocaloric properties of Mn-rich Mn-Ni-Sn Heusler alloys: Experiment and theory

Here, we study the effect of external pressure on martensitic transition, magnetic, and magnetocaloric properties of Mn-rich ${\mathrm{Mn}}_{50}{\mathrm{Ni}}_{\text{41-}x}{\mathrm{Sn}}_{\text{9}+x}$ $(x=\mathrm{0} \mathrm{and} 2)$ Heusler alloys by using a combined experimental and first principles simulation. The $x=0$ alloy exhibits martensitic transition around room temperature (RT), which increases appreciably under external pressure for both the alloys. External pressure and magnetic field show opposite effects on martensitic transition $({\mathrm{T}}_{M})$. The $x=0$ alloy shows a maximum isothermal magnetic entropy change $(\mathrm{\ensuremath{\Delta}}{S}_{M})$ of 6.5 J/kg $\mathrm{K}$ under ambient pressure at RT, which is comparatively larger than that reported in many other Heusler systems at RT. Interestingly, $\mathrm{\ensuremath{\Delta}}{S}_{M}$ decreases with pressure for $x=0$, while it shows an increasing trend for $x=2$. A maximum refrigeration capacity of around $79\phantom{\rule{0.16em}{0ex}}\mathrm{J}/\mathrm{kg}$ is observed for $x=0$. Similar to the magnetic entropy change, the net magnetization for $x=0$ and $x=2$ show opposite trend under external pressure. This is explained by our ab initio simulation by closely inspecting the consequence of nonuniform strain along three crystallographic directions on the net magnetization. This actually arises due to considerable magnetocrystalline anisotropy in these alloys. The unconventional mechanism behind the influence of pressure on magnetic properties is also discussed in the light of varying bond lengths between different magnetic species, and hence on the antiferromagnetic/ferromagnetic exchange coupling strengths under pressure.