Optimization of magnetic refrigerant capacity of Mn5Ge3 alloy near room temperature

Abstract

In search of potential replacement of highly expensive rare-earth metal (Gd)and its alloys [1-2] for environment friendly magnetic refrigeration technology [3] near room temperature , the earth-abundant Mn-based alloys, particularly Mn5Ge3 (TC =297 K) [4], have got much attention with reasonably high relative cooling power (RCP). In this work, we undertake Mn5Ge3 alloy and optimize the synthesis, particularly excess Mn (to compensate evaporation loss) in arc melting method, followed by annealing conditions. The maximum magneto-caloric effect is observed for Mn5Ge3 alloy with ferromagnetic ordering at TC = 299(2) K and MS (at 5 K) =142 emu/g (by dc magnetization study) from arc melting synthesis with 5 % excess Mn and vacuum annealing at 800oC for 7 days. The magnetic ordering for optimized sample is further evaluated by detailed critical exponent study using modified Arrot plot (MAP) and scaling method gives second order phase transition at TC = 297.5(1) K with exponents β = 0.348(1) and γ=1.193(2), which lies in close proximity to 3D Heisenberg model. The magnetic isotherms are further employed to study the magneto-caloric effect. Using Maxwell equations, the change in magnetic entropy, -△Sm│max is calculated as 4.7 J/kgK at 5 T applied field; the field dependence of -△Sm|max follow power law behaviour, -△Sm│max ∝ Hn with exponent, n = 0.62(1), in well consistent with n = 0.638 for 3D Heisenberg model. Finally, the material parameter for practical utilization in magnetic refrigeration i.e. RCP = -△Sm│max x δTFWHM is obtained as 450 J/kg for a field change of 5 T, which is significantly larger than earlier reported 388 J/kg for Mn5Ge3 synthesized with annealing at 750oC for 4 days. Although obtained RCP value is nearly 70 % that for the commonly used rare-earth Gd, importantly with production cost maximum 32 % only [see Table I] that makes Mn5Ge3 alloy highly promising for magnetic refrigeration application near room temperature