Magnetocaloric and Critical Behaviors of ${\rm Ni}_{0.5}{\rm Mn}_{0.5-{\rm x}}{\rm Sn}_{\rm x}$ Heusler Alloys

This work presents the magnetocaloric effect (MCE) and critical behaviors of bulk Ni
_0.5
Mn
_0.5-x
Sn
_x
alloys (
x
=0.14 and 0.15) prepared by an arc-melting method. Under an external field of 50 kOe, we have observed the coexistence of the inverse MCE with a maximum magnetic entropy change (Δ
S
_max
) of 1.39 J·kg
^-1
·K
^-1
at 218 K (associated with the martensitic-austenitic transition), and normal MCE with a minimum magnetic entropy change (Δ
S
_min
) of -4.97 J·kg
^-1
·K
^-1
at 308 K (associated with the ferromagnetic-paramagnetic transition in the austenitic phase) in
x
=0.14 . Meanwhile, the magnetic entropy changes for
x
=0.15 keeps normal negative, with Δ
S
_min
=-4.55 J·kg
^-1
·K
^-1
at 315 K. Interestingly, the ferromagnetic-paramagnetic transition in the austenitic phases for both alloys is found to be second-order type. Having used the Arrott-Noakes method, the critical exponents obtained for
x
=0.14 are β = 0.451 and γ = 1.217, and for
x
=0.15 are β = 0.501 and γ = 0.963, which are close to those expected for the mean-field model (β = 0.5 and γ = 1). This reveals that a long-range-order ferromagnetic interaction dominates in the austenitic phase. A small deviation from the theoretical values, particularly for
x
=0.14, is assigned to magnetic inhomogeneity induced by the martensitic-austenitic transition.