Magnetic analysis of martensitic and austenitic phases in metamagnetic NiMn(In, Sn) alloys

Highlights: • NiMnIn austenite and martensite have similar Ising-type critical exponents. • NiMnIn critical exponents rule out disordered states as spin-glass in martensite. • In NiMnIn alloys, magnetism arises mainly from moments localized at Mn atoms. • NiCoMnSn critical exponents are close to the ones from tricritical mean field model. • NiCoMnSn complex magnetic state results from three different magnetic atoms. - Abstract: Two different metamagnetic shape memory alloys of nominal composition Ni₅₀Mn₃₆In₁₄ and Ni₄₂Co₈Mn₃₉Sn₁₁ have been studied by means of modified Arrott plots to give insight into the magnetic states of both the austenitic and martensitic phases. For Ni₅₀Mn₃₆In₁₄ alloy, the same critical exponents (β = 0.32 and γ = 2.0) are obtained in austenite and martensite. They suggest that localized moments at Mn atoms are responsible for the magnetism of both phases according to the Ising model. The martensite, however, displays a rather complex behavior because β continuously changes with temperature. In Ni₄₃Co_6.5Mn₃₉Sn_11.5, critical exponents in the austenite are β = 0.27 and γ = 1.0. They are close to the tricritical mean field model, but no reliable fits were obtained in the martensite. The results are discussed in terms of microscopically different magnetic states in two alloys reflecting a complex interplay between the ferromagnetic and antiferromagnetic contributions