Magnetic properties and possible martensitic transformation in equiatomic quaternary Heusler alloy CoMnNiSn

•L21B disordered structure has a higher stability in EQHA CoMnNiSn comparing with ordered Y-type structure.•L21B disorder can suppress the martensitic transformation predicted in ordered CoMnNiSn.•CoMnNiSn bulk and ribbon samples are both ferromagnets with Ms closing to 5.00 μB/f.u.•Influence of atomic disorder is important when predicting the properties of Heusler alloys. Equiatomic quaternary Heusler alloy CoMnNiSn has been synthesized in both bulk and ribbon form to explain the contradictions in the magnetic properties and possible martensitic transformation between different literature. The structure, magnetic properties and electronic structure of CoMnNiSn were investigated both theoretically and experimentally. A cubic Heusler phase can be identified in the XRD patterns. A small amount of secondary phase is observed in bulk CoMnNiSn while it is eliminated in the melt-spun ribbons. The saturation magnetization Ms at 5 K of bulk and ribbon CoMnNiSn are 4.85 μB/f.u. and 4.76 μB/f.u., respectively. This and also the calculation results suggest that ferromagnetic ground state has been established in CoMnNiSn samples. There is no trace of martensitic transformation in CoMnNiSn bulk and ribbon samples. The L21B atomic disorder can be responsible for this, which comes from the random occupation of Co/Ni at A, C sites in the cubic lattice of CoMnNiSn and has a higher stability comparing with the ordered Y-type structure. This suppression effect is further explained by the weakening of the Jahn-Teller effect in the austenite DOS of L21B type CoMnNiSn. All this calls for more attention to the effects of atomic disorder in the design of magnetic shape memory alloys in equiatomic quaternary Heusler alloys.