Ordering tendencies and electronic properties in quaternary Heusler derivatives

Abstract

The phase stabilities and ordering tendencies in the quaternary full-Heusler alloys NiCoMnAl and NiCoMnGa have been investigated by in situ neutron diffraction, calorimetry, and magnetization measurements. NiCoMnGa was found to adopt the $\mathrm{L}{2}_{1}$ structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the $\mathrm{B}2$ phase with disorder also between Mn and Ga was observed at $1160\phantom{\rule{0.28em}{0ex}}\text{K}$. In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient $\mathrm{L}{2}_{1}$ ordering, otherwise the system displays only $\mathrm{B}2$ order. Linked to $\mathrm{L}{2}_{1}$ ordering, a drastic increase in the magnetic transition temperature was observed in NiCoMnAl, while annealing affected the magnetic behavior of NiCoMnGa only weakly due to the low degree of quenched-in disorder. First principles calculations were employed to study the thermodynamics as well as order-dependent electronic properties of both compounds. It was found that a near half-metallic pseudogap emerges in the minority spin channel only for the completely ordered Y structure. However, this structure is energetically unstable compared to a tetragonal structure with alternating layers of Ni and Co, which is predicted to be the low-temperature ground state. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies.