# Ordering tendencies and electronic properties in quaternary Heusler derivatives

## TL;DR

This study investigates the phase stability, ordering tendencies, and electronic properties of quaternary Heusler alloys NiCoMnAl and NiCoMnGa using experimental and theoretical methods, revealing the impact of ordering on magnetic and electronic behavior.

## Contribution

It combines neutron diffraction, calorimetry, magnetization, and first-principles calculations to analyze ordering and electronic properties, highlighting kinetic limitations in achieving fully ordered structures.

## Key findings

- NiCoMnGa adopts the L2$_1$ structure with a phase transition at 1160 K.
- NiCoMnAl requires slow cooling or annealing to develop L2$_1$ order, affecting magnetic properties.
- A near half-metallic pseudo-gap appears only in the fully ordered Y structure, which is energetically unstable.

## 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 L2$_1$ structure, with distinct Mn and Ga sublattices but a common Ni-Co sublattice. A second-order phase transition to the B2 phase with disorder also between Mn and Ga was observed at 1160 K. In contrast, in NiCoMnAl slow cooling or low-temperature annealing treatments are required to induce incipient L2$_1$ ordering, otherwise the system displays only B2 order. Linked to this L2$_1$ 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 pseudo-gap emerges in the minority spin channel only for the completely ordered Y structure, which however is energetically unstable compared to the predicted ground state of a tetragonal structure with alternating layers of Ni and Co. The experimental inaccessibility of the totally ordered structures is explained by kinetic limitations due to the low ordering energies.