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.
Highlights
Linked to L21 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
The predicted contraction at the B2-L21 transition in NiCoMnGa of 0.012 Aagrees perfectly with the experimental value as obtained by integrating the excess thermal expansion coefficient between 1000 and 1200 K, while in NiCoMnAl the contraction with ordering is estimated as 0.003 Aby the differences in the heating and cooling curves evaluated at 600 K and 900 K to be compared with the predicted value of 0.005 A. This small discrepancy implies that the experimental lattice constant of NiCoMnAl at low temperatures on cooling is increased compared to the theoretical predictions, which is consistent with a reduced degree of L21 long-range order in NiCoMnAl due to kinetical reasons
Employing in situ neutron diffraction, magnetic measurements, and calorimetry, we studied the ordering tendencies in the quaternary Heusler derivatives NiCoMnAl and NiCoMnGa
Summary
The class of Heusler alloys, with the ternary system Cu2MnAl as the prototypical representative [1], hosts a variety of systems displaying intriguing properties [2]. NiCoMnAl [10] and NiCoMnGa [11], have been suggested by ab initio calculations to be half-metals in their fully ordered Y structure [12]. In assessing the application potential of a given material following from its electronic structure, theoretical and experimental investigations have contrasting characteristics: In ab initio calculations, the distribution of electronic charge is the fundamental quantity that is considered, which depends in principle only on the positions of the ions and their atomic numbers. In the systems of NiCoMnAl and NiCoMnGa we study the degrees of equilibrium long-range order and the associated order/disorder phase transitions by in situ neutron diffraction and the kinetics of order relaxation during isothermal annealing by way of its effect on magnetization and Curie temperature. The associated ordering energies are small, which explains the experimentally observed stability of disorder among Ni and Co
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