An Ab Initio Study of Pressure-Induced Changes of Magnetism in Austenitic Stoichiometric Ni2MnSn

Abstract

We have performed a quantum-mechanical study of a series of stoichiometric Ni2MnSn structures focusing on pressure-induced changes in their magnetic properties. Motivated by the facts that (i) our calculations give the total magnetic moment of the defect-free stoichiometric Ni2MnSn higher than our experimental value by 12.8% and (ii) the magnetic state is predicted to be more sensitive to hydrostatic pressures than seen in our measurements, our study focused on the role of point defects, in particular Mn-Ni, Mn-Sn and Ni-Sn swaps in the stoichiometric Ni2MnSn. For most defect types we also compared states with both ferromagnetic (FM) and anti-ferromagnetic (AFM) coupling between (i) the swapped Mn atoms and (ii) those on the Mn sublattice. Our calculations show that the swapped Mn atoms can lead to magnetic moments nearly twice smaller than those in the defect-free Ni2MnSn. Further, the defect-containing states exhibit pressure-induced changes up to three times larger but also smaller than those in the defect-free Ni2MnSn. Importantly, we find both qualitative and quantitative differences in the pressure-induced changes of magnetic moments of individual atoms even for the same global magnetic state. Lastly, despite of the fact that the FM-coupled and AFM-coupled states have often very similar formation energies (the differences only amount to a few meV per atom), their structural and magnetic properties can be very different.

ANOMALOUS PRESSURE-INDUCED CHANGES IN MAGNETISM OF FERRIMAGNETIC Ni-Mn-Sn STRUCTURES

“…As a continuation of our previous research [11], we have computed pressure-induced changes in the case of two ferrimagnetic Ni-Mn-Sn states. One was the stoichiometric Ni2MnSn case with two Ni-Mn swaps and the second quite off-stoichiometric Mn-rich and Sn-poor alloy with the stoichiometry Ni7Mn8Sn1.…”

“…Its atomic distribution is that of the full Heusler structure, see Figure 1. The lowest energy configuration exhibits local magnetic moments of atoms as listed in Figure 1(a); further details can be found in our recent paper [11]. The magnetic state is, in principle, ferrimagnetic due to the orientation of local magnetic moment of Sn atoms that is antiparallel to those of Mn and Ni atoms.…”

“…Refs. [7,11]). In order to shed a light on our findings, we have evaluated pressure-induced changes in the magnitudes of local magnetic moments and we present them in Figure 2(d).…”

“…Our study was motivated by the current interest in pressure-induced changes in Ni-Mn-Sn structures [7] when an unexpected complexity is often found depending on the type and distribution of internal defects as well as their magnetic state, see, e.g. [11].…”

“…Therefore, the state shown in Figure 1(a) is often called ferromagnetic reflecting the ferromagnetic orientation of local magnetic moments of Mn atoms, as well as of Ni atoms, with respect to the orientation of the total magnetic moment. Our previous study [11] of various magnetic and structural variants of stoichiometric Ni2MnSn with different types of swapped atoms revealed that the response to the hydrostatic pressures is highly sensitive to both atomic configuration and magnetic state.…”