We present a quantum-mechanical study of a B2-phase of Fe70Al30 alloy
with and without antiphase boundaries (APBs) with the {001} crystallographic
orientation of APB interfaces. Our motivation is to explain experimental
findings demonstrating a significantly higher magnetic flux density from 
A2-phase interlayers at the thermally-induced APBs in this material
and suggesting that the ferromagnetism is stabilized by the disorder in
the A2 phase. Our investigation of sharp APBs (without any A2-phase interlayer) 
indicates that they have moderate APB energies (~0.1 J/m2) and cannot explain the 
experimentally detected increase in the ferromagnetism because they 
often induce a ferro-to-ferrimagnetic transition. When studying
thermal APBs, we introduce a few atomic layers of A2 phase of Fe70Al30 into
the interface of sharp APBs. The averaged computed magnetic moment of Fe atoms
in the whole B2/A2 nanocomposite is then increased by 11.5% w.r.t. the B2 phase.
The A2 phase itself (treated separately as a bulk) has the total magnetic moment
even higher, by 17.5%, and this increase also applies if the A2 phase at APBs
is sufficiently thick (the experimental value is 2–3 nm). We link the changes
in the magnetism to the facts that (i) the Al atoms in the first nearest
neighbor (1NN) shell of Fe atoms nonlinearly reduce their magnetic moments
and (ii) there are on average less Al atoms in the 1NN shell of Fe atoms
in the A2 phase. These effects synergically combine with the influence
of APBs which provide local atomic configurations not existing in
an APB-free bulk. The identified mechanism of increasing the magnetic
properties by introducing APBs with disordered phases can be used as
a new designing principle when developing new magnetic materials.

Enhancing magnetism in Fe-Al alloys by increasing chemical disorder

There is an ever-increasing need for new magnetic materials. When developing them, it is advantageous to improve our understanding of mechanisms that are decisive for the existing materials. Iron and iron-based materials are prototype magnetic systems and the question is whether their properties have already reached their limits or can be further improved beyond the current horizons. We have employed quantum mechanical calculations to shed a new light on relations between the extended defects and magnetic properties in Fe-Al-based materials. We show that there is a very strong competition between the thermodynamic stability, that favors Al atoms becoming the first nearest neighbors of Fe atom, and iron magnetic moment that is reduced by this trend [1-3]. We analyze how extended defects, in particular antiphase boundaries, alter local atomic configurations of Fe atoms and, consequently, increase the magnetic moment. The increase can be as massive as by 140% in off-stoichiometric Fe2AlTi intermetallic com-pound with the Heusler L21 structure [4]. We suggest that defects can be used as designing tool when developing new magnetic materials. Additional studies of point defects are under way [5]. 

References
[1] I. Miháliková et al., Nanomaterials 8 (2018) 1059; doi:10.3390/nano8121059.
[2] I. Miháliková et al., Materials 12 (2019) 1430; doi:10.3390/ma12091430. 
[3] M. Friák et al., Nanomaterials 10 (2020) 044, doi:10.3390/nano10010044.
[4] M. Friák et al., Crystals 9 (2019) 299; doi:10.3390/cryst9060299.
[5] P. Šesták et al., manuscript in preparation (2021). 



Designing improved magnetic materials by exploiting extended defects

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.

The impact of pressure and point defects on magnetism in austenitic stoichiometric Ni2MnSn

**Session Chair: **Yoshiyuki Kawazoe

SYM N - Multiscale Simulation, Defects, High Throughput Calculation, Magnetism

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Martin Friak

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Enhancing magnetism in Fe-Al alloys by increasing chemical disorder

Designing improved magnetic materials by exploiting extended defects

The impact of pressure and point defects on magnetism in austenitic stoichiometric Ni2MnSn

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