arXiv subjects

Martin Heczko

Publications and source records attributed to Martin Heczko.

3 recordsLinked to original sources

arXiv2026

The effect of grain boundaries on magnetic exchange interactions in iron

This work investigates how grain boundaries (GBs) modify magnetic exchange interactions in bcc iron, with particular focus on the effect of phosphorus segregation. Using density-functional theory combined with the Liechtenstein-Katsnelson-Antropov-Gubanov Green's-function approach, we calculate Heisenberg exchange parameters for three symmetric tilt GBs, $\Sigma5(310)$, $\Sigma13(510)$, and $\Sigma13(320)$, and use these parameters in Monte Carlo simulations to evaluate finite-temperature magnetic behavior. All clean GBs exhibit strong local deviations from bulk exchange interactions, including antiferromagnetic coupling across the boundary plane. These negative exchange interactions are not governed by interatomic distance alone, but arise primarily from the altered local coordination and symmetry breaking at the GB. Phosphorus segregation, modeled in both substitutional and interstitial configurations at the $\Sigma5(310)$ GB, suppresses the antiferromagnetic couplings and significantly redistributes the local exchange landscape through chemical and electronic effects. Monte Carlo results show that, despite pronounced local perturbations, realistic GB densities cause only a small reduction in the Curie temperature because bulk-like regions dominate the global magnetic transition. A substantial decrease in Curie temperature appears only when the GB volume fraction is artificially increased. The results demonstrate that GBs strongly influence local magnetic interactions while having a limited effect on global magnetic ordering, and they establish a general framework for linking atomistic interfacial structure and chemistry to mesoscale magnetic behavior in Fe-based materials.

cond-mat.mtrl-sci↗

arXiv2026

First-principles study of doping influence on twin formation in Ni-Mn-Ga nonmodulated martensite

We investigate how chemical substitution reshapes the energetics of twin formation in non-modulated (NM) Ni-Mn-Ga martensite. Using density functional theory, we compute generalized planar fault energy (GPFE) curves for the $(101)[10\bar{1}]$ shear system in stoichiometric Ni$_{2}$MnGa and in a set of doped supercells containing Cu, Co, Fe, or Zn on different sublattices. The GPFE landscape is used as a microscopic descriptor of twinning behavior: the first barrier reflects intrinsic stacking-fault formation (twin nucleation), whereas subsequent barriers govern twin thickening and boundary motion. We show that the impact of dopants is strongly site dependent. Substitutions Cu$\rightarrow$Mn, Cu$\rightarrow$Ni, Co$\rightarrow$Ni, and Zn$\rightarrow$Mn lower the nucleation barrier and generally soften the GPFE profile, indicating more favorable conditions for twin formation and propagation; these cases also correlate with a reduced tetragonality $c/a$, which implies a smaller twinning shear and a reduced energetic cost of twin formation. In contrast, Cu$\rightarrow$Ga, Co$\rightarrow$Mn, Co$\rightarrow$Ga, Fe$\rightarrow$Ga, and Zn$\rightarrow$Ga increase GPFE barriers and hinder twinning, even though such substitutions are often used to enhance martensite stability and raise $T_{m}$. Fe$\rightarrow$Mn leaves barrier heights largely unchanged, while Fe$\rightarrow$Ni produces an anomalous GPFE response indicative of unstable twin configurations. Finally, inspired by the nanotwinning characterisation of 10M/14M modulation, we link the depth of the two-layer nanotwin minimum to modulation stability. The substitutions Fe$\rightarrow$Mn, Cu$\rightarrow$Ni, and Zn$\rightarrow$Mn result in a lower energy minimum compared to the structure without the double-layered twin. The other substitutions favor the twin-free NM structure.

cond-mat.mtrl-sci↗

arXiv2024

Shear Deformation of Nonmodulated Ni$_2$MnGa Martensite: An Ab Initio Study

The impact of shear deformation in $(1\,0\,1)[1\,0\,\bar{1}]$ system of non-modulated (NM) martensite in Ni$_2$MnGa ferromagnetic shape memory alloy is investigated by means of ab initio atomistic simulations. The shear system is associated with twinning of NM lattice and intermatensitic transformation to modulated structures. The stability of the NM lattice increases with increasing content of Mn. The most realistic shear mechanism for twin reorientation can be approximated by the simple shear mechanism, although the lowest barriers were calculated for pure shear mechanism. The energy barrier between twin variants further reduces due to spontaneous appearance of lattice modulation or, in other words, the nanotwins with thickness of two atomic planes. Such nanotwins appear also on the generalized planar fault energy (GPFE) curve calculated using a newly developed advanced procedure and exhibits even lower energy than the defect free NM structure. These nanotwin doublelayers are also basic building blocks of modulated structures and play an important role in intermartensitic transformation.

cond-mat.mtrl-sci↗