arXiv subjects

Denys Musiienko

Publications and source records attributed to Denys Musiienko.

3 recordsLinked to original sources

arXiv2026

Finite-element reconstruction of molten-zone dynamics and chemical segregation in optical floating-zone grown Ni--Mn--Ga

Homogeneous single-crystal growth of Ni--Mn--Ga magnetic shape-memory alloys is limited by incongruent melting, Mn reactivity, and longitudinal composition segregation. We report a modified optical floating-zone method in which growth is performed inside a transparent sapphire crucible, stabilising the molten zone while preserving optical heating and visual monitoring. The resulting crystals show improved growth reproducibility and controlled macroscopic geometry. To interpret the measured segregation profiles, a transient finite-element heat-transfer model in COMSOL Multiphysics was coupled to an effective molten-zone mass-balance calculation. The model reveals that the local solidification velocity is not fixed by the imposed translation speed but varies strongly as the molten-zone geometry evolves. The calculated solidification-front geometry is independently supported by EBSD analysis of a quenched growth interface, while the coupled mass-balance calculation reproduces the dominant features of the measured composition profiles. These results show that deviations from ideal Scheil--Pfann behaviour arise from the evolving molten-zone geometry and solidification conditions. The experimentally constrained model therefore provides a practical means of reconstructing otherwise inaccessible growth conditions and a basis for optimisation of sapphire-confined floating-zone growth of incongruently melting alloys.

cond-mat.mtrl-sci↗

arXiv2026

Mn-deficient Ni-Mn-Ga alloys: phase transitions, magnetic properties, and disappearance of martensitic transformation

Composition gradients formed during directional solidification are generally considered undesirable in Ni-Mn-Ga magnetic shape-memory alloys. However, these gradients offer a highly effective platform for systematically investigating composition-property relationships under identical growth and heat-treatment conditions. To exploit this approach, one Mn-deficient and two near-stoichiometric Ni-Mn-Ga oligocrystals were grown by optical zone remelting and characterized by X-ray fluorescence mapping, optical microscopy, X-ray diffraction, low-field magnetic measurements, and vibrating-sample magnetometry. The ingots contained elongated grains and axial compositional gradients, primarily associated with the redistribution of Mn and Ga, whereas Ni remained relatively uniformly distributed. The martensitic and premartensitic transformation temperatures generally increased with increasing Mn content. Neither transformation was detected between 10 and 400 K in specimens containing 20.5-22.0 at.% Mn, all of which had $e/a < 7.40$. In contrast, both transformations were observed in the investigated specimens containing at least 22.1 at.% ($e/a \geq 7.46$). This abrupt change may indicate a possible electron-concentration threshold for the occurrence of these transformations. Saturation magnetization increased with Mn content up to 25.8 at.%, reaching 96.9 A m$^2$/kg at 10 K and 71.2 A m$^2$/kg at 300 K. Post-growth annealing increased the transition temperatures and changed the low-field magnetic response. The results indicate that composition-gradient oligocrystals serve as an effective platform for investigating the relationships between composition, structure, and magnetic properties in Ni-Mn-Ga alloys with the same processing history.

cond-mat.mtrl-sci↗

arXiv2025

Broadband ferromagnetic resonance in Ni-Mn-Ga single crystal

We present a broadband ferromagnetic resonance study in single crystalline Ni$_{50}$Mn$_{28.1}$Ga$_{21.9}$ in the temperature range from room temperature to 120 °C in which the transformation from martensite to austenite phase takes place. Our measurements demonstrate that a large change (an order of magnitude) in the magnetocrystalline anisotropy at the martensitic phase transformation results in a sharp change of the resonance magnetic field. In a single variant martensite phase, the resonance fields satisfy the Kittel's resonance condition for a thin film with the gyromagnetic factor g = 2.0. With the magnetic field parallel to the easy c-axis of the single variant martensite, the resonance is observed only for frequencies larger than 22 GHz. For the multivariant martensite case, the magnetic coupling between the twin variants can be taken into account for the satisfactory Kittel's fit. We observe a weak magnetocrystalline anisotropy in the austenite phase, just above the reverse martensite transformation, comparable to the previous reports based on different magnetic measurements.

cond-mat.mtrl-sci↗