arXiv subjects

L. Straka

Publications and source records attributed to L. Straka.

3 recordsLinked to original sources

arXiv2025

Antiphase boundaries in Ni-Mn-Ga single crystal - experiment and model

Thermally induced antiphase boundaries (APBs) in ferromagnetic, ordered Ni-Mn-Ga single crystal exhibit complex, irregular shapes and closed loops without any lattice plane preferences. The APBs were visualized on polished (100) surface using magnetic force microscopy (MFM) at the same location in parent cubic austenite and monoclinic martensite with uniaxial magnetic anisotropy. Based on ab-initio calculation we suggest that one APB curve with dark and light contrast consists of a pair of APB interfaces with narrow, only one-layer thick, core with structural partial B2' order in contrast to full L21 order of bulk. Calculated magnetic contrast using magnetostatic continuum simulations agrees well with MFM observation.

cond-mat.mtrl-sci↗

arXiv2025

Duality of Wave Modulation and Nanotwinning in Ni-Mn-Ga Martensite via Long-Period Commensurate States

Structural modulation is a key ingredient behind the extraordinary (magneto)elastic response of Ni-Mn-Ga martensite, yet its link to fine microstructural features and twin-boundary supermobility remains unresolved. Here we analyse martensitic single crystals of Ni50.0Mn27.7Ga22.3 and Ni50.0Mn28.1Ga21.9. Neutron and X-ray diffraction reveal an anharmonic five-layer structural modulation, evidenced by high-order satellite reflections, that evolves from commensurate (q = 2/5) to incommensurate (2/5 < q < 5/12) upon cooling. Interpreting the refined modulation displacements as a basal-plane stacking sequence links the wave description to the microstructural evolution on cooling. In this view, evolving incommensurability produces periodic nanodomains interpreted as emerging a/b-nanotwins with a characteristic size of approximately 20 nm at approximately 290 K. With further cooling, the modulation can lock into long-period commensurate (LP-C) states, such as 34O (q = 7/17), 24O (q = 5/12), and 14O (q = 3/7), whose orthorhombic unit cells can be viewed as a/b-nanotwins. Ab initio calculations show that LP-C structures are energetically competitive with the initial commensurate state, supporting a shallow martensitic energy landscape. We propose a physical picture in which the martensitic transformation selects a commensurate state with q = 2/5 in the Mn-rich compositions studied here, while subsequent cooling drives relaxation within the martensitic landscape toward LP-C states, particularly 24O in the present alloys. The resulting structure is neither purely wave-like nor purely nanotwinned; rather, it reflects coupling between a coherent modulation wave and local accommodation via NM-like tetragonal distortions, nanotwinning, and LP-C lock-ins, providing a structural basis for the wave-nanotwin duality in Ni-Mn-Ga martensite.

cond-mat.mtrl-sci↗

arXiv2024

Anharmonic Incommensurate Structure Modulation in Ni-Mn-Ga Martensite Exhibiting Highly Mobile Twin Boundaries

Understanding the crystal structure of magnetic shape memory alloys is crucial for insights into their unique properties, such as the high mobility of twin boundaries and magnetic field functionality. The complex neutron diffraction patterns betweeen 10-300 K indicate an incommensurate and simultaneously anharmonic modulation function (AMF) in Ni50.0Mn27.7Ga22.3 10M martensite. Our identification of the dominant Fourier components in the AMF allowed for a comparison between calculated diffraction patterns and experiments. The AMF explains the appearance of peculiar small-intensity diffraction peaks when nearly commensurate AMF at 300 K turns to incommensurate AMF upon cooling. Further analysis reveals that divergent periodicity between the incommensurate modulation and the lattice leads to the formation of additional nanodomains. Interpreting the modulation displacements within the nanodomains in the terms of the (2-3)2 stacking sequence of basal (110) planes enables to understand that these nanodomains are emerging a/b nanotwins. The nanotwinning interlinked with incommensurate modulation also explains the transition from the tetragonal to the orthorhombic symmetry upon cooling. We identify specific low-temperature orthorhombic structures, like 34O, 24O, 14O, which are significant as their unit cell simultaneously represents an a/b-nanotwin. The ab initio calculations confirm the stability and low and comparable energy of all found nanotwinned structures. Based on the presumed stability of the nanotwinned state and literature comparisons, we propose that the low temperature state of the martensite nominally marked as five-layered modulated, 5M or 10M, is one of the specific a/b-nanotwinned configurations such as 34O, 24O, 14O. The exact choice depends on the composition, but 14O resulting from q = 3/7 is the ultimate limit with a corresponding smallest twin domain size of 3.5 nm.

cond-mat.mtrl-sci↗