| 1. |
Hiroshi Akamine, Yohei Soejima, Tadaaki Nakamura, Sahar Farjami, Takashi Fukuda, Tomoyuki Kakeshita, Minoru Nishida, Isothermal martensitic transformations in an aged Ni-rich Ti-Ni alloy containing coherent Ti3Ni4 particles, Materials Transactions, 2020.03. |
| 2. |
Hiroshi Akamine, Sahar Farjami, Masatoshi Mitsuhara, Minoru Nishida, Takashi Fukuda, Tomoyuki Kakeshita, Electron microscopy study of preferential variant selection in CoPt alloy ordered under a magnetic field, Materials Transactions, 10.2320/matertrans.MAW201313, 54, 9, 1715-1718, 2013.08, Transmission electron microscope (TEM) and scanning transmission electron microscope (STEM) observations were carried out to investigate microstructure formation and variant selection process in L10-type ordered CoPt alloy upon a two-step ordering heat-treatment. The first step corresponds to nucleation process carried out under a magnetic field of 10 T and the second step represents growth process without magnetic field. After the first step of ordering, ordered domains of about 5 nm in size were observed and fraction of the preferred variant with the c-axis parallel to applied magnetic field was slightly higher than that of the other two variants. Formation of tweed microstructure along {011}L10 was confirmed at the initial stage of ordering. This structure is considered to be derived from the periodic alignment of interface between two ordered variants with twin relation. At the early stage of the second step of ordering, numerous micro-twins were formed through tweed microstructure and the volume fraction of the preferred variant was increased accompanying with modulation of twins, while that of other two variants was decreased. After the second step of ordering, the twins were vanished and single variant was obtained.. |
| 3. |
Hiroshi Akamine, Sahar Farjami, Hidetsugu Sakaguchi, Minoru Nishida, Phase-field simulation of magnetic field-induced preferential variant selection in A1-L10 disorder-order transformation, International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, PTM 2015
PTM 2015 - Proceedings of the International Conference on Solid-Solid Phase Transformations in Inorganic Materials 2015, 425-426, 2015.06, Near-equiatomic Co-Pt, Fe-Pd and Fe-Pt alloys undergo cubic-tetragonal disorder-order transformation and form three orientation variants. It has been reported that magnetic fieldinduced preferential nucleation of one variant in the initial stage of ordering introduces preferential growth of the same variant in the later magnetic field-free stage [1]. In this study, the effect of preferential nucleation on microstructure evolution in the later stage is studied by phasefield method. The results indicate that the increase in fraction of preferential variant depends on nucleation ratio between the variants, and the stress field originated from the macroscopic shape deformation is the most possible factor affecting preferential growth in the later stage.. |
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Hiroshi Akamine, K. H. W. van den Bos, N. Gauquelin, S. Farjami, S. Van Aert, D. Schryvers, Minoru Nishida, Determination of the atomic width of an APB in ordered CoPt using quantified HAADF-STEM, Journal of Alloys and Compounds, 10.1016/j.jallcom.2015.04.205, 644, 570-574, 2015.05, Anti-phase boundaries (APBs) in an ordered CoPt alloy are planar defects which disturb the ordered structure in their vicinity and decrease the magnetic properties. However, it has not yet been clarified to what extend the APBs disturb the ordering. In this study, high-resolution HAADF-STEM images are statistically analysed based on the image intensities estimated by the statistical parameter estimation theory. In the procedure, averaging intensities, fitting the intensity profiles to specific functions, and assessment based on a statistical test are performed. As a result, the APBs in the stable CoPt are found to be characterised by two atomic planes, and a contrast transition range as well as the centre of an inclined APB is determined. These results show that the APBs are quite sharp and therefore may have no notable effect on the net magnetic properties due to their small volume fraction.. |
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Hiroshi Akamine, So Okumura, Sahar Farjami, Yasukazu Murakami, Minoru Nishida, Imaging of surface spin textures on bulk crystals by scanning electron microscopy, Scientific Reports, 10.1038/srep37265, 6, 37265-1-37265-8, 2016.11, Direct observation of magnetic microstructures is vital for advancing spintronics and other technologies. Here we report a method for imaging surface domain structures on bulk samples by scanning electron microscopy (SEM). Complex magnetic domains, referred to as the maze state in CoPt/FePt alloys, were observed at a spatial resolution of less than 100 nm by using an in-lens annular detector. The method allows for imaging almost all the domain walls in the mazy structure, whereas the visualisation of the domain walls with the classical SEM method was limited. Our method provides a simple way to analyse surface domain structures in the bulk state that can be used in combination with SEM functions such as orientation or composition analysis. Thus, the method extends applications of SEM-based magnetic imaging, and is promising for resolving various problems at the forefront of fields including physics, magnetics, materials science, engineering, and chemistry.. |
