ABSTRACT
Heusler-type magnetic shape memory alloys (MSMAs) exhibit a martensitic transformation (MT) accompanied by a complex magnetic reordering, strongly affected by an intricate martensitic microstructure. The hierarchic twin structure of martensite, formed as a result of minimization of elastic energy down to atomic scale, is under intensive study nowadays. On the other hand, the much more sophisticated problem of the relationship between nanoscale twin structure and the magnetism in MSMAs has being tackled only recently. It will be shown in this topical review that the nanotwin structure affects not only the basic magnetic parameters of MSMAs, but also can change qualitatively its magnetic nature and related magnetodynamic and magnetoresistance properties. This will be primarily illustrated, both theoretically and experimentally, on the prototype Ni-Mn-Ga and Ni(Co)-Mn-Sn MSMAs in the form of epitaxial thin films, but the conclusions are also valid for other Heusler-type MSMAs, both in the form of thin films, ribbons and bulk single crystals and polycrystals. The following new and remarkable phenomena will be highlighted. (i) A strong ferromagnetic exchange coupling is observed between the submicron twin components in Ni-Mn-Ga ferromagnetic martensite. It results in the modification of the average magnetic anisotropy and the formation of a non-collinear magnetic structure, whereby a negative magnetoresistance appears in a wide temperature range. (ii) Weak antiferromagnetic coupling occurs between the ferromagnetically ordered twin components in Ni(Co)-Mn-Sn martensite. This coupling enabled to explain the exchange bias and magnetic resonance spectra in the same terms as for artificial antiferromagnetically coupled multilayered structures.
ABSTRACT
The article reviews literature on optical coherence tomography (OCT) - one of the modern methods of early diagnostics of primary open-angle glaucoma, which has won great popularity among Russian and foreign ophthalmologists. In addition to a summary of the history of the emergence and development of this method, the main purpose was to compare the investigation results and the analysis of the optic disc as the main target subjected to pathological alterations in glaucoma development. Most of the studies comparing the diagnostic capabilities of OCT and Heidelberg retinal tomography (HRT) in the early detection of glaucoma show equivalence of these methods. On the other hand, OCT allows performing subtler structural analysis of the retina, whereas HRT provides more accurate analysis of internal parameters of the optic disc.
