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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.

Standing spin waves in triangular dots (truncated pyramids) were investigated both experimentally and theoretically. Arrays of nickel triangular pyramids with the base side of 270 nm and height of 70 nm were deposited on Si (111) substrate. The spectra of ferromagnetic resonance were obtained at room temperature with an external saturating magnetic field directed perpendicular to the array plane. The theoretical approach, which allows to describe standing spin wave modes both in perpendicular magnetized regular prisms and in close to prisms truncated pyramids, was developed. Theoretically calculated resonance fields for the observed modes are in a good correlation with the experiment.

The influence of film thickness on the formation of twinning structure, martensitic transformation and magnetoelastic properties of epitaxial films of Ni(Co)MnSn magnetic shape memory alloy is investigated by means of ferromagnetic resonance spectroscopy, synchrotron X-ray diffraction and standard magnetic measurements. It is found that constraints from the film/substrate interface block the martensitic transformation in the 20 nm thick film. The increase of the film thickness results in a progressive stress relaxation and, as a result, the martensitic transformation becomes possible starting from 50 nm. Twinning of the films is required to conserve the films surface area. The elastic energy balance between the film/substrate interface and the twin boundaries leads to the formation of a submicron wide, stripe like, periodical structure of twins, which is of interest for spintronic or magnonic applications. The width of the twin variants increases with the film thickness growth, resulting in the dramatic modification of magnetic properties.

In this work, a detailed report on the growth of elongated bar-like crystallites with a definite crystallographic orientation, forming a self-organized patterned surface layer, with a thickness of about 100 nm, onto 900 nm thick ferromagnetic shape memory alloy Ni–Mn–Ga films epitaxially grown on MgO(001) is presented. The phenomenon appears above a threshold of lattice mismatch (≈2%) between the film and substrate. The structural, magnetic and transformation properties of such films are described and analyzed. Particularly, the different magnetic states of the two film layers are discerned by ferromagnetic resonance. The feasibility of creating a ferromagnetic shape memory alloy with a nanoscale self-patterned surface, during the very same process of film deposition, can be interesting for potential applications where control of the surface architecture would be needed.

Magnetic shape memory alloys are under intensive investigation due to their unusual physical properties, such as magnetic shape memory effect, magnetic field induced superelasticity, direct and inverse magnetocaloric effect etc., promising for novel applications. One of the intriguing properties of these materials in a single phase state is a giant magnetoresistance. Here we report the remarkable results about the magnetoresistive properties of epitaxial films of Ni52.3Mn26.8Ga20.9 magnetic shape memory alloy in the temperature range of 100–370 K, well below the martensitic transformation temperature. It was found that the formation of non-collinear magnetic structure due to a nanotwinning of the film results in electron scattering on such a structure and noticeable negative magnetoresistance in the entire investigated temperature range.

1 µm-thick polycrystalline thin films of the ferromagnetic shape memory alloy, Ni49.7Fe17.4Co4.2Ga28.7 (at.%), were sputter-deposited on MgO(0 0 1) substrates heated at 773 K. The films' structure, magnetic properties and transformation behavior have been characterized. They exhibit both forward and reverse martensitic transformations at 218 K and 232 K, respectively. Ferromagnetic resonance and x-ray diffraction measurements discerned three ferromagnetic phases: cubic L21-ordered austenite, tetragonal martensite and disordered cubic γ-phase. The latter phase showed a unique spin-waves resonance spectrum, enabling estimation of the value of the exchange stiffness constant equal to A = 0.7 × 10−6 erg cm−1.