Magnetism of nanotwinned martensite in magnetic shape memory alloys

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 lms, but the conclusions are also valid for other Heusler-type MSMAs, both in the form of thin lms, 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 modi cation 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 arti cial antiferromagnetically coupled multilayered structures.

Magnetic Shape-Memory Heuslers Turn to Bio: Cytocompatibility of Ni–Mn–Ga Films and Biomedical Perspective

“…Coherently, the high-resolution backscattered electron (BSE) image in Figure d shows the dominant presence of Y-type twining configurations and the small regions of X-type twining configurations, some of which are labeled in the figure. X-type and Y-type are characterized by different orientations of the twins (i.e., at 45° and 90° degrees to the MgO(001) substrate, respectively) in the martensitic phase. − As a consequence, they are also characterized by different magnetic anisotropy contributions, all in-plane for the Y-type and both in-plane and out-of-plane for the X-type. The blurred regions in Figure d are expected to be due to the material undergoing the martensitic transition at room temperature.…”

Mechanical and Magnetic Properties of Porous Ni50Mn28Ga22 Shape Memory Alloy

“…Porous alloys based on Ni-Mn-Ga exhibit various properties and possess an interesting characteristic-the capability to tune the martensitic-austenitic transition over a broad temperature range through adjustments in the Ni, Mn, and Ga content, or the addition of substitute elements [48][49][50]. Therefore, these alloys with porous structures have low costs, high magnetic entropy changes under relatively low external fields, easy-to-control transition temperatures, and significant surface areas that facilitate effective thermal medium conduction and heat exchange, making them powerful candidates for magnetic refrigerants.…”

Magnetic domain engineering in SrRuO3 thin films

“…In order to further reveal the relationship between the structural domains and magnetic domains, we performed the cryogenic MFM and in situ transport measurements in a Hall-bar geometry at various magnetic fields. Since the anisotropic field of SRO is extremely large, e.g., 12 T, the dipole-dipole interaction, which determines the magnetic domain structures in low-anisotropy materials, such as martensites 37 , can be negligible in SRO. Therefore, the structurally pinned magnetic domains would follow local anisotropy and the magnetic domains can be mapped to structural domains.…”