The magnetostrictive materials exhibit a strain caused by the orientation of the magnetic moment when exposed to a magnetic field. A particular class of magnetostrictive materials is called magnetic shape memory (MSM) alloys or ferromagnetic shape memory alloy (FSMA) materials, which can change their shape remarkably when subjected to magnetic field. In the following, the MSM materials are introduced and their extraordinary structure, properties, and performance are described.

Giant Magnetostrictive Materials

Martensitic transitions in shape memory Ni-Mn-Ga Heusler alloys take place between a high temperature austenite and a low temperature martensite phase However, intermartensitic transformations have also been encountered that occur from one martensite phase to another To examine intermartensitic transitions in magnetic shape memory alloys in detail, we carried out temperature dependent magnetization, resistivity, and x-ray diffraction measurements to investigate the intermartensitic transition in Ni50Mn50–xGax in the composition range 12≤x≤25 at % Rietveld refined x-ray diffraction results are found to be consistent with magnetization and resistivity data Depending on composition, we observe that intermartensitic transitions occur in the sequences 7M→L10, 5M→7M, and 5M→7M→L10 with decreasing temperature The L10 non-modulated structure is most stable at low temperature

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Extended investigation of intermartensitic transitions in Ni-Mn-Ga magnetic shape memory alloys: A detailed phase diagram determination

The all-d-metal Mn2-based Heusler ferromagnetic shape memory alloys Mn50Ni40−xCoxTi10 (x = 8 and 9.5) are realized. With a generic comparison between d-metal Ti and main-group elements in lowering the transformation temperature, the magnetostructural martensitic transformations are established by further introducing Co to produce local ferromagnetic Mn-Co-Mn configurations. A 5-fold modulation and (3, −2) stacking of [00 10] of martensite are determined by X-ray diffraction and HRTEM analysis. Based on the transformation, a large magneto-strain of 6900 ppm and a large volume change of −2.54% are observed in polycrystalline samples, which makes the all-d-metal magnetic martensitic alloys of interest for magnetic/pressure multi-field driven applications.

Magnetostructural martensitic transformations with large volume changes and magneto-strains in all-d-metal Heusler alloys

A Ni-Mn-Ga single crystal with a modulated five-layered martensite structure is reported, demonstrating giant magnetic field induced strain (MFIS) of 7.1% at room temperature and of 6% at temperatures close to the austenite transformation (TA = 71 °C). The room temperature MFIS clearly exceeds the best results of around 6% measured earlier in 10M martensites. The larger MFIS is connected to the huge (>1%) change in the lattice distortion of the 10M structure, obtained within a narrow temperature interval of 47 K, which has been previously observed only during intermartensitic transformation. The present material shall effectively reduce the size of magnetic shape memory actuators.

Over 7% magnetic field-induced strain in a Ni-Mn-Ga five-layered martensite

The magnetic shape memory effect can be classified as an example of the multiferroic effect combining ferro-elasticity and (ferro)magnetism. After short overview of all known effects the focus is on magnetic field induced structure reorientation (MIR) in the martensite of Ni–Mn–Ga. In this material giant deformations of up to 12% have been observed in moderate magnetic fields. The phenomenology of the effect is first discussed and a model presented. The properties of Ni–Mn–Ga relevant to MIR are then considered. One necessary condition for MIR is a highly mobile twin boundary or interface between two differently oriented martensite variants (ferroelastic domains). In 10M modulated martensite, two types of mobile twin boundary (type I and type II) are observed with complex layered microstructures consisting of a hierarchy of twinning systems. The boundaries strongly differ according to the magnitude and temperature dependence of the twinning stress. Finally, the nature of these boundaries and their...

Magnetic shape memory effect and highly mobile twin boundaries

Magnetic field-induced strain (MFIS) of 12% is reported in ferromagnetic Ni 46Mn24Ga22Co4 Cu 4 martensite exhibiting non-modulated (NM) tetragonal crystal structure with lattice parameter ratio c / a > 1 . The strain was measured at ambient temperature in a magnetic field of the order of 1 T. The twinning stress σ T W and the magnetic stress σ M A G were also measured and the condition for a giant MFIS observation σ T W < σ M A G was confirmed. The MFIS was achieved in NM Ni 46Mn24Ga22Co4 Cu 4 martensite by considerable lowering of the σ T W value as compared to the values for NM martensites in ternary Ni-Mn-Ga system.

W. Zou

Papers

12% magnetic field-induced strain in Ni-Mn-Ga-based non-modulated martensite