Magnetic Shape Memory Properties Research Articles

Modelling Shape-Memory Effects in Ferromagnetic Alloys

We develop a combined Ginzburg–Landau/micromagnetic model dealing with conventional and magnetic shape-memory properties in ferromagnetic shape-memory materials. The free energy of the system is written as the sum of structural, magnetic and magnetostructural contributions. We first analyse a mean field linearized version of the model that does not take into account long-range terms arising from elastic compatibility and demagnetization effects. This model can be solved analytically and in spite of its simplicity allows us to understand the role of the magnetostructural term in driving magnetic shape-memory effects. Numerical simulations of the full model have also been performed. They show that the model is able to reproduce magnetostructural microstructures reported in magnetic shape-memory materials such as Ni2MnGa as well as conventional and magnetic shape-memory behaviour.

Understanding the martensitic phase transition ofNi2(Mn1−xFex)Gamagnetic shape-memory alloys from theoretical calculations

By using first principles in combination with atomistic spin dynamics calculational methods, we determine the temperature-dependent free energies of the $L{2}_{1}$- and $L{1}_{0}\ensuremath{-}{\mathrm{Ni}}_{2}({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\text{Ga}\phantom{\rule{0.28em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$, including phonon vibrational and magnetic energies. The $x$-dependent martensitic phase transformation (MPT) temperature $({T}_{M})$ and the Curie temperature $({T}_{C})$ are well represented. It is found that, the abnormal nonmonotonic behavior of ${T}_{M}\ensuremath{\sim}x$ mainly originates from the phonon vibrational free energy. The magnetic energy, which does not change the trend of ${T}_{M}$ against $x$ but decreases the driving force of the MPT, is indispensable as well to get reasonable ${T}_{M}$ values. This insight provides a good understanding of the physical mechanisms driving the MPT of ${\mathrm{Ni}}_{2}({\mathrm{Mn}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\text{Ga}$, and promotes the improvement of their magnetic shape memory properties.

Effects of the substitution of gallium with boron on the physical and mechanical properties of Ni–Mn–Ga shape memory alloys

The effects of the substitution of gallium with boron on the physical, mechanical and magnetic shape memory properties of Ni51Mn28.5Ga20.5−x B x (at.%) (x = 0, 1, 2, 3) polycrystalline alloys are investigated. It has been found that transformation temperatures are decreasing while hardness is increasing with boron addition. B-doping of NiMnGa alloys results in the formation of a second phase that increases its ductility and strength in compression. Moreover, saturation magnetization of austenite is decreasing, while Curie temperature of austenite is increasing with B-doping.

Recent Progress and Future Perspectives in Magnetic and Metamagnetic Shape-Memory Heusler Alloys

Magnetic shape-memory properties refer to the ability of certain materials to show strong response in strain to an applied magnetic field. This strain is caused by either inducing the martensitic transition or rearranging martensitic variants. In the first, case a superelastic effect is possible, while in the second, the system is able to show the shape-memory effect. The complex behaviour displayed by these materials is mainly a consequence of a strong interplay between magnetism and structure which is driven by a martensitic transition. This interplay is the source of many other observed effects such as giant magneto-resistance, exchange bias and magnetocaloric effects. In this paper, we will overview the present state of the art, discuss present challenges and outline some future perspectives in the field.

Temperature and field induced strain in polycrystalline Ni 50Mn 35In 15− xSi x magnetic shape memory Heusler alloys

Magnetic shape memory properties of polycrystalline Ni 50Mn 35In 15− x Si x were investigated. A reversible strain of more than 0.4% was observed for x = 0 at a magnetic field H = 5 T that was found to be associated with a field induced reverse martensitic transformation. The strains were found to increase with the substitution of In by Si and strains larger than 1% were observed for x = 2 at H = 5 T. Both the positive and negative strain changes were observed in the vicinity of martensitic transition temperatures. The strain in Ni 50Mn 35In 15− x Si x was found to depend on silicon concentration, and on samples texture.

Bulk and thin films of magnetic shape memory Ni–Mn–Ga alloys deposited by multi-dipolar plasma-assisted sputtering

Ni–Mn–Ga alloys of Heusler type, with potential magnetic shape memory properties, were synthesized as bulk materials. They were characterized both for their structural and magnetic properties at and above room temperature. Thin film deposits were realised by using a microwave-controlled multi-dipolar plasma-assisted sputtering technique. The properties of all resulting materials are discussed in terms of composition of the alloys owing to the method used to elaborate the materials.

Magnetocaloric effect in Heusler shape-memory alloys

We present a comparative study of the magnetocaloric properties of Ni–Mn–X Heusler shape-memory alloys with X = Ga , Sn and In. In these materials, the magnetocaloric effect is a consequence of the magnetostructural coupling that enables the magnetic shape-memory properties. We show that inverse magnetocaloric effects can occur in these materials. The origin of this anomalous behavior is different in stoichiometric Ni 2 MnGa and in Ni–Mn–Sn/In. In the former case it is related to the strong uniaxial magnetic anisotropy of the martensitic phase, while in the later it is an intrinsic effect associated with an incipient antiferromagnetism.

Magnetic field and stress induced martensite reorientation in NiMnGa ferromagnetic shape memory alloy single crystals

Magnetic shape memory properties of a single crystal Ni 2MnGa alloy were characterized through monitoring magnetic field induced strain (MFIS) as a function of compressive stress, and applied stress induced strain as a function of magnetic field. Compressive stress and magnetic field were applied perpendicular to each other along the [1 0 0] and [0 1 1] axes, respectively. The critical magnetic fields for variant reorientation, first cycle effect and cyclic evolution of MFIS are reported as a function of stress level. It was revealed that increasing constant magnetic field level significantly increases the stress required for the reorientation, i.e., magnetostress and leads to superelasticity in martensite. Possible microstructural mechanisms, considering the interplay between stress and magnetic field favored martensite variants, magnetic domains and magnetization rotation, are proposed. Moreover, it was observed that the MFIS evolution is field rate dependent as was evidenced by a rate dependent two-stage reorientation where the maximum MFIS magnitude increases as the field rate increases. This effect was attributed to the difference between the nucleation and propagation barrier strength for twin boundary motion in NiMnGa alloys. The magnetostress (5.7 MPa), blocking stress (5 MPa) and maximum MFIS (5.8%) combination observed in this study is the highest reported to date in NiMnGa alloys. The high blocking and magnetostresses are a consequence of the low test temperature (−95 °C) where the magnetocrystalline anisotropy energy is high and detwinning stress is low. Thus, for magnetic shape memory alloys, the selection of the operating temperature with respect to martensite start and Curie temperatures is critical in optimizing actuator performance since both magnetocrystalline anisotropy energy and detwinning stress are a strong function of temperature below the characteristic temperatures.