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Magnetic Shape Memory (MSM) Alloys are a class of Shape memory alloys which can change their shapes under a magnetic field by undergoing a martensitic transformation. The shape change is caused by structure reorientation of the twin boundary movement or the martensite twin variant redistribution under magnetic field. This effect occurs within certain temperature range, which is below the martensite transformation temperature and above the critical temperature for MSM effect. Ni-Mn-Ga is now the most extensively investigated material in terms of the MSM properties. Magnetic field-induced (MFI) strains of up to 10% have been observed in Ni2MnGa. Such large MFI strains qualify the MSM alloys as a promising candidate for actuator materials. Besides the conversion of magnetic stimuli to mechanical response, the reverse operation of the material is also gaining increasing interest due to its potential applications in sensors and power generation.

History and Development
While the discovery of the shape memory effect can be traced back to the 1930s, the first observation of the magnetic shape memory (MSM) effect was not reported until several decades later.In 1984, the stoichiometric Ni2MnGa compound was extensively studied by Webster, et al in an attempt to determine the influence of structural instability on the ferromagnetic properties of the Heusler alloys. Further collaborative effort was made by research groups from Ukraine and Spain to develop the off-stoichiometric Ni–Mn–Ga alloy system and to study ferromagnetic ordered shape memory alloys. In 1996, K. Ullakko and his coworkers at MIT first reported magnetic field induced strains (MFIS) of nearly 0.2% in unstressed crystals of Ni2MnGa under magnetic fields of 8 kOe applied at 265 K. Extensive research was then carried out to investigate the MSM properties and MFIS in Ni-Mn-Ga system and other alloys. It was found that pre-stressed MSM alloys show significantly increased MFIS under applied magnetic field, and the strain up to 6% was reported in the case of Ni-Mn-Ga system with orthogonal magnetic field and external load. The percentage strain depends greatly on the composition of the alloy. By tuning the proportion of the Ni,Mn and Ga in the alloy, it is possible to increase the electron to atom ratio, which increases the martensitic transition temperature from 220K in stoichiometric Ni2MnGa so that the MSM material can work at room temperature. After a decade’s effort, it has made possible to achieve magnetically controlled variations in the linear size of crystals up to 10% by A. Sozinov, using 7M modulation.

However, the brittleness of Ni-Mn-Ga alloys is the biggest obstacle for its practical applications. It has been reported that proper addition of rare earth elements can improve the mechanical properties of various alloys and intermetallic compounds. Therefore, the modification of Ni-Mn-Ga alloy by adding rare earth elements then became a new research field. For example, the addition of Tb, Sm, have been reported to increase the bending strength of Ni-Mn-Ga alloys to some extent. And the compressive ductility of the alloy can be significantly improved by adding Nd. Moreover, the shape memory effect(e.g. martensitic transformation temperature) of Ni–Mn–Ga alloys are very sensitive to some rare earth elements. Researches are also going on to add elements, e.g. Gd, Dy , Ge to improve the MSM effect.

Another possible way of overcoming the disadvantages of single-crystalline MSMs (e.g. brittleness, difficult preparation and cost) is to make the MSM–polymer-composite, which is to embed the single-crystalline MSM particles (single-crystalline when in the austenitic state) in a stiffness-matched matrix, which is soft enough to allow the particles to deform. The MSM–polymer-composite is basically made by mixing MSM particles with a polymer and curing the polymer within a desired mould with an applied magnetic field For applications, the use of matrix can also reduce eddy current losses caused by an alternating magnetic field as well as enhance the fracture of the alloys. The matrix material for an actuator composite must 1) allow the particles to undergo the large MSM-related strain, and 2) must transmit this strain such that the whole composite exhibits an appreciable strain. The matrix materialused so far for these MSM composites are polyurethane for dampers and epoxy for SM actuators.

While ferromagnetic single crystals display excellent MSM effect (~10% strain in Ni–Mn–Ga alloys near the stoichiometric composition Ni2MnGa), their polycrystalline counterparts show comparatively much poorer properties. This is because the MSM effect is frequently inhibited by incompatibilities at grain boundaries. In polycrystals, the grains cannot easily deform by twin boundary motion, as the surrounding grains constrain them, which leads to the necessity for higher critical stresses or magnetic fields for the movement of twin boundaries compared with single crystals. However, due to the difficulties in fabricating large-size single-crystal alloys, it is crucial to develop polycrystalline MSM alloys with high performance for practical interest.Studies have been done in polycrystalline foams (resulting in 0.115% strain), in polycrystalline melt-spun ribbons (resulting in 0.025% strain) and in thin films (no macroscopic strain, as film was constrained). Textured polycrystalline MSM-polymer-composites are also under study. Polycrystalline Ni2MnGa fibres embedded in epoxy are reported to reach large MSM strain of ~1.0%.

Lattice type
Magnetic shape memory alloys have usually been single crystal systems which are usually based off of the Ni2MnGa system (See Figure 1). This is a subset of a Heusler alloy, which is an intermetallic alloy with a specific ordering of atoms in the lattice.

In the cubic form, Ni2MnGa is paramagnetic, and lacks any shape memory properties. The shape memory effect arises when the alloy undergoes a martensitic transformation from the cubic structure to a martensitic orthorhombic or tetragonal phase. These are both layered phases, where the tetragonal phase has a reshuffling of atoms with a period of five (5M) while the orthorhombic phase has a seven layer structure (7M). This transformation occurs upon cooling the crystal from high temperature to below the transition temperature, which can range from 200K for Ni2MnGa and increasing with an increase in Ni concentration up to 625K with Ni2+xMn1−xGa where x = .36.

Mechanism
During the martensitic transformation, several crystal twins form, which are divided by the twin boundaries. The unit cells between two twin boundaries are considered to belong to the same twin variant, which are able to undergo reorientation under external force. Before the external magnetic field is applied, the magnetization vectors lie along certain crystallographic axes called directions of easy magnetization. When the magnetic field is applied, usually the magnetization vectors will turn from the easy direction of the unit cell to the direction of the external magnetic field. However, with the presence of the twin boundaries, it is possible to align the magnetization vectors along with the external magnetic field by changing the orientation of the unit cells without moving the magnetization vectors away from the easy axis inside the unit cell. If the anisotropy energy is high (which means the energy required to turn the magnetization vectors away from the easy axis is high), while the energy of the motion of the twin boundaries is low enough at the same time, it will be more energetically favorable to reorient the unit cell, which will eventually cause shape change in the direction parallel to the twin boundaries.

Crystal Training
There are two types of strain that Ni2MnGa crystals can undergo,magnetoelasticity and magnetoplasticity. Magnetoelasticity is a strain which is only present during the application of a magnetic field, found in crystals which have not been trained and have small twinned areas. Magnetoplasticity can only be attained in properly trained crystals, which undergo thermo-magneto-mechanical training; which is a cyclical set of deformation followed by the application of a magnetic field. The total attainable strain increases with the amount and effectiveness of the training put into the crystal.

Actuators
The main application of magnetic shape memory (MSM) alloys is for actuators. The commercial MSM actuators can be found from Adaptamat Ltd. Usually, the MSM actuator consists of the MSM element, ferromagnetic core and coils surrounding the ferromagnetic core. The operation mechanism could be explained as below: before the application of the magnetic field, the MSM element is aligned with the short martensite crystallographic along the direction of pre-stress loading. When the magnetic field perpendicular to the MSM element is imposed, the MSM element elongates in the direction perpendicular to the field. After the removal of the field, the MSM element returns to its previous position with the help of mechanical spring. The pre-stress on the sample ranged from 0.5 to 1MPa. Sometimes, biasing permanent magnets are used to increase field strength and improve the efficiency of the coils. The key elements of the mechanical circuit of the MSM actuator include the MSM element, the moving mass and the mechanical spring. Sometimes, the resonance frequency of the system is reached since the MSM actuator can work at a high frequency (5000Hz or even higher). Hence, in some cases the mechanical resonance frequency can be used to increase the motion of the system. The core type of the magnetic circuit influences the properties of the MSM actuators significantly. Specifically, eddy currents should be minimized in high frequency applications.

One advantage of MSM actuator is the fast response. For example, response time of 0.2ms was demonstrated to reach strokes of 5% for non-stoichiometric Ni2MnGa alloy. The operation speed of the MSM actuators only depends on both the eddy currents in the core and the inertia of the moving parts.

Hysteresis between the strain and the actuator input current is caused by the internal properties of the MSM material, e.g. twinning. Hysteresis could cause the loss in material and ease the control of some positioning system application. However, it reduces vibrations and overshooting of the actuator in rapid shape changes of the element.

The strain of the actuator is related with the load on it. The optimal load to reach maximal strain is around 1-1.5Mpa. Fatigue test results demonstrated that the stroke of the actuator was stable even after 200 million cycles of the alternating magnetic field. The performances of various actuators were summarized in. According to the performances on maximum operating frequency and maximum work output, MSM actuators bridge the gap between traditional and “smart” actuators. For example, the highest operating frequency of MSM actuators (~104Hz) is lower than that of magnetostriction and piezo actuators (~107Hz). The maximum work output of MSM actuators (~5*104J/m3) is lower than that of shape memory actuators (~5*105J/m3). Compared to conventional actuator technologies (hydraulics, solenoid and pneumatics), the advantages of MSM actuators include fast response, reduced size, high operation frequencies, reliability and efficiency.

Sensors
While the general interest of the MSM materials study is focused on the conversion of magnetic signal into mechanical response, the inverse application of the MSM property has attracted increasing attention. The principle of the inverse application of MSM property is based on the fact that like under a magnetic field, when external mechanical force is applied to the MSM materials, the reorientation of the twin variants also occurs, accompanied by the consequential magnetization of the material. It has been reported that under constant magnetic field, strain induced changes in the magnetic flux density can be observed in Ni-Mn-Ga single crystals. Applications of this property may be applied to sensor design and energy harvesting.

The sensing behavior of the MSM material can be demonstrated in a commercially-available Ni-Mn-Ga single crystal. As the initial state, a constant magnetic bias of 368 kA/m is applied to the MSM material to align all the magnetization vectors in the same direction and convert the entire material into the single magnetic-field-preferred variant. The loading stress of 4.4 MPa, which serves as the stimulus, will compress the crystal to induce the twin variants reorientation, during which the magnetization vectors are no longer aligned with the external field. Such change in the magnetization vector direction causes the change in the magnetic flux density of 145 mT. If the magnetic bias is large enough, the material will return to its initial length after the stress is unloaded, so that the sensor would stand by for next stimulus.

Energy harvesters
The change in the flux density induced by external stress can be further extended to voltage generation and energy harvesting. Induced voltages close to 100 V can be achieved in the MSM actuator with a non-stoichiometric alloy Ni49Mn26Ga25, even the measurement system is small and nonoptimized. Since the stress induced shape change is reversible under external magnetic field, it is possible to create cyclic twin variant reorientation, which has the potential to convert free vibrations to electrical power. With a non-stoichiometric alloy Ni51.1Mn24Ga24.9, a few milliwatts of power output can be obtained under slow fluctuating loads of 10 Hz. This can be increased to over 1 watt at frequencies over 100 Hz by optimizing the power conversion hardware. Such energy harvesting behavior is comparable and possibly better than that of magnetostrictive and piezoelectric materials at low frequencies. Several relevant factors can also be optimized including loading frequency, strain range and magnetic field strength to achieve maximal energy output.