Magnetic Shape Memory Materials Research Articles

Where are we at with shape-memory alloys in this ‘high-tech’ world?

Some of the basic physics of shape-memory materials is outlined. Shape memory relies on a change in the crystallography of the material via a martensitic transformation. While many alloys show shape-memory properties, only certain materials have ‘made it’ to technological applications. The most notable of these is <i>Nitinol</i>, the commercial name for a nickel‒titanium alloy which was discovered in the US Navy Laboratories almost by accident. The most important current and future applications for <i>Nitinol</i> are, and will continue to be, as various medical devices. The material is most favourable for medical applications, firstly, because it exhibits shape-memory behaviour at very close to body temperature (37°C) and, perhaps equally importantly, only an extremely small percentage of human beings have any allergic reaction to either nickel or titanium. Not so important are magnetic shape-memory materials for which, particularly the material Ni<sub>2</sub>MnGa, there are increasing numbers of applications requiring the shape-memory behaviour to be controlled by an applied magnetic field. The properties of some shape-memory materials relevant to current applications are summarised and, consistent with the theme of sustainability, some comment is made on the likely future of shape-memory materials in the market place.

Fabrication and Actuation of Magnetic Shape-Memory Materials.

Soft actuators are deformable materials that change their dimensions or shape in response to external stimuli. Among the various stimuli, remote magnetic fields are one of the most attractive forms of actuation, due to their ease of use, fast response, and safety in biological systems. Composites of magnetic particles with polymer matrices are the most common materials for magnetic soft actuators. In this paper, we demonstrate the fabrication and actuation of magnetic shape-memory materials based on hydrogels containing field-structured magnetic particles. These actuators are formed by placing the pregel dispersion into a mold of the desired on-field shape and exposing it to a homogeneous magnetic field until the gel point is reached. At this point, the material may be removed from the mold and fully gelled in the desired off-field shape. The resultant magnetic shape-memory material then transitions between these two shapes when it is subjected to successive cycles of a homogeneous magnetic field, acting as a large deformation actuator. For actuators that are planar in the off-field state, this can result in significant bending to return to the on-field state. In addition, it is possible to make shape-memory materials that twist under the application of a magnetic field. For these torsional actuators, both experimental and theoretical results are given.

Twin boundary mobility in additive manufactured magnetic shape memory alloy 10M Ni-Mn-Ga

The additive manufacturing of magnetic shape memory (MSM) alloys is attracting increasing interest as a promising method for manufacturing the active parts of small devices with complex geometries and tailored properties. Recently, laser powder bed fusion (L-PBF) was demonstrated as a successful approach for manufacturing magnetically actuatable samples from Ni-Mn-Ga MSM alloys, presenting a giant repeatable magnetic-field-induced strain of 5.8%. The most important characteristics of an MSM material that define its application potential include its magneto-structural and twin boundary (TB) mobility properties. This research presents for the first time a detailed characterization of TB mobility in a mm-sized single crystalline grain of five-layered (10M) martensite of Ni-Mn-Ga manufactured via L-PBF. Both type 1 and type 2 TBs, which are characteristic of 10M Ni-Mn-Ga, were identified and their mobility was characterized under mechanical stress and magnetic actuation. The results show that the TBs of both types exhibit behavior similar to that observed in conventionally grown single crystals. However, their mobility is decreased, which is attributed to keyhole porosity defects characteristic of the L-PBF process. The average twinning stress for type 1 and type 2 TBs was measured as 1.4 and 0.6 MPa, respectively, resulting in a maximum output stress of about 1.3 MPa for type 1 and 2.1 MPa for type 2 TBs. The presented results demonstrate that L-PBF has good potential for manufacturing active parts from Ni-Mn-Ga. The stabilization of desired TB types and patterns in an active region to ensure reliable and repeatable device operation is required.

Modelling and design of low-power, non-conventional current sensors based on smart materials for high-voltage transmission lines

This paper presents a new technique for measurement of current based on magnetic shape memory (MSM) smart alloys. MSM alloys undergo shape changes when exposed to magnetic fields. The non-conventional instrument transformer (NCIT) proposed in this paper utilises this property to measure current. There is a correlation between the magnetic field produced by a current and the shape change of an MSM material (MSM sensor). By exploiting this correlation, we have shown that it is possible to measure alternating currents (a.c.) in high voltage overhead transmission lines. A change in the length of the MSM element causes voltage output in a linear variable differential transducer. The design of the NCIT was optimised for transmission lines. Several designs of its magnetic circuit were simulated using finite element package ANSYS APDL. Several key parameters were investigated to evaluate their effects on the sensitivity of the NCIT. Results are presented as the relationship between the current in the conductor and strain (linear elongation) of the MSM element. A commonly used conductor in high-voltage transmission lines was modelled together with the MSM element and the magnetic circuit. Recommendations have been made on the design of NCITs considering various parameters. In addition, analyses of errors in ANSYS models for the magnetic circuit have been presented. The developed methodology and obtained results are verified by comparing them to the results obtained through an experiment done by a manufacturer of MSM materials.

Nanotwinned (inter)martensite transformation interfaces in Ni50Mn25Ga20Fe5 magnetic shape memory single crystal foil

Using common and high resolution transmission electron microscopy (TEM), we study the magnetic and crystal phase structures as well as their evolution in Ni50Mn25Ga20Fe5 magnetic shape memory material with high Curie point. The particular alloying by Fe and changing thickness of the TEM foil enable us to observe all martensitic phases known in Ni-Mn-Ga Heusler alloy system and their respective transitions simultaneously. Starting from cubic austenite at about 10 nm foil thickness, the structure evolves via peculiar interleaved stripes of austenite and five-layered modulated 10M martensite to pure 10M phase with a low density of stacking faults at 40 nm thickness. With further increasing thickness, the 10M phase transforms gradually to seven-layered modulated 14M martensite with an increased density of stacking faults. Finally, the non-modulated tetragonal NM phase appears within the 14M phase by detwinning of nanotwins forming the modulated phases. High resolution TEM further confirms that nanotwinning and stacking faults are inherent structure features tightly connected with the lattice modulation and intermartensite transformations. Overall, the evolution of average lattice shows the same general trend, an increasing simple shear with (11¯0) shuffling plane, across the whole phase sequence. Additionally, we found large local variation of lattice parameters in all phases, which is is ascribed to strong lattice softening in the vicinity of the martensitic transformation and high density of stacking faults in 14M martensite lattice.

The nature of the metamagnetic transition in Heusler alloy Ni44.9Mn43In12.1 studied for magnetic refrigeration application

The nature of the phases, phase transitions, and their magnetic characteristics all play an essential role in the performance of Ni-Mn-In Heusler alloys, both as magnetic shape memory and giant magnetocaloric effect (MCE) materials. The details of the structural and magnetic transformations in Ni 44.9 Mn 43 In 12.1 Heusler alloy are reported in this work, using synchrotron and neutron diffraction and AC-SQUID susceptibility results. Of special interest is the affirmation of an antiferromagnetic (AFM) phase and the exclusion of a spin-glass state as the cause of the observed drop in magnetization upon cooling. This is of critical importance for magnetocaloric applications because of the dominant contribution to MCE of the magnetic entropy change upon phase transformation. • The magnetic and crystallographic nature of Ni 44.9 Mn 43 In 12.1 , were studied using synchrotron and neutron diffraction and AC-SQUID. • Cooling below 304 K results in ferromagnetic to antiferromagnetic transition; critically important for the magnetocaloric effect. • The crystallographic and magnetic transformations from austenite to martensite and from ferromagnetic to antiferromagnetic, coincide. • The hysteresis of the crystallographic transition is narrow, an advantage for MCE and other applications.

Direct and Indirect Determination of the Magnetocaloric Effect in the Heusler Compound Ni1.7Pt0.3MnGa.

Magnetic shape-memory materials are potential magnetic refrigerants, due the caloric properties of their magnetic-field-induced martensitic transformation. The first-order nature of the martensitic transition may be the origin of hysteresis effects that can hinder practical applications. Moreover, the presence of latent heat in these transitions requires direct methods to measure the entropy and to correctly analyze the magnetocaloric effect. Here, we investigated the magnetocaloric effect in the Heusler material NiPtMnGa by combining an indirect approach to determine the entropy change from isofield magnetization curves and direct heat-flow measurements using a Peltier calorimeter. Our results demonstrate that the magnetic entropy change in the vicinity of the first-order martensitic phase transition depends on the measuring method and is directly connected with the temperature and field history of the experimental processes.

Improvement of magnetostriction performance by doping Mg in spinel MnV2O4

By a combination of magnetization M(T), M(H) and strain ΔL/L700K, ΔL/L0Oe, the positive magnetostrictions up to ∼5000 ppm originating from the rearrangement of tetragonal domains were observed in spinel Mn0.85Mg0.15V2O4, which exhibits two successive magnetic transitions at TC ∼ 42 K and T* ∼ 28 K. An anomalous magnetic hysteresis loop under a high field occurs below T*, caused by the rearrangement of tetragonal domains. We found that the Mg-doping at the Mn site can effectively promote the positive magnetostriction in Mn1-xMgxV2O4 systems. Moreover, the remnant strain can be significantly lowered by Mg-doping due to the enhancement of magnetic anisotropy. These results provide a possible approach for further optimizing the performance of magnetic shape-memory materials.

Low-temperature martensite relaxation in Co–Ni–Ga shape memory alloy monocrystal revealed using in situ cooling, transmission electron microscopy and low rate calorimetry

This work presents the results of research on a Co49Ni21Ga30 magnetic shape memory single crystal. Based on a literature review, it was identified that analyses of phase transformations have been limited to specific heating and cooling rates, which could lead to an incomplete description of the resulting phenomena. Differential scanning calorimetry (DSC) performed with different heating/cooling rates enabled the precise determination of enthalpy values, which deviate from literature values. Weak and previously unnoticed thermal phenomena at temperatures below 190 K were also observed. Their presence was confirmed by low-temperature in situ transmission electron microscopy (TEM). Through DSC measurements and TEM observations, a model of the discovered phenomenon was proposed, which may have an impact on a better understanding of the physics of magnetic shape memory materials.

Magnetic shape-memory material holds great promise

Magnetic shape-memory material holds great promise

Experimental Characterization of Geometric Aspects of the Behavior of Magnetic Shape Memory Materials and Theoretical Interpretation

Small samples of a Ni-Mn-Ga single crystal of three different geometries were subjected to bending by applying a rotating magnetic field. The magneto-mechanical behavior of the sample in cantilever was analyzed and special attention was given to elongations and curvature along the deformation process. A sequence of 3000 images was made using a high-resolution camera and the data was analyzed using a code in Matlab. Furthermore, the geometric analysis showed that, when the magnetic field is equal to cero, the sample do not recover its original shape totally and the presence of a pseudo-elastic behavior was observed. Analysis and interpretation of the data allows the presentation of some hypotheses concerning to the crystalline structure and the role of dislocations, represented by a dislocation density, in the martensite phase of these materials. These hypotheses are discussed more formally in the second part of this paper. Some experiments are proposed that would give the opportunity to a wider theoretical knowledge of MSMM.

Experimental Characterization of Geometric Aspects of the Behavior of Magnetic Shape Memory Materials and Theoretical Interpretation

Small samples of a Ni-Mn-Ga single crystal of three different geometries were subjected to bending by applying a rotating magnetic field. The magneto-mechanical behavior of the sample in cantilever was analyzed and special attention was given to elongations and curvature along the deformation process. A sequence of 3000 images was made using a high-resolution camera and the data was analyzed using a code in Matlab. Furthermore, the geometric analysis showed that, when the magnetic field is equal to cero, the sample do not recover its original shape totally and the presence of a pseudo-elastic behavior was observed. Analysis and interpretation of the data allows the presentation of some hypotheses concerning to the crystalline structure and the role of dislocations, represented by a dislocation density, in the martensite phase of these materials. These hypotheses are discussed more formally in the second part of this paper. Some experiments are proposed that would give the opportunity to a wider theoretical knowledge of MSMM.

Magnetic Shape Memory Turns to Nano: Microstructure Controlled Actuation of Free-Standing Nanodisks.

Magnetic shape memory materials hold a great promise for next-generation actuation devices and systems for energy conversion, thanks to the intimate coupling between structure and magnetism in their martensitic phase. Here novel magnetic shape memory free-standing nanodisks are proposed, proving that the lack of the substrate constrains enables the exploitation of new microstructure-controlled actuation mechanisms by the combined application of different stimuli-i.e., temperature and magnetic field. The results show that a reversible areal strain (up to 5.5%) can be achieved and tuned in intensity and sign (i.e., areal contraction or expansion) by the application of a magnetic field. The mechanisms at the basis of the actuation are investigated by experiments performed at different length scales and directly visualized by several electron microscopy techniques, including electron holography, showing that thermo/magnetomechanical properties can be optimized by engineering the martensitic microstructure through epitaxial growth and lateral confinement. These findings represent a step forward toward the development of a new class of temperature-field controlled nanoactuators and smart nanomaterials.

Magnetically Driven Pump for Solid-State Microfluidic Flow Control

Microfluidics is a rapidly developing field due to the many advantages that handling fluids in the microliter and nanoliter scale. This includes reduced volumes of samples and reagents used, quicker processing times and more precise and consistent results. A key requisite to exploit these advantages is fluid control at the nanoliter scale. Fluid handling systems that can accurately dose the volumes needed in microfluidics are underdeveloped, particularly options that are small and easy to manufacture. Some options include capillary flow, electroosmotic, piezoelectric and peristaltic technologies. The magnetic shape memory (MSM) pump is similar in principle to a peristaltic pump except that there are no mechanical parts such as gears or valves. The MSM material itself replaces the classical mechanical components found in traditional pumping mechanisms. The material is the machine. The MSM material is an alloy of nickel, manganese and gallium (Ni-Mn-Ga). The MSM effect, which is the coupling of the material’s atomistic and magnetic structures, is responsible for the large shape change (up to 10%) that has been observed in the single crystalline material. Through a process known as twinning, the crystallographic structure of the material can be manipulated by converting energy from an applied magnetic field. This is advantageous in that the material’s shape can be precisely changed without contacting the device itself. The MSM pump is realized when this material is integrated into a microfluidic channel and controlled using a localized magnetic field. The material becomes a metal muscle. The MSM pump is a solid-state microfluidic flow control device that operates similar to a peristaltic pump. The material’s shape changes to create a cavity at the inlet channel due to a locally applied, external magnetic field. The formation of this cavity creates a negative pressure which is then filled with a bolus of fluid from the outlet. This cavity is then translated across the material until it reaches the outlet channel, at which it is then ejected from the material and into the microfluidic system. There are no moving parts, such as gears or check valves, in the micropump itself. The current peristaltic MSM micropump has a resolution of 40 – 150 nL per complete pumping cycle. New techniques have been developed to achieve a resolution nearing 1 – 2 nL. The MSM pump has repeatable, stable performance even at pumping pressures exceeding 100 kPa and can pump both gas and liquids (and is thus self-priming). Furthermore, the material acts simultaneously as both the pumping mechanism and a valve. This multi-functionality, and the simplicity of the overall MSM pump, make it a competitive technology for devices requiring flow rates from nearly zero to 1,500 µL/min, particularly devices where the pumping mechanism must be integrated directly into the microfluidic device such as a lab-on-a-chip, as shown in the figure. Using the MSM pump, we created and manipulated a droplet lipid bilayer and delivered sub-microliter volumes of the electrically controlled nano-valve lysenin. We will present the performance of the MSM pump as well as the results from these recent experiments. This demonstrates the capabilities of this technology and illustrates its capacity for other applications within the field of microfluidics. Figure 1

Integratable magnetic shape memory micropump for high-pressure, precision microfluidic applications

Precisely controlling the flow of fluids on a microscopic scale has been a technological challenge in the field of microfluidics. Active microfluidics, where a defined manipulation of the working fluid is necessary, requires active components such as micropumps or microvalves. We report on an optimized design of an integratable, wireless micropump made from the magnetic shape memory (MSM) alloy Ni–Mn–Ga. An external magnetic field generates a shape change in the MSM material, which drives the fluid in a similar fashion as a peristaltic pump. Thus, the pump does not need electrical contacts and avoids the mechanical parts found in traditional pumping technologies, decreasing the complexity of the micropump. With a discrete pumping resolution of 50–150 nL per pumping cycle, which is further scalable, and a pumping pressure well exceeding 2 bar, the MSM micropump is capable of accurately delivering the fluids needed for microfluidic devices. The MSM micropump is self-priming, pumping both liquid and gas, and demonstrates repeatable performance across a range of pumping frequencies. Furthermore, it operates simultaneously as both a valve and reversible micropump, offering superior possibilities compared to existing technologies within the flow rate range of 0–2000 µL/min. Due to its simplicity, this technology can be scaled down easily, which lends itself for future integration into lab-on-a-chips and microreactors for life science and chemistry applications.

Magneto-optical Kerr effect of a Ni2.00Mn1.16Ga0.84 single crystal across austenite and intermartensite transitions

We carried out magneto-optical Kerr effect (MOKE) and magnetization measurements on a single crystal of Ni2.00Mn1.16Ga0.84, which is a magnetic shape memory material with application potential for actuator devices or for energy recuperation. Up to the time of our study, there had been reports of MOKE measurements in polar geometry. Against earlier predictions, we show that surface magnetic states of the martensite and the austenite can be also probed efficiently via longitudinal MOKE. A single-variant magnetic state prepared at room temperature is characterized by square-shaped ferromagnetic hysteresis loops yielding coercive fields, which are key material properties for future applications. Temperature dependencies of Kerr rotation were found to be linearly proportional to magnetization for martensitic phases. After passing through an inter-martensitic structural transition below room temperature in zero magnetic field, the coercive fields are more than doubled in comparison with the room temperature values. Above room temperature where an austenite structure is formed, MOKE signals are dominated by quadratic contributions and the magnitude of Kerr rotation drops due to changes in the electronic and magnetic domains structure.

Antiferromagnetic coupling between martensitic twin variants observed by magnetic resonance in Ni-Mn-Sn-Co films

Magnetic properties of $\mathrm{N}{\mathrm{i}}_{46.0}\mathrm{M}{\mathrm{n}}_{36.8}\mathrm{S}{\mathrm{n}}_{11.4}\mathrm{C}{\mathrm{o}}_{5.8}/\mathrm{MgO}(001)$ epitaxial thin film, which undergo a martensitic phase transformation from cubic austenitic phase to a twinned orthorhombic martensitic phase at 270 K, were studied by the magnetic resonance at the microwave frequency of 9.45 GHz. It was found that the single resonance line observed in the austenite splits into three lines in the martensitic phase. A theoretical approach was developed to show that the additional resonance lines are caused by the weak antiferromagnetic coupling of the ferromagnetic twin components across twin boundaries. Fitting of the experimental resonance lines to model gives an effective field of antiferromagnetic coupling of about 1.5 kOe, which is two or three orders of magnitude lower than in the conventional antiferromagnetic solids because the number of magnetic ions interacting antiferromagnetically through the twin boundary is much less than the total number of magnetic ions in the twin. This feature shows a strong resemblance between the submicron twinned martensite and artificial antiferromagnetic superlattices, whereby providing a distinctive insight into magnetism of the studied magnetic shape memory material.

Magnetic Shape Memory - Polymer Hybrids

Martensitic Ni-Mn-Ga based alloys are known for the Magnetic Shape Memory (MSM) effect, which upon application of an external magnetic field can generate a strain up to 12 % depending on the microstructure of the martensite. The MSM effect occurs by rearrangement of the martensite variants, which is most advantageous in single crystals. Single crystals are, however, rather tedious to produce and there has been attempts to achieve MSM effect in polycrystals. However, in polycrystals the magnetic field induced shape change remains low as compared to single crystals. As an alternative to the former, hybrid MSM materials offer several advantages. When compared to single crystals, hybrids have extended freedom of shaping, lower raw material price, relatively large MSM strain and easier manufacturability. Embedding MSM particles into a suitable polymer matrix results in actuation function or good vibration damping performance. In the present study we report on the mechanical, structural and magnetic properties of MSM polymer hybrids, which are prepared by mixing gas-atomized Ni-Mn-Ga MSM powder into epoxy matrix and aligning the magnetic particles in a magnetic field.

Design and Pre-Implementation of Micro-Movements Sensor with Magnetic Shape Memory Material

The paper describe design and built the micro-displacement sensor demonstrator with the use of the active material MSMA. It shows the selection of individual components of the system and its construction. Article presents problems arising during experiments and suggests possible ways of their solution. Shortly characterized other technologies, which are used in commonly used micro-displacement sensors. It also formulated a few comments that may simplify the further development of this project.

NiMn-based metamagnetic shape memory alloys

Metamagnetic shape memory alloy (MMSMA), in which the martensitic transformation temperature can be controlled by a magnetic field, is a new type of magnetic shape memory material. In MMSMAs, because unique physical properties, such as giant magnetoresistance, giant magneto-thermal conductivity and inverse magnetocaloric effects, are observed to be associated with the martensitic transformation, these materials are good candidates for application as actuators and sensors acting in mixed external fields of temperature, stress and magnetic field. In the present Viewpoint, some interesting and unique properties in the MMSMAs are introduced, and some criteria of materials design for practical use are discussed.