Field-induced Twin-boundary Motion Research Articles

Study of the critical parameters for magnetic field-induced strain in high temperature Ni-Mn-Ga-Co-Cu-Fe single crystals

Three single crystals of Ni(Co)-Mn(Fe)-Ga(Cu) high-temperature magnetic shape memory alloys exhibiting a tetragonal non-modulated martensite have been produced. Extensive measurements of the key parameters responsible for the magnetic field induced twin boundary motion, such as magnetocrystalline anisotropy, twinning stress, magnetostress and tetragonality ratio have been carried out as a function of the composition and temperature. Universal relations between the indicated parameters have been disclosed. The feasibility of high temperature magnetic actuation is supported in, at least, one of the crystals, indicating that these materials are very attractive for high temperature applications.

Microstructure and magnetic properties of as-cast Ni2MnGa alloys processed by twin roller melt spinning

The magnetism and the microstructure of Ni2MnGa alloys, processed for the first time by twin roller melt spinning at tangential wheel speeds of 10m/s (V10), 15m/s (V15) and 20m/s (V20) are investigated. At room temperature, the major phase in the as-cast alloys is the cubic L21 Ni2MnGa ordered austenitic phase, with a lattice parameter only ∼0.1% larger than the tabulated value. The order domain size in the austenitic phase decreases from (40±1) nm in samples V10 to (19±1) nm in V20. Mn(S,Se) small precipitates are also found uniformly embedded in the ribbons with mean size of (26±2) nm (V10) and (7±2) nm (V20), exhibiting a definite orientation relation with the austenitic matrix, <100>P// <100>a. The as-cast alloys transform to an intermediate cubic phase I at about 220–230K depending on the quenching rate and to a martensitic phase at about 130K. The high temperature austenitic phase and the low temperature martensitic phase are ferromagnetic; in both cases the saturation polarization is lower in samples quenched at higher rates. The demagnetization curves measured from saturation in the martensitic state show two marked steps: a first one for positive fields in V15 and V10 (∼46mT) and a larger second one for relatively large inverse fields (∼130–250mT) in all the samples. These steps are likely to arise from a demagnetization mechanism involving a field induced twin boundary motion in the few martensite variants selected by the crystallographic texture and the quenched stresses in the ribbons.

NiMn‐Based Alloys and Composites for Magnetically Controlled Dampers and Actuators

AbstractAs a new class of magnetically driven active materials, magnetic shape memory (MSM) alloys show large strains of up to 10% in Ni2MnGa single crystal with a modulated martensitic structure. The magnetic field induced twin boundary motion can be exploited for actuators, sensors, and as it is an energy dissipating process, also for vibration damping devices. However, single crystals of Ni2MnGa are difficult to prepare and are inherently very brittle. One solution is the use of aligned MSM particles suspended in a stiffness‐matched polymer matrix. In this review, we focus on the preparation and characterization of textured NiMn‐based MSM composite. Essential issues, e.g., the criterion of polymer selection and the interface of MSM particles and polymer are discussed. Second aspect of the paper is to review recent developments in metamagnetic shape memory NiMn‐(In,Sn,Sb) systems which exhibit field‐driven multifunctional properties such as magnetocaloric effect, magnetoresistant change, magnetoelectricity, and thermoelectricity.

Effects of magnetostatic interaction on domain microstructure evolution in magnetic shape memory alloys: Phase field simulation

Domain microstructure evolution during magnetic field-induced twin boundary motion in magnetic shape memory alloys was investigated using phase field micromagnetic microelastic modeling. The computer simulations show bending and splitting of magnetic domain walls, magnetization rotation due to external and internal magnetic fields and pinning of magnetic domain wall motion at twin boundaries. The study reveals the important roles of magnetostatic interaction in the domain processes, providing insights into the complex domain phenomena in magnetic shape memory alloys and explaining experimentally observed domain microstructures.

Magnetic field-induced twin boundary motion in polycrystalline Ni–Mn–Ga fibres

Magnetic field-induced twin boundary motion leading to large magnetic field-induced strain of ∼1.0% was established in polycrystalline Ni50.9Mn27.1Ga22.0 (at.%) fibres at room temperature (∼60–100 μm in diameter and ∼3 mm in length). The fibres' grains are as large as the fibre diameter and of random orientation. At room temperature, a ferromagnetic 5M martensite is found. Magnetic field-induced twin boundary motion was indicated by magnetic measurements and validated by electron backscatter diffraction (EBSD). The application of a magnetic field shifts the equilibrium temperature of martensite and austenite by ∼0.4 K T−1, which agrees with calculations using the Clapeyron–Clausius approach.

Absence of magnetic domain wall motion during magnetic field induced twin boundary motion in bulk magnetic shape memory alloys

A detailed study of twin boundary motion in NiMnGa single crystals together with in situ magnetic domain observation is presented. Optical polarization microscopy in connection with a magneto-optical indicator film technique was used to investigate the reorganization of the magnetic domains during twin boundary motion over a wide magnetic field range. Images at different field strengths demonstrate that no magnetic domain wall motion within the twins takes place, even during the structural reorientation by twin boundary movement. This absence of interaction of magnetic and structural domains is different from currently proposed models, which assume domain wall movement under an external field.

Structure formation in martensitic Ni 50Mn 30Ga 20 MSM alloy

Ni–Mn–Ga single crystals are known for showing strains of up to 10% originating from magnetic field induced twin boundary motion. It is known that among the possible martensitic phases the 5M and 7M structure show magnetic-field-induced strain (MFIS) of 6% and 10%, respectively. For the composition Ni 50Mn 30Ga 20 different martensitic structures can develop, as it has been shown in our previous work. Both the 7M and NM structures can be found at room temperature. Therefore, the preparation routes to achieve one or the other structure were investigated. This study shows that thermal as well as mechanical treatments influence the final structure. The structural change observed after mechanical treatment is a stress-induced intermartensitic transformation. This stress-induced NM phase can be transformed back to the 7M phase by annealing in the austenite state. Finally, for the composition Ni 50Mn 30Ga 20 either the 7M or the NM structure can be achieved by appropriate thermomechanical treatment.

Model for temperature dependence of field-induced strain in ferromagnetic shape memory alloys

Magnetic field-induced twin-boundary motion in Ni–Mn–Ga and other magnetic shape memory alloys is typically observed over a limited range of temperature, 10–120 K below the martensite transformation. Twin-boundary motion requires shuffling of atoms between two stable minima in {1 0 1} planes, a process that occurs only above a threshold yield stress. A model is proposed to account for this phenomenon. It is based on the magnetic torque and thermal activation acting on the atoms about a twin dislocation that separates the two stable twin variants. Field-induced twin-boundary motion is predicted to occur when the thermally activated, temperature-dependent torque exceeds the twin-boundary yield stress (itself, temperature dependent). Activation energies of order (2.4 to 5) × 10 −21 J/atom (0.15–0.3 eV) give a good agreement with observations.

Giant magnetic-field-induced strain in Ni–Mn–Ga crystals: experimental results and modeling

Strains up to 6% have been induced by application of magnetic fields of 320–400 kA/m (4–5 kOe) to Ni–Mn–Ga single-crystal ferromagnetic shape memory alloys at room temperature. The blocking stress (stress at which field-induced strain is zero) is in the range 1.5–2 MPa. These strains are not magnetostrictive in origin but arise rather from field-induced motion of twin boundaries in the martensitic phase. A thermodynamic model for field-induced twin-boundary motion describes the main features of the field dependence and stress dependence of the strain. All parameters of the model can be measured from experimental stress vs. strain and magnetization vs. field curves.