Magnetically Induced Reorientation Research Articles

Rapid floating zone growth of Ni2MnGa single crystals exhibiting magnetic shape memory functionality

To obtain a material exhibiting magnetically induced reorientation (MIR), one of the magnetic shape memory effects, a well-developed method for single crystal growth is crucial. A [100] oriented Ni2MnGa single crystal exhibiting MIR was successfully prepared using the floating-zone technique with a high speed of 80 mm/h. The comparison with a sample grown by Bridgman method and a reference sample grown in AdaptaMat Ltd. by directional solidification is provided based on elemental homogeneity, crystallographic quality by x-ray diffraction, transformation behaviour and magnetic properties. All these methods indicated that the single crystal grown by floating zone with high speed is comparable or superior to the samples prepared by other methods. Importantly, the magnetization measurement revealed that the single crystal prepared this way exhibits MIR down to 10 K.

Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

Magnetic Domain Structure and Magnetically-Induced Reorientation in Ni-Mn-Ga Magnetic Shape Memory Alloy

Direct observation of a-b twin laminate in monoclinic five-layered martensite of Ni-Mn-Ga magnetic shape memory single crystal

Full hierarchy of twinning including a-b twin laminate was directly observed by optical and scanning electron microscopy (SEM) in the vicinity of macroscopic, mobile a-c twin boundary in five-layered monoclinic (10M) martensite of Ni50Mn28Ga22 single crystals exhibiting magnetically induced reorientation (MIR). The submicron a-b twin laminate was identified by backscattered electrons in SEM. The a-b laminate terminated straight on Type I and by laminate branching on Type II twin boundary in agreement with the model.

Ni–Mn–Ga Single Crystal Exhibiting Multiple Magnetic Shape Memory Effects

Both magnetically induced phase transformation and magnetically induced reorientation (MIR) effects were observed in one Ni50Mn28Ga22 single crystal sample by direct measurement of the magnetic field-induced strain. We investigated various twinning microstructures ranged from single twin interface to fine twinning and crossing twins to evaluate what controls the apparent twinning stress crucial for MIR. The main challenges for the applications of these effects are outlined.

Temperature-dependent magnetostriction as the key factor for martensite reorientation in magnetic field

The magnetic field application leads to spatially inhomogeneous magnetostriction of twinned ferromagnetic martensite. When the increasing field and magnetostrictive strain reach certain threshold values, the motion of twin boundaries and magnetically induced reorientation (MIR) of twinned martensite start. The MIR leads to giant magnetically induced deformation of twinned martensite. In the present article, the threshold field (TF) and temperature range of observability of MIR were calculated for the Ni–Mn–Ga martensite assuming that the threshold strain (TS) is temperature-independent. The calculations show that if the TS is of the order of 10−4, the TF strongly depends on temperature and MIR can be observed only above the limiting temperature (~220 K). If the TS is of the order of 10−6, the TF weakly depends on temperature and MIR can be observed at extremely low temperatures. The obtained theoretical results are in agreement with available experimental data.

Influence of aging and thermomechanical cycling on the magnetostriction and magnetic shape memory effect in martensitic alloy

An influence of internal stress created by the crystal defects on the magnetically induced reorientation (MIR) of martensite variants in the ferromagnetic shape memory alloy (FSMA) has been analyzed. Using the internal stress conception, a noticeable influence of the spatial reconfiguration of crystal defects on the ordinary magnetostriction of FSMA and magnetic shape memory (MSM) effect has been predicted. It has been shown that the defect reconfiguration, which stabilizes the martensitic phase during martensite aging, increases the shear elastic modulus. The increase of shear modulus reduces the magnetostriction value and in this way suppresses the MSM effect. The magneto-thermo-mechanical training of aged alloys destabilizes the martensitic phase, restores the initial magnetostriction value, and promotes the MSM effect.

Understanding Motion of Twin Boundary—A Key to Magnetic Shape Memory Effect

High mobility or the ease of movement of twin boundary in magnetic field, necessary for magnetically induced reorientation (MIR) is studied. In modulated 10M martensite two types of a-c twin boundaries, Type I and Type II are mobile in magnetic field. Here, we focus on the behavior of a single twin boundary in magnetic field at different temperatures. Using Type II, twin boundary MIR resulting in 6% field-induced deformation can occur in the field ~10 kA/m. For Type I, in contrast, the field more than ten times larger is needed. The effect of demagnetization is evaluated. In addition, the temperature dependence of the field needed to initiate twin motion is flat for Type II compared with sharp increase for Type I. The explanations for this sharply differing behavior are proposed.

Transformation behaviors, structural and magnetic characteristics of Ni—Mn—Ga films on MgO (001)

Ferromagnetic Ni—Mn—Ga films were fabricated by depositing on MgO (001) substrates at temperatures from 673 K to 923 K. Microstructure, crystal structure, martensitic transformation behavior, and magnetic properties of the films were studied. With increasing deposition temperature, the surface morphology of the films transforms from granular to continuous. The martensitic transformation temperature is not dependent on deposition temperature; while transformation behavior is affected substantially by deposition temperature. X-ray analysis reveals that the film deposited at 873 K has a 7M martensite phase, and its magnetization curve provides a typical step-increase, indicating the occurrence of magnetically induced reorientation (MIR). In situ magnetic domain structure observation on the film deposited at 873 K reflects that the martensitic transformation could be divided into two periods: nucleation and growth, in the form of stripe domains. The MIR occurs at the temperature at which martensitic transformation starts, and the switching field increases with the decrease of temperature due to damped thermal activation. The magnetically induced martensitic transformation is related to the difference of magnetization between martensite and austenite. A shift of martensite temperature of dT/dH = 0.43 K/T is observed, consistent with the theoretical value, 0.41 K/T.

Why Magnetic Shape Memory Alloys?

Why Magnetic Shape Memory Alloys?

Microstructure of freestanding single-crystalline Ni 2MnGa thin films

The complex crystal structure and variant distribution of single-crystalline freestanding Ni–Mn–Ga films was studied in detail using X-ray diffraction in two- and four-circle geometry. Selective chemical etching of a chromium buffer layer facilitates the release of the rigid MgO substrate that would inhibit magnetically induced reorientation (MIR) of variants. The substrate-constrained as well as the freestanding films possess identical crystal structure featuring a seven-layered (pseudo-)orthorhombic modulation (7 M/14 M). Of the 12 different variants observed before and after releasing the film from the substrate, four are predominantly represented. These have the short c-axis and the long a-axis aligned in the film plane. This allows for a reduction of the stray field energy, since the c-axis is the magnetic easy axis. The variant distribution induces a distinct mesoscopic surface pattern that can be observed by light microscopy. On a freestanding microbridge, prepared by photolithography in combination with selective chemical etching, the phase transformation can thus be traced directly. After several heating cycles the surface pattern remains basically unchanged. Application of a magnetic field of 0.5 T while the film transforms from austenite to martensite leads to a considerable change of the structure.

Magnetically resettable 0.16% free strain in polycrystalline Ni–Mn–Ga plates

We investigated the microstructure, mechanical training and free strain due to magnetically induced reorientation (MIR) of a polycrystalline Ni 50 Mn 29 Ga 21 alloy prepared by directional solidification, which showed columnar grains with a strong A fibre texture. Two-side mechanical training greatly decreased the twinning stress level of the properly cut plates. Consequently, a free strain of 0.16% has been observed in the trained plates and this strain is magnetically resettable more than 20 times by rotating the magnetic field by 90°.

Self-activated reversibility in the magnetically induced reorientation of martensitic variants in ferromagnetic Ni-Mn-Ga films

Magnetically induced reorientation (MIR) of twin variants for [101]-oriented martensite has been studied for Ni-Mn-Ga thin films deposited on (001) YSZ substrates. The MIR effect displayed by these films is self-reversible, is characterized by a small increase in the magnetization, and shows a temperature-dependent activation field. These properties can be described in terms of a magnetic microstructure comprised of MIR-active and MIR-inactive twin variants, where the degree of activity depends on the relative orientations of the externally applied magnetic field and the easy axis of the magnetization of the variants. The MIR-active variants are responsible for the detwinning, while the MIR-inactive variants exert a restoring force on the MIR-active variants as they reorient. This interplay between the active and inactive variants is highly advantageous as it allows for the complete self-activation of the reversible MIR effect, a response which is desired for actuation applications. The results further demonstrate that the magnetic domain structure formed in the films is inherited from a substrate-induced crystallographic texture and microstructure. It is also suggested that a tailored MIR response could prove possible through an optimization of the film texture and microstructure.

Magnetically induced reorientation of martensite variants in constrained epitaxial Ni–Mn–Ga films grown on MgO(001)

Magnetically induced reorientation (MIR) is observed in epitaxial orthorhombic Ni–Mn–Ga films. Ni–Mn–Ga films have been grown epitaxially on heated MgO(001) substrates in the cubic austenite state. The unit cell is rotated by 45° relative to the MgO cell. The growth, structure texture and anisotropic magnetic properties of these films are described. The crystallographic analysis of the martensitic transition reveals variant selection dominated by the substrate constraint. The austenite state has low magnetocrystalline anisotropy. In the martensitic state, the magnetization curves reveal an orthorhombic symmetry having three magnetically non-equivalent axes. The existence of MIR is deduced from the typical hysteresis within the first quadrant in magnetization curves and independently by texture measurement without and in the presence of a magnetic field probing microstructural changes. An analytical model is presented, which describes MIR in films with constrained overall extension by the additional degree of freedom of an orthorhombic structure compared to the tetragonal structure used in the standard model.

An Introduction to Actuation Mechanisms of Magnetic Shape Memory Alloys

Magnetic shape memory alloys attain strains up to 10% when applying a magnetic field. The phenomenology of two actuation mechanisms is described in the following introduction. The first is called Magnetically Induced Reorientation (MIR) and the second Magnetically Induced Martensite (MIM) . Material requirements and advantages of both effects are discussed.