Mn Ga Alloys Research Articles

Enhancing the elastocaloric effect in Ni–Mn–Ga alloys through the coupling of magnetic transition and two-step structural transformation

Elastocaloric effect driven by uniaxial stress in the Ni–Mn–Ga alloys can be greatly enhanced through introducing magnetic transition or inter-martensitic transformation to martensitic transformation. Here, we present large elastocaloric effect in a ⟨0 0 1⟩A textured Ni55Mn19Ga25Ti1 polycrystalline alloy prepared by directional solidification by exploiting the coupled multiple phase transformations, i.e., paramagnetic-ferromagnetic transition, martensitic transformation, and inter-martensitic transformation. Owing to such magneto-multistructural transformation, the transformation entropy change related to the inverse transformation is enhanced to 29.6 J kg−1 K−1. Consequently, on unloading from a compressive stress of 180 MPa, a large adiabatic temperature change of −12.9 K and specific adiabatic temperature change of −72 K GPa−1 are achieved, being much superior over those in the Ni–Mn–Ga based alloys obtained previously.

Modeling on the dynamic mechanical response of single-crystalline Ni–Mn–Ga alloys based on Hamilton’s principle

In this paper, a variational approach is proposed to study the dynamic mechanical response of a single-crystalline Ni–Mn–Ga sample. First, some constitutive assumptions are adopted to describe the material properties of single-crystalline Ni–Mn–Ga alloys. Hamilton’s action integral is then formulated for the mechanical system being studied, which depends on the position vector field and the variant state distribution in the sample. By calculating the variation of the action integral with respect to the position vector field, the equation of motion, as well as the boundary condition and the twin interface connection condition, can be obtained. By further calculating the variation of the action integral with respect to the variant state distribution (through twin interface movements), the expression of the driving force on the twin interfaces is derived, based on which the twin interface movement criterion is established. Combining the equation of motion and the twin interface movement criterion, the governing system for modeling the dynamic response of the single-crystalline Ni–Mn–Ga sample can be formulated. To show the validity of the governing system, a simple example is studied and some analytical results are constructed. Especially, the relation between the external mechanical load and the twin interface velocity is revealed, which is consistent with the experimental observations.

Liquid Metal-Based Magnetorheological Fluid with a Large Magnetocaloric Effect.

Herein, liquid metal and Mn0.6Fe0.4NiGe0.54Si0.46 particles with a large magnetocaloric effect are adopted to prepare a novel magnetocaloric suspension named liquid metal-based magnetorheological fluid (LM2RF), which can well solve the problems of brittleness, low thermal conductivity, and poor machinability in classical magnetocaloric materials. The low melting point and high boiling point of liquid metal could significantly widen the operating temperature range of such a fluid. As a carrier, the high thermal conductivity, low viscosity, and large density of liquid metal display advantageous to heat transfer. The maximum loading fraction is 19.5 wt %, while LM2RF features the liquid state. A series of tests are conducted to investigate the alloying behavior in LM2RF. It is found that galinstan will react with Mn0.6Fe0.4NiGe0.54Si0.46 particles and form MnGa alloy. However, the reaction rate is very slow and the generated MnGa alloy is passivating. Consequently, the quantity of MnGa alloy is too sparse to affect the magnetocaloric performance of LM2RF. Overall, the LM2RF exhibits a large MCE at around room temperature with a lower magnetic hysteresis loss, and the transition temperature (Tm) remains constant in 60 days. This work demonstrates the outstanding performance of LM2RF and provides a promising alternative to MCE materials for practical magnetic refrigeration.

Study on magnetoresistance of Ni48Mn22Ga22Co4Cu4 microwires

Ni–Mn-Ga(Co, Cu) alloys have implicated magnetoresistance mechanisms as ferromagnetic shape memory alloys. The relationship between magnetoresistance and the martensitic transformation was researched. The magnetoresistance mechanism of microwires was related to the reorientation of martensite induced by magnetic fields. It is found that a negative MR of 4.81% was obtained in 20 kOe field in Ni48Mn22Ga22Co4Cu4 microwires with diameters of 0.28 mm at 327 K. The effect of diameters on the martensitic transformation observed in ρ-T curves could be explained as the effect of rapid quenching during the preparation process, that smaller diameters wires have faster cooling rate and lower atomic order.

Theoretical Approach to Investigation of the Magnetic and Magnetocaloric Properties of Heusler Ni–Mn–Ga Alloys

The magnetic and magnetocaloric properties of Heusler Ni2 + xMn1 – xGa alloys (x = 0.16, 0.18, and 0.3) have been studied using a model based on the Malygin theory of smeared phase transitions, the Bean–Rodbell theory of first-order phase transitions, and the mean-field theory. The temperature dependences of strain, magnetization, and isothermal change in entropy of these alloys have been analyzed. It is shown that the largest change in the magnetic entropy is observed in a Ni2.18Mn0.82Ga alloy, in which the martensitic transition is accompanied by a change in the magnetic ordering. The smallest change in the entropy is observed in a Ni2.3Mn0.7Ga alloy, which exhibits the magnetocaloric effect in a martensitic phase with a change in the magnetic ordering. However, the refrigeration capacity of this alloy is twice as high as that of the other considered compositions.

Application of the Ritz–Rayleigh method for Lamb waves in extremely anisotropic media

We propose a numerical approach for the calculation of frequency–dispersion curves in a flat anisotropic waveguide based on the Ritz–Rayleigh method, offering several significant benefits over commonly used analytical and numerical models. The problem is linearized using a tailored functional basis based on Legendre polynomials, utilizing all symmetries provided by the problem to reduce the computational demands, which brings significant benefits for an inverse procedure, and thus for material characterization. The approach is completely general in terms of elastic properties and direction of propagation. As an exemplary calculation, frequency–dispersion curves of phase and group velocity are calculated for a (001)-oriented free-standing film of the Ni–Mn–Ga alloy exhibiting a strong cubic anisotropy. In the case of propagation along the [100] direction, the dispersion curves oscillate strongly, creating pairs of symmetric and antisymmetric modes. Taking advantage of the generality of the approach, the curves are also presented for the case of extreme anisotropy. Sets of curves for directions [110] and [1 0.9 0] (i.e., 3∘ off [110]), where the pseudo-surface wave exists, are also calculated. High-frequency asymptotes are shown to correspond with surface and bulk modes propagating along the plate.

Microstructure of Ni2MnGa Alloys Solidified by Suction Casting

Microstructure of Ni₂MnGa Alloys Solidified by Suction Casting

Over 2% magnetic-field-induced strain in a polycrystalline Ni50Mn28.5Ga21.5 alloy prepared by directional solidification

Ni–Mn–Ga single crystal alloys can exhibit giant magnetic shape memory effect through variant reorientation induced by the magnetic field. However, such effect is greatly weakened in polycrystalline alloys, due to the orientation differences of martensite variants and the constraints of grain boundaries. Here, to improve the magnetic shape memory effect, a polycrystalline Ni50Mn28.5Ga21.5 alloy with coarse columnar shaped grains and strong <0 0 1>A texture is prepared by directional solidification. With the aids of mechanical training, the twinning stress of five-layered modulated (5 M) martensite in the directionally solidified Ni50Mn28.5Ga21.5 alloy is successfully lowered to ~0.9 MPa. A giant magnetic field induced strain up to ~2.1% is achieved under the magnetic field of 1 T, being much higher than those reported previously in polycrystalline alloys. In addition, a reversible magnetostrain of ~0.4% is also attained without the assistance of an external stress or a magnetic field. It is demonstrated that the microstructure control through directional solidification as well as mechanical training could be an effective solution to enhance the magnetic field induced output strain in polycrystalline Ni–Mn-Ga alloys.

Variant reorientation in a single-crystalline Ni–Mn-Ga sample induced by a quasi-static rotating magnetic field: Mechanism analyses based on configurational forces

In this paper, we introduce a modeling work on the variant reorientation in a single-crystalline Ni–Mn-Ga sample induced by a quasi-static rotating magnetic field. First, some constitutive assumptions are proposed for the variant state distribution, the effective magnetization and the elastic energy density of Ni–Mn-Ga alloys. Then, the total energy functional for a Ni–Mn-Ga sample subject to coupled magneto-mechanical loads is formulated. Through a variational approach, the governing equation system of the model can be formulated, which is composed of the magnetic and mechanical field equations, the evolution laws of some internal variables and the twin interface movement criteria. An efficient numerical scheme is adopted to solve the governing system. With the obtained numerical solutions, we first analyze the evolution properties of the configurational force on the twin interfaces. Especially, the effects of the application directions of magnetic fields on the configurational force are investigated. The influences of the different energy terms on the configurational force are also analyzed. Based on these results, we further conduct numerical simulations on the magneto-mechanical behavior of the Ni–Mn-Ga sample subject to a quasi-static rotating magnetic field. The predicted global responses of the Ni–Mn-Ga sample show good consistency with the experimental result. The configurations of the sample and the distributions of some important physical quantities in the sample during the different loading stages can also be described. Furthermore, by considering the properties of the configurational forces, the underlying mechanisms responsible for the rotating field-induced variant reorientations can be revealed.

Optimizing the Caloric Properties of Cu-Doped Ni-Mn-Ga Alloys.

With the purpose to optimize the functional properties of Heusler alloys for their use in solid-state refrigeration, the characteristics of the martensitic and magnetic transitions undergone by Ni50Mn25−xGa25Cux (x = 3–11) alloys have been studied. The results reveal that, for a Cu content of x = 5.5–7.5, a magnetostructural transition between paramagnetic austenite and ferromagnetic martensite takes place. In such a case, magnetic field and stress act in the same sense, lowering the critical combined fields to induce the transformation; moreover, magnetocaloric and elastocaloric effects are both direct, suggesting the use of combined fields to improve the overall refrigeration capacity of the alloy. Within this range of compositions, the measured transformation entropy is increased owing to the magnetic contribution to entropy, showing a maximum at composition x = 6, in which the magnetization jump at the transformation is the largest of the set. At the same time, the temperature hysteresis of the transformation displays a minimum at x = 6, attributed to the optimal lattice compatibility between austenite and martensite. We show that, among this system, the optimal caloric performance is found for the x = 6 composition, which displays high isothermal entropy changes (−36 J·kg−1·K−1 under 5 T and −8.5 J·kg−1·K−1 under 50 MPa), suitable working temperature (300 K), and low thermal hysteresis (3 K).

An evaluation of the Mn–Ga system: Phase diagram, crystal structure, magnetism, and thermodynamic properties

The Ni–Mn-Ga alloy system is attractive due to its functional properties with potentials in various applications. However, the fundamental alloy thermodynamics of the binary Mn–Ga system is still lack of investigation. Therefore, a comprehensive evaluation of the Mn–Ga is performed in this work. Different versions of Mn–Ga phase diagrams available in the literature are reviewed. A new version of the Mn–Ga phase diagram is recommended along with possible invariant reactions. The crystal structure, magnetic transition, and thermochemical properties of intermetallic compounds are reviewed by considering available experimental and modeling such as ab initio calculations. In fact, more experimental information on the Mn-rich side is required in order to perform CALPHAD thermodynamic modeling for a reliable database. Further experiments are recommended to study the high-temperature phase equilibria between liquid, (γMn), (δMn) and Mn2Ga(h), phase reactions between Mn8Ga5 and Mn7Ga6, and invariant reactions involving the MnGa phase. Nevertheless, the summarized information on phase equilibria, phase diagram, crystallography, magnetic transition temperature, magnetic moment, heat capacity, and enthalpy of formation can support the future thermodynamic investigation of the Mn–Ga system, which is critical for the materials design and discovery of Ni–Mn-Ga alloys.

Secondary phase strengthened high coercivity in sputtered D022 Mn3−xGa films

Rare-earth-free permanent magnet Mn-Ga alloys have attracted wide interest owing to their large magnetocrystalline anisotropy and high Curie temperature. In this work, we reported a large coercivity of ~22 kOe at room temperature in the Mn3−xGa film, deposited by magnetron sputtering on the Si(0 0 1)/SiO2 substrate. Combined analyses of the x-ray diffraction, magnetization and high-resolution transmission electron microscopy revealed a cubic α-Mn phase coexisting with the main D022 ferrimagnetic phase in the film. It is proved that the mixed structures can refine the magnetic domains, thus enhancing the coercivity in the high temperature annealed sample.

Magnetostructural coupling induced magnetocaloric effects in Ni–Mn-Ga-Fe microwires

Ferromagnetic shape memory Ni–Mn-Ga alloys exhibit magnetostructural coupling during the Curie points, which may alter their magnetocaloric effect (MCE). Here MCE of Ni–Mn-Ga alloy microwires are tuned by Fe-doping. The chemical ordering annealed Ni49·6Mn25.5Ga22.6Fe2.3 (Fe2), Ni49·6Mn25Ga21Fe4.4 (Fe4) and Ni48Mn26Ga19.5Fe6.5 (Fe6) microwires have diameters of ~30–50 μm and grain sizes of ~2–5 μm. The magnetic entropy changes (ΔSm) of the microwires increase with external magnetic fields, then reach a positive peak and finally decrease to negative peak at a high field. The direct (negative ΔSm, DMCE) and inverse (positive ΔSm, IMCE) MCEs are attributed to microscopic and mesoscopic couplings in microwires. In addition, the specific features of MCEs are also controlled by the magnetization difference (ΔM) between martensite and austenite phases. Interestingly, the DMCE and IMCE are component dependent. At high ΔM, the linearly decreasing region is dominant and Fe4 and Fe6 microwires produce a negative ΔSm at high magnetic fields due to the microscopic coupling with free electron valence ratio e/a = 7.692 and 7.725. On the other hand, the ΔM is small in Fe2 microwire, which has a positive ΔSm because mesoscopic coupling is dominant with e/a = 7.607; the ΔSm is highest when a single variant martensite occurs, then decreases linearly with increasing magnetic fields. The transition from positive to negative values in ΔSm with the applied fields is related to the counterbalance of mesoscopic and microscopic couplings at e/a = 7.66.

Uniaxial magnetocrystalline anisotropy of tetragonal Mn Ga100− (50 ≤ x ≤ 75) alloys

Abstract Mn-Ga alloys have attracted considerable interests due to the high uniaxial magnetocrystalline anisotropy (MCA) arising from their tetragonal structure, but the evolution and influence mechanism of the uniaxial MCA associated with composition variations is still unclear. In this paper, experiments and first-principles calculations are combined to study the uniaxial MCA over a wide composition range of MnxGa100−x (50 ≤ x ≤ 75) alloys, ranging from MnGa to Mn3Ga. By adopting suitable annealing processes, the single tetragonal phase is obtained in all studied alloys. From the law of approach to saturation, it is deduced that the magnetocrystalline anisotropy constant (K1) firstly increases from 1.59 MJ m−3 for x = 53.5 to a maximum value of 2.24 MJ m−3 for x = 65, and then decreases to 1.3 MJ m−3 for x = 75. By comprehensive first-principles calculations, we find that under the effect of the crystal field, the Mn atom occupying 4d site of the tetragonal lattice (Mn-4d) has easy axis anisotropy, which is positive to the uniaxial MCA; while Mn-2b has a soft in plane tendency, which is negative to the uniaxial MCA. As x increases from 53.5 to 65, the itineracy of the d electrons is increased through d-d hybridization between Mn-2b and Mn-4d, simultaneously the spin-orbit coupling is enhanced due to c axis shortening of the tetragonal lattice, both improve the overall easy axis behavior, resulting in the increase of K1. As x further increasing from 65 to 75, the soft in plane tendency of Mn-2b leads to the decrease of K1. The analysis can be applied to other 3d alloys, and offers a guidance for improving their uniaxial anisotropy, especially for Mn based permanent magnet.

Optical properties of Fe–Mn–Ga alloys

The first-principles calculations of the electronic structures and the interband optical conductivity (OC) spectra have been performed for the stoichiometric Fe2MnGa alloy with L21 and L12 types of atomic ordering. The calculated optical properties of Fe2MnGa alloy for the L21 and L12 phases are complemented by the experimental OC spectra for bulk and thin film Fe–Mn–Ga alloy samples near the stoichiometry 2:1:1 with L21 and L12 (for bulks) as well as the body-centered-cubic and face-centered-cubic (for films) structures, respectively. A reasonable agreement between experimental and calculated interband OC spectra was obtained for both phases of the alloy. The experimental data show no significant difference in the OC spectra with respect to the degrees of atomic and magnetic orders of the samples.

MnGa and (Mn,Ga)N-like alloy formation during annealing of Mn/GaN(0001) interface

In this paper the behaviour of the Mn/GaN system under the influence of annealing is presented. The interface is formed by Mn vapour deposition onto the GaN crystal, (0001)-oriented, non-doped under ultrahigh vacuum. Physicochemical properties and structural changes of the interface induced by annealing are investigated in situ by employing X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). The impact of annealing on the subsurface layer morphology is observed. Annealing effects, which have been confirmed by XPS and UPS, demonstrate Ga diffusion into the Mn film and Mn dissolution in the bulk GaN. LEED patterns confirm MnGa alloy formation, which in the plane parallel to the substrate surface is arranged in a square lattice with lattice constant equal to about 3.9 Å, and exists in epitaxial configuration relative to the substrate. Incorporation of Mn atoms into the GaN lattice results in the creation of (Mn,Ga)N-like alloy.

Microstructure evolution and kinetic laws for the motion of multiple twin boundaries in Ni–Mn–Ga

The motion of individual twin boundaries in ferromagnetic Ni–Mn–Ga alloys is the basic mechanism by which large reversible strains are generated. The resulting mechanical response makes these alloys potential candidates for magneto-mechanical actuation applications. In this work, we characterize the relations between the magnetically induced strain response of Ni–Mn–Ga single crystals and their internal twin boundary arrangements and kinetics. The macroscopic response is measured under constant magnetic fields, while the kinetics of individual twin boundaries is measured by the pulsed magnetic field method. By testing two different crystals with different twinning microstructures, we show that the main characteristics of the macroscopic response can be controlled by the number of mobile twin boundaries. In addition, we show that the magnitudes of controlling kinetic parameters of different twin boundaries vary with their location within the sample.

Mn valence state mediated room temperature ferromagnetism in nonpolar Mn doped GaN

In this work, we demonstrate room temperature ferromagnetism in nonpolar, m-plane manganese doped gallium nitride (m-GaN:Mn) epitaxial thin films synthesized using plasma-assisted molecular beam epitaxy on nonpolar, m-plane sapphire (Al2O3) substrates. We observed that Mn doping concentration plays a key role in stabilizing the valence state of Mn ions thereby influencing the magnetic and transport properties. Chemical characterization by X-ray photoelectron spectroscopy, X-ray absorption near edge spectra, and energy dispersive X-ray spectroscopy affirm homogeneous valence state Mn+3 in lightly doped samples, while higher doping levels cause a heterogeneous Mn+0 and Mn+2 mixed valence states. Magnetic measurements indicate that films with low doping levels show the signature of room temperature ferromagnetism, while films with higher doping show antiferromagnetic/spin glass behavior besides the indication of MnGa alloys. Temperature dependent resistivity measurements show that both the samples are semiconducting with deep level defect states. Our findings suggest the possibility of incorporating spin functionality in non-polar optoelectronic devices.

Magnetic and Structural Properties of L10 Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Magnetic and Structural Properties of L1₀ Mn–Ga–Al Epitaxially Grown Thin Films of Island Structure

Magnetoelastic Waves in Ferromagnets in the Vicinity of Lattice Structural Phase Transitions

The dispersion laws for coupled magnetoelastic waves in ferromagnets with uniaxial or cubic symmetry have been calculated. The features of obtained dispersion laws in the vicinity of spin-reorientation phase transitions are analyzed. The interaction between elastic and spin waves is shown to depend on the direction of the ferromagnet magnetic moment. The influence of the magnetoelastic interaction on the dispersion law of quasispin waves in the degenerate ground state of a uniaxial “easy plane” ferromagnet is studied. The results of calculations show that the magnetoelastic interaction eliminates the degeneration and leads to the appearance of a magnetoacoustic gap in the ferromagnet spectrum. The behavior of the spectra of coupled magnetoelastic waves in the vicinity of lattice phase transitions, namely, in the vicinity of martensitic phase transformations in materials with the shape memory effect, is analyzed. The obtained results are used to interpret experimental data obtained for the Ni–Mn–Ga alloy. The phenomenon of a drastic decrease of the elastic moduli for this alloy, when approaching the martensitic phase transition point is explained theoretically. It is shown that the inhomogeneous magnetostriction is the main factor affecting the elastic characteristics of the material concerned. A model dissipative function describing the relaxation processes associated with a damping of coupled magnetoelastic waves in ferromagnets with cubic or uniaxial symmetry is developed. It takes the symmetry of a ferromagnet into account and describes both the exchange and relativistic interactions in the crystal.