Ferromagnetic Austenite Research Articles

Magnetocaloric effect and phase transformation of Zn-doped Ni–Mn–Ti all-d metal Heusler alloys

Ni–Mn–Ti Heusler alloys have gained a great consideration recently due to their important magnetoresistance and magnetocaloric properties. Herein, the structure, martensitic transition (MT), and magnetocaloric effect (MCE) of Ni50-xZnxMn35Ti15 (x = 0, 2, 4, and 6) Heusler alloys have been studied. XRD results show that the studied alloy with x = 0 display a monoclinic (5 M) martensite phase. Besides, the examined alloy with x = 2 presents a combination of monoclinic (5 M) martensite and cubic (B2) austenite phases, whereas the alloys with x = 4 and x = 6 exhibit only a cubic (B2) austenite phase. By increasing Zn content x up to 6, the Curie temperature of austenite decreases from 283 K to 132 K, and the MT temperatures decrease from ferromagnetic austenite to weak-magnetic martensite, which is apparently due to the exchange interaction of Mn–Mn and the low power of d-d hybridization between the A/C and B(D) sites. The best maximum magnetic entropy change (ΔSM, max) and refrigeration capacity (RC) are for the studied alloy with x = 4 (5.22 J kg−1K−1 and 217 J kg−1 at 120 kOe, respectively). The Zn addition not only facilitates the formation of B2-type Heusler phases but also offer the possibility to tune the MT and establish strong ferromagnetic coupling.

Magnetic properties, phase diagram and low-temperature specific heat of Ni50Mn50-xSbx alloys

This study examines the magnetic properties and temperature-composition phase diagram of the Ni50Mn50-xSbx alloys. The addition of Sb into the antiferromagnetic Ni50Mn50 alloy introduces ferromagnetic interaction, resulting in a sudden decrease in the Néel temperature. The characteristic regions of ferromagnetic (FM) and antiferromagnetic (AFM) interactions were determined on temperature-composition phase diagram, as well as compositional dependence of Néel temperature was computed. Increase of Sb content gives rise to an interplay between AFM and FM interactions, leading to the emergence of a spin glass-like state. We identified six distinct magnetic phases in the Ni50Mn50-xSbx system, encompassing AFM, paramagnetic, and FM martensite, as well as spin glass, and paramagnetic and FM austenite. Importantly, this behavior may extend to other Ni50Mn50-xZx (Z = In, Sn) alloys, where addition of Z elements leads to the introduction of FM interaction. Moreover, a significant impact of magnetic ordering on low-temperature specific heat is demonstrated. Neglecting the magnetic contribution in ferromagnetic phase leads to the overestimation of electronic specific heat coefficient by factor of 2.

Room temperature magnetocaloric effect and electrical transport properties of Co50Fe7V25Ga18Heusler alloys undergoing martensitic transformation

The metamagnetic martensitic transformation from the ferromagnetic austenite phase to the paramagnetic martensite phase was excited in the Co50Fe7V25Ga18 Heusler alloy near room temperature. Accompanying the transformation, the alloy exhibits a good magnetocaloric effect near room temperature with a cooling capacity of 17.02 J/kg under 3 T magnetic field at 302 K and the cusp-type anomalous of the resistivity with the magnetoresistance effect of ∼ 2.27% under a 3 T magnetic field. The power law equations for the temperature dependence of resistivity indicates the actuality of half-metallic properties. Our experimental results indicate that Co50Fe7V25Ga18 Heusler alloy is likely to become a potential candidate for room-temperature magnetic refrigeration and magnetic sensors applications.

Magnetostress and multicaloric effect in Ni44.1Co5.0Mn36.1In13.4Cu1.4 polycrystalline alloy

In this Letter, we report on a large magnetic field-induced stress of 14.4 MPa/T and a low stress hysteresis of 50 MPa in a polycrystalline Ni44.1Co5.0Mn36.1In13.4Cu1.4 alloy, rendering it as a promising candidate for high-energy output and high-efficient actuators. The large magnetostress mainly originates from the large transformation entropy change of 15.4 J/(kg K) and the high sensitivity of phase transformation temperature to the magnetic field of −9.2 K/T. Furthermore, we have explored the possibility by adopting multicaloric strategy to enhance the transformation reversibility and widen the cooling temperature window. Unexpectedly, the application of the magnetic field has little impact on the elastocaloric temperature change, which might be ascribed to the large magnetization character of ferromagnetic austenite phase and the non-synergic spin-lattice transformation behavior in the Ni44.1Co5.0Mn36.1In13.4Cu1.4 alloy.

Thermo-magneto-mechanical properties of near stoichiometric Ni2MnGa (Mn > Ga) thin films deposited by radio-frequency magnetron sputtering on Si substrate

Near stoichiometric annealed thin films (1.5 to 3 µm) of Ni2MnGa alloy and whose compositions are such that the Martensitic Transformation Temperatures TM(e/a) are in the range -60 °C to 200 °C have been deposited by radio-frequency magnetron sputtering on SiO2/Si substrate. Crystallographic structure characterization such as planar spacing d(i), Thermal Expansion Coefficient CTE and structural domain size L(i)*, has been realized by X-ray diffraction over a wide temperature range from 25 to 205 °C. The mechanical properties such as the Young's modulus (E) and hardness (HB), have been measured by ultra-nano-indentation tests carried out from ambient temperature to 175 °C. SQUID-VSM magnetometer measurements from -263 °C to 127 °C provide some magnetic characteristics: evolution of the magnetization M versus the temperature and the Curie temperature TC. Three diffraction peaks which can be attributed to the tetragonal (NM) or modulated (5 M or 7 M) martensitic structures of polycrystalline films have been identified. At ambient temperature Ta, the calculated planar spacing d(i) associated with at least two diffraction peaks present a linear decreasing with TM(e/a). As a function of the temperature and for the austenitic state only one value of the CTE has been determined. For the martensitic state, two values have been extracted and whose change is close to TM(e/a). No evident evolution has been observed at the vicinity of the Curie temperature. The Young’ modulus and particularly the hardness are very dependent on the film composition, the temperature and the state of the microstructure. At room temperature HB linearly decreases with TM(e/a). As a function of the increasing temperature, for the martensitic state, E slightly decreases whereas HB is almost constant. In the austenitic domain, E slightly increases and HB linearly increases with the temperature. A phenomenological model has been proposed. The whole of the annealed films show a ferromagnetic behavior and from the thermomagnetic curves it has been shown that four distinct states can be observed: para or ferromagnetic austenite and para or ferromagnetic martensite. A transition on the magnetization, certainly due to a change of the martensitic crystal structure, occurs at the neighborhood of TC. Mechanical (HB) and magnetic (M) properties are strongly linked and at Ta it has been shown that M is a linear decrease function of HB.

Large magnetocaloric and magnetoresistance effects during martensitic transformation in Heusler-type Ni44Co6Mn37In13 alloy

NiMn-based Heusler alloys are promising materials in the multi-functional applications due to their ample magneto-responsive effects across martensitic transformation, such as magnetocaloric effect, magnetostriction and magnetoresistance. In this work, the evolution of microstructure, large magnetocaloric and magnetoresistance effects during martensitic transformation are systematically studied in a Ni44Co6Mn37In13 alloy. Magnetic and caloric measurements verify that the martensite in weak magnetic state would gradually transform to be ferromagnetic austenite upon heating. The temperature dependence of microstructure observed in an in-situ optical microscopy unambiguously verify the ‘kinetic arrest’ of martensitic transformation. Most importantly, due to the high sensitivity of transformation to the magnetic field, large magnetocaloric and magnetoresistance effects near room temperature are obtained, including magnetic entropy changes of 8.4 Jkg-1K−1, effective refrigerant capacity of 181 Jkg−1 and magnetoresistance change of 75 % under the field change of Δμ0H = 0–5 T, making the studied sample promising for room temperature magnetic refrigeration and magnetic storage.

Large reversible magnetocaloric effect and magnetoresistance by improving crystallographic compatibility condition in Ni(Co)-Mn-Ti all- d -metal Heusler alloys

Recently, all-$d$-metal Ni(Co)-Mn-Ti Heusler systems have become the research hotspot due to their magnetoresponsive properties associated with a tunable first-order magnetostructural transformation (MST) and excellent mechanical stability for potential applications. However, the presence of large thermal hysteresis acts as an obstacle to the cyclic operation of this novel material. In this present paper, we investigate a large reversible magnetocaloric effect (MCE) and magnetoresistance (MR) in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ all-$d$-metal Heusler alloys that undergo a first-order MST accompanied by a large magnetization change between ferromagnetic austenite and antiferromagnetic martensite phases. Tuning the small at.% of Co doping in ${\mathrm{Ni}}_{37\ensuremath{-}x}{\mathrm{Co}}_{13+x}{\mathrm{Mn}}_{34.5}{\mathrm{Ti}}_{15.5}$ systems, we achieved an optimum composition with $x$ = 1 where low thermal hysteresis of $\ensuremath{\sim}4.7$ K, a narrow transformation interval of $\ensuremath{\sim}11.2$ K, and improved sensitivity of transformation temperature $\ensuremath{\sim}2.8$ K/T is observed. In addition, the origin of small hysteresis is studied based on geometric compatibility between cubic austenite and monoclinic martensite phases, calculated from the powder x-ray diffraction data. These optimized parameters lead to a reversible magnetic field-induced inverse martensitic transition under the field cycling, yielding a large reversible magnetic entropy change ($\mathrm{\ensuremath{\Delta}}{\mathrm{S}}_{M}$) of $\ensuremath{\sim}17.78$ J ${\mathrm{kg}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 277 K in a field change of 5 T. Moreover, a large reversible magnetoresistance (MR) of $\ensuremath{\sim}14%$ out of 32.6% is also obtained under 5-T magnetic field for $x$ = 1 sample in all-$d$-metal Heusler alloys. These reversible magnetoresponsive properties are comparable to other Ni-Mn-based Heusler alloys and have not been reported so far in the all-$d$-metal Heusler system. Therefore our present work demonstrates a pathway to design new cyclically stable multifunctional materials in all-$d$-metal Heusler systems for solid-state cooling devices and magnetic recording applications.

Magnetic glassy martensite induced reversible magnetocaloric effect in Heusler alloys

Heusler alloys hold great potential to act as cooling medium for next-generation solid state refrigeration due to their substantial entropy changes at field-driven magnetostructural transitions. One of the key impediments in the use of Heusler alloys for efficient cooling lies in the poor reversibility of magnetocaloric effect (MCE). In this work, a highly reversible MCE with large isothermal heat Q of ∼ 4.83–5.3 kJ/kg near room temperature, is achieved in Ni50Mn34In12Ga4 Heusler alloys at the transition between unfrozen canonical spin-glassy martensite and ferromagnetic austenite, crosschecked by indirect and direct methods. Phase field simulations reveal that the presence of spin glass in martensite state can result in reversible field-induced magnetostructural transitions in Heusler alloys, thus effectively circumventing the irreversibility of large MCE at strong first-order transitions without the need of any external stimuli such as mechanical load. The occurrence of such field-induced first-order transitions arose from the negative coupling between magnetic moment and the volume of the material. This work suggests the great power of ferroic glasses in improving the reversibility of promising MCE at first-order phase transitions and may push forward more Heusler alloys close to the practical refrigerant in highly efficient solid-state cooling protocols.

Large magnetocaloric effect and magnetoresistance in Fe-Co doped Ni50-(FeCo) Mn37Ti13 all-d-metal Heusler alloys

The effect of substituting FeCo in Ni site maintaining their same ratio on structural, magnetocaloric, and magneto-transport properties of all- d -metal Ni 50- x (FeCo) x Mn 37 Ti 13 ( x = 16, 18, and 20) Heusler alloys are investigated. These alloys undergo a first-order magnetostructural transition (MST) between low temperature weak magnetic martensite (monoclinic or orthorhombic structure) to a high temperature ferromagnetic (FM) austenite (cubic structure) phase around room temperature. The MST temperature is observed to shift towards the lower temperature with increasing (FeCo) x content. These alloys with x = 16, 18, and 20 exhibit isothermal magnetic entropy change (∆S M ) as large as about ~ 13.8 , ~ 12.7 and ~ 11.8 JkgK −1 across their MST associated with an effective refrigerant capacity (RC eff ) of about 119.9 , ~ 126.2 and ~ 194.4 J/kg respectively due to an application of 5 T magnetic field. It is interesting to note that ∆S M remains almost identical over a large temperature region with (FeCo) x content from 16 to 20 at%. In addition, the maximum values of large magnetoresistance (MR)~ − 26.1 % for x = 16, − 25.4 % for x = 18, and − 24.3 % for x = 20 are achieved at the field of 5 T. These materials with large magnetocaloric response in an extended temperature window around room temperature as well as large negative MR can be considered as potential candidates for multifunctional application. • We report the co-doping effect on magnetocaloric effect and magnetoresistance in all- d -metal Heusler alloys. • The magnetostructural transition (MST) shifts towards lower temperature with doping contents. • We observe large isothermal entropy change ( ∆ S M ) , and large magnetoresistance near room temperature.

Defect Engineering: Electron-Exchange Integral Manipulation to Generate a Large Magnetocaloric Effect in Ni41Mn43Co6Sn10 Alloys.

A promising magnetocaloric effect has been obtained in Ni-(Co)-Mn-X (X = Sn, In, Sb)-based Heusler alloys, but the low isothermal magnetic entropy change ΔSM restricts the further promotion of such materials. Defect engineering is a useful method to modulate magnetic performance and shows great potential in improving the magnetocaloric effect. In this work, dense Ni vacancies are introduced in Ni41Mn43Co6Sn10 alloys by employing high-energy electron irradiation to adjust the magnetic properties. These vacancies bring about intense lattice distortion to change the distance between adjacent magnetic atoms, leading to a significant enhancement of the average magnetic moment. As a result, the saturation magnetization of ferromagnetic austenite is accordingly improved to generate a high isothermal magnetic entropy change ΔSM of 20.0 J/(kg K) at a very low magnetic field of ∼2 T.

Magnetic properties, martensitic transformations and magnetocaloric performances in Ni44Mn45-xFexSn11 (x = 0–3) Heusler alloys

The structural, magnetic, and magnetocaloric properties of Ni44Mn45-xSn11Fex (x = 0, 1, 2 and 3) Heusler alloys have been studied detailly by means of X-ray diffraction, differential scanning calorimetry, and magnetization measurements. The martensitic transition temperature (TM) and austenite Curie temperature (TAC) increase gradually with increasing Fe content. Large differences in magnetization between the ferromagnetic austenite and weakly magnetic martensite phases and obvious thermal hysteresis have also been observed during the martensite transformation. With the magnetic field change up to 0–7 T, the maximum value of magnetic entropy changes reaches 26.7 and 23 J kg−1K−1 for x = 1 and 2, respectively, which are much larger than the undoped sample (9.8 J kg−1K−1). Whereas, the effective refrigerant capacity RCeff reduced due to Fe doping increased the hysteresis loss. The alloy for x = 1 has highest RC value of 172 (255) J kg−1 for a magnetic field change of 0–5 T (0–7 T), furthermore, a large magnetic entropy change and moderate RCeff has been obtained around room temperature.

5M and 7M martensitic stability and associated physical properties in Ni50Mn35In15 alloy: first-principles calculations and experimental verification

The excellent magnetic-driven properties of Ni-Mn-based ferromagnetic shape memory alloys are intrinsically related to the presence of the modulated martensite; however, the metastability of modulated martensite hinders the availability of these properties. Herein, the phase stability and associated physical properties of the 5M and 7M modulated martensites in Ni50Mn35In15 alloy are studied by the first-principles calculations and experiments. Results show that this alloy undergoes a magneto-structural coupling transformation (i.e., ferromagnetic austenite → ferrimagneitc 5M martensite → ferromagnetic 7M martensite). The stability of the possible phases can be understood based on both the density of states and differential charge densities analysis. A strong bonding ability between Ni and Mn atoms in the 7M martensite is responsible for it being the most stable phase.

Microstructure characterization, structure and magnetic properties of Ni–Mn–Sn shape memory alloys

In this study, three shape memory alloys, Mn50Ni50-xSnx (x = 5, 7.5, and 10), were elaborated by melt spinning. The structure, microstructure, and magnetic properties of these alloys were carried out by X-ray diffraction, scanning electron microscopy, and physical property measurement system, respectively. The experimental results showed that the increase in Sn ratio caused a phase transformation. The structure of the martensite phase (for x = 5 and 7.5) was seen as a monoclinic 14 M structure, while the structure of the austenite phase (for x = 10) was observed as a cubic L21 structure. We revealed that the addition of Sn led to an almost linear decrease in martensitic transition temperatures, due to the decrease in valence electron ratio (e/a). We explored that for x = 10, the martensitic transition that occurs from the ferromagnetic austenite to weakly magnetic martensite was realized due to the strong magnetostructural coupling in this alloy. Also, it is found that there is a slight decrease in temperatures and a slight increase in the thermal hysteresis range, due to the increase in the magnetic field. While, for x = 7.5 and x = 5, they have the transitions at high temperatures above 400 and 550 K, respectively. This fact might limit these alloys in their applications. So, the present results indicate that the amount of Sn content in the MnNiSn alloys plays an important role to modify the structural and magnetic properties.

Revealing the role of site occupation in phase stability, magnetic and electronic properties of Ni-Mn-In alloys by ab initio approach

The effects of site occupation on the phase stability, martensitic transformation, and the magnetic and electronic properties of a full series of Ni-Mn-In alloys are theoretically studied by using the ab initio calculations. Results indicate that the excess atoms of the rich component directly take the sublattices of the deficient components of the Ni2Mn1+xIn1-x, Ni2-xMn1+xIn, and Ni2+xMn1-xIn alloys. Nevertheless, the mixed and indirect site occupations may coexist in the Ni2+xMnIn1-x system. The relevant magnetic configurations of the austenite for the four alloy systems have also been determined. The results show that, except for the austenite in the Ni2-xMn1+xIn alloys, which tend to be ferrimagnetic, the other alloys all present ferromagnetic austenite. Thus, the site occupation and associated magnetic states are the crucial influencing factors of the phase stability, martensitic transformation, and the total magnetic moment. The electronic structure of the austenite phase also shows that the covalent bonding plays an important role in the phase stability. The key finding of this work is both Ni2Mn1+xIn1-x and Ni2+xMnIn1-x alloys serve as the potential shape memory alloys.

Influence of the Cu substitution on magnetic properties of Ni\u2013Mn\u2013Sn\u2013B shape memory ribbons

The Heusler alloy Ni50-xCuxMn38Sn12 + By (x = 0, 1, 3 and 5) was successfully produced in ribbon form using melt spinning technique. The magnetic properties of the obtained ribbons were analyzed in detail. In all ribbons, it was detected that the ferromagnetic austenite phase transformed into the weak magnetic martensite phase. A separation between FC and ZFC curves at lower temperatures was found. An increase in the magnetization in FC mode can be attributed to the coexistence of ferromagnetic (FM)/antiferromagnetic (AFM) at martensitic phase. It was found that the transition temperatures shifted to low temperatures with increasing the Cu content. The magnetization results under high magnetic field (10 kOe and 50 kOe) showed a thermal hysteresis between the cooling and heating cycles, which is clear evidence for a first-order transformation in the ribbons. From M–H data, all the ribbons exhibited ferromagnetic behavior at low temperatures below the martensitic transition temperature and paramagnetic behavior at high temperatures above the transition temperature. The results provide us a comprehensive view to reveal the effect of the Cu substitution on the magnetic properties of Ni–Mn-based shape memory ribbons.

Relative contributions of the electron-lattice and the electron-spin scatterings to the giant baroresistance effect in Ni–Co–Mn–In system

The evolutions of electrical transport property and Baroresistance (BR) effect at the martensitic transformation (MT) with the change of Co content have been systematically investigated in the Ni50-xCoxMn35In15 (0 ≤ x ≤ 7) Heusler system. The structure measurements show that every sample undergoes first-order MT from a L21 cubic structure to a 3 M monoclinic structure. According to magnetic measurements, it is also found that the samples with lower Co content (0 ≤ x ≤ 3) exhibit a paramagnetic MT, i.e. the MT occurs between the paramagnetic austenite and the paramagnetic martensite. At higher Co content, however, the samples display a metamagnetic MT, i.e. the MT occurs between the ferromagnetic austenite and the paramagnetic martensite. Due to the enhancement of the ferromagnetic ordering for the austenitic phase with Co increasing, the difference in resistivity between austenitic and martensitic phases is enlarged significantly. This leads to a giant BR effect with a value of ∼250% associated with the hydrostatic pressure-induced MT can be observed in the sample with x = 6. Moreover, the Hall measurement results indicate that the difference in electrical transport behavior between both phases for the studied alloys mostly relies on the mobility of carriers related to the electro-lattice and the electro-spin scatterings. In this case, the underlying mechanism of the relative contributions originating from these two different scattering processes to the saturated BR effect has been separated quantitively based on the electrical transport measurements under applied different hydrostatic pressures. It demonstrates that the electron-spin scattering acts as a dominant role in improvement of the BR effect for this system.

Glassy Magnetic Transitions and Accurate Estimation of Magnetocaloric Effect in Ni-Mn Heusler Alloys.

In this work, the structural and magnetic transitions of Heusler alloy Ni50Mn34In14Ga2 have been carefully studied through measurements of heat flow and magnetization under DC and AC magnetic fields. This alloy undergoes the transition sequence of spin-glassy martensite (SPM) → ferromagnetic austenite (FA) → paramagnetic austenite at ∼225 and ∼305 K, respectively, during heating. Splitting of zero-field-cooling (ZFC)/field-cooling (FC) curves in martensite is caused by the slowdown dynamics of spin glass as evidenced by frequency dispersion and aging effects. The development of a spin-glass state is believed to be the result of strain relaxation and interaction of ferroelastic twin walls in the martensite. The magnetocaloric effect (MCE) at the SPM-FA transition was then measured using indirect, quasi-direct, and direct methods. The MCE magnitudes are controlled by the entropy changes associated with the first-order martensite transition and magnetic ordering of austenite under the magnetic field. The existence of a spin-glass state in martensite can also improve the reversibility of the magnetostructural transitions, which is beneficial for the improvement of the reversibility of associated MCE. These results provide an in-depth understanding of the transitions and magnetic properties of the Ni-Mn Heusler alloys and suggest that the MCE at the first-order magnetostructural transitions estimated solely using indirect methods may need some revision.

Investigation of Magnetic Entropy Change and Critical Behavior Analysis of Cu- and Si-Doped Ni–Mn–Sn Heusler Alloys

We have studied the magnetic property as a function of temperature and magnetic field (μ0.H) and critical exponent on Ni–Mn-based Heusler alloys such as Ni46Mn41Sn13 (NMS), Ni46Mn41Sn10Si3 (NMSS), and Ni46Mn38Cu3Sn13 (NMCS). Magneto entropy change and the order of phase transition are estimated around the martensite and austenite phases of all these samples. Magnetic critical behavior of each sample is analyzed around second-order ferromagnetic (FM) austenite phase transition employing Arrott and Kouvel–Fisher plots. The reliability of estimated critical exponents is verified by Widom scaling relation and M3 vs H plot. Furthermore, one more local exponent n which depends on the magnetic state is also estimated through the isothermal magnetization of magnetic entropy change (∆SM) plots of all these samples. The obtained values for all these alloys are well close to the mean-field theoretical model and it is proposed that the existence of long-range magnetic interaction around austenite transition of these materials.

Ferromagnetic martensitic transformation and large magnetocaloric effect in Ni35Co15−xFexMn35Ti15 (x = 2, 4, 6, 8) alloys

The crystal structure, ferromagnetic martensitic transformation, and magnetocaloric effect of Ni35Co15−xFexMn35Ti15 (x = 2, 4, 6, 8) alloys have been investigated. All the alloys crystallize in a B2-type cubic structure at room temperature. The Curie temperature of austenite between paramagnetic and ferromagnetic states decreases from 349.0(7) K to 287.2(8) K with increasing Fe content x up to 8, whereas the corresponding martensitic transformation temperature from ferromagnetic austenite to weak-magnetic martensite increases continuously from 182 K to 230 K, and the accompanied thermal hysteresis decreases gradually. The maximum values of magnetic entropy change (ΔSM)/refrigeration capacity reach 9.5(8) J kg−1 K−1/79.4(5) J kg−1 (0–20 kOe) and 24.0(4) J kg−1 K−1/206.8(4) J kg−1 (0–50 kOe), respectively. The present results indicate that the Ni35Co15−xFexMn35Ti15 alloys are considerable for magnetic refrigeration.

Significant enhancement of magnetocaloric effect in a NiMnCuGa Heusler alloy through textural modification

Magnetocaloric materials are of increasing interest to bring magnetic refrigeration to everyday households and drastically impact the energy demands for temperature control devices. In this work, a polycrystalline Heusler alloy of composition Ni2Mn0.76Cu0.24Ga with coinciding structural and magnetic transformation temperatures was subjected to compressive stress assisted thermal cycling (SATC) to enhance the magnetic properties by inducing a preferred orientation in the martensite. Isofield magnetization measurements showed a sharpening of the transformation between ferromagnetic martensite and paramagnetic austenite due to SATC. In isothermal magnetization measurements, SATC was seen to increase the magnetostructural coupling. With a 2 T applied magnetic field, the magnetocaloric effect (MCE) increased from ∼10 to ∼25 J/kg K and the refrigeration capacity (RC) almost doubled due to SATC. Heat capacity measurements were largely unaffected by SATC. The change in adiabatic temperature was estimated by using Cp and change in magnetic entropy (ΔSM) calculations. SATC was seen to increase ΔTad from ∼1.2 K to 2 K for an applied magnetic field of 2 T. Neutron diffraction measurements revealed highly textured martensite in the as received state that rotated to a more ideal preferred orientation after SATC that enhanced the magnetostructural transformation; and thus, improving the MCE and ΔTad.