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In the present study, polycrystalline Co50V34Ga16−xFex ( 1 ≤ x ≤ 2 ) quaternary Heusler alloys were fabricated by the arc-melting method. It was found that they undergo a paramagnetic martensitic transformation (MT) from the L21-type cubic austenitic structure to the D022 tetragonal martensitic structure. With the increase of the Fe concentration, the MT shifts towards a higher temperature range, which is strongly related to the variation of the valence electron concentration. Moreover, it was also found that the MT exhibited by every alloy is sensitive to the applied hydrostatic pressure due to a relatively high difference in volume between the two phases. By using the quasi-direct method based on caloric measurements, the barocaloric effect (BCE) associated with the hydrostatic pressure-driven MT was estimated in the studied alloys. The results demonstrated that the sample with x = 1.5 performs an optimal BCE among these three alloys. [ABSTRACT FROM AUTHOR]

In this work, a non-spontaneous martensitic transformation (MT) activated by the hydrostatic pressure has been observed in polycrystalline Ni43Co7Mn35In15 Heusler alloy. This alloy can experience a nearly complete metamagnetic MT with the assistance of an enough pressure. As the change of pressure reaches 20 kbar, the maximum baroresistance (BR) of the studied alloy achieves ~ 825% at 50 K and remains ~ 280% even in the vicinity of room temperature revealing a giant BR effect over a wide temperature range. More interestingly, the perfectly reversible isothermal BR effects as high as over 650% have been also found at the temperatures below 130 K by overcoming a thermal barrier between direct and reverse MTs. Such an interesting behavior could be ascribed to a pressure-induced continuous first-order MT accompanied by a prominent change of spin ordering, which brings about a combined contribution of electron-spin and electron-lattice scatterings. [ABSTRACT FROM AUTHOR]

In the present work, polycrystalline Co52V29+xGa19-x(0 ≤ x ≤ 2) Heusler alloys have been prepared. The related experimental results confirm that each sample undergoes a first-order martensitic transformation (MT) from the L21 cubic austenitic state to the D022 tetragonal martensitic state. With increase of V concentration, both transformation temperature and strain are heightened simultaneously, thus giving rise to an increment of transition entropy change Δ S tr . In addition, the influence of hydrostatic pressure on MT and transition strain have been also carefully studied. With increase of pressure, the transformation temperatures of these samples move to high values apparently, but their reversible strain only exhibit a slight enhancement and quickly approach the saturated values. For different compositions, however, there is no obvious difference in the rate of transformation temperature change with pressure. Such a behavior opens up the potential applications of the functional properties associated with the hydrostatic pressure-induced MT for this system. [ABSTRACT FROM AUTHOR]

In this work, we have developed a ferromagnetic shape memory alloy Co50V34Ga16 with a metamagnetic martensitic transformation (MT) from the high-magnetization austenitic phase to the low-magnetization martensitic phase. As a consequence of a strong coupling between structure and magnetic degrees of freedom, the metamagnetic MT of this alloy is relatively sensitive to the external magnetic field, thus giving rise to a field-induced reverse MT. Associated with such a unique behavior, both considerable inverse magnetocaloric effect (9.6 J/kg K) and magnetostrain (0.07%) have also been obtained under the magnetic field change of 3 T. Our experimental results indicate that this kind of Co-V based alloy probably becomes an alternatively promising candidate for applications in magnetic sensors and magnetic refrigeration. [ABSTRACT FROM AUTHOR]

The electrical transport properties at martensitic transformation (MT) in polycrystalline Ni43.7Fe5.3Mn35.4In15.6 have been intensively investigated under different hydrostatic pressures. For this alloy, the experimental results show that applying a higher hydrostatic pressure can convert its MT from the metamagnetic type into the paramagnetic type. It provides a unique opportunity to separate the relative contributions of electron-spin and electron-lattice scatterings across the metamagnetic MT based on the dynamical Clausius-Clapeyron equation, which delivers a deeper insight into the resistivity change of metamagnetic MT for the Mn-rich Ni-Mn based Heusler alloys. In addition, the studied alloy also reveals a giant positive baroresistance (BR) effect with a saturated value of 115% at 242 K. This performance originates from the combined effect of electron-spin and electron-lattice scatterings associated with a prominent hydrostatic pressure-induced MT, which contribute 46% and 69% to the overall BR ratio, respectively. [ABSTRACT FROM AUTHOR]

In this study, we reported the influence of replacing Cu for Ni on the crystal structure, martensitic transformation, and magnetic properties for polycrystalline Ni50 −xCu xMn37In13 ( x = 5 and 7) quaternary alloys. It was found that the studied alloys undergo a first-order martensitic transformation from ferromagnetic austenite to paramagnetic martensite near room temperature. With increasing Cu content, the martensitic transformation significantly shifts toward a lower temperature, and fully splits off from the Curie transition of austenite, which enhances the difference in magnetization between the two phases for the alloy with x = 7. Moreover, both inverse magnetocaloric and magnetoresistance effects during reverse martensitic transformation were also studied in these two alloys. In comparison to Ni50− xCu xMn37In13 with x = 5, an optimized refrigerant capacity (∼100 J kg−1) and a considerable magnetoresistance (47%) were obtained in Ni43Cu7Mn37In13 alloy for the magnetic field change of 30 kOe. In addition, the magnetic measurements with hydrostatic pressure demonstrate that the magnetoresistance in this system mainly arises due to the electron-lattice scattering triggered by structural transition. [ABSTRACT FROM AUTHOR]

The evolutions of phase transformation, magnetic and electrical transport properties with the change of Fe concentration have been systematically investigated in a series of Ni 60- x Fe 13+ x Ga 27 (x = 0–10) Heusler alloys. The structure and heat flow measurements reveal every sample undergoes a disordered-ordered transition from a disordered B 2 structure to a L 2 1 ordered structure, and a first-order structural transformation from the L 2 1 cubic structure to a L 1 0 tetragonal structure is corresponding to the martensitic transformation (MT). With increasing Fe concentration, the disordered-ordered transition and ferromagnetic transition shift to higher temperature, while the MT shifts to lower temperature. According to phase diagram established in the present investigation, it is also found that the increase of Fe content can convert the first-order MT from a paramagnetic type to a ferromagnetic one, which is accompanying by an obvious increase of both effective and spontaneous magnetic moments. Furthermore, the calculated magnetocrystalline anisotropy constant (K 1) of the studied alloys monotonically increases as Fe increases. It is evident that the magnetic degrees of freedom in these two MTs remain almost unchanged so that the electron-spin scattering doesn't change, which results in the resistivity difference between the austenitic and martensitic states is feeble. Such an intrinsic nature leads to a weak magnetoresistance (MR) and baroresistance (BR) effects in the studied samples because of only contribution of electron-lattice scattering change on the resistivity. This demonstrates from a negative perspective that the electron-spin scattering change originated from the variation in the magnetic degrees of freedom acts as a dominant role in improvement of the MR and BR effects for the Heusler system possessed the first-order MT. • The phase diagram including the structural ordering transition for Ni-Fe-Ga alloys was established. • Ni 60- x Fe 13+ x Ga 27 alloys undergo a disordered-ordered transition from a disordered B 2 structure to a L 2 1 ordered structure. • The magnetocrystalline anisotropy constant monotonically increases as Fe increases. • The baroresistance and magnetoresistance effect of Ni-Fe-Ga has been obtained around the room temperature. • The electron-spin scattering change acts as a crucial role to improve the MR and BR effects. [ABSTRACT FROM AUTHOR]

An experimental investigation was conducted to understand the influence of hydrostatic pressure on phase transition, thermal expansion and barocaloric property of a polycrystalline Ni 54 Mn 23 Ga 23 alloy. Two pressure ranges of 0–1.28 GPa and 0–0.74 GPa were selected for thermal expansion and barocaloric property measurements due to the different test limits of instrumentals. The pressure dependence of fourteen parameters was studied systematically. It was found that the sample experiences a martensitic transformation (MT) near room temperature and the response of MT peak temperature (T M) to pressure is about 15.89 K/GPa. The phase transition strain (λ tr), transformation width (Δ T tr) relative volume change (|Δ V / V |) and maximum adiabatic temperature change (Δ T ad) max increase linearly with pressure and their shifting rates are about 0.071 %/GPa, 7.5 K/GPa, 0.211 %/GPa and 12.2 K/GPa, respectively. Simultaneously, applying hydrostatic pressure can also enhance the barocaloric property of the sample, leading to a large increase in the absolute value of the maximum entropy change (|Δ S | max), the refrigerating capacity (RC) and their reversible values. On cooling, the sensitivities of the reversible RC and its proportion to pressure are around 185.52 J/kg GPa and 16.7 %/GPa, respectively. Finally, the average linear expansion coefficient (α ave) of both martensite and austenite phases keeps unchanged while in the MT region it decreases from -99.84×10–6/K to -86.04×10–6/K with increasing pressure from 0 to 1.28 GPa. These findings indicate that applying hydrostatic pressure can effectively adjust the phase transformation, thermal expansion and barocaloric property of Ni-Mn-Ga alloys, which are meaningful for their application in solid-state refrigeration. • Pressure can effectively adjust phase transformation, thermal expansion and barocaloric property of Ni 54 Mn 23 Ga 23 alloy. • A large response of martensitic transformation (MT) peak temperature to pressure is reported. • The barocaloric property and its reversibility are highly sensitive to pressure. • The average linear expansion coefficient during MT decreases with increasing pressure. [ABSTRACT FROM AUTHOR]

• Applying hydrostatic pressure can push the MT to a higher temperature. • Curie transition almost remains unchanged under hydrostatic pressure. • The average magnetic moment and mass susceptibility of SPM cluster are reduced. • The strength of FM/AFM interactions are suppressed by hydrostatic pressure. • The application of hydrostatic pressure suppresses the exchange bias effect. The effect of hydrostatic pressure on martensitic transformation (MT), superparamagnetic (SPM) and exchange bias (EB) behaviors in polycrystalline Ni 45 Cu 5 Mn 35 In 15 has been intensively investigated by various experimental methods. The experimental results show that the studied alloy experiences a MT from the austenitic state with a L 2 1 -type cubic structure to the martensitic state with a seven-layer (7 M) modulated orthorhombic structure with decreasing temperature, accompanied by a significant change in magnetization. It is found that applying hydrostatic pressure can induce the MT to occur in the higher temperature, while the Curie transition almost remains unchanged due to the suppression of ferromagnetic (FM) coupling. In addition, the applied hydrostatic pressure can suppress the EB field and coercivity of the studied alloy due to the reduction of the competition between FM and AFM interactions. This phenomenon can be attributed to the fact that the application of hydrostatic pressure reduces the size of the SPM cluster and its average magnetic moment. [ABSTRACT FROM AUTHOR]

A martensitic transformation (MT) with paramagnetic behavior has been developed in Co 50 Fe 2.5 V 31.5 Ga 16 quaternary Heusler alloy near room temperature. Accompanying the transformation, this alloy shows a good two-way shape memory effect, which results in a considerable relative volume change between two phases. Because of this, it has been also found that the MT is strongly sensitive to the applied hydrostatic pressure. By utilizing an indirectly estimated method, the obtained maximum reversible isothermal entropy (Δ S T) and adiabatic temperature change (Δ T ad) resulted from hydrostatic pressure induced MT, respectively, achieve ∼31 J/kg K and 6 K under 5 kbar, performing a large barocaloric effect. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Effects on substitution of Ge by In on structural and magnetic properties are studied in MnCoGe 1- x In x compounds. A small amount of In (2% and 5%) dopant can shift the martensite transformation temperature to room temperature and make it overlap with Curie temperature window, which results in a spin-lattice coupling and a large conventional magnetocaloric effect. Giant values of magnetic entropy change ( △S M ) are obtained in both magnetizing and demagnetizing processes in the vicinity of the magnetostructural transition. The maximum values of | △S M | for MnCoGe 0.95 In 0.05 reach 21.4 and 25.4 J/kg K for a small magnetic field change △H  = 2–0 and 0–2 T, respectively, which make it promising for room temperature magnetic refrigeration applications. [ABSTRACT FROM AUTHOR]

Both the barocaloric effect (BCE) and its reversible behavior during martensitic transformation (MT) have been experimentally investigated in Ni 42.3 Co 7.9 Mn 38.8 Sn 11 quaternary Heusler alloy. The experimental results show that the studied alloy experiences the MT from the ferromagnetic L 2 1 -type cubic structure to the paramagnetic-like three-layered (3 M ) modulated monoclinic structure, resulting in a value of ∼1.1% for the relative volume change ε between two phases. Due to appearance of an enhanced ε , the migration rate of martensitic transformation with respect to the hydrostatic pressure reaches ∼4.7 K/kbar, while a high transition entropy change (∼28 J/kg K) is still maintained. Based on an indirectly estimated method, the calculated maximum value for isothermal entropy change Δ S T achieves ∼ -23 J/kg K with the pressure change of 6.2 kbar, which yields the values of ∼10 K for adiabatic temperature change Δ T a d and ∼786 J/kg for relative cooling power (RCP), performing a large BCE. According to the change of phase fraction at selected hydrostatic pressures, a detailed schematic diagram has been established to evaluate the reversibility of this effect. With applying an external cyclic pressure of 6.2 kbar, the maximum repeatable values calculated for both Δ S T and Δ T a d are decreased to ∼ -15 J/kg K and ∼5 K, respectively. This is attributed to an insufficient driving force, which can only partially transform this material, leading to a reversible MT appeared in the mixed state. [ABSTRACT FROM AUTHOR]

We report effect of hydrostatic pressure on martensitic transformation in polycrystalline Ni 44.6 Co 5.5 Mn 35.5 In 14.4 . With applying pressure, the transformation moves to high temperatures at a rate of 4.4 K/kbar. Utilizing an indirect method, the calculated maximum isothermal entropy changes achieve ~ 14.3 J/kg K at 2.76 kbar and ~ 15.6 J/kg K at 5.98 kbar, which respectively output the values of ~ 5 K and ~ 6 K for the adiabatic temperature change near room temperature. After deducting the average hysteretic loss (~ 152 J/kg), the effective refrigeration capacities still reach ~ 38 J/kg at 2.76 kbar and ~ 247 J/kg at 5.98 kbar, performing an enhanced barocaloric effect. [ABSTRACT FROM AUTHOR]

The crystal structures, martensitic transformations (MT) and magnetic properties of polycrystalline Ni 56- x Co x Mn 18 Ga 26 ( x = 0, 1, 2) alloys have been investigated comprehensively. These alloys exhibit an unmodulated martensitic structure ( L 1 0 ) at room temperature. In the Co free alloy, a magneto-structural coupling MT is presented. With increasing Co content, however, the MT moves to a higher temperature and finally partly splits off from ferromagnetic transition at x = 2. Simultaneously, two successive magneto-structural transformations were observed at x = 1, which could be ascribed to the existence of an intermartensitic transition (7 M ↔ L 1 0 ). Owing to the evolution of MT with Co doping, the magnetization of paramagnetic (PM) austenite is increased slightly, while the ferromagnetic (FM) martensite is almost unchanged. The multiple functional properties associated with the field-induced MT were also investigated in these alloys. It was found that the optimized magnetostrain amounts to ∼0.12% in the alloy with x = 2. In addition, the refrigerant capacity (RC) with the values of 50 J/kg and 49 J/kg were obtained in the alloy with x = 0 and x = 1, respectively. [ABSTRACT FROM AUTHOR]

Combined thermal strain and magnetization measurements were performed in polycrystalline alloys Ni 49.31 Mn 22.76 Ga 27.93 (Ni 49), Ni 53.00 Mn 21.04 Ga 25.96 (Ni 53) and Ni 55.62 Mn 17.52 Ga 26.86 (Ni 55.5) to understand the influence of magnetic field (B) on transformation strain (Δλ) and relative volume change (ω) at martensitic transformation (MT). It is found that during cooling and heating both Ni 49 and Ni 53 samples undergo a MT while the Ni 55.5 sample experiences a coupled magnetostructural transformation (MST). Such a MST can enhance the Δλ and ω whereas thermal hysteresis weakens them, which results in a non-monotonic variation in Δλ: 0.211% (Ni 49) → 0.166% (Ni 53) → 0.185% (Ni 55.5) under B = 0 T. Similar tendency was observed in ω. Besides, the values of Δλ and ω increase with increasing field, partially due to the contribution of magnetic field-induced strain. The maximum value ω = 0.674% was obtained in Ni 49 sample under 3 T, indicating that an extra volume space of ∼1% should be needed to avoid the destruction of refrigeration chamber. These findings provide key information for future use of Ni–Mn-Ga alloys in refrigeration engineering field. • Ni–Mn-Ga alloys exhibit a large transformation strain (Δλ) at martensitic transformation (MT). • Magnetostructural transformation can enhance transformation strain and relative volume change (ω). • Thermal hysteresis weakens the magnitude of Δλ and ω. • Magnetic field has a positive effect on the increase in Δλ and ω due to the magnetic field-induced strain. • A large relative volume change of 0.674% was obtained in Ni 49.31 Mn 22.76 Ga 27.93 under 3 T. [ABSTRACT FROM AUTHOR]

The magnetic field effect on phase transition and thermal expansion properties were investigated in Ni 55 Mn 18 Ga 27 alloy. The results show that the alloy has an excellent isotropy, reproducibility and thermostability in thermal strain and exhibits a large negative thermal expansion (NTE) effect in the direct and reverse martensitic transformations (MTs). For the direct MT, the average linear expansion coefficient α is about −225.21 ppm/K and the corresponding temperature span is around 8.2 K. Additionally, the sensitivities of the transformation strain and peak temperatures to field are about 88.5 ppm/T and 1.31 K/T, respectively. The rates of α and temperature spans of NTE to magnetic field are about 8.88 ppm/K T and 0.82 K/T, indicating that the magnetic field can improve the NTE property of the Ni–Mn-Ga alloy. Finally, the NTE mechanism has also been discussed. Our findings are beneficial to improving the thermal expansion and NTE properties of Ni–Mn-Ga alloys. • The thermal strain of the Ni55Mn18Ga27 alloy is highly isotropic, reproducible and thermostable. • The alloy exhibits a large negative thermal expansion (NTE) effect during the martensitic transformations (MTs). • The average linear expansion coefficient α is −225.21 ppm/K for the direct MT and the rate of α to field is ∼8.88 ppm/K T. • The NTE mechanism has also been discussed. • Applying magnetic field can improve the NTE property of the Ni–Mn-Ga alloy. [ABSTRACT FROM AUTHOR]

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 Ni 50- x Co x Mn 35 In 15 (0 ≤ x ≤ 7) Heusler system. The structure measurements show that every sample undergoes first-order MT from a L 2 1 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. • The MT from a paramagnetic type into a metamagnetic one has been found. • A giant BR effect (∼250%) has been obtained around the room temperature. • The change of electrical transport during MT depends on the mobility of carriers. • Both electro-lattice and electro-spin scatterings has been separated quantitively. • The spin-dependent scattering plays a crucial role to improve the BR effect. [ABSTRACT FROM AUTHOR]

The crystal structure, transformation property and magnetocaloric effect (MCE) were investigated in Ni 55.5 Mn 17.8 Ga 26.7 alloy. The sample at room temperature exhibits an unmodulated martensitic structure with a small residual trace of 7 M modulated martensitic structure. Two successive magneto-structural transformations (MSTs) consisting of martensitic and intermartensitic transformations are observed. It is found that the maximum isothermal entropy change |Δ S | max of 7.65 J/kg K at 309 K and refrigeration capacity (RC) of 78.84 J/kg are attained for a field change μ 0 Δ H = 3 T under ambient pressure (p = 0 GPa). The enhanced MCE can be ascribed to the two successive MSTs. Much importantly, the |Δ S | max is basically independent of hydrostatic pressure and equals to 7.74 J/kg K at 314 K for p = 0.622 GPa. Besides, the application of hydrostatic pressure improve the width of phase transition and a large RC of 101.57 J/kg has been observed for p = 0.622 GPa under μ 0 Δ H = 3 T. Consequently, the response of RC to magnetic field increases from 26.86 J/kg T for p = 0 GPa to 34.13 J/kg T for p = 0.622 GPa. The reversible RC is also improved by pressure. Such an enhanced MCE and the non-toxic raw materials make this alloy particularly appealing for solid-state cooling technologies. • Ni 55.5 Mn 17.8 Ga 26.7 alloy exhibits two successive magneto-structural transformations. • An enhanced MCE associated with the two successive MSTs was obtained. • The maximum isothermal entropy change almost keeps constant under ambient and high pressures. • Application of hydrostatic pressure can effectively broaden the phase transforming temperature span. • A considerable refrigerant capacity was obtained under hydrostatic pressure. [ABSTRACT FROM AUTHOR]