Magnetic refrigeration has drawn great attention from researchers because of its advantages over the conventional cooling technique, such as the compact size, low mechanical vibration, environmental friendliness, and high efficiency.^[1–3] Materials exhibiting the large magnetic entropy changes ( are potential candidates for magnetic refrigeration applications.^[4] Till now, the large has been widely studied in materials experiencing the magnetic-field-induced first-order transition, e.g., La–Fe–Si,^[5–7] Gd–Si–Ge,^[8] Mn–Fe–P–As,^[9] and MMnX (M= Co or Ni, Si or Ge) alloys.^[10–12] Besides these materials, Heusler-type NiMn-based magnetic shape memory alloys also attract considerable attention for its large and tunable magnetocaloric effect.^[13–20]

The magnetocaloric effect in NiMn-based magnetic shape memory alloys is generated from the magnetic-field-induced first-order magnetostructural transformation from martensite to austenite.^[21–23] According to Liuʼs report, the adiabatic temperature change with a magnetic field variation of 0–2 T is as large as −6.2 K in Ni–Mn–In–(Co) alloy,^[24] which is larger than the magnetocaloric effect of Gd. Additionally, due to the fact that the magnetostructural transformation temperature is sensitive to the valence electron concentration ( ,^[25] cell volume,^[26] grain size,^[27] and external parameters (magnetic field or applied hydrostatic pressure),^[28] the magnetostructural transformation can be tuned by changing the composition or preparation conditions.^[29–35] For instance, the magnetostructural coupling and magnetocaloric effect can be effectively tailored through changing the ratio between Ni and Mn magnetic elements or introducing a fourth magnetic element.^[30–32] The doping effect of non-magnetic elements on the magnetostructural transformation and the magnetocaloric properties in Ni–Mn–In alloys has been theoretically studied by Sokolovskiy et al.^[33] They predicted that the adiabatic temperature change can be enhanced by a factor of 3 when the Mn atoms are partially replaced by non-magnetic Cu atoms in the Ni₅₀Mn₃₄In₁₆ alloy.^[33] Recently, Kaya et al. also reported the improvement of magnetocaloric properties through introducing non-magnetic Al element in the Ni₄₃Mn₄₆In₁₁ alloys.^[35] Based on these previous experimental and theoretical reports, it is expected that nonmagnetic V element may also be able to tune the magnetostructural transformation and magnetocaloric properties in NiMn-based Heusler alloys.

In this work, we investigate the magnetostructural transformation and magnetocaloric properties of Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) ferromagnetic shape memory alloys. The martensitic transformation temperature decreases when Mn is substituted by V. High field M–T and isothermal magnetization curves confirm the magnetic-field-induced magnetostructural transformation. The tunable martensitic transformation temperature, enhanced magnetic field driving capacity, and large magnetic entropy changes are observed.

The polycrystalline Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) alloys were prepared by arc melting the appropriate amount of constituents (99.98% purity) in an argon atmosphere using a water-cooled copper crucible. The ingots were remelted for three times to ensure the homogeneity. After that, the ingots were annealed at 1073 K for three days in vacuum quartz tubes, and then quenched into cold water. X-ray energy-dispersive spectroscopy (EDS, Thermo system 7) was used to determine the elemental composition of the samples. The structural transitions were investigated by differential scanning calorimetry (DSC, Mettler Toledo) with a rate of 10 K/min during heating and cooling cycles. The crystal structures of samples were identified by x-ray diffraction (XRD, Bruker, D8 Advance) using Cu-Kα radiations. Magnetic measurements were carried out on physical property measurement system (PPMS, Quantum Design, Dynacool) and magnetic property measurement system (MPMS-7, Quantum Design). To avoid the irreversibility, the isothermal magnetization curves (M–B) were measured using a so-called loop method.^[36]

Figure 1 shows the XRD patterns of Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) alloys at room temperature. For x = 0 and 1, a small fraction of martensitic phase is detected in addition to the dominant cubic austenite phase at room temperature. With further increase of x, (022) peak from the martensite shifts to the higher angle and overlaps with the (220) peak from the austenite. It is known that the structure of austenite in Ni–Mn–Sn magnetic shape memory alloy is sensitive to the composition. It could be cubic L2₁, Hg₂CuTi, or B2 structure.^[37] In this case, the superlattice (111), (222), (400), and (422) reflections confirm L2₁ cubic structure. As the size of V (0.134 nm) is larger than that of Mn (r=0.126 nm), with V substitution, the lattice parameter (a) and unit cell volume increase, indicating that V substitution may produce the lattice distortion in these alloys.

Fig. 1.

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	Fig. 1. (color online) XRD patterns of Mn48−xVxNi42Sn10 (x = 0, 1, 2, and 3) alloys at room temperature.

Figure 2 shows the DSC curves of Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) alloys during the heating and cooling processes. The large endothermic/exothermic peaks represent the occurrence of martensitic and reverse martensitic transformations. The thermal hysteresis, which represents the irreversibility between the heating and cooling processes, identifies the first-order nature of the transformation.^[38] According to the DSC data, the start and finish temperature of reverse martensitic transformation (A_s and A_f) and the start and finish temperature of martensitic transformation (M_s and M_f) are listed in Table 1. It can be seen that with the V-substitution, the structural transformation temperature (T_t decreases. Here, T_t in heating ( and cooling ( processes are defined by and , respectively. The entropy change accompanied with the complete martensitic transformation ( can be determined from the DSC measurements.^[39] The estimated values of are 32, 30, 29, and for x = 0, 1, 2, and 3, respectively.

Fig. 2.

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	Fig. 2. (color online) DSC curves of Mn48−xVxNi42Sn10 (x = 0, 1, 2, and 3) alloys during heating and cooling processes.

Table 1.

Table 1.

Table 1. The measured elemental compositions and corresponding electronic concentrations for the present Mn48−xVxNi42Sn10 (x = 0, 1, 2, and 3) samples. . x Mn/at.% Ni/at.% Sn/at.% V/at.% e/a 0 47.88 42.17 9.95 00 7.961 1 46.85 42.09 9.91 1.15 7.942 2 45.90 42.11 10.07 1.92 7.923 3 44.88 42.16 10.11 2.85 7.904

Table 1. The measured elemental compositions and corresponding electronic concentrations for the present Mn48−xVxNi42Sn10 (x = 0, 1, 2, and 3) samples. .

The temperature dependence of magnetization (M–T) curves for Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, and 2) alloys were measured to investigate the magnetic phase transitions. Figure 3 shows the M–T curves of Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, and 2) alloys under an applied field of 0.01 T. With increasing the temperature, all the samples transform from a weak magnetic martensite to a ferromagnetic austenite, as shown in Fig. 3. A thermal hysteresis of about 11 K is observed for all the three samples accompanied with the martensitic transformation, indicating the first-order nature for the magnetostructural transformation. The estimated values of A_s, A_f, M_s, M_f, , and from M–T curves are listed in Table 2. These characteristic temperatures for the transitions from the M–T curves agree well with the DSC measurement (Fig. 2). It is known that T_t is related to the value of e/a.^[31] In this case, e/a decreases with the substitution of V for Mn, as T_t decreases. Additionally, T_t in NiMn-based magnetic shape memory alloy is also sensitive to the cell volume. The partial replacement of Mn by V atoms induces the expansion of the unit cell, which also shifts the magnetostructural transformation to lower temperatures.

Fig. 3.

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	Fig. 3. (color online) The temperature dependence of magnetization (M–T) curves for Mn48−xVxNi42Sn10 (x = 0, 1, and 2) alloys under an applied field of 0.01 T. (a) x=0, (b) x = 1, and (c) x = 2.

Table 2.

Table 2.

Table 2. Phase transformation temperatures, effective refrigeration capacity (RCeff), field driving capacity (d dB), and magnetic entropy change ( for the Mn48−xVxNi42Sn10 (x = 0, 1, and 2) alloys. . Sample As Af Mf d dB RCeff ) /K /K /K /K /K /K / / / x = 0 261 268 259 251 266 256 −1.4 37.21 15.2 x = 1 245 254 242 233 249 237 −1.7 75.4 18.8 x = 2 231 238 288 218 235 223 −2.01 82 24.3

Table 2. Phase transformation temperatures, effective refrigeration capacity (RCeff), field driving capacity (d dB), and magnetic entropy change ( for the Mn48−xVxNi42Sn10 (x = 0, 1, and 2) alloys. .

In order to investigate the magnetic-field-induced structural transformation, the isothermal magnetization (M–B) curves for Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, and 2) alloys in the field of 0–5 T were measured by the so-called loop method.^[36] Initially the samples were cooled down to complete the martensitic state and then slowly heated to the target temperature before starting each M–B measurement. As shown in Figs. 4(a)–4(c), the curves show an obvious metamagnetic behavior around T_t, which corresponds to the magnetic-field-induced structural transformation from a week magnetic martensite to a ferromagnetic austenite. The magnetic hysteresis reflects the first-order nature of the transition. The values of magnetization difference ( under the field of 5 T are 41.6, 43.9, and 46.7 Am²/kg for x = 0, 1, and 2, respectively. The increase of in Mn₄₆V₂Ni₄₂Sn₁₀ is attributed to the enhanced field driving capacity.

Fig. 4.

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	Fig. 4. (color online) M–B curves at various temperatures around the martensitic transformation for the Mn48−xVxNi42Sn10: (a) x = 0, (b) x = 1, and (c) x = 2. (d) High field M–T curves for Mn48−xVxNi42Sn10 (x = 0, 1, and 2) alloys with a magnetic field of 3 T (open symbols) and 5 T (solid symbols) in the heating process.

Owing to the magnetization difference between the martensite and austenite, the structural transformation can be driven by the magnetic field. With increasing the magnetic field from 3 to 5 T, the M–T curves of Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, and 2) alloys shift to the lower temperature, as shown in Fig. 4(d). The rate of A_s shifted by magnetic field, i.e., dB, is −1.4, −1.7, and for the x = 0, 1, and 2 samples, respectively, confirming the field-induced magnetostructural transformation. The enhanced magnetic field driving capacity with the V-substitution in Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, and 2) alloys can be attributed to the larger magnetization difference. It is also well known that the maximum value of magnetic entropy change ( can be estimated by M–T curves measured at different constant fields. We calculated the value for Mn₄₆V₂Ni₄₂Sn₁₀ alloy using Clausius–Clapeyron equation ( from M–T curves at 3 and 5 T, as shown in Fig. 4(d). The calculated maximum value of is (using Am²/kg, T, and K), and it is in a good agreement with that obtained from M–B curves using Maxwell relations.

According to the M–B curves, the during the magnetic-field-induced structural transformation is calculated by Maxwell relation

Fig. 5.

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	Fig. 5. (color online) The temperature dependence of under an applied magnetic field of 2 and 5 T for Mn48−xVxNi42Sn10 (x = 0, 1, and 2) alloys measured upon heating. Inset: HL of Mn46V2Ni42Sn10 alloy.

The effective refrigeration capacity (RC_eff) of Mn_48−xV_xNi₄₂Sn₁₀ (x= 0, 1, and 2) alloys is calculated by subtracting the average hysteresis loss (HL) from the refrigeration capacity (RC) values. Here, the value of RC is obtained by integrating the area under –T curves using the temperature at half maximum of the peak as the integration limits.^[38] The average HL is calculated by the method mentioned in Ref. [45]. The temperature dependence of HL for Mn₄₆V₂Ni₄₂Sn₁₀ alloy is shown as the inset of Fig. 5. The values of RC_eff around the temperature of martensitic transformation under the field change of 0–5 T are 37.21, 75.4, and for x = 0, 1, and 2, respectively. Consequently, the substitution of V for Mn leads to an increase in the magnetization jump at the magnetostructural transition, the enhanced magnetic field driving capacity (d dB), and effective refrigeration capacity (RC_eff).

The magnetostructural transformation and the coupled magnetocaloric properties for Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) ferromagnetic shape memory alloys are investigated using both structural and magnetization measurements. By replacing Mn with V, the martensitic transformation temperature can be tuned within a wide temperature range. The magnetic-field-induced magnetostructural transformation is confirmed by the high field M–T curves and isothermal magnetization curves. The observed peak values of under an applied magnetic field of 5 T for x = 0, 1, and 2 are 15.2, 18.8, and , respectively. Large values of , enhanced field driving capacity, and tunable martensitic transformation temperatures suggest that Mn_48−xV_xNi₄₂Sn₁₀ (x = 0, 1, 2, and 3) alloys are promising candidates for magnetic refrigeration applications.