In recent decades, magnetic cooling refrigerators based on the magnetocaloric effect (MCE) have received considerable attention. Compared with the traditional gas compression for refrigeration, magnetic cooling offers advantages in terms of low cost, safety, size, low mechanical vibration, high cooling efficiency, and environmental friendliness.^[1–4] The application of magnetic refrigeration is strongly dependent on the MCE of materials. The large MCE observed in coupled materials has led to the widespread investigation of compounds that exhibit a magnetic first-order transition, e.g., LaFe Si _x ,^[5] MnFeP As ,^[6] Gd₅Si Ge _x ,^[7] and especially Ni-Mn-based ferromagnetic shape memory alloys.^[8,9]

Recent experimental and theoretical work demonstrates that the MCE in Ni-Mn-based magnetic shape memory alloys is related to the thermal and magnetic-field-induced magnetostructural transformation (MT), which can be tuned by changing the valence electron concentration ( . The MT and coupled MCE have been widely investigated in Ni-rich Ni-Mn-based Heusler alloys, such as Ni₅₀Mn₃₇Sn₁₃,^[10] Ni₄₃Co₇Mn₃₉Sn₁₁,^[11] Ni₅₀Mn Sb B _x ,^[12] Ni₄₄Co Mn In ,^[13] Ni Cu _x Mn₃₁Ga₁₉,^[14] and Ni Co _x Mn Al .^[15] The MT and MCE are also observed in high-Mn-content Mn₂NiX ( , Sn, and Ga) inverse Heusler alloys.^[16–18] It is known that Mn₅₀Ni₄₀In₁₀ alloy undergoes an MT from ferromagnetic austenite to weak magnetic martensite during cooling, and Co-doping leads to an increase in the magnetization difference ( between the two phases.^[17,18]

A similar transition is observed in Mn₅₀Ni₄₀Sn₁₀ alloy. The MT in Mn₅₀Ni₄₀Sn₁₀ alloy is very sensitive to the p-d hybridization, which can be influenced by atomic substitution, resulting in a tunable MCE.^[19,20] Attempts have been made to tune the MT through the partial substitution of Ni by Cu or Co in Mn₅₀Ni₄₀Sn₁₀ alloys.^[21,22] However, the influence of sp-elements like antimony (Sb) on the MT and magnetic properties of Mn-rich Mn₂NiSn inverse Heusler alloys has not yet been reported. In the current work, Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys are prepared by arc-melting. The effects of Sb-substitution on the structure, magnetic phase transition, and MCE around the MT are investigated and discussed in detail. Our results reveal that the MCE and transition temperature of Mn₅₀Ni₄₀Sn Sb _x alloys are strongly dependent on the Sb concentration, and demonstrates that Sb-doped Mn₂NiSn alloys are competent candidates for refrigeration applications.

A series of polycrystalline Mn-rich Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys were prepared by arc-melting the appropriate amounts of high-purity Mn, Ni, Sn, and Sb (99.95 wt.% purity) in an argon atmosphere. The samples were re-melted three times to enhance their homogeneity. The resultant samples were then sealed in vacuum quartz tubes and annealed at 1073 K for three days before being quenched in cold water. The martensitic transformations in the Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys were identified by differential scanning calorimetry (DSC) in the temperature range 150–350 K with a cooling and heating rate of 10 K/min. The crystal structure was investigated by x-ray powder diffraction (XRD) using Cu- radiation at room temperature (RT). The target compositions were confirmed through x-ray energy dispersive spectrometry (EDS), and the results are presented in Table 1. The magnetic measurements were carried out on a physical property measurement system (PPMS).

Table 1.

Table 1.

Table 1. The measured compositions and structural parameters of Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys. . x Mn/at.% Ni/at.% Sn/at.% Sb/at.% a/Å Unit cell volume/Å3 1 50.82 39.06 9.20 0.92 5.998 215.784 2 51.10 38.78 8.10 2.02 5.997 215.676 3 50.72 39.41 6.82 3.05 5.993 215.245 4 50.21 39.82 6.04 3.93 5.990 214.922

Table 1. The measured compositions and structural parameters of Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys. .

The RT XRD patterns of the Mn-rich Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys are shown in Fig. 1. The presence of (220), (400), and (422) peaks indicates that the Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys exhibit a Heusler-type B2 structure with the space group at RT. Using the XRD data, the lattice parameter a and unit cell volume were calculated (listed in Table 1). Because the atomic size of Sb ( Å) is smaller than that of Sn ( Å), both a and unit cell volume decrease with increasing Sb content.

Fig. 1.

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	Fig. 1. (color online) XRD patterns of Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys at room temperature.

The thermal-induced structural transformations in the Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys were investigated by DSC in heating and cooling cycles. As shown in Fig. 2, large endothermic and exothermic peaks are observed, corresponding to the occurrence of the first-order structural transformation between martensite and austenite. As x increases from 1 to 4, the endothermic and exothermic peaks in the heating and cooling processes increase by 37 K and 38 K, respectively. It has been reported that the structural transformation temperature in Ni–Mn-based magnetic shape memory alloys is strongly related to e/a. The calculated e/a for the Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys are given in Table 2. Here, the 3d and 4s electrons are considered as valence electrons in transition metals, and the 5s and 5p electrons are considered to be valence electrons in main-group elements. Because Sn has fewer 5p electrons than Sb, e/a increases with the Sb content. With the increase in e/a, the structural transformation shifts to higher temperatures. A similar relation between the structural transformation temperature and e/a has been observed in other Ni–Mn-based magnetic shape memory alloys.^[22–25] Additionally, previous reports on Ge-doped NiMnSn and Ga-doped NiMnIn alloys indicate that the structural transformation temperature increases as the unit cell volume decreases.^[26,27] In the present study, the Sb substitution causes the lattice to shrink, which may enhance the hybridization between Ni and Mn/Sn, leading to an increase in the structural transformation temperature.^[28]

Fig. 2.

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	Fig. 2. (color online) DSC curves of Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys.

Table 2.

Table 2.

Table 2. The martensitic start temperature ( , martensitic finish temperature ( , austenitic start temperature ( , austenitic finish temperature ( , and valence electron concentration ( for Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys. . x /K /K /K /K e/a 1 207.8 193.4 206.8 220.7 7.877 2 215.8 197.6 213.8 230.8 7.882 3 228.3 212.1 232.3 243.9 7.916 4 242.1 230.9 244.9 254.1 7.935

Table 2. The martensitic start temperature ( , martensitic finish temperature ( , austenitic start temperature ( , austenitic finish temperature ( , and valence electron concentration ( for Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys. .

Figure 3 shows the temperature dependence of magnetization ( for the samples under an applied magnetic field of 0.01 T in the heating and cooling processes. In the heating process, the Mn₅₀Ni₄₀Sn₉Sb₁ alloy experiences the magnetic order–disorder transition of martensite at approximately 170 K, followed by a sharp MT to ferromagnetic austenite at around 198 K. With further heating to 313 K, the magnetization in this sample gradually decreases to zero, corresponding to the Curie temperature of austenite. An obvious irreversibility between the heating and cooling curves is observed during the MT, indicating the first-order nature of this transition. The characteristic temperatures of the MT, austenitic start temperature , austenitic finish temperature , martensitic start temperature , and martensitic finish temperature , are listed in Table 2. As compared in Table 2 and Fig. 3, the MT shifts to higher temperatures asxincreases, which agrees with the DSC measurements. As the transformation temperature is tuned to be close to the Curie temperature of austenite, the magnetization of austenite decreases remarkably with Sb substitution, leading to low values of .

Fig. 3.

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	Fig. 3. (color online) M − Tcurves for Mn50Ni40Sn Sb x alloys: (a) x = 1, (b) x = 2, (c) x = 3, and (d) x = 4, under an applied magnetic field of 0.01 T during the heating and cooling processes.

To study the magnetic properties around MT in more detail, the isothermal magnetization (M–B) was measured by the so-called loop method.^[29] Before applying the magnetic field, the samples were cooled to full martensitic state, then slowly heated to the measurement temperature with a ramp rate of 3 K/min. As shown in Fig. 4, the metamagnetic behavior can be observed when the temperature is close to , which corresponds to the magnetic field-induced MT from martensite to austenite. The existence of magnetic hysteresis indicates the first-order nature of the transition. The observed magnetic field-induced MT is related to between martensite and austenite.^[30] For x = 1, 2, 3, and 4, the values of under 5 T are 42 A·m²/kg, 40 A·m²/kg, 26 A·m²/kg, and 28 A·m²/kg, respectively. The decrease of leads to the reduced magnetic field driving capacity in Mn₅₀Ni₄₀Sn₆Sb₄ alloy.

Fig. 4.

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	Fig. 4. (color online) M–B curves for Mn50Ni40Sn Sb x alloys: (a) x = 1, (b) x = 2, (c) x = 3, and (d) x = 4.

According to the M–B curves, the isothermal magnetic entropy change ( as a function of temperature was estimated under different magnetic field variations for Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys using Maxwell's equation:^[31]

(1)

Fig. 5.

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	Fig. 5. (color online) Temperature dependent for Mn50Ni40Sn Sb x (x = 1, 2, 3, and 4) alloys.

In summary, we investigated the effect of Sb substitution on the martensitic transformation and magnetocaloric properties of Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys. The Mn-rich Mn₅₀Ni₄₀Sn Sb _x (x = 1, 2, 3, and 4) alloys have a B2-type structure at RT. As the Sb content increases, the MT shifts to higher temperatures. The magnetization difference between martensite and austenite means that the MT can be induced by an applied magnetic field. Obvious metamagnetic behavior can be observed in the M–B curves. The coupled maximum values with a magnetic field variation of 0–5 T are 19.16 J·kg K , 17.29 J·kg K , 12.63 J·kg K , and 8.65 J·kg K for x = 1, 2, 3, and 4, respectively. The tunable MCE indicates the potential for promising applications as a magnetic cooling refrigerant.